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Department of Animal Sciences and Division of Nutritional Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801
* To whom correspondence should be addressed. E-mail: dhbaker{at}uiuc.edu.
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ABSTRACT |
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 TOP ABSTRACT IntroductionLITERATURE CITED |
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Introduction |
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TOPABSTRACTIntroductionLITERATURE CITED |
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There were elements of great creativity and keen observation in the discovery of essential nutrients, but in many cases serendipity was involved as well. This was true for many of the hormone discoveries also. Research in the late 1800s that led to the discovery of insulin began with studies on the role of the pancreas in digestion (2). After removal of the pancreas of dogs, it was noticed that flies were attracted to their urine. On analysis, the urine was found to be loaded with sugar. This led to pancreatic secretions of animals being used to treat human diabetes and later (1953) to determination of the 51-amino acid sequence of bovine insulin.
The review that follows focuses on the contributions of experimental animal models to what is known today about nutrient-nutrient interactions, bioavailability of nutrients and nutrient precursors, and tolerance levels for excessive intakes of nutrients.
Nutrient-nutrient interactions
There are roughly 60 physiologically essential nutrients, and most of those that are present in foods and in the body do not exist as the nutrient per se but instead as components of proteins, fats, enzymes, coenzymes, nucleic acids, phytates, or bone components. Work with experimental animals has led to information on the structure of many of these nutrient-containing compounds. Clearly, digestive and (or) metabolic processes are required to transform these compounds into a form that the body can absorb from the gut, metabolize in the body, and use.
Animal models were used to characterize most of the classic mineral interactions. We now know that excess dietary P decreases Ca absorption, and likewise, that excess dietary Ca decreases P absorption. Also, vitamin D is required for efficient absorption of both Ca and P. The interactions between Zn and Cu; Mo and Cu; Zn and Fe; cysteine or ascorbate and Cu, Zn, or Fe; Mn and P; Se and vitamin E; I and Br; As and cysteine; phytates or oxylates and several macro- and micromineral elements have been identified and help explain the utilization efficiencies of mineral elements in various foods (3). Among the vitamins, animals were used to help us understand the tryptophan x niacin and lysine x niacin interactions (4,5) as well as the effect of Fe deficiency on the conversion efficiency of tryptophan to niacin (6). Also, progress in solving pellagra in humans was held back for many years because of lack of a suitable animal model. Ultimately, the dog (black tongue) became the best animal model for learning which foods were pellagragenic and which were ameliorative. The rat and pig were used to prove that excess dietary methionine (also S-methylmethionine) could spare, and even eliminate, the dietary need for preformed choline. The close functional relationship between folic acid and vitamin B-12 was also worked out using animal models, as was the establishment that both riboflavin and Zn were required to convert vitamin B-6 vitamers to pyridoxal phosphate. Before vitamin B-12 became available in crystalline form, clues as to its presence in animal or fermentation products but not plant products came from the "animal protein factor" work done with chickens and pigs in the period 1940 to 1950.
The many amino acid interactions/interrelationships also profited from animal research. Thus, the methionine-cysteine, phenylalanine-tyrosine, arginine-proline and glycine-serine interactions are all examples of 1 amino acid sparing the dietary (or physiological) need for another amino acid (7). In other cases, negative effects of 1 or more amino acids were shown to affect utilization of other amino acids. This is illustrated with antagonisms among the branched-chain amino acids in rats and chicks (8), lysine antagonizing arginine in chicks and dogs but not in pigs or cats (9–13), and supplemental glycine being capable of partially reversing growth depressions in chicks and rats caused by excess methionine consumption (8,14). The negative interactions among amino acids have, for the most part, not been studied in humans.
Bioavailability
There are probably no better examples of how experimental animals have contributed basic nutritional information than studies conducted on bioavailability of nutrients and nutrient precursors (3,7,15). For the most part, energy values of foods, digestibility and protein quality of proteins, absorption efficiencies of mineral salts and the minerals in foods, absorption efficiencies of the vitamin components of foods and various vitamin precursors, and amino acid utilization from various amino acid precursors, analogs, and metabolites have been derived from studies employing animal models. Today, however, stable isotope techniques are allowing more and more nutrient bioavailability studies to be done using human subjects.
Minerals. There are vast differences in the absorption efficiency of mineral elements provided either as mineral salts or as components of food (16). The halogen elements Cl, I, and F are almost completely absorbed, and Na and K are generally absorbed with over 90% efficiency. In contrast, the essential mineral elements Cr and Mn are absorbed poorly, most estimates being in the range of 2% to 4%. Phosphorus from inorganic sources is absorbed with 70–90% efficiency, depending on P status, but much of the P in plant-based foods exists as a phytate complex, and in this form the efficiency of P absorption may be as low as 10%. The P in foods existing in either phospholipid or nucleic acid form, however, is absorbed as efficiently as inorganic P (3,15). Phytic acid in foods also has negative effects on the absorption of other mineral elements such as Ca, Zn, Fe, and Mn. Oxalates in foods such as spinach have similar effects. The negative effects of phytic acid on trace-mineral bioavailability are exacerbated by excessive Ca ingestion. Reducing agents such as cysteine and ascorbate enhance the absorption of Zn and Fe but decrease the absorption of Cu and Co.
Animal studies have led the way in providing answers to questions of bioavailability of mineral supplements ingested as mineral salts (3,15). Sulfates, chlorides, phosphates, acetates, citrates, and gluconates are generally the best sources of supplemental elements, but carbonates of Ca and Zn are well utilized also. Oxides and sulfides are generally poor sources of mineral elements, although food-grade ZnO and MnO are well utilized as bioavailable sources of Zn and Mn. Oxides of Fe and Cu have relative bioavailability values (i.e., efficacy value relative to a standard such as FeSO4·7H2O or CuSO4·5H2O) of near zero (3,17).
Vitamins. Bioavailability of vitamins in foods and food ingredients has received little research attention, probably because it is easy and economical to supplement diets with crystalline forms of the vitamins. Nonetheless, in poor countries of the world, crystalline vitamins are often unavailable or unaffordable, such that multiple vitamin deficiencies are not uncommon. Animal models were instrumental in solving vitamin-deficiency diseases such as beri-beri (chick thiamin deficiency), scurvy (guinea pig ascorbic acid deficiency), pellagra (dog, rat, pig, and chick deficiency of niacin and tryptophan), rickets (dog, rat, and chick deficiency of Ca, P, and/or vitamin D), night blindness (rat deficiency of vitamin A), dermatitis (rat deficiency of vitamin B-6), low fertility and muscle dystrophy (rat deficiency of vitamin E), hemorrhagic disease (chick deficiency of vitamin K), and anemia (monkey, rat, and chick deficiency of folate and/or vitamin B-12). Many of these diseases were thought initially to be infectious and of bacteriologic origin. However, when an association with diet was noted, and when this was followed by development of animal-model bioassays with defined purified diets, progress was quickly made in defining the disease condition and in reversing or preventing it with the proper vitamin-containing food or (later) with the vitamin itself.
Today we know that certain vitamins are extremely heat labile (thiamin, folate, pyridoxal forms of B-6) because of Maillard-type reactions. Also, the niacin in most cereal sources is poorly available, biotin is antagonized by avidin in raw egg white, and the biotin in wheat is not well absorbed; vitamin D bioactivity is enhanced by UV light, whereas UV light can inactivate riboflavin that is in solution; (excess) methionine can spare the requirement for preformed choline, raw fish may contain antithiamin (thiaminase) factors, and oilseed products (e.g., soybeans) may contain a substantial portion of their vitamin B-6 in glycosylated forms such as pyridoxine glucoside, a poorly bioavailable source of vitamin B-6 (18).
Among the fat-soluble vitamins, vitamin A and vitamin D have received the most attention. Although preformed vitamin A (esters of all-trans retinol) is found in foods of animal origin, carotenoids are the source of vitamin A activity in plant-based foods. At least 600 different carotenoids exist as yellow, orange, and red pigments in foods, but <10% of these have meaningful vitamin A bioactivity. β-Carotene is the carotenoid with the greatest vitamin A activity, but its cleavage efficiency in the gut by 15,15'-dioxygenase is very different among animal species: the chick and rat are as much as 4 times more efficient than the pig in converting β-carotene to retinol (18–20). There are 2 main factors that affect the bioavailability of β-carotene and other carotenoids as vitamin A precursors: 1) release of carotenoids from the food matrix and 2) efficiency of the dioxygenase enzyme in splitting all-trans β-carotene into 2 retinal molecules. These 2 factors, however, are affected by 3 other factors: 1) species, 2) nutrient status, and 3) intake level of provitamin A carotenoids (21). In the human, it is assumed that 12 µg all-trans β-carotene in a mixed diet will yield 1 µg of all-trans retinol (22). Because various animal species absorb carotenoid compounds with greatly different efficiencies, selection of a suitable animal model for human vitamin A studies is problematic. Lee et al. (20) concluded that gerbils and preruminant calves were the best models for evaluating β-carotene conversion to vitamin A in humans. Other species, however, are more appropriate for studies of the effects of certain xanthophyll compounds (e.g., lutein, zeaxanthin) on macular degeneration and cataract formation.
Vitamin E deficiency exacerbates vitamin A deficiency in that vitamin E is necessary for cleavage of β-carotene to retinal. Vitamin E exists in foods in 8 different forms, 4 of which are tocopherols and 4 tocotrienols. D-
-Tocopherol has the highest vitamin E bioactivity, but although β- and 
-tocopherols contain substantially lower vitamin E bioactivity values, these isomers of tocopherol nonetheless contain good antioxidant activity. The rat antisterility assay and beef cattle tissue tocopherol uptake assay have been compared with serum vitamin E uptake values in humans. The beef cattle estimates for the bioactivity of D--tocopherol, DL--tocopheryl, D--tocopherol acetate, and DL--tocopheryl acetate are more in line with the human estimates than are the rat bioactivity estimates.
There is little information on the bioavailability of vitamin D in food sources vs. that furnished via UV sunlight exposure. However, it is now known that the active form of vitamin D-3 (cholecalciferol) is actually a hormone, 1,25-dihydroxycholecalciferol. This compound is available for clinical use, and it is the most biologically active form of vitamin D. 1-Hydroxycholecalciferol, however, is a precursor synthetic form of cholecalciferol that the liver can efficiently convert to the 1,25 hydroxylated form of cholecalciferol (23). 25-Hydroxycholecalciferol is the principal blood form of cholecalciferol, and this compound, available commercially, has more bioactivity than cholecalciferol itself but lower bioactivity than the 1-hydroxylated forms of cholecalciferol.
Virtually nothing is known about vitamin K bioavailability in foods (18). All 3 forms of vitamin K—phylloquinones (K-1) in plants, menaquinones (K-2) in fermentation products, and menadiones (K-3), i.e., water-soluble synthetic forms of vitamin K—are bioactive. Bioavailability comparisons among the several menadione sources have been done exclusively in animal models, primarily chicks, using prothrombin time as a response criterion.
Upper tolerance levels for nutrients
There is a paucity of human information on excess ingestion of both nutrients and nonnutrients (23). Studies in animals have shown that most nutrient and drug toxicities are exacerbated when purified diets based on casein or amino acids (e.g., enteral and parenteral formulas) are consumed. This is illustrated by work in our laboratory on Cu toxicity in the young chick (24) and methotrexate toxicity in adult rats (25,26). The National Research Council has compiled information on both mineral (27) and vitamin (28) tolerances of animals, and these publications are very helpful in summarizing upper tolerance limits for these nutrients. For amino acids, FASEB issued a report in 1992 (29) that dealt with safety issues surrounding excess amino acid intakes, but it was obvious from this report as well as the 2002 Food and Nutrition Board report (30) that most of what is known about amino acid tolerance levels has come from animal research.
The safety margin for excess consumption of vitamins is large, even for vitamin A, which is considered perhaps the most dangerous of the vitamin excesses. Animal studies have predominated over human studies on vitamin excesses, with 2 notable exceptions: niacin and vitamin C. Because pharmacologic niacin doses of 1 to 3 g/d, 67 to 200 times the adult requirement, are used clinically to treat hypercholesterolemia and schizophrenia, the human literature is extensive on side effects (e.g., flushing) of excess niacin ingestion (23,28,31). In terms of growth depression in chicks, dietary levels of niacinamide above 5000 mg/kg cause a linear decrease in weight gain, but niacin levels as high as 20,000 mg/kg (600 mg/d; 3 g·kgBW–1·d–1) are without effect (32). Regarding vitamin C, with the exception of guinea pigs, apes, and a few birds and fish, animals do not require a dietary source of ascorbic acid. Thus, human studies of vitamin C excess are as prevalent as animal studies (28).
With mineral excesses, there is concern about excess Se and F among the essential mineral elements. Also, there is extensive animal literature on excess consumption of nonnutrient elements such as Cd, Hg, Pb, and As (27). The chick has been particularly helpful as an animal model for the Cu toxicity problems associated with Wilson's Disease and Indian Childhood Cirrhosis. Supplements of L-cysteine, L-ascorbate, and Zn are used by hepatologists to treat these Cu toxicity diseases. Chick work has shown that L-cysteine is more effective than either Zn or L-ascorbate in ameliorating Cu toxicity; also, cysteine is more effective than either cystine or methionine (33). The chick was also used to define I toxicity and its reversal by administration of supplemental Br (34,35). Both rats and humans have been used to evaluate pharmacologic doses of Se for cancer prevention (36).
There is keen interest in the potential benefits of consuming amino acid supplements at levels beyond nutritional requirements (37). Unfortunately, little evidence exists for tolerable upper amino acid limits in humans (29,30). Most of what is known about pharmacologic amino acid dosing comes from studies in animals. It is clear from these studies that individual amino acid excesses are more anorexigenic when provided in low-protein than in high-protein diets (8,38,39). Most of this early work was done with rats fed casein-based purified diets. Use of diets containing mixed ingredients and with normal protein levels is probably more relevant in terms of extrapolation to humans. Our work with chicks and pigs, therefore, involved individual amino acid excesses in diets containing 20% to 23% protein from corn and soybean meal (11,12,14,40–43). This work showed that a large dietary excess (4 g/100 g) of methionine, cysteine, or tryptophan was more growth depressing than the same excess of other amino acids. Also, branched-chain amino acids were better tolerated than excesses of other amino acids.
Species comparisons
Animal research has contributed heavily to what we know today about nutrition and metabolism. In recent years, the young pig has come into particular prominence as an animal model, i.e., for studies of amino acid metabolism (9,44), total parenteral nutrition (45–48), rotavirus infection (49), and bacterial and viral pneumonia (50). Other than monkeys, pigs are thought to be the most nearly like humans of any animal model (51,52). Among the animal species that have contributed useful nutrition information, many exhibit well-documented differences in how they use, metabolize, and excrete nutrients. Some species-specific features are listed below:
-hydroxylated cholecalciferol compounds to chick diets containing surfeit vitamin cholecalciferol (70,71), but neither pigs nor laying hens respond in a similar fashion (72,73). It is obvious from this (incomplete) list of species differences that it is important to choose the right animal model for predictions of what might happen in humans. Other considerations include availability of facilities and cost of the experiments to be performed. Clearly, research with animal models has been valuable in advancing our knowledge of nutrition. The first 50 y of the 20th century might be thought of as the qualitative era of nutrition wherein most of the essential nutrients and their functions were discovered. The last 50 y could be thought of as the quantitative era, a time when nutrient requirements, nutrient-nutrient interactions, and pharmacologic aspects of nutrients were the focus. A 2006 Experimental Biology (History of Nutrition) symposium provided excellent reviews of how research with food animals has contributed to our knowledge of nutrition concepts and principles in energetics (75), carbohydrates and lipids (57), proteins (76), and body composition and growth (77).
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FOOTNOTES |
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2 Author disclosures: D. H. Baker, no conflicts of interest.
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LITERATURE CITED |
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TOPABSTRACTIntroductionLITERATURE CITED |
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1. Combs GF Jr. The vitamins. San Diego: Academic Press; 1992.
2. Roberts RM. Serendipity: accidental discoveries in science. New York: John Wiley & Sons; 1989.
3. Ammerman CB, Baker DH, Lewis AJ. Bioavailability of nutrients for animals: amino acids, minerals, and vitamins. San Diego: Academic Press; 1995.
4. Augspurger NR, Baker DH. Excess dietary lysine increases growth of chicks fed niacin-deficient diets, but dietary quinolinic acid has no niacin-sparing activity. Poult Sci. 2007;86:349–55.
5. Oduho G, Baker DH. Quantitative efficacy of niacin sources for the chick: nicotinic acid, nicotinamide, NAD and tryptophan. J Nutr. 1993;123:2201–6.
6. Oduho GW, Han Y, Baker DH. Iron deficiency reduces the efficacy of tryptophan as a niacin precursor for chicks. J Nutr. 1994;124:444–50.
7. Baker DH. Utilization of precursors for L-amino acids. In: D'Mello JPF, editor. Amino acids in farm animal nutrition. Wallingford, Oxon: CAB International; 1994, p. 37–64.
8. Harper AE, Benevenga NJ, Wohlhueter RM. Effects of ingestion of disproportionate amounts of amino acids. Physiol Rev. 1970;50:428–558.
9. Ball RO, Urschel KL, Pencharz PB. Nutritional consequences of interspecies differences in arginine and lysine metabolism. J Nutr. 2007;137:1626S–1641S.
10. Austic RE, Scott RL. Involvement of food intake in the lysine-arginine antagonism in chicks. J Nutr. 1975;105:1122–31.
11. Czarnecki GL, Hirakawa DA, Baker DH. Antagonism of arginine by excess dietary lysine in the growing dog. J Nutr. 1985;115:743–52.
12. Edmonds MS, Baker DH. Failure of excess dietary lysine to antagonize arginine in young pigs. J Nutr. 1987;117:1396–401.
13. Fascetti AJ, Maggs DJ, Kanchuk HE, Clarke HE, Rogers QR. Excess dietary lysine does not cause lysine-arginine antagonism in adult cats. J Nutr. 2004;134:2042S–45S.
14. Baker DH. Comparative species utilization and toxicity of sulfur amino acids. J Nutr. 2006;136:1670S–75S.
15. Baker DH. Bioavailability of minerals and vitamins. In: Lewis AJ, Southern LL, editors. Swine nutrition. London: CAB International; 2001, p. 357–79.
16. Groff JL, Gropper SS, Hunt SM. Advanced nutrition and human metabolism. Minneapolis: West Publishing; 1995, p. 284–416.
17. Baker DH. Cupric oxide should not be used as a copper supplement for either animals or humans. J Nutr. 1999;129:2278–80.
18. Baker DH. Vitamin bioavailability. In: Ammerman CB, Baker DH, Lewis AJ, editors. Bioavailability of nutrients for animals: amino acids, minerals and vitamins. San Diego: Academic Press; 1995, p. 399–431.
19. Erdman JW, Poor CL, Dietz JM. Factors affecting the bioavailability of vitamin A, carotenoids, and vitamin E. Food Technol. 1988;42:214–21.
20. Lee CM, Boileau AC, Boileau TWM, Williams AW, Swanson KS, Heintz KA, Erdman JW. Review of animal models in carotenoid research. J Nutr. 1999;129:2271–7.
21. Lindshield BL, Erdman JW. Carotenoids. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition, Volume 1, 9th edition. Washington, DC; International Life Sciences Institute; 2006, p. 184–97.
22. Institute of Medicine. Dietary reference intakes: the essential guide to nutrient requirements. Otten JJ, Hellwig JP, Meyers LD, editors. Washington (DC): National Academy Press; 2006.
23. Biehl RR, Baker DH, DeLuca HF. 1-Hydroxylated cholecalciferol compounds act additively with microbial phytase to improve phosphorus, zinc and manganese utilization in soy-based diets fed to chicks. J Nutr. 1995;125:2407–16.
24. Funk MA, Baker DH. Toxicity and tissue accumulation of copper in chicks fed casein and soy-based diets. J Anim Sci. 1991;69:4505–11.[Abstract]
25. Funk MA, Baker DH. Effect of fiber, protein source and time of feeding on methotrexate toxicity in the rat. J Nutr. 1991;121:1673–83.
26. Funk MA, Baker DH. Effect of soy products on methotrexate toxicity in the rat. J Nutr. 1991;121:1684–92.
27. National Research Council. Mineral tolerance of animals. 2nd revised edition. Washington (DC): National Academy Press; 2006.
28. National Research Council. Vitamin tolerance of animals. Washington (DC): National Academy Press; 1987.
29. FASEB Life Sciences Research Office. Safety of amino acids used as dietary supplements. Washington (DC): FDA Contract no. 223–88–2124, Task Order no. 8; 1992.
30. Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for energy, carbohydrate, fat, fatty acids, cholesterol, protein and amino acids (macronutrients): preliminary report. Washington (DC): National Academy Press; 2002.
31. Andersson RGG, Aberg G, Brattsand R, Ericsson E, Lundholm L. Studies on the mechanism of flush induced by nicotinic acid. Acta Pharmacol Toxicol (Copenh). 1977;41:1–10.[Medline]
32. Baker DH, Yen JT, Jensen AH, Teeter RG, Michel EN, Burns JH. Niacin activity in niacinamide and coffee. Nutr Rep Int. 1976;14:115–20.
33. Persia ME, Parsons CM, Baker DH. Amelioration of oral copper toxicity in chicks by dietary additions of ascorbic acid, cysteine and zinc. Nutr Res. 2003;23:1709–18.
34. Baker DH, Parr TM, Augspurger NR. Oral iodine toxicity in chicks can be reversed by supplemental bromine. J Nutr. 2003;133:2309–12.
35. Baker DH. Iodine toxicity and its amelioration. Exp Biol Med (Maywood). 2004;229:473–8.
36. Clark LC, Combs GF Jr, Turnbull BW, Slate EH, Chalker DK, Chow J, Davis LS, Glover RA, Graham GF, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. JAMA. 1996;276:1957–63.
37. Cynober L. Introduction to the 5th amino acid workshop. J Nutr. 2006;136:1633S–35S.
38. Sauberlich HE. Studies on the toxicity and antagonism of amino acids for weanling rats. J Nutr. 1961;75:61–72.
39. Muramatsu K, Odagiri H, Morishita S, Takeuchi H. Effect of excess levels of individual amino acids on growth of rats fed casein diets. J Nutr. 1971;101:1117–26.
40. Edmonds MS, Baker DH. Comparative effects of individual amino acid excesses when added to a corn-soybean meal diet: effects on growth and dietary choice in the chick. J Anim Sci. 1987;65:699–705.
41. Edmonds MS, Baker DH. Amino acid excesses for young pigs: effects of excess methionine, tryptophan, threonine or leucine. J Anim Sci. 1987;64:1664–71.
42. Edmonds MS, Gonyou HW, Baker DH. Effect of excess levels of methionine, tryptophan, arginine, lysine or threonine on growth and dietary choice in the pig. J Anim Sci. 1987;65:179–85.
43. Dilger RN, Toue S, Kimura T, Sakai R, Baker DH. Excess dietary L-cysteine (but not L-cystine) is lethal for chicks but not for rats or pigs. J Nutr. 2007;137:331–8.
44. Riedijk MA, Stoll B, Chacko S, Schierbeek H, Sunehag AL, van Goudoever JB, Burrin DG. Methionine transmethylation and transsulfuration in the piglet gastrointestinal tract. Proc Natl Acad Sci USA. 2007;104:3408–13.
45. Brunton JA, Bertolo RFP, Pencharz PB, Ball RO. Proline ameliorates arginine deficiency during enteral but parenteral feeding in neonatal piglets. Am J Physiol. 1999;277:E223–31.[Medline]
46. Bartholome AL, Albin DM, Baker DH, Holst JJ, Tappenden KA. Supplementation of total parenteral nutrition with butyrate acutely increases structural aspects of intestinal adaptation after an 80% jejunoileal resection in neonatal piglets. JPEN J Parenter Enteral Nutr. 2004;28:210–22.
47. Donovan SM, Zijlstra RT, Odle J. Use of the piglet to study the role of growth factors in neonatal intestinal development. Endocr Regul. 1994;28:153–62.[Medline]
48. Park YK, Monaco MM, Donovan SM. Delivery of total parenteral nutrition (TPN) via umbilical catheterization: development of a piglet model to investigate therapies to improve gastrointestinal structure and enzyme activity during TPN. Biol Neonate. 1998;73:295–305.[CrossRef][Medline]
49. Zijlstra RT, McCracken BA, Odle J, Donovan SM, Gelberg HB, Petschow BW, Zuckermann FA, Gaskins HR. Malnutrition modifies pig small intestinal inflammatory responses to rotavirus. J Nutr. 1999;129:838–43.
50. Escobar J, Van Elstine WG, Baker DH, Johnson RW. Decreased protein accretion in pigs with viral and bacterial pneumonia is associated with increased myostatin expression in muscle. J Nutr. 2004;134:3047–53.
51. Tumbleson M. Swine in biomedical research. New York: Plenum Press; 1986.
52. Miller ER, Ullrey DE. The pig as a model for human nutrition. Annu Rev Nutr. 1987;7:361–82.[CrossRef][Medline]
53. Rippel RH, Rasmussen OG, Jensen AH, Norton HW, Becker DE. Effect of level and source of protein on reproductive performance of swine. J Anim Sci. 1965;24:203–8.
54. Pond WG, Wagner WC, Dunn JA, Walker EF. Reproduction and early postnatal growth of progeny in swine fed a protein-free diet during gestation. J Nutr. 1968;94:309–16.
55. Baker DH, Becker DE, Jensen AH, Harmon BG. Protein source and level for pregnant gilts: a comparison of corn, opaque-2 corn and corn-soybean meal diets. J Anim Sci. 1970;30:364–7.
56. Nelson MM, Evans HM. Relation of dietary protein levels to reproduction in the rat. J Nutr. 1953;51:71–84.
57. Nafikov RA, Beitz DC. Carbohydrate and lipid metabolism in farm animals. J Nutr. 2007;137:702–05.
58. Baker DH. Toxicity of sucrose and fructose for neonatal pigs. J Nutr. 1997;127:1049S–50S.[Medline]
59. Rutter WJ, Krichevsky P, Scott HM, Hansen RG. The metabolism of lactose and galactose in the chick. Poult Sci. 1953;32:706–15.
60. Molitoris BA, Baker DH. Choline utilization in the chick as influenced by levels of dietary protein and methionine. J Nutr. 1976;106:412–18.
61. Pond WG, Lowrey RS, Maner JH, Loosli JK. Parenteral iron administration to sows during gestation or lactation. J Anim Sci. 1961;20:747–50.
62. Baker DH, Sugahara M, Scott HM. The glycine-serine interrelationship in chick nutrition. Poult Sci. 1968;47:1376–7.[Medline]
63. Graber G, Baker DH. The essential nature of glycine and proline for growing chickens. Poult Sci. 1973;52:892–6.[Medline]
64. Wolf AV, Prentiss PG, Douglas LG, Swett RJ. Potability of sea water with special reference to the cat. Am J Physiol. 1959;196:633–41.
65. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annu Rev Nutr. 1984;521–62.
66. Baker DH, Czarnecki-Maulden GL. Comparative nutrition of cats and dogs. In: Olson RE, Bier DM, McCormick DB, editors. Annual Review of Nutrition. Palo Alto, CA: Annual Reviews, Inc. 1991;11:239–63.
67. Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutr Res Rev. 2002;15:153–68.[Medline]
68. Sturkie PD. Avian physiology. New York: Springer-Verlag, 1986; p 320.
69. Baker DH. Tolerance for branched-chain amino acids in experimental animals and humans. J Nutr. 2005;135:1585S–90S.
70. Edwards HM Jr. Dietary 1,25-dihydroxycholecalciferol supplementation increases natural phytate phosphorus utilization in chickens. J Nutr. 1993;123:567–77.
71. Biehl RR, Baker DH, DeLuca HF. 1-Hydroxylated cholecalciferol compounds act additively with microbial phytase to improve phosphorus, zinc and manganese utilization in soy-based diets fed to chicks. J Nutr. 1995;125:2407–16.
72. Biehl RR, Baker DH. Efficacy of supplemental 1-hydroxycholecalciferol and microbial phytase for young pigs fed phosphorus- or amino acid-deficient corn-soybean meal diets. J Anim Sci. 1996;74:2960–66.[Abstract]
73. Snow JL, Persia ME, Biggs PE, Baker DH, Parsons CM. 1-Hydroxycholecalciferol has little effect on phytate phosphorus utilization in laying hen diets. Poult Sci. 2003;82:1792–96.
74. Reeves PG, Nielsen FH, Fahey GC. AIN-93 purified diets for laboratory rodents: final report of the American Institute of Nutrition ad hoc writing committee on the reformulation of the AIN-76A rodent diet. J Nutr. 1993;123:1939–51.
75. Johnson DE. Contributions of animal nutrition research to nutrition principles: energetics. J Nutr. 2007;137:698–701.
76. Bergen WG. Contributions of research with farm animals to protein metabolism concepts: a historical perspective. J Nutr. 2007;137:706–10.
77. Mitchell AD. Impact of research with cattle, pigs, and sheep on nutritional concepts: body composition and growth. J Nutr. 2007;137:711–14.
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