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History of Nutrition

A Short History of Nutritional Science: Part 4 (1945–1985)¹

Department of Nutritional Sciences, University of California, Berkeley, CA 94720-3104

² To whom correspondence should be addressed. E-mail: kcarp{at}uclink.berkeley.edu.

Because of the huge number of papers published from 1945 to 1985 (250,000 were abstracted in Nutrition Abstracts and Reviews), I have restricted coverage in the first section of this paper to those dealing most directly with the discovery of new nutrients, the effects of deficiency and the interaction of other factors affecting their availability. This omits biochemical mechanisms of absorption and function except where they directly affect nutritional requirements. This is an important restriction because so many interesting mechanisms were worked out during this period, for example, the conversion of vitamin D to an active hormone and the role of vitamin A in the visual cycle. But space is a limiting factor here.

Less than one in a thousand of the papers published can be listed here, with the selection inevitably influenced (and therefore biased) by the author’s greater exposure to some areas than to others. Some senior scientists may therefore see that their work in this period has not received the attention that it deserves. For this I can only apologize. I have also cited reviews as well as original papers, especially where several groups have contributed to a particular development.

So much was discovered in the period from 1912 to 1944, covered in Part 3 (3), that it was not surprising that some people thought that the subject had been exhausted. Oxford University closed its nutrition group soon after World War II with that justification. In fact, important findings were still to come from the lines of research that were already in progress, which we can classify roughly as the discovery of new nutrients, to be reviewed first. But there was also a whole new approach to nutritional questions that could be described as "A critique of the affluent diet" and "more not always being better," and this proved to be of great practical importance for the adult population as well as in stimulating new lines of research.

	Vitamins

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
Folic acid.

As described in Part 3, it had been found by 1944 that the material named "folic acid" was a vitamin required by monkeys and chickens to prevent macrocytic anemia and was also a growth factor for some bacteria. It was then discovered that other bacteria could synthesize the factor, and this was made use of in the preparation of larger quantities for the study of its chemistry. In 1946 it was identified by Robert Stokstad and colleagues at the Lederle Laboratories in the U.S. as N-[(6-pteridinyl)-methyl]-p-aminobenzoic acid conjugated with one or more L-glutamic acid residues (4). This nomenclature was simplified to pteroyl mono (or poly) glutamate. After absorption it was reduced to tetrahydrofolic acid (TH₄F)² and also methylated (MeTH₄F). Humans and animals can use all these forms, with intestinal enzymes removing excess glutamates before absorption, but some microorganisms require the monoglutamate specifically (5). This work involved many people, including an active group at the Parke Davis Company.

Folic acid, in relatively large doses and given either by mouth or injection, was also found to be active in stimulating red cell production in pernicious anemia patients, although it was realized that it could not be the active factor in the liver extracts being used for the same purpose. However, by 1948, it had also been found that patients treated in this way for several months typically began to show neurological disturbances that responded to liver extracts (6). We will return to this puzzle in the following section.

In general, young rats would thrive without any source of folic acid in their diet, but, when a relatively insoluble sulfonamide drug was added to their diet, they rapidly developed leucopenia (a shortage of white cells in the blood) and this condition did respond to folic acid. It was determined that the sulfonamide structure selectively inhibited the introduction of the rather similar p-aminobenzoic acid into the molecules of folic acid that were normally being synthesized by bacteria in the large intestine, so that the rat no longer obtained its requirement of the vitamin by coprophagy (feces eating) (7).

Making further use of the concept of the antagonism between similar molecules, analogues of folic acid were synthesized and several, but particularly methotrexate (4-amino-10-methyl folic acid), by 1950 were found to antagonize the functions of folic acid in animal tissues, and to be useful in the chemotherapy of cancer in which the rapid growth of tumors needed to be inhibited (7).

Another line of interest in folic acid came from observations in the UK indicating an unusually large proportion of mothers whose babies were born with neural tube defects (spina bifida, etc.) seemed to come from lower income groups. Obviously, this could have a multitude of causes. One of the first suggestions receiving follow-up was that a high level of potato consumption might increase the risk (8). Another was that only a small proportion of the mothers had been taking vitamin supplements before or during their pregnancy.

In view of the finding of malformations in the embryos of rats deficient in folic acid, there was a study of the folic acid status of mothers who had been pregnant with malformed embryos, compared with that of mothers who had delivered healthy babies. Of 35 mothers who had infants with malformations of the central nervous system, 24 were judged to be folate deficient, whereas of the same number of otherwise similar mothers with healthy babies, only 6 were judged deficient by the FIGLU test, an important difference (9). Studies were then set up with women who had already had a malformed baby, because they were known to have a higher risk of its recurrence. Women who volunteered to join the study were given a multivitamin supplement that included folic acid before their next conception. The timing was important because neural tube defects occur within the first 21 d of conception, before a woman would normally know that she is pregnant. The results from the trial (Table 1) were that the incidence of malformed embryos was 3.6 times greater among women who had not taken the multivitamin supplement than in those who had, i.e., 4.2% compared with 1.2% (10).

Vitamin B-12.

As stated previously, it was realized that folic acid, although it produced a response in pernicious anemia patients, was not the active factor in the increasingly concentrated liver extract preparations being used for this condition. It had also been known for many years that sufferers from the disease lacked an "intrinsic factor" secreted into the stomach that normally in some way activated an "extrinsic factor" present in meat and particularly in liver. This latter factor was still effective if given to patients by injection, but only to a very slight extent if given by mouth (11).

No animal model had been discovered that could be used to assay for the activity of liver extracts. So every fraction had to be tested by measuring the response in the blood picture of a number of pernicious anemia patients. However, in 1948 a group at the Merck Laboratories in New York State announced the successful isolation of the "liver factor." This had been aided by assays for a microbial growth factor that they had correctly guessed would turn out to be the same thing, as well as by the color of the more potent extracts. They named their crystals "vitamin B-12" (12). Another group in the UK followed them a few weeks later, having also guessed that the increasingly pink color of their more potent extracts could serve as a guide (13). Analysis of the crystals gave a sensational result—the presence of the trace metal cobalt! The vitamin then received the alternative name "cobalamin." It also proved active in extraordinarily small quantities. Even 5 µg was found to give a response in patients and it was realized that some of the injectable liver concentrates in use had contained only one part of the vitamin in a million.

Microbiological assays indicated that no plants synthesized the vitamin, only certain microorganisms (14,15). This in itself was something entirely new. Up until that time, it appeared that the grand scheme of nature on Earth was that the Animal Kingdom as a whole lived on the Plant Kingdom, that could utilize solar irradiation as the energy source for its syntheses. The only known role for microorganisms in the grand system was that they broke down dead tissues from both kingdoms and prepared them for reuse by plants. But here, for the first time, animals were found to require something that they could only obtain from microbial synthesis.

As described in Part 3, cobalt had already been found to be required by cattle, but it had not been proven possible to obtain a response to cobalt in the usual small animal species, rats or chicks. A lot of things now began to come together. The organisms living in the rumen (or forestomach) of cattle and of other ruminants such as sheep were determined to be particularly active. This explained why these animals benefited from a supply of inorganic cobalt salts; the fermentation occurred ahead of the ordinary system for digestion and absorption in the true stomach and small intestine, and this made the microbial cobalamin available for transfer to the tissues of the host animal.

In poultry, on the other hand, it appeared that the cobalamin produced by fermentation in the large intestine was not absorbed into the blood stream. It had already been found that intensively farmed chickens would not thrive indefinitely on a purely plant diet, and there had been studies of their need for an "animal protein factor" known to be supplied by fish meal, but also by dried cow manure. However, chicks hatched from eggs laid by a hen well provided with cobalamin could survive on stores for a long period, and it was only the next generation that showed severe deficiency. Poultry, and also pigs, kept outdoors appeared to obtain sufficient cobalamin from soil organisms and worms (16).

It appeared that rats too were unable to absorb the cobalamin produced by organisms in their large intestine. However, they managed to indulge in coprophagy even when housed on raised wire screens and they could obtain some vitamin in this way, though not always enough for maximum growth (17,18). Monogastric herbivores, like the horse or elephant, have hindgut fermentation that adds to the energy that they can extract from feed. It appeared that significant quantities of cobalamin were produced by this fermentation, and it was found that labeled cobalamin injected into a horse’s cecum was absorbed into the blood stream (19,20). Humans also have some cobalamin production in the cecum but it is unclear as to whether or not any significant quantity is absorbed. Vegans, who obtain none from their food, have low levels of the vitamin in their blood, but some have remained healthy for a decade or more, despite declaring that they have remained on a strictly vegan diet (21).

There are enough cobalamin-synthesizing microorganisms in the sea to account for the presence of this vitamin in the zoo-systems living in water, with the larger fish living on smaller organisms right down to the microscopic level (22).

Pernicious anemia.

Now we return to the puzzle of why two such different molecules as folate and cobalamin should both reverse pernicious anemia. It was soon realized that only cobalamin prevents the slower development of neurological complications with degeneration of the spinal cord in pernicious anemia patients and most vegans (6). Because of this, folic acid supplements were seen as a possible danger in that they hid the existence of a vitamin B-12 deficiency.

It was reported in 1962 that in vitamin B-12-deficient patients, folic acid accumulated in the blood in the methylated form, MeTH₄F (23). Further biochemical studies led to the concept that cobalamin had an essential role in allowing MeTH₄F to transfer its methyl group to a second homocysteine-methionine cycle, so that its deficiency led essentially to a functional folic acid deficiency unless larger quantities of folic acid were supplied in the diet (24). The transferred methyl groups were normally used through a succession of cycles for the synthesis of DNA components needed for the continual production of blood cells.

Another effect of the deficiency of either vitamin is a rise in the concentration of homocysteine in the blood. This has been another matter of concern because people with very high levels of this compound in the blood as result of a genetic effect are particularly subject to early onset arteriosclerosis (25). This has become an active field of research, but almost entirely with publications after 1985.

Pellagra and niacin.

By 1945 there were already reasons for thinking that pellagra might not be the result of a straightforward deficiency of niacin. Using a newly developed method of analysis for niacin, it had been found that poor rice diets from India (where pellagra was not a problem) contained less niacin than Rumanian corn-based diets, where it was a problem (26,27). In addition, rats fed a standard purified diet with 15% casein as the protein source thrived without any source of the vitamin and actually excreted niacin and a methylated derivative in their urine.

It was then found that rats would become niacin deficient if their purified diet was diluted with 40% corn meal, the food that had traditionally been associated with the disease in Europe. Furthermore, growth could be restored if the diet was supplemented with 0.05% of the amino acid tryptophan that is present at a lower level in corn than in other grains (28). It was then shown that tryptophan could also be used to treat humans afflicted with pellagra, with some 3% of the amino acids being converted to niacin under the conditions of one trial (29). Cats showed no conversion, and dogs much less than rats because they would become niacin deficient on diets containing relatively well balanced protein (30).

At first it was thought that the additional tryptophan might be stimulating microbial synthesis of niacin in the small intestine, but later it was found by isotopic labeling that there was actually an enzymic route for the conversion of a portion of the tryptophan molecules to niacin (31).

There are still problems in understanding pellagra. No animal model of niacin deficiency has shown the sun-induced areas of dermatitis seen in pellagrins. It is now realized that Goldberger’s own trial with convicts had actually produced riboflavin deficiency and a later attempt to induce niacin deficiency specifically in men failed (32). There is also the question of why pellagra has not been a traditional problem in Mexico where corn has been the staple, with little in the way of animal foods for the poorest section of the population. Evidence, mainly with experimental animals, has indicated that niacin is released from a nutritionally unavailable bound form during the alkaline treatment of corn during the traditional preparation of tortillas, and this contributes at least some additional niacin to the diet (33–35).

	Minerals

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
Factor 3.

Several lines of work had been leading to the idea that there might be one more vitamin to be discovered. It had been realized that alcoholics suffering from cirrhosis of the liver typically had very imbalanced diets of low protein content and were greatly helped by having their diet improved. This led to studies with rats of the effect of such diets on their livers even in the absence of alcohol, and by 1944 it had been found that supplementation with methionine was effective against the liver necrosis produced with such diets (36,37). At the same time, Klaus Schwarz at Heidelberg University was hoping to find a new vitamin by working with such rat diets. His protein source was casein purified by boiling in mild alkali and reprecipitating with acid. He found that liver necrosis was produced only with casein pretreated in this way, and that it could be prevented with wheat germ and then with vitamin E specifically (38). It is surprising to think of Schwarz being able to carry on this basic research in the final years of World War II when Germany was being so heavily bombed, but Heidelberg was spared by the Allied Air Forces, as were Oxford and Cambridge by the Luftwaffe.

After the war, Schwarz moved to the U.S. and continued his work at NIH. There he found that diets based on yeast as the protein source failed to induce liver necrosis, although they had done so in Germany. In 1951 he reported that this was explained, not by the varieties of yeast in use, but by the medium on which they were grown. Corn steep liquor consumed in the U.S. seemed to be supplying a protective factor not present in the sulfite liquors consumed in Europe (39). In his next paper he went back to his earlier experience in which treatment with alkali had caused casein to lose its protective factor that he now called "Factor 3," because it could not have been either vitamin E or methionine, and he described how he had been able to make concentrates from casein fractions (40).

I myself learned of this puzzle through the frustrations of an older friend and colleague at the Rowett Institute in Scotland. He had been studying environmental conditions that affected whether or not rats would actually develop necrosis on a supposedly necrogenic casein-based diet, but suddenly he could no longer produce the condition. One possible explanation was that he had to change his source of casein, because the original company was no longer marketing a vitamin-free grade. After many trials this was found to be the factor, independent of the degree of purification of the casein (41). All he knew was that the first company had obtained its crude casein from New Zealand and the second from Europe. He seemed to be getting nowhere, and this very experienced pathologist abandoned his research career and moved abroad to a routine clinical position. With the benefit of hindsight, we can see that if his friend had been a soil scientist, "New Zealand" might have rung a bell and led to his making the decisive breakthrough.

In fact, the breakthrough came three years later from Schwarz himself and from the Lederle research group, both of whom discovered in 1957 that Factor 3 contained the element selenium and that its activity could be replaced by sodium selenite (42,43).

Selenium.

Up to this time, selenium was known only as a toxic element that made it impossible to graze animals on areas where the soil, and thus the plants growing on it, were particularly rich in it (44). It was also suspected of being a carcinogen, so that it was illegal to add it to animal feed; but it now appeared likely that this element was a hitherto unrecognized essential trace element. It had been found to prevent liver necrosis in rats and exudative diathesis in chicks, both of which were already known to be preventable with vitamin E, but in the form of sodium selenite, it was 500 times as active as the vitamin (42,43). This was reminiscent of the puzzle over rickets being prevented by such different treatments as irradiation and dosing with cod liver oil.

Although the disorders in rat and chicks had been induced with special diets designed to be deficient, it was reported from New Zealand, where volcanic pumice soils are particularly low in selenium, that specific diseases in sheep, cattle, pigs, horses and poultry on farms were all responding to supplementation with selenite (45). "White muscle disease" in lambs and "hepatosis dietetica" in pigs, which were practical problems in many parts of Scandinavia and the U.S., also responded to supplementation of their rations with 0.1 µg/g of sodium selenite (46). Most of these conditions also responded to dosing with vitamin E, but usually to a lesser extent (47,48). That both might be having an antioxidant role seemed more likely when selenium was found to be an essential component of the enzyme glutathione peroxidase (49,50)

A group at Cornell carried out the first chick bioassays of the relative potency of selenium in different forms and foods, but results were found to differ according to the procedure used (51).

The next question was whether humans were also at risk from selenium deficiency. This was first studied in New Zealand, but no clear evidence of a problem was obtained (52). However, in the Keshan area of China, where selenium levels are equally low, a characteristic cardiac myopathy (Keshan disease) was seen that appeared to respond to selenium supplements, but was not a simple cause and effect. To examine this fascinating problem, limited to the work done by 1985, could occupy this whole article, but good reviews are available (52,53).

Chromium.

During their study of Factor 3, Klaus Schwarz and Walter Mertz discovered that rats fed some of their experimental diets showed an impaired glucose tolerance that was not reversed with concentrates of Factor 3 (54). Eventually the deficiency was found to be reversed with trivalent chromium that appeared to act as a cofactor with insulin (55). Hexavalent chromium was inactive.

Zinc.

In the period covered in Part 3 it had been difficult to produce an experimental zinc deficiency. Rats and mice appeared to need only about 1 µg/g of zinc in their purified diets. However, in the 1950s it was discovered that parakeratosis, a fairly common problem in growing pigs characterized by dermatitis, diarrhea and anorexia, was the result of a zinc deficiency (Fig. 1) (56). This was despite their diets containing some 40 µg/g of the element. It appeared to occur when plant protein concentrates were being used in the rations and particularly when high levels of either bone meal or calcium carbonate were also included; but when the level of zinc was doubled, growth and health were excellent (57).

View larger version (146K): [in this window] [in a new window]   FIGURE 1 Pigs with parakeratosis before supplementation (3), and 8 wk after their rations had been supplemented with 0.02% zinc carbonate (4,56). (By permission of the Society for Experimental Biology and Medicine)

In the 1960s, Anasta Prasad and colleagues, under a grant to Vanderbilt University, began the investigation of dwarfing and hypogonadism among teenage boys in Egypt and found that, in addition to their anemia being corrected with iron salts, they had low levels of plasma zinc (59). They then found that supplementation with zinc salts stimulated growth and maturity (60). Two further trials failed to give clear-cut results, but positive findings were observed from a controlled study of a comparable group in Iran using a higher level (40 mg/d) of supplemental zinc and with both control and supplemented groups receiving a range of other trace minerals (Table 2) (61).

Bioavailability.

It had become clear with zinc, as in the previous sections dealing with niacin and selenium, that simple analysis for the total concentration of a nutrient in the diet was not a sufficient measure of its adequacy, and that one had to face the more difficult question of its bioavailability (64). Evidence had mounted that increasing the intake of one mineral could result in impaired absorption of others (65). Also, for selenium, chromium and niacin (as well as other B- vitamins), availability was influenced by the chemical forms or combinations in which the nutrient was present.

It was found that copper-deficient animals failed to synthesize normal elastin, with the result that their arterial walls were greatly weakened (66). It has been suggested that many human diets, as a consequence of their high zinc:copper ratios, have a relative copper deficiency and that this could be a factor contributing to the high incidence of ischemic heart disease (67).

	Protein

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
Amino acid patterns.

I have not tried to cover work designed to measure the quantitative requirements for individual nutrients; however, protein is not in this category. No two proteins are identical, nor is the mix of proteins from one food identical to that from another. Therefore, the question remained as to how closely the amino acid pattern of our food needed to match that of our body proteins, which in practice related to the extent to which either animal protein or synthetic amino acids were needed to balance vegetable proteins for a diet to be ideal.

Workers hoped to overcome this problem by stating requirements in terms of individual amino acids, but in 1945 it had not been demonstrated that mixtures of amino acids could completely replace protein in the human diet. William Rose and his colleagues at the University of Illinois had been working to resolve this since 1942 and they reported their findings from 1948 to 1955 (Table 3) (68–70), with a final discussion in 1957 (71).

A group at the Massachusetts Institute of Technology suggested that, although people on relatively low protein intakes were in nitrogen balance, their equilibrium might be at the expense of lower rates of protein turnover and of potential synthesis of antibodies when exposed to infection (75). This was an important question and by 1985 procedures were being developed for its measurement using turnover and oxidation studies with isotope-labeled amino acids (76). However, no definitive answer had been obtained and another worker recommended caution in justifying the need for increased protein on the basis of such measurements (77).

It was also demonstrated that the requirement of growing chicks for the limiting essential amino acid, lysine, was increased from 0.85% of the diet to 1.1% when the total level of protein in the diet was raised from 20% to 30% (78). Alfred Harper and colleagues at Wisconsin then demonstrated that adding a single amino acid at a fairly high level, for example 2% L-histidine to a diet containing 12% casein (plus methionine) that supported rapid weight gains (56 g in 9 d) in young rats, could inhibit their appetite and performance (in this example to 45 g) (79). Attempts have been made to divide effects of this general kind into toxicities, antagonisms and imbalances (80). However, there was no evidence that they were likely to occur in practice with humans. One possible concern was that high protein Western diets might be causing an acidosis that resulted in a compensating loss of calcium, and thus of bone (81).

The world protein problem.

The period had begun therefore with studies indicating that the supply of protein, at least for diets based on cereals, was not a problem (82). Nevertheless, in 1960, a senior nutritionist said, "We have moved from the era of vitamin research to protein research," and the head of the Nutrition Division of FAO (the Food and Agriculture Organization of the United Nations) wrote that, "deficiency of protein in the diet is the most serious and widespread problem in the world" (83–85).

This idea grew from the finding that a serious disease, called "kwashiorkor" in West Africa and recognized by flaky dermatitis, hair changes, edema and apathy, was also common among 1–4-y-old children in other parts of the developing world (Fig. 2) (86). It was found to respond to concentrated relatively high protein nutritional supplements such as skim milk powder, and the previous idea that it was an infantile form of pellagra was abandoned because it did not respond to nicotinic acid or other B-vitamins (87).

View larger version (138K): [in this window] [in a new window]   FIGURE 2 Kwashiorkor characterized by edema of the face (left), and by skin lesions on the back (right). (Courtesy of Dr. R. G. Whitehead)

Much work was carried out in areas where the problem existed, for example at the Institute for Nutrition in Central America and Panama, to develop and test cheaper alternatives to milk powder based on locally available cereals and oilseed flours. These could prevent the condition from developing and also cure it, although not quite as quickly as with milk powder (89). Individual babies could also be deficient in electrolytes and vitamins as well as in protein and energy (90). Others suggested that essential fatty acids might also be deficient (91,92).

In 1968 the United Nations published a paper entitled International Action to Avert the Impending Protein Crisis (93). By then, several projects had been set up, with substantial funding (some from governments and foundations), to develop processes and machinery in advanced countries for the preparation of stable, solvent-extracted high protein powders from fish [fish protein concentrate (FPC)] and other materials. This was encouraged by enthusiastic international conferences, even though the original idea had been to devise new crops or simple methods of food processing that could be adopted in underdeveloped villages. In addition, even more sophisticated processes were being planned to produce "single cell protein" (SCP) from yeasts, fungi and bacteria, grown on media ranging from molasses waste to petroleum (94). Doris Calloway drew attention to the poor tolerance and even toxicity of some SCP materials, and that their high content of nucleic acids was also a problem for humans, who metabolize purines only to the relatively insoluble uric acid, so that they were more suited for animal species that do not have this problem (95).

"Hi-tech" projects, which were supposed to be aiding relatively primitive communities, received particularly bitter criticism from the faculty at the London School of Hygiene and Tropical Medicine who referred to "a continuing process of justifying scientific enthusiasms by the drawing of facile and tenuous links between research which is intellectually exciting to the investigator and problems which are of sufficient public concern to make it politically attractive to devote funds to them" (96). Even in the U.S., where scientists are usually less willing to risk giving offense, the leader of the government-financed FPC project was to say later (in a book worth reading) that, "Much of the motivation for FPC development had little or nothing to do with the ostensible and well-publicized humanitarian goal" (97).

This is an episode in our history that nutritional scientists would probably like to forget, but one use of history is to learn from our mistakes and to not repeat them. It was brought to an end by the realization that most kwashiorkor victims had been receiving diets that were as deficient in energy as they were in protein, and too bulky for the youngsters to take in sufficient amounts (98). The general need was to provide more concentrated foods and correct electrolyte deficiencies rather than concentrate just on protein (99–101). It was especially difficult to improve diets based on roots such as cassava that were very bulky as well as low in protein (102). The United Nations Organization, which had previously emphasized its concerns about "a world protein problem," made no mention of it at its 1974 World Food Conference (103).

The synthetic production of essential amino acids likely to be first-limiting in Third World diets was also stimulated in the 1960s (104). Although results with human trials were generally disappointing, these compounds have found practical uses in intensive pig and poultry feeding (105).

	Essential fatty acids

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
Linoleic acid.

As described in Part 3 (3) it was already known that rats would show scabby tails, have excessive water loss and fail to breed successfully if reared fed a diet lacking the polyunsaturated linoleic acid 18:2(n-6). The symbol ‘n’, or ‘’ in the earlier literature, refers to the number of carbon atoms, counting from the hydrocarbon end of the molecule, before an unsaturated bond is reached.

The next obvious question was whether humans had a similar requirement. In 1963, as the result of a large study of over 400 infants fed formulas for 6 mo differing in fat content, it was concluded that children receiving <0.1% of their energy as linoleic acid developed dry thickened skin and in many cases showed unsatisfactory growth. When more linoleic acid was provided, the problems promptly disappeared (106). The same signs of deficiency were seen after quite short periods in infants that had to be fed parenterally and were not receiving lipid (Fig. 3) (107). In one case, rubbing the skin with oil rich in linoleic acid proved sufficient to correct the condition (108).

View larger version (147K): [in this window] [in a new window]   FIGURE 3 The foot of an infant suffering from a deficiency of linoleic acid as a consequence of fat-free intravenous alimentation (107). (By permission of the American Journal of Clinical Nutrition)

When animals were limited in the supply of linoleic acid there was a correspondingly larger synthesis of eicosotrienoic acid 20:3(n-9) from the nonessential oleic acid 18:1(n-9) (110,111). The eicosotrienoic:arachidonic ratio in the blood was found to be an indicator of marginal linoleic acid deficiency, even before the appearance of clinical symptoms, in rats and pigs, and then in humans (106,112,113).

Cats gave quite different results. They were found to require arachidonic acid in their diet and to lack the enzymes needed to lengthen and desaturate linoleic acid (114). This was analogous to their inability to use carotene as a source of retinol. As carnivores they can, of course, obtain both preformed retinol and arachidonic acids from the animal tissues that they consume and do not have to produce the molecules required by the Animal Kingdom from a precursor found in plants. This seemed to suggest that linoleic acid served for other species only as a precursor for arachidonic acid. However, later work demonstrated that the waterproofing effect of linoleic acid on rat skin was caused by epidermal sphingolipids incorporating linoleic acid as such (115).

Alpha-linolenic acid.

Because the gamma molecule 18:3(n-6) is also considered a linolenic acid, one should perhaps always use the prefix alpha for the 18:3(n-3) form, but in practice it is assumed that is what is meant by linolenic acid. In the Burrs’ pioneering work with rats it appeared that linolenic acid could fully replace linoleic acid, but later workers, perhaps using purer preparations, did not find this to be the case (116,117). In a critical trial lasting three generations, rats grew well and appeared to behave normally with linoleic, but without linolenic acid (118). However, it was noticed that the rats strongly retained docosohexanoic acid 22:6(n-3) that was formed from linolenic acid, and others found it to be particularly concentrated in the outer segments of retina rods (119). In contrast, there was no doubt that fish required n-3 acids for normal growth (120).

In 1982 it was reported that a 6-y-old girl, who had for several years been receiving only parenteral feeding with a formula containing very little linolenic acid, began to show neurological abnormalities including blurred vision. These disappeared when the formula was changed to include more linolenic acid (121). When rhesus monkeys were fed mixes free of linolenic acid and their babies received the same mix, it was observed that at 12 wk of age they responded poorly in a visual acuity test of preferential looking compared with controls (122). It seemed reasonable to conclude therefore that linolenic acid, or longer chain n-3 acids, were required in the diet.

Prostaglandins

It had long been known that extracts from male accessory glands of animals had a vasodepressor effect, and in 1963 Sune Bergström and colleagues in Stockholm identified some of these active compounds, which had been called ‘prostaglandins,’ as derivatives of C20-cyclopentanoic acid (123). Two years later they showed that they could be synthesized in the body from arachidonic acid, and suggested that at least some of the effects of essential fatty acid (EFA) deficiency could be because of inadequate synthesis of prostaglandin hormones (124). Later it was shown that increasing the linoleate content of a diet increased prostaglandin synthesis (125). We will return to this subject in a later section headed "Polyunsaturated fats" where their importance had reappeared as an end point of quite a different sphere of investigation.

	Shortcomings of the affluent diet

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
Was more always better?

The following is a quotation from 1957: "For practically all of the first half of this century ... problems of deficiency disease and undernutrition have been emphasized. Public nutrition programs have been dedicated largely toward increased consumption of milk, meat, eggs ... and practically everything in the usual diet. ... The term ‘allowance for safety’ is predicated upon the idea that excess is preferable to limitation ... . It is worthwhile considering how much responsibility the nutritionist has for the incidence of obesity in this country. ... The evidence that ‘good diets’, by past definition, are an important causative factor in atherosclerosis, diabetes and other diseases that are largely of the upper income group, is so strong as to be nearly conclusive." Those who have known it will recognize the clear and independent voice of Mark Hegsted here (126).

After World War II it was realized that in some ways the most affluent countries did not have the best health records, and as stated above, their dietary patterns were actually conducive to chronic, noninfective diseases in middle age. The evidence for long-term effects in adults had, of course, to come from epidemiological studies. One surprise was that during the war, when supplies of food, and particularly of animal foods in European countries were severely restricted, the incidence of some diseases was generally reduced (127–129).

The first aspects of the affluent diet to be under suspicion were the high levels of saturated fat and the low levels of fiber (coupled, in the minds of some workers, with the high intakes of sugars). We will consider first the studies related to dietary lipids.

Dietary fat and cholesterol.

One important cause of death in Western countries was ischemic heart disease (or IHD). This is caused by a diminution in blood supply to a portion of the heart muscle that may result in necrosis or myocardial infarction. It was known that autopsies of IHD cases typically showed a narrowing of coronary blood vessels by atherosclerosis, i.e., deposition of plaques rich in cholesterol on the walls. This had long been a subject of interest among pathologists and in 1934 one wrote, "The literature contains a multitude of reports correlating atherosclerosis with diet, blood pressure, race etc." and later: "... where the neutral fat intake is low atherosclerosis is not prevalent" (130), but it does not seem to have caught the attention of nutritionists prior to WWII.

A characteristic of most Western diets was that a high proportion of energy came from fat, much of it animal. The first thought therefore was that the IHD problem came from ingesting too much cholesterol found only in animal foods. Feeding cholesterol to guinea pigs, and to other herbivores that did not receive it in their normal diet, had been found to result in increased levels of serum cholesterol and in the formation of plaques. However, in human studies, the feeding of different types of fat did not affect serum cholesterol levels in proportion to the contribution of cholesterol, although the level of dietary cholesterol did have some effect (131–133).

From 1950 on, Ancel Keys, working with numerous collaborators in different countries, studied many environmental aspects of the IHD epidemic in the U.S. and different parts of Europe. He began work in Naples where the percentage of dietary energy coming from fat was only half of that in the U.S., i.e., 20% vs. 40%, and the death rate of men aged 30–50 y from degenerative heart disease was only one third or less (134). Classification of cause of death can differ from country to country, but all other major causes seemed to be recorded at similar rates in Italy and the U.S., so that the lower total death rate in Italy could be mainly attributed to degenerative heart disease (135). Serum cholesterol levels did not rise in middle age in Naples to the same extent as in the U.S. or England, and it seemed that total fat intake was the determining factor and much more important than either physical activity or degree of obesity (136).

In 1954 it was reported from the Rockefeller Foundation in New York that a mix of plant fats supported a considerably lower level of serum cholesterol than did the same diet with fat from animal sources (1.55 vs. 1.96 g/L) (137). The group then found, after feeding different plant and land animal fats, that the resulting cholesterol levels were lowest with fats of higher iodine value (i.e., more unsaturated), regardless of which kingdom they came from (138).

Contrasting national diets.

It was reported in 1959, that when published national IHD death rates were compared with different dietary characteristics, the closest correlation was with "the percentage of energy coming from saturated fat" (139). Keys too, in his "Seven Countries Study" of 16 different communities, found the closest correlation (r = 0.84) between deaths from IHD and this measure (134). His data, further compressed into single groups of the same nationality, are summarized in Table 6. We see again that deaths in communities in the U.S. and other countries with about the same high intakes of saturated fat were similar, and roughly three times as great as in the Mediterranean countries which had much lower intakes. This was exemplified to me in 1956 when a friend told me that his Greek-American, businessman father had just died in Boston of a heart attack, while his grandfather was still living in good health on his farm back in Greece.

No one suggested that Americans could be persuaded to adopt a Japanese diet, and the "Mediterranean diet" began to be considered as a more practicable ideal, but what did it consist of in the 1950s and 60s? There were some big differences. In one study area in Yugoslavia 96 g/d of fish was eaten, and in another none, but the IHD rates were similar. Only 35 g/d of meat were eaten in the Greek communities, but over 200 g/d in one Yugoslav community where the IHD rate was again similar. The Greek and Italian communities all had a high consumption of fruit and vegetables, but the Yugoslavs had no more than the Dutch and American groups (142,143).

There remains the question posed by Philip James and colleagues as to whether the Mediterranean diets are "protective or simply nontoxic" (144). Were the more affluent diets elsewhere worsened by the presence of trans fatty acids from hydrogenated oils or a lack of antioxidants from leafy vegetables? In 1985 all these points remained to be investigated further.

Lipoproteins.

Cholesterol is carried in the blood in combinations with proteins and as early as 1950, a Berkeley group that had devised a method of separating different classes of serum lipoproteins suggested that high levels of the low density or ß-fraction might indicate an increased risk of atherosclerosis (145). In 1959 Robert Olson presented additional evidence for the importance of this fraction (146). Then in 1977 it was reported from the NIH’s large Framingham study that the analysis of 79 coronary heart disease deaths, among subjects whose lipoproteins had been studied, showed that the greatest risk was among those with the lowest levels of cholesterol in HDL or -lipoproteins. Of those with 44 mg/100 mL or less the incidence was 105/1,000, and among those with more it was only 48/1,000 (147). In other words, HDL was apparently a good form of cholesterol. However, cholesterol in this combination made up only a small proportion of the total, so that earlier studies in which just total cholesterol was measured still had value.

A committee set up by the NRC to study the influence of diet and nutrition on cancer reported in 1982 that, "of all the dietary components studied, the combined epidemiological and experimental evidence is most suggestive for a causal relationship between fat intake and the occurrence of cancer, ... particularly breast and colon cancer. ... Experimental data on cholesterol and cancer risk are too limited to permit any inferences to be drawn" (148).

Polyunsaturated fats.

As discussed in an earlier section, PUFA, after lengthening to C-20 forms, had been shown to be the source from which a variety of prostaglandins with very varied hormonal actions were synthesized. In particular, some stimulated the aggregation of platelets, but the major action was inhibition (149). This could explain the reduced formation of arterial plaques in subjects consuming more polyunsaturated fats.

It was also suggested that the observed effect of polyunsaturated acids in reducing blood pressure was mediated by the synthesis of a favorable balance of prostaglandins (150). However, in experiments in which prostaglandins were injected into rats, only short-term effects were observed, presumably because they are unstable and perhaps need to be synthesized within the tissues in which they are active (125).

When unsaturated vegetable fats, in contrast to more saturated animal fats, were first found to reduce serum cholesterol levels in both humans and rats, it was assumed to be due to their higher content of the recognized EFA, i.e., linoleic and perhaps linolenic acids. Since fish oils had little of these two acids, although they were highly unsaturated, it had not been expected that they too would reduce serum cholesterol levels. In fact, they were found to be equally or even more effective than vegetable oils in both humans and rats (151–153).

It was then suggested that fish oils might have an additional beneficial effect on the incidence of IHD, in addition to reducing serum lipids. It was found that eicosapentaenoic acid, the C20:5(n-3) polyunsaturated acid in fish oils, is metabolized to prostaglandins that antagonize platelet aggregations to a greater extent than those derived from the C20:4(n-6) derivative of the linoleic acid in vegetable oils (154). The value of fish oils was confirmed in further work, but it was also observed that bleeding times were somewhat increased (155).

At the very end of this period, results were reported from the Netherlands of a follow-up study of over 800 middle-aged men whose diet varied greatly in fish consumption, from none to 45 g/d. In the 20 y of the study 78 died from IHD with the risk rate for the ‘high-consumers’ being only 42% that of the abstainers, and with a significant trend for decreasing risk with higher consumption (156).

Dietary fiber.

Two medical men, with long experience in different parts of Africa, had each become impressed by seeing there so little of some of the noninfective diseases most common in industrialized countries. In 1969 Denis Burkitt pointed out that cancer of the colon and rectum among 35–64-y-old men was 10 times as frequent in Connecticut as it was in East Africa, and intermediate in Puerto Rico and most Asian countries. He commented, "Bowel cancer and other noninfective diseases of the bowel are rare in every community examined which exists on high-residue diet, and common in every country where a low-residue diet has been adopted. It seems likely that carcinogens produced by the action of an abnormal bowel flora, when held for a prolonged period in contact with the bowel mucosa, may account for the high incidence of these diseases ... (157). In 1972 Hugh Trowell suggested that it was a higher intake of dietary fiber that also helped to protect people in less developed countries from IHD (158). Following these and other papers, there was increased interest in the possible role of fiber in the diet, but it was also realized how heterogeneous it was and difficult to define. In addition, people who obtained more fiber by eating more fruit and vegetables tended to do so at the expense of foods richer in protein and fats. To quote from just three papers, Ancel Keys and colleagues at the University of Minnesota found that giving subjects large quantities of fruits and vegetables in place of cereals and sugars, with fat intake kept constant, still resulted in a 10% reduction in serum cholesterol levels, though it was not necessarily the extra fiber that was responsible (159). In Australia it was found that giving 15 g/d of citrus pectin (so-called "soluble fiber") resulted in an 13% reduction in plasma cholesterol concentration, with no laxative effect, whereas cellulose had shown only a laxative effect (160). Fiber fermented in the large intestine raised its content of SCFA and it was suggested that this might possibly have an anti-carcinogenic effect (161).

Over the period, the evidence strengthened that communities eating a Western diet were more subject to several noninfective adult diseases (162,163). It was also observed that, within Western communities, vegetarians had a lower risk of IHD than others, but it was uncertain whether the advantage could be attributed to their higher fruit and vegetable (and thus fiber) intakes or the lower intakes of animal fat (164). Although dietary fiber continued to be valued for its laxative action, it seemed to remain unproven that the lack of it was the specific factor directly responsible even for the high incidence of bowel cancer in Western populations (148,165). Also, in South Africa it was noted that Africans who had moved to townships and adopted a more Western diet of lower fiber content still had a very low incidence of colon cancer (166).

	Epilogue

TOP Vitamins Minerals Protein Essential fatty acids Shortcomings of the affluent... Epilogue LITERATURE CITED

 
If it seemed to some people that nutritional science was virtually complete in 1945, it was certainly not the case 40 years later.

The incidence of ischemic heart disease was falling in the affluent countries, presumably in part as a result of people taking nutritional advice, but also from smoking less and the availability of improved drugs. However, the problem of obesity with its attendant diabetes was still growing, and nutritional science had not been able to come up with an easily adopted solution for people with a sedentary lifestyle. By 1985, as technology advanced but the human machine remained the same, a few people were reversing the traditional work-rest cycle, i.e., doing their work (and associated travel) sitting down, but spending their breaks on a treadmill.

We can look back with respect on the labors of our predecessors over a period of 200 y and on their remarkable accomplishments, while recognizing that there remained, and still remains, much more to be done in terms of the particular individual as well as of the statistically average member of the population. Only the surface has been scratched in investigating the effects of the nonnutrient chemicals in foods on resistance to disease.

	ACKNOWLEDGMENTS

 
I thank the anonymous reviewers who have commented on the drafts in this series (particularly in relation to the topic of lipids in Part 4) and Patricia Swan, whose editing throughout the series balanced criticism and encouragement.

	FOOTNOTES

 
¹ This is the last of four invited papers on the history of nutritional science. The previous papers (1–3) were published in the March, April and October 2003 issues of The Journal of Nutrition.

³ Abbreviations used: EFA, essential fatty acids; FPC, fish protein concentrate; IHD, ischemic heart disease; MeTH₄F, methyltetrahydrofolic acid; TH₄F, tetrahydrofolic acid.

Manuscript received 25 July 2003.

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