Dietary fat and obesity: an epidemiologic perspective

The observation that dietary fat has an effect on weight gain and the development of obesity that is larger than would be expected on the basis of fat's energy value is mainly experimental. Several methodologic problems limit the interpretation of epidemiologic studies of the association between dietary fat intake and obesity. Among the issues relevant in this context are underreporting of energy and fat intakes, dieting behavior, inadequate control for variables such as energy expenditure, and limited between-subject variation in fat intake in developed countries. Different types of epidemiologic studies suffer from different types and magnitudes of bias and many conflicting results can be obtained. Ecologic and cross-sectional studies especially suffer from these biases. The more appropriate type of study (ie, prospective studies of fat intake and subsequent weight gain) was carried out in several countries but conflicting results were obtained. Overall, the observed associations seem to depend on the stage of cultural transition of the population studied (eg, modernization compared with postmodernization). Current epidemiologic methods are inadequate for performing valid studies of the relation between percentage of energy from dietary fat and obesity. Specifically designed prospective studies of unbiased estimates of energy balance are necessary. Appropriate control for confounders and emphasis on the possibility that genetic predisposition plays a role will also be necessary. At this stage there is no conclusive evidence from epidemiologic studies that under isoenergetic conditions dietary fat intake promotes the development of obesity more so than other macronutrients.