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Carbohydrates, Appetite and Feeding Behavior in Humans¹

Aberdeen Centre for Energy Regulation and Obesity, Rowett Research Institute, Bucksburn, Aberdeen, Scotland

²To whom correspondence should be addressed. E-mail: j.stubbs{at}rri.sari.ac.uk

	ABSTRACT

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
The view of carbohydrates in relation to obesity has changed over the past few decades from being conducive to overconsumption and weight gain to being protective. This article reviews the mechanisms by which carbohydrate is purported to protect against weight gain. Although carbohydrate is metabolized and stored in the body less efficiently than fat, when de novo lipogenesis is invoked on very high carbohydrate diets, the beneficial effect on energy balance is likely to be minimal when typical high fat Western diets are consumed. However, it has been suggested that high carbohydrate foods may influence energy balance by reducing food intake through greater satiety effects, reducing energy density and displacing fat from the diet—the fat-sugar seesaw effect. To date, there seem to be few differences between sugars and starches on satiety and energy intake, but few studies have examined this. Some reduced-fat, and, therefore, higher carbohydrate, foods are highly energy dense. High carbohydrate foods do not necessarily have a low energy density. Evidence from recent studies suggests that adding carbohydrate, and especially sugar, to the diet neither displaces fat from the diet nor protects against elevated energy intake. Although it is easier to overeat on high fat than low fat foods, simply replacing fat with carbohydrate in the diet may not be as protective against overconsumption as the energy density or fat-sugar seesaw arguments suggest.

KEY WORDS: • carbohydrate • satiety • energy balance

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
This review critically discusses the current evidence (and lack of evidence) for the role of dietary carbohydrates in appetite and energy balance control.

	Perspective

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
The public perception of carbohydrates has oscillated in recent decades. Throughout the 1970s, there was a tendency for some nutritionists to view carbohydrates (especially refined forms) as conducive to weight gain. This reached its logical extreme with perceptions of sugars as "pure, white and deadly" by Yudkin (1) . Since then, the nutritional perception of carbohydrates has improved dramatically. By the mid-1990s, dietary fat had developed a reputation of near demonic proportions as the dietary villain of the late 20th century and was squarely blamed as the major dietary constituent promoting excess energy intake (2 ,3) . During this time, dietary carbohydrates were generally viewed in generic terms as a beneficial nutrient whose ingestion could promote all manner of positive outcomes with reference to weight control (4 ,5) . The positive effects of carbohydrates on energy balance were enshrined in the predictions of Flatt (6) in his splendid glycogenostatic model of energy balance control.

It was generally accepted that carbohydrates are absorbed, metabolized and stored with less bioenergetic efficiency than dietary fat and per unit of energy ingested were protective against weight gain. A general perception was developing that because de novo lipogenesis seems limited when humans feed on Western diets, carbohydrate ingestion does not promote fat storage (4 ,5) . At the same time there was a renaissance of interest in carbohydrate-specific models of feeding (7) . The notion that carbohydrate metabolism or stores exert powerful negative feedback on energy intake became quite firmly established in the field of energy and nutrient balance. The simultaneous focus of researchers and health professionals on dietary fat as the pivotal nutrient promoting high levels of energy intake reinforced carbohydrate-specific models of feeding. High fat hyperphagia was seen as being due to the tendency for subjects to eat to carbohydrate balance rather than energy balance (7) . Thus, hyperphagia on high fat diets (which are by definition low in the percentage of energy from carbohydrate) was seen as being driven by the need to ingest a certain level of carbohydrate. By the same reasoning, diets high in carbohydrates were deemed to be more satiating, specifically because they were high in carbohydrates.

The extension of this logic led to the notion that it was difficult, if not virtually impossible, to overeat on a high carbohydrate diet (4) . Epidemiological observations showed that in subjects self-recording their food intakes, the percentages of energy intake from fat and carbohydrates are reciprocally related. In particular, there seemed to be a strong negative relationship between percentage energy from fats and sugars. This relationship has been termed the fat-sugar seesaw. One seminal study noted that high sugar consumers also tend to be thinner than high fat consumers (8) . This led to the suggestion that sugar displaces fat energy from the diet, and because fat is conducive to weight gain, high sugar intakes may well protect against obesity (8 ,9) .

Carbohydrates had never had it so good. These messages percolated through the scientific community to governments, consumers and industry who were (and still are) deeply concerned about diet-induced obesity in developed and developing countries. Fat reduction became the order of the day and the low fat food market rapidly expanded (10 ,11) . The fat reduction message has now become so strong that UK consumers report positive attitudes and intentions to reduce their fat intakes (12) and seem to actively select lower fat versions of some foods when foraging their local supermarkets. The food industry has gone to great lengths to diversify products in the direction of low fat, lower fat and high carbohydrate foods, which have sufficient sensory appeal that consumers will continue to select and ingest them. A major sensory attribute of high carbohydrate foods, which is almost ubiquitously appealing, is sweetness.

	Doubts about the role of carbohydrates in protecting against weight gain

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
In recent years some doubts have arisen about the paramount role of carbohydrates as the central nutrient around which energy balance is regulated and body weight controlled. Although evidence exists that excess carbohydrate is stored with less efficiency than fat, the relevance of these effects to free-living Western consumers has been questioned (13) . Several rigorous tests of carbohydrate-specific models of feeding have suggested that carbohydrate oxidation or stores per se do not exert powerful unconditioned negative feedback on energy intake (14) . Rather, as macronutrients come in the diet (where fat is disproportionately energy dense), there seems to be a hierarchy in the satiating efficiency of the macronutrients protein, carbohydrate and fat. Per unit of energy ingested, protein induces supercaloric compensation; carbohydrate generates approximately caloric compensation and fat precipitates subcaloric compensation and, hence, often excess energy intakes (14) . Simple statistical modeling has shown that models that include all three macronutrients explain far more of the variance in energy intake—either in the laboratory or free-living subjects self-recording their own intakes. When energy density is controlled, protein is still far more satiating than carbohydrates or fats (at least when ingested in excess of 1- to 1.5-MJ loads). Under these conditions, differences in the satiating efficiency of carbohydrates and fats become more subtle (14) .

High fat hyperphagia can be explained by the high palatability and energy density of high fat foods (which facilitate greater levels of intake) and the low postabsorptive satiety value of fat (which prevents subsequent compensatory decreases in energy intake) (15) . Although carbohydrate is more satiating than is fat, excess fat intake is not necessarily driven by a need to eat to maintain carbohydrate balance. High carbohydrate foods are usually, but not always, less energy dense than high fat foods and contain dietary fiber, which limits rates of ingestion and digestion, both of which can have a limiting effect on energy intake. High carbohydrate foods that are dry will tend to exert a higher osmotic load in the gut than will high fat foods of similar moisture content. When the energy content and energy density of high carbohydrate and high fat foods are compared, readily assimilated carbohydrate is more satiating than fat. This difference in the satiating capacity of fat and carbohydrate can be deemed to be independent of energy density or palatability (16) . However, this effect is weaker than when high fat, energy dense foods are compared with lower fat, less energy dense foods that are high in carbohydrate. Thus, the nutrient-specific differences in the satiating effects of fats and carbohydrates need to be considered in relation to the structure and composition of the foods in which those nutrients abound.

It has frequently been stated that there is no evidence that foods high in carbohydrates promote overconsumption (5) . This may be because most studies examining the effects of high carbohydrate and high fat foods on feeding behavior compare high fat, more energy dense foods with high carbohydrate (and, therefore, low fat) less energy dense foods. Most studies that demonstrate the effects of fat in promoting excess energy intake examine how adding fat to the diet influences feeding. Very few studies have examined how adding carbohydrates to food affects feeding behavior or energy intake. In one study at least, increasing the energy density of the diet by dramatically increasing the maltodextrin content led to marked elevations of energy intake over 14 d (17) .

Although the fat-sugar seesaw has become a well-recognized phenomenon (8 ,9) , the phenomenon itself has been harder to pin down. The fact that fat and sugar, or even fat and carbohydrate, are reciprocally related to each other in the diet is almost inevitable because they are the main energy-providing macronutrients. We have found a strong fat-sugar seesaw in 1032 ready-to-eat foods taken from the British food tables. Gram for gram, the reciprocal relationship between fat and carbohydrate (or fat and sugar) is far less evident (15) . Furthermore, until recently there have been few if any interventions in which incremental amounts of fats and sugars have been added to the diet to ascertain the effects on total energy and macronutrient intake.

A reasonable amount of evidence exists to support the conclusion that carbohydrates are more satiating than fat and that, on average, high fat diets will tend to promote higher levels of intake than will low fat diets. However, it has also been argued that the low fat food revolution has flooded the diet with high glycemic index carbohydrate foods that are not particularly low in energy density (18 ,19) . This statement raises two issues that have not been given enough attention. Are all carbohydrates the same in relation to appetite and energy balance? Can carbohydrates promote excess energy intakes? If so, which ones do so and when do they do it?

	Current issues relating to carbohydrates, appetite and energy balance

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
The remainder of this review focuses on the doubts raised above to shed more light on the role of carbohydrates in appetite and energy balance control and to highlight what we do not yet know and, hence, where we need to focus our research efforts. From a bioenergetic perspective, are carbohydrates more protective than fat against weight gain? What relationship exists between dietary carbohydrates and energy density? How easy is it to overeat on a high carbohydrate diet? Does an increased sugar intake displace fat from the diet? Do carbohydrates influence the sensory stimulation to eat and are sweet carbohydrates a vehicle for fat intake? Which carbohydrates protect against or promote weight gain?

	Bioenergetic perspective: are carbohydrates more protective than fat against weight gain?

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
Carbohydrates can be used by the body with less energetic efficiency than other nutrients in three ways. They may have a different metabolizable energy coefficient; be metabolized or stored less efficiently than other nutrients, especially fat; and influence physical activity or nonexercise thermogenesis differently from fat. The two former areas have been considered in detail elsewhere (13 ,20) ; the definitive experiments to test the latter have yet to be done. The general conclusion is that carbohydrates seem to exert some beneficial effects on energy balance from a bioenergetic standpoint. However, except under extreme conditions of fiber ingestion or carbohydrate overfeeding, these effects are unlikely to influence energy balance by more than a few percent. Furthermore, higher fiber foods tend to be more bulky, of a lower energy density and less palatable than more energy dense foods (21) and less likely to be selected and ingested. In the current food market, it is remarkably easy to consume a diet high in fiber, high in starch or high in sugars. It is, therefore, important to focus on how types of carbohydrates influence appetite and energy balance.

	Relationship between dietary carbohydrates and energy density

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
We have examined the relationship among dietary macronutrients, water and energy density in ready-to-eat foods from the British food tables, ready-to-eat snack foods and the whole diet ingested by 102 free-living subjects (15) . The 1032 ready-to-eat foods were derived from the British food composition tables (22) . These were all of the foods that a person could readily ingest without any cooking or preparation and included foods from all of the categories, including single foods such as oils, butter and sugar. When water and nutrients are plotted as g/100 g of food, as predictors of dietary energy density all nutrients contribute positively to energy density, and water content contributes negatively. However, only fat (energy density [ED (KJ/100 g)] = 462.6 + 35.4 x fat, R² = 75.0) and water (ED(KJ/100 g) = 2034 - 21.2 x water, R² = 66.7) have a large R² value; those for protein and carbohydrate indicated a generally poor relationship. When the relationship between nutrient composition (expressed as a percentage of total food energy) and energy density is examined, fat again correlates positively with energy density whereas protein and carbohydrate show a weak negative relationship to energy density. The relationship for fat under these conditions is relatively weak (energy density = 320.1 + 16.3 x fat, R² = 35; Fig. 1 ).

View larger version (15K): [in this window] [in a new window]   Figure 1. Relationship between percentage weight from dietary macronutrients and water (predictor variables) and energy density (outcome variable) of 1032 ready-to-eat foods, taken from the British food composition tables. Fat: energy = 320.1 + 16.3 x fat (R2 = 35.7); carbohydrate: energy = 1175 - 6.6 x carbohydrate (R2 = 8.6); protein: energy = 1117.5 - 11.3 x protein (R2 = 9.5); water: energy = 2033.8 - 21.2 x water (R2 = 66.7). From Stubbs et al. (22) .

When snack foods or foods designed to be specifically high in protein, carbohydrate or fat are examined, it becomes apparent that some high carbohydrate foods can have a considerable energy density. Furthermore, most commercially available snack foods are not high in fat or high in carbohydrate alone but are mixtures of both. There is considerable overlap in energy density between high fat and high carbohydrate foods (50%). Most snack foods are characterized as low in protein, low in moisture, high in carbohydrate and with a moderate amount of energy coming from fat. These relationships did not change much when the whole diet of 102 British adults was considered (15) . It does not, therefore, follow that an increase in the consumption of low fat foods or virtually fat free foods (especially those absolutely high in carbohydrates) will always lower dietary energy density. Thus, when examining the effect of different types of carbohydrates, and indeed fats, on feeding behavior, it is important to take energy density into account, especially in short-term studies, because diets of a low energy density will constrain intake and perhaps more so if they are also high in fiber.

	How easy is it to overeat on a high carbohydrate diet?

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Clearly, overeating depends on the energy density, palatability and type of carbohydrate in the diets concerned. Excess energy intakes are possible when normal weight men feed ad libitum on high carbohydrate diets for 14 d (17) . The sensory attribute primarily associated with short-chain carbohydrates (sweetness) is known to stimulate energy intake, especially when combined as mixtures with fats (23) . Do the sensory attributes of some sweet foods elevate palatability and, hence, intake of those foods? Dissolving short-chain carbohydrates in solution seems to be an effective means of supplementing energy intake. Most snack foods produced (rather than sold) tend to be high in carbohydrate and energy density (R. J. Stubbs, unpublished data). These considerations suggest that there is considerable scope for high carbohydrate foods to promote high levels of energy intake and, in some cases, energy balance. The exact conditions under which this occurs, and in whom, are presently unclear.

In a recent series of studies, Mazlan (16) showed that the addition of mandatory snacks rich in sugar, fat or starch (energy density: 550 kJ/100 g) led to elevated energy intakes over 7 d in men, women and lean and overweight subjects. Although short-term studies show that low energy density preloads that are high in sugar induce energy compensation (24) , a longer study showed that a less energy dense, high sugar diet consumed ad libitum led to energy intakes in women similar to those from a higher fat, more energy dense diet because subjects ate more food on the high sugar diet (25) . The high sugar diet appeared to stimulate the greatest food intake, presumably because of its high levels of sweetness. It also appears that sugar can stimulate appetite and leads to excess energy intake (and obesity) in rats.

Data from several studies in rats showed that overeating simple sugars, such as glucose and sucrose, has led to obesity, especially if those sugars are in solution (26 ,27) . Few studies have addressed this issue in humans in the longer term. Raben et al. (Raben, A., Vasilaras, T. H., Møller, A. C. & Astrup, A., personal communication, 2001) recently investigated the effect of long-term supplementation with either sucrose or artificial sweeteners (primarily as drinks) on ad libitum food intake and body weight in overweight subjects. Two groups of overweight subjects (36 women and 6 men) consumed dietary supplements containing sucrose or artificial sweeteners for 10 wk in a parallel design. On average, the sucrose intervention supplemented 3.4 MJ/d and 152 g/d of sucrose and the sweetener intervention added 1.0 MJ/d and 0 g/d sucrose per day. After 10 wk of sucrose supplementation, energy intake increased (2.6 MJ/d) as did energy density and the percentage of energy from sucrose (to 28%) and carbohydrate; the percentage of energy from fat and protein decreased. On the sweetener-supplemented diet, the only change was a small decrease (by 4% of energy intake) in sucrose intake.

Taken together, these studies suggest that excess energy intake can readily occur when subjects consume sweet short-chain carbohydrates in foods that are rich in readily assimilated energy. We may have to revise our assessments of the capacity of certain carbohydrates to elevate energy intake. This raises the question of whether sweet carbohydrates displace fat from the diet.

	Does an increased sugar intake displace fat from the diet?

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
It has been suggested that the existence of the fat-sugar seesaw means that fat and carbohydrate (especially sugar) reciprocally affect each other’s intake (8 ,9) . It has been further suggested (primarily) from epidemiological data that increasing intake of sugars may, through the operation of the fat-sugar seesaw, displace fat energy from the diet (8 ,9) and so decrease the risk of excess energy intakes. However, these simple arguments that fat is a risk for, and carbohydrate (especially sugar) a protector against, weight gain have limitations and inconsistencies.

The major limitation of the epidemiological studies is that they rely heavily on self-reported energy intake. Indeed, the fat-sugar seesaw disappears when misreporters are excluded from the dataset (28 ,29) . Ostensibly, the above relationships seem to be supported by data from laboratory studies. In a review of animal studies, most of the data showed that high fat diets are more likely to promote higher energy intake and obesity than are high carbohydrate diets (30) . However, as discussed above, the evidence relating to the effects of sugar on energy intake is far more controversial.

The literature considering how adding specific nutrients to the diet affects appetite and energy intake contains a large bias. In most of the studies, high fat, energy dense foods have been compared with low fat, less energy dense foods. Few studies have examined the effects of increasing the energy density of the diet using readily hydrolyzed or short-chain carbohydrates (14) . This is important because the explosion of the low fat food market has increased the availability of more energy dense, high carbohydrate foods (19) .

Fats and sugars are generally not consumed in isolation, and both animals and humans tend to show a strong preference for fat-sugar mixtures (23 ,31) . The relationships derived from epidemiological data separate and compare high fat with high sugar (and low fat) consumers. These studies do not consider the effects of fats and sugars in combination. Although the epidemiological data have suggested a fat-sugar seesaw, it has been noted that high sugar consumers are not low energy consumers and may have more active lifestyles (29) . Few if any studies have been specifically designed to detect changes in macronutrient selection.

Therefore, to have a clearer picture on how adding sugars and fats into the diet affects risk of excess intake, it is pertinent to examine how incrementally adding sugar and fat into the diet will affect food and energy intake. Mazlan (16) recently did this in a series of 7-d studies. In these studies, high sugar, high fat mandatory snacks were incrementally added to the diet at 0, 1.5 and 3.0 MJ/d. Energy density, palatability, taste, texture and appearance of these foods were equalized across treatments. The high fat snacks contained 80% fat, the remaining energy being evenly split between protein and carbohydrate. The high sugar snacks were of a similar design comprising 80% carbohydrate with 65% of total energy from sugar. Energy density was 550 kJ/100 g. The remainder of the diet was consumed ad libitum and consisted of a counterbalanced selection of high protein, high carbohydrate and high fat foods. These studies were conducted in lean and overweight men. The general effect of incrementally increasing these mandatory snacks was to elevate total energy intake. Subjects did not compensate at all for the high fat snacks and only compensated by 20–30% for the high sugar snacks (Fig. 2 ).

View larger version (49K): [in this window] [in a new window]   Figure 2. Effect of 0, 1.5 and 3.0 MJ of sugar-rich or fat-rich mandatory snacks of identical energy density on ad libitum energy intake in six lean and six overweight men over 7-d intervention periods. Energy from mandatory snacks shown as . Total daily energy intake increased significantly with increasing energy from mandatory snacks (fat increment: F(2,20) = 15.70; P < 0.001, sugar increment: F(2,20) = 17.79; P < 0.001). There were no statistically significant differences between the fat and sugar increments.

	Do carbohydrates influence the sensory stimulation to eat and are sweet carbohydrates a vehicle for fat intake?

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
The concept of the fat-sugar seesaw is important in the relationship between fat, carbohydrate and energy intake: if sugar displaces fat from the diet, it is unlikely to act as a vehicle for fat intake. The above discussions have led us to believe that sugar intake per se does not exert a strong leverage effect on fat intake. As mentioned earlier, many foods comprise mixtures of sugars and fats. Are these foods more potent at stimulating intake than foods individually rich in sugars or fats? The evidence is extremely limited and this is an area for further focus.

Drewnowski (31) provided compelling evidence that the sensory attributes of fats and sugars in combination are a potent stimulus to energy intake. In short-term studies, Green and Blundell (32) showed that sweet high fat foods promote almost twice the energy intake of savory high fat foods despite the savory foods being more energy dense. However, a longer-term intervention using these foods did not show much elevation of energy intake in habitual consumers of snacks (33) . We recently found that such foods elevate energy intake in subjects who do not claim to snack regularly (S. Whybrow et al., unpublished data).

Sweet, high fat foods are theoretically one of the most likely types of foods to stimulate energy intake. However, there is a dearth of medium- and long-term interventions examining this issue. Such studies need to be carefully designed because they involve a consideration of both the physiological effects of the nutrients concerned and their nutrient-associated sensory properties. It is highly likely that different types of subjects will behave differently in response to such foods, a case in point being the phenotypes of consumers of high and low fat diets.

	Which carbohydrates stimulate and which carbohydrates protect against excess energy intakes?

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 
As more foods rich in readily assimilated carbohydrates become available, the overall energy density of high carbohydrate foods is, on average, increasing. Considerable controversy exists regarding which carbohydrates promote and which protect against excess energy intake. The term carbohydrate includes a range of structures from monosaccharides and disaccharides at one end of the spectrum to a range of unavailable complex carbohydrates at the other. Shorter-chain carbohydrates are likely to have more of an intake-promoting effect and unavailable complex carbohydrates are likely to have more of an intake-restraining effect. A broad range of starches and shorter-chain carbohydrates occurs between these extremes (34) . Their sensory and physiological effects cannot be predicted from chain length alone because other factors such as the ratio of - to ß-links and branching of chains are critically important. The effects on appetite and energy balance of the vast range of specific carbohydrate subtypes are almost unknown at this level. Some carbohydrates may constrain intake because they limit digestibility of foods (35) . They may not elevate satiety, which adds further to the heterogeneity of feeding and motivational responses.

As has been discussed throughout this article, numerous confounding effects need to be controlled when comparing the effects of different carbohydrates on appetite, feeding behavior and energy balance. These effects include the energy density of foods; presence of other nutrients in foods; moisture content of foods being compared; sensory attributes of foods; and psychological, physiological and genetic predisposition of the subjects being studied. Given the number of confounding issues, it is not surprising that anything more than a preliminary assessment of which carbohydrates promote and which protect against weight gain is difficult.

Sugars vs. starch.

A number of studies have compared the satiating effects of preloads containing different hexoses and found relatively few differences between them in terms of appetite responses (13) . It is unclear whether this uniform response relates to the constraints of the preloading method or whether the monosaccharides simply have similar satiating efficiencies. There is little evidence from preloading studies or the 7-d studies of Mazlan (16) that sugars and high GI starches are different in terms of satiety or energy intake. Because there is a large range of starch subtypes, much work would be required to completely characterize the effects of starch structure on appetite and energy balance. Some work has been done on resistant starch, although with conflicting results. In the Raben et al. (25) study where higher-starch diets contained more fiber and where the high-sugar diet was sweeter, differences did occur (25) . The intake-promoting effect of the high-sugar diet was likely due to the sensory stimulation to eat associated with sweet foods.

GI of carbohydrates.

This area is equally controversial, probably because high- and low-GI foods often differ in more than just the GIs of the carbohydrates concerned. Roberts (36) recently reviewed this issue. Of six studies reviewed, four low-GI treatments were found to promote satiation within a meal whereas two did not. Between-meal satiety was less easy to predict because three studies showed low-GI foods to enhance postmeal satiety and two studies showed that high glycemic index carbohydrates had the same effect. There was a similar lack of consensus about whether high or low glycemic index foods delay the return of hunger. The most consistent finding was that low glycemic index foods reduced subsequent energy intake in short-term interventions. The reasons for this effect have been suggested by a most elegant study conducted by Harber et al. (37) in 1977. They took the simple approach of comparing the same weight of ingested whole apples, puréed apples (disrupted but without the structural fibers) or apple juice (without the fiber). Ten subjects ingested test meals based on the intact, puréed or juiced apples, each containing 60 g of available carbohydrate. Juice could be consumed 11 times faster than intact apples and 4 times faster than purée. With the rate of ingestion equalized, juice was significantly less satiating than purée and purée was significantly less satiating than intact apples. The authors noted that plasma glucose concentration rose to similar levels after all three meals, but a striking rebound fall occurred after juice and to a lesser extent after purée but not after intact apples. Serum insulin concentration rose to higher levels after juice and purée than after intact apples (Fig. 3 ).

View larger version (16K): [in this window] [in a new window]   Figure 3. The effects of disruption or removal of dietary fiber from apples on plasma glucose and insulin concentrations. Juice and purée were consumed in a time similar to that for the whole apples. To convert insulin values from µ/L to pmol/L, multiply by 7.175; to convert glucose values from mg/dL to nmol/L, multiply by 0.05551. Reprinted with permission from Harber et al. (37) .

Once carbohydrates are ingested and any primary interconversions (e.g., fructose to glucose) are made, there is little reason to suppose that there is much difference among carbohydrates in their effects on satiety. Most differences are likely to be due to sensory, preabsorptive and absorptive events. The study by Harber et al. (37) illustrates how simple mechanical disruption or removal of structural carbohydrates from a food can influence all three.

Dietary fiber.

Most work on the effects of different carbohydrates on energy intake has been done with unavailable complex carbohydrates (UCC)³ or fiber. The time-energy displacement concept has been invoked to suggest that the addition of UCC to the diet enhances satiation and limits meal sizes. This effect is apparent in some of the studies discussed above and the phenomenon has been used to limit weight gain in farm animals consuming single feeds.

Over 50 studies have been conducted to examine the effects of dietary fiber on food intake and body weight (38) . These have been extensively covered in four recent reviews to which the reader is referred for a detailed discussion of this issue (38 –41) . In summary, various loads of UCC or fiber at one meal have been shown to decrease hunger and energy intake at the next meal, but the effects are relatively modest. Levine and Billington (38) note that 26 of 38 long-term studies examined the effects of increased UCC ingestion on body weight. The results of this seemingly large number of trials are equivocal because of the different forms of fiber used, different vehicles chosen (i.e., ranging from real foods to tablet formulations), different subject populations and the various degrees of experimental control ranging from overt to double-blind manipulations. The conclusion seems to be that supplementing the diet with tolerable levels of extracted UCC seems to have, at best, modest effects in decreasing body weight over several months or more. However, fiber-rich bulky diets of low energy density may have different effects; the reader should consider the methodological issues detailed in a number of references (38 –41) before drawing firm conclusions. A major problem with the notion of using dietary fiber to limit intake is that people do not enjoy very fibrous foods and, therefore, tend not to select them.

Wet vs. dry carbohydrates.

Mattes (42) recently conducted a meta-analysis of feeding responses to either liquid or solid manipulations of the nutrient and energy content of the diet. The analysis suggests that the physical state of the ingested carbohydrate may be important in influencing subsequent energy compensation. The reasons for this are unclear but may relate to the rate, timing and density at which the energy is ingested. A threshold may exist below which energy is poorly detected. In 1955, Fryer (43) supplemented the diet of college students for 2 mo with a high carbohydrate drink containing 1.8 MJ/d. Compensation was incomplete (50%) after 8 wk. The recent study by Raben et al. (personal communication, 2001) also confirms that supplementing the diet with wet carbohydrates can lead to elevations of both energy intake and body weight in the long term.

A large range of carbohydrate subtypes have specific structures that either alone or in combination with other nutrients are likely to influence appetite and energy balance. The effects on some aspects of appetite and energy balance of some of these carbohydrates are summarized in Table 1 . At present very little is known about which aspects of carbohydrate structure are most likely to influence motivation to eat and feeding behavior. Here lies an expanse of virgin territory for research into the development of functional foods.

Very little is known about the vast range of different starches and their various structures in relation to appetite and energy balance. These are still uncharted landscapes on the research horizon. The foods most capable of limiting energy intake (both voluntary and metabolizable) are those rich in UCC. However there is a catch: in general, humans are not too fond of these foods. As typified by the average Western diet, when given the choice, we tend to select a diet comprising 37–42% fat, 10–20% sugar and a variable amount of high glycemic index starches. The average fiber intake of Western adults is spectacularly low. There is growing evidence that wet carbohydrates are particularly conducive to weight gain in humans as well as rodents. The evidence relating to the glycemic index of carbohydrates currently cannot be interpreted because of the heterogeneity of study designs, vehicles and treatments used. This should be an area of future research. Given the range of carbohydrate structures available to the food market and the different physiochemical properties that they possess, it is particularly important to identify how these potentially beneficial effects of carbohydrate structure can be used to enhance preabsorptive and absorptive satiety signals. We have only just scratched the surface.

View larger version (0K): [in this window] [in a new window]   Figure .

	FOOTNOTES

³ Abbreviation used: UCC, unavailable complex carbohydrates.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION Perspective Doubts about the role... Current issues relating to... Bioenergetic perspective: are... Relationship between dietary... How easy is it... Does an increased sugar... Do carbohydrates influence the... Which carbohydrates stimulate... LITERATURE CITED

 

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