Thermogenic responses induced by nutrients in man: their importance in energy balance regulation

The regulation of body weight depends upon the control of food intake and the regulation of energy expenditure. In man, the control system for food intake may be overwhelmed by psychological or social influences and the thermogenic response to a variable energy input may play an important role in the energy regulatory system. Energy expenditure can be divided into 3 components: basal metabolic rate, thermogenesis and physical activity. Of these 3 components, thermogenesis, (i.e. the energy expended above the metabolic rate in the resting state) is the expenditure. The two main factors which contribute to thermogenesis, i.e food intake and cold exposure, elicit diet-induced thermogenesis (DIT) and non-shivering thermogenesis (NST), respectively. It is of interest to study thermogenesis in individuals who present a tendency to gain weight, in order to assess whether the thermogenic responses may be lower in these subjects than in lean controls. It has recently been shown that DIT consists of two separate components which can be described as "obligatory" and "regulatory" thermogenesis. The former is due to the energy costs of digesting, absorbing and converting the nutrients to their respective storage forms. The latter is an energy dissipative mechanism, mainly studied in animals. There is good experimental evidence showing that brown adipose tissue (BAT) is involved in the adaptive thermogenesis observed in rats fed a varied and palatable "cafeteria" diet. In addition, a thermogenic defect in BAT has been demonstrated in adult as well as young genetically obese animals, and this defect is present not only in adult, but also in young (12 day old) ob/ob mice, i.e. before the development of obesity. Thus, a defective thermogenesis seems to be a cause, rather than a consequence, of obesity in these animals. In man, the role of thermogenesis in energy balance regulation is not yet understood. Some conflicting results may have arisen from inadequate techniques to measure energy expenditure. In our laboratory, we have developed three different techniques to measure energy expenditure in man, namely direct calorimetry, indirect calorimetry using an open-circuit ventilated hood system, and a respiratory chamber. Data from recent studies on DIT in man support the concept that a defect in thermogenesis may contribute to energy imbalance and weight gain in obese individuals.(ABSTRACT TRUNCATED AT 400 WORDS)