It is not overeating, but under-activity that causes obesity. We are paid not to exercise now. There is persuasive evidence that much obesity is due to under-exercising rather than overeating. In a series of randomized, controlled trials it was found that sedentary men who take up jogging lose body fat in proportion to miles run, increase their energy intake, and improve their lipoprotein pattern. In a 1-year comparison of fat loss by dieting vs. fat loss by exercise without dieting, both methods were found to be effective in moderately overweight men, and both approaches raised plasma HDL cholesterol. It has also been demonstrated in overweight men and women losing weight on a prudent diet (low fat, low cholesterol) that adding exercise to energy restriction further increased loss of body fat and reduced waist-to-hip girth ratio, especially in men. Risk of coronary heart disease was also substantially further reduced by addition of exercise, in both sexes. These studies suggest that regular exercise is a valuable addition to dietary change for weight control and reduction of risk of chronic disease in people of all ages.
The energy demands of physical activity must be sufficiently intense to trigger these changes. Few modern humans engage in activity vigorous enough to do this. And there must be enough muscle tissue relative to fat tissue for the muscle to generate a sufficiently high energy demand to “draw down” on fat reserves. If there is a high volume of fat relative to muscle tissue, then, even though muscle is more insulin sensitive than fat, the fat volume will contain more receptors than the muscle
tissue. Consequently, energy will flow to fat on balance. The body composition of modern humans makes it easier for them to “get fatter”. They have so much fat that their fat out competes their muscle (and organs) for energy. In addition, they are incapable of the vigorous activity that would make inroads into their fat inventory.