Dietary fat and obesity: a review of animal, clinical and epidemiological studies.

The First Law of Thermodynamics provides a framework for understanding the imbalance between energy intake and expenditure that produces obesity, but it does not help understand the role of genetics, the regulation of food intake, the distribution of body fat, the mechanisms by which diets work or the mechanism by which portion control has gotten out of control. In animals, increasing dietary fat increases body fat, and it is unlikely that humans escape this important biological rule. In epidemiological studies, increasing dietary fat is associated with increased prevalence of obesity probably by increasing the intake of energy dense foods. In the National Weight Loss Registry, three things were associated with weight loss: continued monitoring of food intake, lowering dietary fat intake, and increased exercise. The relation of dietary fat is most evident when physical activity is low. The speed of adaptation to dietary fat is increased by exercise. When dietary fat is reduced, weight is lost, but weight loss eventually plateaus. The rate of weight loss during the initial phase is about 1.6 g/day for each 1% decrease in fat intake. When dietary fat is replaced with olestra to reduce fat intake from 33% to 25% in obese men, weight loss continues for about 9 months reaching a maximum of nearly 6% of body weight and a loss of 18% of initial body fat. In the control group with a 25% reduced-fat diet, weight loss stopped after 3 months and was regained over the next 6 months, indicating the difficulty of adhering to a conventional low-fat diet. Thus, dietary fat is an important contributor to obesity in some people.