Exercise and Macronutrient Balance

Abstract

Reduced physical activity represents one of the most significant changes in lifestyle that has been observed during the twentieth century. Our sedentary lifestyle and the reduced energy requirements of the majority of our jobs has been a source of comfort in a business world where efficiency and productivity are sought. The impact of the transition from a traditional to a modern lifestyle on daily energy needs can be estimated by variousmeans. By using the doubly labelled water technique and indirect calorimetry, Singh et al. (1) showed that the energy cost of living at the peak labor season was as high as 2.35 resting metabolic rate (RMR) in Gambian women. When this value is compared to results usually obtained in women living in industrialized countries, 1.4 to 1.8 RMR (2,3), it can be estimated that for a given body weight, a modern lifestyle may have reduced the energy cost of living by as much as 1 to 4MJ/day. Accordingly, a recent analysis by Prentice and Jebb (4) has emphasised the contribution of sedentariness to the increased prevalence of overweight in the United Kingdom. Despite these observations, the contribution of exercise to the prevention and treatment of obesity is still perceived as trivial by many health professionals. The perception of many of themwas recently well summarized by Garrow (5) who stated that exercise is a remarkably ineffective means of achieving weight loss in obese people, mainly because their exercise tolerance is so low that the level of physical activity that they can sustain makes a negligible contribution to total energy expenditure. When one looks at the currently available literature, it is difficult to disagree with this statement. Indeed, numerous studies have demonstrated that when exercise is used alone to treat obesity, body weight loss is generally small (6). In addition, the further weight loss generated by adding an exercise program to a reduced-calorie diet is also often small if not insignificant (7). Traditionally, the study of the impact of exercise on body weight control has focused on its energy cost and on the hope that the body energy loss will be equivalent to the cumulative energy cost of exercise sessions. In practical terms, this means for instance that if a physical activity program induces an excess of energy expenditure of 2000 kcal/week, a similar energy deficit should be expected in the active obese individual. Recent experimental data show that such a view is not realistic since it does not take into account the compensations in other components of energy balance which may either attenuate or amplify the impact of exercise on body energy stores. It thus appears preferable to consider exercise as a stimulus affecting regulatory processes which can ultimately affect all the components of energy balance instead of only focusing on its energy cost. The objective of this chapter is to