The role of high-fat diets and physical activity in the regulation of bodyweight

The prevalence of obesity is increasing in westernized societies. In the US A the age-adjusted prevalence of BMI >30 kg/m(2) increased between 1960 and 1994 from 13 % to 23 % for people over 20 years of age. This increase in t he prevalence of obesity has been attributed to an increased fat intake and a decreased physical activity. However, the role of the impact of the leve l of dietary fat intake on human obesity has been challenged. High-fat diet s, due to their high energy density, stimulate voluntary energy intake. An increased fat intake does not stimulate its own oxidation but the fat is st ored in the human body. When diet composition is isoenergetically switched from low to high fat, fat oxidation only slowly increases, resulting in pos itive fat balances on the short term. Together with a diminished fat oxidat ion capacity in pre-obese subjects, high-fat diets can therefore be conside red to be fattening. Another environmental factor which could explain the i ncreasing prevalence of obesity is a decrease in physical activity. The per centage of body fat is negatively associated with physical activity and exe rcise has pronounced effects on energy expenditure and substrate oxidation. High-intensity exercise, due to a lowering of glycogen stores, can lead to a rapid increase in fat oxidation, which could compensate for the consumpt ion of high-fat diets in westernized societies. Although the consumption of high-fat diets and low physical activity will easily lead to the developme nt of obesity, there is still considerable inter-individual variability in body composition in individuals on similar diets. This can be attributed to the genetic background, and some candidate genes have been discovered rece ntly. Both leptin and uncoupling protein have been suggested to play a role in the prevention of diet-induced obesity. Indeed, leptin levels are incre ased on a high-fat diet but this effect can be attributed to the increased fat mass observed on the high-fat diet. No effect of a high-fat diet per se on leptin levels is observed. Uncoupling proteins are increased by high-fa t diets in rats but no data are available in human subjects yet. In conclus ion, the increased intake of dietary fat and a decreasing physical activity level are the most important environmental factors explaining the increase d prevalence of obesity in westernized societies.