Relevance of energy expenditure and energy intake to the etiology of obesity

UFR Sante Médecine et Biologie Humaine Leonard de Vinci Laboratoire de Physiologie du Comportement Alimentaire 74 Avenue Marcel Cachin 93017 Bobigny France E-mail: comp-alim{at}smbh.univ-paris13.fr

Dear Sir:

The very interesting article by Shepard et al (1) raises a recurrent problem concerning the risk factors of obesity. This study shows that a short period of inactivity leads to a more positive energy balance in persons consuming a diet high in fat than in persons consuming an isoenergetic diet high in carbohydrate. However convincing the results, it must be remembered that energy intake was fixed in this study. No evidence is provided that this positive energy balance would not have resulted in a decrease in the spontaneous energy intake of subjects consuming an ad libitum diet, either via adiposity-induced leptin secretion (2, 3) or an increase in fatty acid release and its glucose-sparing effect (4). The fact that the replacement of fat in the diet with another nutrient was shown to induce a weak compensatory response in many other studies (5) does not obviate the need to assess the consequences of physical activity and dietary composition on eating behavior. For example, the differences in fat-mass gain between animals fed diets with the same macronutrient content and energy density or between animals fed diets with different fat contents and energy densities have an energy-intake determinant (6–8). Moreover, in humans, the role of energy density on adiposity is suspected to be mediated via an increased energy intake (9). This is not surprising because eating behavior is driven by physiologic processes that are highly sensitive to any modification in metabolism.

Although the consequences of consuming diets with a fixed energy content but with different fat and carbohydrate contents on energy expenditure, oxidative pathways, and adiposity—such as in the study by Shepard et al—are important, we propose that before concluding that the macronutrient content of the diet is relevant to the etiology of obesity, further studies of the consequences of such a diet on spontaneous energy intake should be conducted.

REFERENCES

1. Shepard TY, Weil KM, Sharp TA, et al. Occasional physical inactivity combined with a high-fat diet may be important in the development and maintenance of obesity in human subjects. Am J Clin Nutr 2001;73:703–8.
2. Meister B. Control of food intake via leptin receptors in the hypothalamus. Vitam Horm 2000;59:265–304.
3. Chapelot D, Aubert R, Marmonier C, Chabert M, Louis-Sylvestre J. An endocrine and metabolic definition of the intermeal interval in humans: evidence for a role of leptin on the prandial pattern through fatty acid disposal. Am J Clin Nutr 2000;72:421–31.
4. Randle PJ. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years. Diabetes Metab Rev 1998;14:263–83.
5. Poppitt SD, Prentice AM. Energy density and its role in the control of food intake: evidence from metabolic and community studies. Appetite 1996;26:153–74.
6. West DB, Boozer CN, Moody DL, Atkinson RL. Dietary obesity in nine inbred mouse strains. Am J Physiol 1992;262:R1025–32.
7. Chang S, Graham B, Yakubu F, Lin D, Peters JC, Hill JO. Metabolic differences between obesity-prone and obesity-resistant rats. Am J Physiol 1990;259:R1103–10.
8. Warwick ZS, Schiffman SS. Role of dietary fat in calorie intake and weight gain. Neurosci Biobehav Rev 1992;16:585–96.
9. Prentice A. Manipulation of dietary fat and energy density and subsequent effects on substrate flux and food intake. Am J Clin Nutr 1998;67(suppl):535S–41S.