MECHANISMS BY WHICH DIETARY CARBOHYDRATES AFFECT ENERGY BALANCE

9.2 MECHANISMS BY WHICH DIETARY CARBOHYDRATES AFFECT ENERGY BALANCE

For body weight changes to occur, energy intake and energy expenditure cannot be equivalent. Thus, if a dietary component such as carbohydrates is to consistently influence body weight, it follows that this component must influence one or more factors that influence the consumption or expenditure side of the energy balance equation. In this section, we review the evidence for an association between carbo- hydrates and several key factors that have the potential to influence energy balance. These factors include satiation and satiety, energy density of foods, palatability and taste preference, the fat:carbohydrate ratio of the diet, metabolic fuel partitioning, and the glycemic index.

9.2.1 S ATIATION AND S ATIETY

To begin to understand the potential role of carbohydrates in energy regulation and the development of overweight, it is necessary to understand the role carbo- hydrates play in appetite and hunger control. The term satiation will refer to the sensation of fullness experienced during a meal or eating episode that controls

meal size. 6 Satiety will refer to the sensation of fullness experienced between meals or eating episodes that inhibits the resumption of eating and extends the duration between meals. 6 Because protein exerts a distinctly high level of satiation and satiety, most studies maintain constant protein levels when studying the effects of the other macronutrients, carbohydrates and fat, on satiation and satiety. 7,8 When lean subjects were allowed to eat from a range of high-fat or high-sucrose foods, passive overconsumption occurred only when high-fat foods were consumed. 9,10 The term passive overconsumption indicates that an individual does not deliber- ately intend to ingest excess calories, but rather signals for satiation do not function effectively to control meal size. Therefore, although dietary carbohydrates do not exert the powerful satiation and satiety effects of dietary protein, they have a greater effect than dietary fat. 7,11–13

Not all carbohydrates, however, exert the same effect on satiation and satiety. It has been hypothesized that sugars and other simple carbohydrates may encourage

Functional Food Carbohydrates

consumption beyond the sensation of fullness, because of their relatively high pal- atability and rapid rate of digestion. The rapid absorption of simple carbohydrates into the bloodstream has been shown to trigger a release of insulin in excess of need, resulting in a rapid and steep decline in blood sugar, and hence a more rapid and intense resumption of hunger (see glycemic index below). A high sugar intake has also been shown to lead to insulin resistance, de novo hepatic fatty acid synthesis,

and visceral fat deposition in rats. 14 Both animal and human studies suggest that high consumption of simple carbohydrates can lead to excess energy intake. 8,15 Dietary fiber, on the other hand, appears to protect individuals from passive overconsumption and subsequent positive energy imbalance by promoting increased satiation and satiety in comparison to digestible complex carbohydrates and simple sugars. 10,16–18 The mechanisms by which dietary fiber promotes satiation and satiety include increased chewing, increased gastric distention, delayed gastric emptying, decreased rate of nutrient absorption and digestion, and unique effects on gut hormones.

The increased chewing time required for foods that are naturally high in dietary fiber may promote satiation by reducing the rate of ingestion as well as distending

the stomach as a result of increased gastric juice and saliva secretion. The ingestion of high-fiber foods, especially those foods that are high in soluble fiber, results in

a viscous gel formation in the stomach, which is thought to increase gastric distention and ultimately reduce the rate of gastric emptying. As a result, nutrient digestion and absorption are also delayed, which tends to increase satiety. 16–18 Postprandial blood glucose concentration also tends to be lower after high-fiber than after low- fiber meals or foods. As a result, insulin secretion is reduced, as is the likelihood of reactive hypoglycemia during the postabsorptive period, consequences which may also promote satiety. Finally, dietary fiber ingestion increases the secretion of such gut hormones as cholecystokinin, glucagon-like peptide-1 (GLP-1), peptide YY, and neurotensin, which may alter glucose homeostasis or act independently of glycemic response as satiety factors. 17,18 For example, when GLP-1 is provided exogenously,

it slows gastric emptying and reduces hunger in the presence of fiber. 17 It is likely that dietary fiber’s unique ability to promote satiety and satiation through a number of different mechanisms may be influential in preventing excess calorie intake, and ultimately positive energy imbalance or overweight development.

9.2.2 E NERGY D ENSITY

Energy density refers to the calorie content of a given weight of food (calories/gram). Comparisons of results from studies examining energy density are difficult because

of differing or poorly defined measures of energy density. 19 However, energy density may be an important determinant of energy intake and thus energy balance. 20 Clinical

studies have demonstrated that total caloric intake is markedly affected by manipu- lations in energy density, independent of the percentages of energy from macronu- trients. 21,22 It has also been found that obese subjects consume a greater proportion of foods high in energy density than lean subjects. 19,23

The energy density of a food is, in part, a function of its macronutrient compo- sition. 24 Both dietary proteins and carbohydrates have a relatively low energy density,

Carbohydrates and Obesity

providing only 4 calories per gram. Dietary fat, on the other hand, is much more energy dense, providing 9 calories per gram. Alcohol is also fairly energy dense,

providing 7 calories per gram. Yao and Roberts 20 concluded that the two most significant determinants of dietary energy density are water and fat content. Figure

9.1 illustrates the association of energy density (kcal/100 g) with fat, water, and fiber contents (g/100 g) of 200 commonly consumed foods.

Dietary fat is a major determinant of energy density as a result of its high caloric density compared to carbohydrates or proteins, and the wide variation in the pro- portion in which it is found in commonly consumed foods. 20 Because carbohydrates have a naturally lower energy density than fat, they are less likely to lead to passive overconsumption and subsequent positive energy imbalance. However, it should not

be assumed that all foods high in carbohydrates are low in energy density. For instance, commercially available low-fat foods or virtually fat-free foods (specifi-

cally those with large amounts of added sugars or other concentrated carbohydrates) can have considerable energy density. 8 In general, nonprocessed foods that are naturally high in carbohydrates and low in dietary fat (e.g., fruits and whole grains) are low in energy density and unlikely to lead to passive overconsumption and a subsequent positive energy imbalance.

The impact of water on the energy density of foods is due to its zero-energy content and the wide variation in the proportion in which it is found in commonly consumed foods. It has been shown in experimental studies that so-called wet carbohydrates (high-carbohydrate foods with high water content, such as soups and fruits) have a higher satiety value than dry carbohydrates, such as pretzels and bagels. An exception to this rule, however, is beverages. Sugars in liquid form have a particularly weak appetite suppressant effect. Calorie intake in the form of beverages is not proportionately compensated for by a reduction in subsequent intake of solid foods and could therefore lead to excess calorie intake. 25

Dietary fiber has the potential to influence energy density because of its minimal energy content. 20 However, in actuality, the influence of fiber on energy density is

modest because the fiber content of foods does not vary widely, and there is an upper limit to the amount of fiber found in foods typically consumed by humans. As seen

in Figure 9.1, 20 fiber content was not significantly related to energy density when studied in 200 commonly consumed foods. Therefore, although dietary fiber can

contribute to a reduction in the energy density, this effect is dwarfed by the much larger impact of water and fat on energy density of commonly eaten foods.

9.2.3 P ALATABILITY AND T ASTE P REFERENCE

Palatability , a subjective measure of the pleasantness of food, has consistently been shown to influence food choice and ultimately dietary intake. 26 Palatability and energy density are inextricably linked; 27 energy-dense foods are generally highly palatable. Highly palatable energy-dense foods, such as those foods high in dietary fat and low in water, are associated with increased caloric intake during single meals

and with increased intake at subsequent meals. 20 Conversely, carbohydrate-rich foods, which are naturally low in energy density, such as foods high in fiber, tend

to be less palatable. However, as previously noted, foods high in carbohydrates are

Functional F FIGURE 9.1 The association of energy density (kcal/100 g) with fat, water, and fiber contents (g/100 g) of 200 common foods randomly

selected from the Fred Hutchinson Cancer Research Center Food Frequency Questionnaire (FHCRC/Block FFQ, version 06.10.88). Nutrient contents of the food were calculated using standard food composition tables (Minnesota Nutrition Data System, software developed by the

ood Carboh Nutrition Coordination Center, University of Minnesota, Minneapolis; Food Database version 11A; Nutrient Database version 26, 1996).

(From Yao, M. and Roberts, S.B., Nutr. Rev., 59, 247, 2001. Permission for use granted by the International Life Sciences Institute.)

ydrates

Carbohydrates and Obesity

not always low in energy density. There are a number of high-carbohydrate foods that are both energy dense and highly palatable (e.g., desserts such as cakes, cookies, doughnuts, and ice cream). Consuming these foods could lead to passive overcon- sumption, and ultimately to a positive energy imbalance.

Taste is usually the number one reason given for eating a specific food, and a decrease in good taste is often given as a reason for terminating or reducing food

intake. 20 The sense of taste, termed taste preference, mediates the relationships among metabolic status, food acceptance, and actual food consumption. 28 Taste preferences appear to have both genetic and acquired components and tend to vary by gender. Women tend to prefer foods such as chocolate, ice cream, doughnuts, cookies, and cake (foods with a high sweet–fat combination), and men tend to prefer salty, meaty foods, such as meatloaf and steak (foods with high protein and fat

content). 29 Taste preferences also tend to vary by weight status; lean subjects have stronger taste preferences for sweet foods, 19 whereas obese subjects tend to have stronger preferences for fat-rich foods. 30

9.2.4 D IETARY F AT :C ARBOHYDRATE R ATIO

There is a high degree of intercorrelation between the percentages of energy derived from dietary fats and carbohydrates in the diet. Observational studies have identified

this significant inverse correlation in a number of populations. 31,32 Intervention trials have also illustrated this inverse relationship, whereby reductions in dietary fat intake

have been typically accompanied by increases in the percentage of energy derived from carbohydrates, while the percentage of energy derived from protein remains

rather stable. 33 The reciprocal relationship between the percentages of energy from dietary fats and carbohydrates has been coined the fat–sugar seesaw. 8

A high dietary fat:carbohydrate ratio has been associated with greater total energy intake and greater body weight. 33,34 The dominant mechanism by which this ratio influences positive energy imbalance is through properties of dietary fat that lead to excess calorie intake and not through properties of dietary carbohydrates.

Dietary fat’s energy density leads to passive overconsumption, 9 and reductions in dietary fat have been accompanied by decreases in total energy intake. 33 Therefore, it can be concluded that a low-carbohydrate diet may increase the risk of positive energy imbalances or overweight development by way of its correlation with a high dietary fat intake.

9.2.5 M ETABOLIC F UEL P ARTITIONING

Use of carbohydrates and proteins as fuel sources by the body varies in accordance with the amounts consumed in a given meal. In other words, the consumption of carbohydrates promotes their oxidation. Carbohydrate consumption also promotes carbohydrate storage. Glycogen, the storage form of carbohydrate in animals, is stored in both the liver and muscle cells. The storage capacity for carbohydrates is limited and, for optimal functioning, is normally maintained within a relatively narrow range (i.e., ~200 to 500 g in adults). In comparison, body fat, the other form of stored energy in the body, is present at highly differing levels among individuals,

Functional Food Carbohydrates

and the capacity to store fat is relatively unlimited. 35 Following meal consumption, the carbohydrate component of the meal is utilized first for energy, and the dietary

fat is taken up predominantly by the adipose tissue and stored until insulin levels fall during the postprandial period. The amount of carbohydrates consumed in a meal determines the extent to which carbohydrates suppress oxidation of dietary fat and promote its storage. De novo lipogenesis, the conversion of carbohydrates into fat, occurs when the body’s total glycogen stores are considerably raised from their usual 4 to 6 g/kg of body weight to >8 to 10 g/kg of body weight. This requires deliberate and sustained overconsumption of large amounts of carbohydrates for 2

to 3 days. 35 It should be noted that dietary protein can similarly be converted to fat. In summary, increased dietary carbohydrate intake increases the rate of carbohydrate oxidation and storage, and an elevated rate of carbohydrate oxidation suppresses fat oxidation and promotes fat storage. 36

The use of fat as body fuel is determined primarily by the gap between total energy expenditure and the amount of energy ingested in the form of carbohydrates and proteins. 35 Since the fraction of total dietary energy provided by proteins is relatively small and relatively constant, and because the body spontaneously main- tains a nearly constant protein content by adjusting amino acid oxidation to amino acid intake, fat oxidation is regulated primarily by events pertaining to the body’s carbohydrate economy. 35,36

9.2.6 G LYCEMIC I NDEX

The glycemic index (GI) is a classification of the blood glucose-raising potential of carbohydrate-containing foods. 37 It is computed by calculating the area under the

glycemic response curve during a 2-h period after consumption of 50 g of carbohy- drates from a test food; the value is expressed relative to the effect of a standard, which is either glucose or white bread. Many dietary factors, such as starch chem- istry, fiber content, fat content, and physical form of food (e.g., liquid vs. solid form),

influence the GI of a food. 38 There is concern that the consumption of high-GI foods increases insulin output from the pancreas. Chronically high insulin output leads to

a number of deleterious effects on the body, such as high blood triglycerides, increased fat deposition in adipose tissue, increased fat synthesis in the liver, and a more rapid return of hunger after a meal.

Some scientists have hypothesized that the consumption of high-GI foods is associated with a positive energy imbalance and the rising prevalence of overweight and obesity. 39,40 They further suggest that the reduction in dietary fat, as advocated by the federal government and various other official medical and health agencies, has led to a compensatory rise in carbohydrate consumption, and the carbohydrates

that tend to replace fat in low-fat diets are typically high in GI. 39 Studies that have examined the relationship between the glycemic index and energy imbalance have been limited by their short-term duration. These studies have investigated the associations between GI and hunger, satiation, and satiety and have had inconsistent

results. 40 Futhermore, higher dietary fiber content is often associated with low GI in foods and meals. It is often difficult to assess if the positive metabolic effects

are a result of the high dietary fiber content or low-glycemic-index nature of a

Carbohydrates and Obesity

meal, as these two characteristics are often present together in foods. 41 Long-term clinical trials are necessary to understand the effects of GI on body weight regu- lation.

9.2.7 S UMMARY OF M ECHANISMS

In conclusion, our current understanding regarding the properties and metabolism of carbohydrates suggests that a higher proportion of carbohydrates in the diet would be protective against a positive energy imbalance, and therefore obesity development, but not all carbohydrates are alike. Different types of carbohydrates have distinct properties and distinct impacts on various factors that impact energy balance. Nonbeverage foods with wet carbohydrates (those with high water con- tent), high fiber content, and low energy density have properties that would tend to protect against obesity development. Foods with more concentrated and pal- atable forms of carbohydrates, especially in beverage form, would be more “obesigenic.”