Energy Balance Equation

To maintain a stable body weight, energy intake must be balanced by an equivalent amount of energy expenditure, as shown by the following equation:

E_in – E_out = E_stored

in which E_in represents energy intake, E_out represents energy expenditure, and E_stored represents energy stored in the body (the difference between energy intake and expenditure). All components of this equation can be characterized in terms of the amount and composition of energy. The macronutrient composition of ingested food becomes particularly important when one considers that maintenance of constant body weight and composition requires not only that energy intake equal energy expenditure but that, on average, the intakes of the major nutrients comprising body tissues—protein, carbohydrate, and fat—are balanced by equivalent oxidation of each. Failure to achieve this nutrient balance results in changes in body stores of protein, carbohydrate, or fat. Significant changes in these compartments as a result of cumulative effects over time can influence metabolic processes (for example, the rate of fatty acid oxidation), which can, in turn, alter the proportions of metabolic fuels oxidized [1–3]. Furthermore, alterations in the fuel mixture can affect the steady-state body weight and body composition that is reached under a given set of dietary conditions [2–4].

Diet Composition, Energy Balance, and Nutrient Balance

In this section, we discuss the effect of diet composition on energy intake, energy expenditure, and nutrient balance in general terms and under conditions of weight reduction.

Effects on Energy Intake

Most research on the effects of diet composition on food intake has focused on the fat and carbohydrate components of the diet, whereas protein has been less studied. Studies in rodents and humans have shown that voluntary energy intake tends to increase, with increases in the proportion of dietary energy supplied by fat [5–7]. Although some reports have indicated that diets high in sweet carbohydrates (for example, sucrose) elevate long-term energy intake in animals [8], a similar long-term effect has not been described in humans. High levels of protein (> 25% of calories) have a suppressive effect on energy intake in animals [9]. In humans, high-protein loads acutely reduce subsequent energy intake relative to low-protein foods [10]. The effects of protein are probably of limited importance in humans because protein makes up a small and relatively constant proportion (10% to 15%) of calories [11, 12].

Effects on Energy Expenditure and Nutrient Oxidation

Energy expenditure (E_out) involves several factors, as shown by the following equation:

E_out = RMR + EE_ACT + TEF

in which RMR is the resting metabolic rate, EE_ACT is the energy expenditure associated with physical activity, and TEF is the thermic effect of food.

Diet composition has not been shown to directly influence resting metabolic rate, which accounts for more than 50% of total daily energy expenditure in sedentary persons [13]. Because the main determinant of resting metabolic rate is fat-free mass (with fat mass as a lesser determinant) [13], diet composition may have secondary effects on resting metabolic rate through long-term effects on body composition.

Studies in humans have yielded no data suggesting that diet composition can directly affect the energy expended in physical activity. To calculate the energy expended in physical activity, both the amount and energy cost of physical activity must be considered. Unlike for resting metabolic rate, the factors that can explain the interpatient variation in amount and energy cost of physical activity are not well understood. Little is known about the influence of diet composition on the amount of physical activity; however, this area deserves further study.

The third component of energy expenditure is the thermic effect of food, which represents the energy expended in digesting, absorbing, interconverting, and storing ingested nutrients. Overall, the magnitude of the thermic effect of food is small (about 6% to 10% of total ingested energy) and is less than the error involved in measuring food intake under outpatient conditions. Both the amount [14] and composition [15, 16] of food can affect the thermic effect of food; however, increasing the proportion of dietary fat from 20% to 60% of total calories while keeping the proportion of protein constant does not produce a measurable difference in energy expenditure during a 24-hour period [17].

Effects on Nutrient Balance

Nutrient balance represents the difference between the intake of a given energy-yielding nutrient and its oxidation during a specified time period. In the case of fat, a person is in fat balance when the oxidation of fat by the body is equal to the intake of fat from the diet. If this condition is not met, fat is either gained or lost. A similar situation is true for the other major dietary energy sources—protein and carbohydrate.

Recent work has shown that the oxidation of protein and carbohydrate is closely tied to their intake, whereas fat oxidation is not as tightly correlated with intake [18–21]. Thus, both protein and carbohydrate are actively metabolized after ingestion, and the balance of these nutrients is closely maintained. Fat oxidation, conversely, is not actively driven by fat intake but occurs passively as the difference between total energy expenditure and the oxidation of protein and carbohydrate [22]. Because fat intake does not promote its own oxidation, maintenance of fat balance requires that other factors promoting fat oxidation, such as negative energy balance or exercise, must compensate for the fat consumed in the diet. The tendency for persons to overconsume energy when the diet is rich in fat and the prevalence of sedentary lifestyles in Western societies undoubtedly contribute to the high incidence of obesity in these populations.

Influence of Obesity Treatment on Energy and Nutrient Balance

By definition, the treatment of obesity involves the creation of a negative energy balance, a condition in which energy expenditure exceeds energy consumption. When this condition is achieved, body energy stores are consumed to sustain metabolic processes, and weight loss ensues. Because a negative energy balance is required for successful obesity treatment, we should consider the effects of this physiologic state on the main components of the energy balance equation described earlier and on nutrient balance.

When a negative energy balance is achieved through reduced energy intake, the primary and short-term effect on energy expenditure is a reduction in the thermic effect of food proportional to the reduction in energy intake. As caloric restriction continues, the resulting decrease in body mass is accompanied by a reduction in the resting metabolic rate. The energy expended in physical activity also decreases because the energy cost of movement is less at lower body masses [23].

Exercise in conjunction with caloric restriction is frequently used in obesity treatment programs. The primary effect of exercise is to increase energy expenditure during periods of activity. Physical activity does not appear to have significant effects on the thermic effect of food [24]. When combined with food restriction, exercise may preserve fat-free mass, but the effect is small and is unlikely to have a measurable effect on the decrease in resting metabolic rate that accompanies weight reduction [25].

The effect of a negative energy balance on nutrient oxidation and balance is necessarily dominated by the metabolic condition associated with energy deprivation. Regardless of how the negative energy balance is achieved, the body is forced into a negative protein and fat balance. This condition occurs because fat is the major stored fuel oxidized under hypocaloric conditions (fasting or dieting) and because some net protein oxidation is necessary due to the loss of tissue supporting the lost fat. In addition, protein loss occurs due to the essential role of protein in providing amino acids that can be converted to glucose. Carbohydrate balance may be negative initially after the imposition of a negative energy balance, but a new steady-state balance (possibly at a reduced level of body carbohydrate stores) is reached within several days and is maintained. Carbohydrate will remain the preferred fuel for several important tissues (for example, brain and formed blood elements) even during a complete fast, and the body is equipped to provide this fuel by converting amino acids into glucose through the process of gluconeogenesis.

Diet Composition and Obesity Treatment

Diet composition is one of many variables that can influence the short- and long-term outcome of an obesity treatment program. In the next section, we address some of the most commonly asked questions about the effect of diet composition in the treatment of obesity.

Effect of Diet Composition on the Amount, Rate, and Composition of Weight Loss

Diet composition has no significant effect on the amount, rate, or composition of weight loss under conditions of a negative energy balance. The most significant factor determining the amount and rate of weight loss is the degree of negative energy balance achieved by the weight-loss regimen. When body weight is lost under conditions of negative energy balance, both fat and lean tissue are lost, and, although diet composition can potentially affect the type of tissue lost, such effects are very small, given the short durations of most obesity treatment programs.

During the weight-loss phase of obesity treatment, the most significant effects of diet composition are on the thermic effect of food and on the degree of behavioral compliance with the prescribed diet. The thermic effect of food is reduced when energy intake is lowered. Table 1 shows the magnitude of the effect of diet composition on the thermic effect of food. In a person eating 2500 kcal/d, the difference in the thermic effect of food between a diet high (40% of calories) and low (20% of calories) in fat content would be about 30 kcal/d. If the same person reduced his or her daily intake to 1200 kcal/d, the thermic effect of food would be reduced by more than 50%. The difference in the thermic effect of food between the high- and low-fat diets in this situation would be only 15 kcal/d. This difference would be even smaller for a person on a very-low-calorie diet. This example shows that diet composition has a small overall effect on energy expenditure and an even smaller effect during caloric restriction. Such an effect could influence body weight regulation only during extended periods (several months at minimum). Considering the relatively short periods during which food restriction is used for the treatment of obesity, diet composition would have an insignificant effect on energy balance.

One of the most difficult challenges faced by an obese person is adherence to a prescribed diet. The stronger the feeling of hunger and deprivation, the greater the urge to break the diet. It is possible that diet composition may affect feelings of hunger and thus influence the ability of patients to adhere to the weight-loss regimen. For example, the inclusion of carbohydrate in very-low-calorie diets has been suggested to decrease feelings of hunger; however, the available data show mixed results [26, 27].

Influence of Diet Composition on the Successful Maintenance of a Reduced Body Weight

Diet composition may affect long-term success in maintaining a reduced body weight. Most obese persons who successfully lose weight regain it within a period of months after the treatment program [28]. Thus, the behavior of the individual after the active weight-loss phase is critical in determining the long-term success of obesity treatment. After weight loss diet composition can have important effects on total energy intake and on its influence on nutrient balance. As previously mentioned, several experiments have shown that energy intake in humans increases in proportion to increased dietary fat [5–7]. Although this rule may not apply to all persons, it is reasonable to accept that the abundant availability of diets rich in fat will increase the probability of occurrence of periods of positive energy balance. Under conditions of positive energy balance, the amount of body fat stored is proportional to the fat content of the diet [29]. This condition occurs because dietary fat does not promote its own oxidation, as do protein and carbohydrate, and because the efficiency of storage of dietary fat as body fat is very high (> 95%).

After a reduction in body weight has been achieved, it can only be maintained if energy intake is equal to energy expenditure. Maintenance energy intake after weight reduction is necessarily less than before weight loss because less energy is required to maintain a reduced body mass. In addition to the need to permanently adjust total energy intake to match the altered total energy requirement of the individual, the composition of the diet must be permanently altered to match the composition of the fuel mixture burned. This condition is necessary for the maintenance of a nutrient balance and of a constant body composition. Because a successful weight-loss regimen results in a disproportionate loss of fat (compared with lean) mass, the composition of the maintenance diet must be lower in fat; that is, less dietary fat is required to maintain a reduced body fat mass. The size of the shift in diet composition needed to maintain a nutrient balance after weight reduction is related to how closely the fuel mixture burned by the person at the reduced weight matches the fuel composition of the person's habitual diet (before weight loss). If the person is attempting to maintain a reduced body weight on a regimen including little or no exercise, then dietary fat intake must be reduced significantly. Conversely, a maintenance regimen including aerobic exercise will require a smaller dietary adjustment because exercise increases fat oxidation.

Permanent alterations in the amount and composition of food eaten are very difficult to achieve, and this difficulty undoubtedly contributes to the high rate of weight regain after successful weight reduction. Increased physical activity is also not easy to maintain but can provide a substantial weapon in the battle against positive fat balance and obesity.

Conclusions

Diet composition is one of many variables that can influence the success of obesity treatment. The effect of diet composition is likely to be greatest after (rather than during) the weight-loss phase of obesity treatment. After weight reduction, maintenance energy requirements are reduced due to a reduction in body mass, and fat oxidation is reduced due to a large decrease in body fat mass. Prevention of weight regain requires permanently reduced total energy intake to match the new maintenance energy requirement and reduced fat intake to match the reduced fat oxidation. American adults typically eat diets containing 35% to 45% of calories from fat [11, 12], and such diets may create a positive fat balance due to the low rate of fat oxidation and the failure of dietary fat to promote its own oxidation. In addition, such diets also promote excess energy intake compared with diets predominated by carbohydrate or protein.

Conversely, aerobic exercise increases fat oxidation and thus mitigates against a positive fat balance. The combination of a diet with a reduced level of fat (probably less than 30% of calories) and a lifestyle that incorporates an increased level of physical activity maximizes the chance of successful maintenance of a reduced body weight after initial weight loss. Such a regimen would also be effective in the prevention of obesity.

Because changes in diet and activity level require permanent changes in behavior, many adults may find these goals difficult to achieve. With this in mind, future work aimed at treating and preventing obesity should focus on the early development of preventive behaviors, preferably during childhood.