Thermogenic foods can increase the thermic effect of food. In addition, they may cause an up regulation of brown adipose tissue and this contributes to adaptive thermogenesis.
The energy balance equation is often used to justify that weight gain and obesity is caused by too much food and too little exercise. This ‘eat-too-much, do-too-little’ hypothesis of weight gain blames the individual for their excessive body fat. Blaming the individual is partly justified because personal responsibility should be taken for nutritional intake. However, allowances should be made for the disinformation that is propagated by the food manufacturers, who obfuscate the real causes of obesity. Evidence now suggests that it is not the amount of food consumed that leads to excessive weight gain, but the type of food consumed. The traditional energy balance equation is therefore not helpful because it misleads the public, because energy balance is far more complex than that portrayed in medical textbooks. Energy expenditure for example, is composed of numerous dynamic components under the regulation of the hypothalamus. A detailed analysis of the regulatory mechanisms of energy balance is therefore justified.
Total 24 hour energy expenditure is subdivided into a number of individual contributory components (figure 1). The biggest contributor to total energy expenditure is the resting metabolic rate (RMR). Resting metabolic rate is defined as the amount of energy expended at rest, several hours after a meal or any physical activity. The RMR is essentially the energy expended in maintaining system functions. A similar component is the basal metabolic rate (BMR) that is measured after a longer period without food or exercise. The BMR is lower that the RMR and usually equates to around 75 % of the RMR. The thermic effect of activity (TEA) is the increase in energy expenditure following exercise, a small proportion of which is due to the actual exercise. The metabolic efficiency of exercise is less than 30% and the rate of energy expenditure can vary depending on the duration and intensity of the exercise. The thermic effect of a sedentary individual can account for 20 % of total energy expenditure but this can double with exercise.
Figure 1. The components of total energy expenditure. RMR = resting metabolic rate, BMR = basal metabolic rate, TEA = thermic effect of activity, TEF = thermic effect of food, AT = adaptive thermogenesis.
The thermic effect of food (TEF) is the increase in energy expenditure above the RMR following ingestion of food (figure 2). This is due to the metabolic cost of processing the food. The TEF accounts for around 10 % of the total energy expenditure, but this can change depending on the exact composition of the diet. However the TEF is always greater than can be predicted, and the difference between the predicted (obligatory) and the actual (facilitative) TEF is not understood, although explanations have been suggested. For example, the discrepancy may due to futile cycles wasting energy as glucose is passed through 3-carbon cycles. In addition, release of catecholamines might cause increases in thermogenesis. Adaptive thermogenesis (AT) is a component of total energy expenditure that is described as energy expenditure that yields heat but without work. Adaptive thermogenesis includes the facultative component of TEF, and the rest is likely explained by non-shivering thermogenesis in brown adipose tissue.
Figure 2. The thermic effect of food.
Energy balance is therefore complex and the simple ‘eat-too-much, do-too-little’ hypothesis of weight gain cannot explain obesity. For example, if the TEA is increased through activity, and the energy intake reduced through calorie restriction, the traditional model states that weight loss will ensue. However, it is possible that faced with caloric restriction and increase energy expenditure, the hypothalamus can simply reduced the expenditure of energy through AT, the TEF or the RMR. In this way weight loss cannot be forced on the body without the necessary compliance of the hypothalamus to allow weight loss. This is pivotal because the energy balance equation is not controlled by the individual, but by the requirements of the individual, and under control of the autonomic nervous system. Setting a caloric intake of 20 % below maintenance and increasing expenditure through exercise often fails to cause long-term weight loss, because the other components of energy expenditure are adjusted to allow no overall net weight loss.
Additional insights into energy balance can be deemed from experiments that force feed subjects high energy intakes. If the energy balance equation is to be believed as valid, this should cause rapid increase in body weight, mainly as fat. However, some individuals do not gain weight on such regimens, and often any weight gain present includes large amount of lean tissue. The lean tissue increase explains the increase in the RMR seen on such forced feeding regimens. However, energy expenditure increases cannot be explained solely by increases in RMR. Therefore it is likely that the additional energy expenditure is made up of increases in the TEF and the TEA. Further, starvation studies do not cause beneficial body composition changes long term, with most weight lost being accounted for by lean tissue. That healthy diets cause weight loss without the need for exercise or caloric restriction also calls into question the traditional energy balance equation and suggests that the ‘eat-too-much, do-too-little’ hypothesis of obesity is wrong.
