The protein leverage hypothesis states that energy intake is determined by the protein component of the food. In this regard the carbohydrate and fat is of secondary importance to dietary protein. Foods high in protein satisfy the protein needs of the organism and so excess calories in the form of fat and carbohydrate are not consumed. However, low protein foods do not provide adequate amino acids for metabolic needs, and so the organism consumes more energy in order to provide the required quantity of protein. This relates to the logical premise that low protein foods are likely high in fat or carbohydrate, or both. The result is an overconsumption of unwanted energy which has deleterious effects on metabolic regulation and homeostasis (figure 1). Therefore in the protein leverage hypothesis, consumption of energy in organism is centred around the need for protein. That high protein diets do increase satiety in humans, suggests that protein may be the driver of feeding behaviour. Certainly evidence strongly supports a role for high protein diets in weight loss.
Figure 1. The protein leverage hypothesis suggests that low protein foods may lead to over consumption of carbohydrate or fat energy.
Research concerning the protein leverage hypothesis has been performed mainly using animals. While these experiments have provided evidence that supports the protein leverage hypothesis, little work has been performed on humans. However, recently a study was published in the American Journal of Clinical Nutrition1 that provided some data to support the relevance of the protein leverage hypothesis to humans. In the crossover randomised study, 40 healthy men and 39 healthy women with a mean body mass index of 23.7 kg/m2 (normal weight) were fed diets containing 5 (low), 15 (moderate) and 30 % (high) energy from milk (whey) or plant (soy) protein. Variation on the protein content of the diet was inversely mirrored by the carbohydrate content of the diet, such that as protein was increased carbohydrate was decreased concomitantly. This produced protein to carbohydrate ratios of 5:95, 15:85 and 30:70 for the low, moderate and high protein diets, respectively. The fat content of the diet was maintained at 35 % of total calories.
Consumption of the diets for 12 consecutive day in an ad libitum fashion resulted in a significantly lower energy intake in the high protein diet (1719.7 kcal per day), compared to the low protein (2228.4 kcal per day) and moderate protein diet (2297.7 kcal per day). In addition, the satiety was greater during the high protein intake, and fluctuations in appetite were also reduced during this time. These results support other studies showing that high protein diets increase satiety. All the subjects lost weight in the study, but regained the lost weight in the 6 week washout period between each treatment diet. This may suggest that the reason they lost weight was not due to the protein but some other factor. For example, the diets were highly controlled and each subject had their meals pre-prepared by the researchers. This may have reduced the consumption of sugar during the study period, and caused weight loss in the subjects. In addition, fat mass was not measured and so it is difficult to draw valid conclusions from the weight loss seen.
The authors suggested that these results support the protein leverage hypothesis and provide evidence that this theory may be valid for humans. Therefore humans, like other animals, may regulate energy balance based on the amount of protein in the diet, which in turn is determined by the nitrogen balance experienced at the time. Exercise for example, could increase nitrogen turnover and raise the requirement for protein. Eating low protein high carbohydrate foods under these circumstances may lead to overeating because the protein needs are not satisfied. If this overeating results in the consumption of fructose containing foods, as are common in the Western diet, weight gain may ensure. Generally data from the nutritional literature showing improved body composition, increased satiety and enhanced fat loss on high protein diets supports the protein leverage hypothesis. Hyperphagia seen in some obese individuals could therefore be thought of as a protein deficiency, in much the same way as pica (cribbing) results from a deficiency of minerals.
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