The satiating power of protein--a key to obesity prevention?

There is an urgent need for effective tools to prevent weight gain in the population at large and weight regain in overweight persons after weight loss. In theory the solution is simple, but implementation will continue to be difficult and ineffective as long as we maintain the view that just telling people that they should eat less and exercise more does the job. This simplistic strategy assumes that humans have conscious control over appetite and body weight regulation, which is certainly not the case for most people; if it were true, there would be no overweight or obese people. I have never met an obese patient who has worked hard to become obese and to maintain an excessive body size. We need to acknowledge that our regulatory systems are geared to prevent depletion of body energy stores and undernutrition effectively, whereas the systems that reduce appetite and increase energy expenditure during periods of excess availability of foods are easily suppressed by palatability and by the social, psychological, and rewarding aspects of foods. In the past, when people expended plenty of calories just by going about their daily business, it was possible to remain slim while eating a diet with nearly any nutritional profile. The problem nowadays is that many people are extremely sedentary, which makes it possible to overeat even when dietary intakes are relatively small. For example, it is easy to prepare tasty, filling meals with drinks for an active person who needs 2500 kcal/d, but it is difficult to do the same for a sedentary person who needs only 1800 kcal/d. This is why the current focus of science is to increase the satiating power of the diet, so that people feel full with fewer calories. An ad libitum diet that reduces the fat content from 40% to 25–30% of energy produces some weight loss (1). It is highly probable that the obesity rate would have increased even more over the past 20 y if the dietary fat content had not decreased during this period. However, because of the concomitant reductions in energy needs, increases in portion sizes, and increases in soft drink consumption that have occurred during the same period, the slight reduction in dietary fat over this time has clearly been insufficient to prevent weight gain and the increasing prevalence of obesity. The high obesity rate has also created a market for alternative, and often unsubstantiated, dietary solutions. The higher than usually recommended protein content of many popular diets, such as the Atkins Diet, The Zone, and The South Beach Diet, seems to point at possible solutions to the obesity epidemic. Many national dietary guidelines have, until recently, recommended that only 10–20% of the calorie content of the diet come from protein; however, 30–40% of the calorie content in the aforementioned diets comes from protein, at the expense of carbohydrates. Newer research indicates that the high-protein content of these diets may actually be the reason for their partial success in inducing weight loss, despite no restrictions in total calories (2). High-protein diets, such as the Atkins diet, may also suppress food intake by producing ketosis. Ketosis results from the depletion of glycogen stores induced by a severe restriction of carbohydrates, to an extent that goes far beyond what is commensurate with a healthy diet. In this issue of the Journal, Weigle et al (3) showed that an increase in dietary protein from 15% to 30% of energy and a reduction in fat from 35% to 20%, at a constant carbohydrate intake, produces a sustained decrease in ad libitum calorie intake and results in significant weight loss. They sequentially assigned 19 persons to the following diet regimens: 2 wk of a weightmaintenance diet (15% of energy as protein, 35% as fat, and 50% as carbohydrate), 2 wk of an isocaloric diet (30% of energy as protein, 20% as fat, and 50% as carbohydrate), or 12 wk of an ad libitum diet (30% of energy as protein, 20% as fat, and 50% as carbohydrate). They found that the subjects felt more satiated with the isocaloric high-protein diet than with the weightmaintenance diet. When the subjects were given the possibility to regulate their energy intake under the ad libitum conditions, spontaneous calorie intake decreased by 441 kcal/d, body weight decreased by 4.9 kg, and fat mass decreased by 3.7 kg. We do not yet understand how protein increases satiety. Weigle et al found that the effect could not be explained by changes in the hunger hormone ghrelin or in the satiety hormone leptin. Weigle et al’s results clearly showed that protein is more satiating than is fat, and previous studies have shown that protein is more satiating than is carbohydrate (4). Moreover, diets with a fat content fixed at 30% of calories produce more weight loss when high in protein (25% of energy) than when normal in protein (12% of energy): 9.4 compared with 5.9 kg after 6 mo; after 1 y, evidence was found to suggest that the high-protein diet, independent of the loss of total body fat, resulted in a significant loss of visceral fat (5). Should we advise overweight and obese patients to increase their protein intakes from 10–20% to 20–30% of calories and reduce their intake of fat and carbohydrates correspondingly? If