High protein diets have been shown to have weight loss effects. The reasons for the beneficial effect of high protein diets on weight loss have been extensively studied. It is know for example that protein is inefficiently metabolised to fatty acid and so is much less likely to be stored as body fat when compared to either dietary carbohydrate or fat. In addition, the requirement for nitrogen by the skeletal muscle may cause a prioritisation of the protein to more the more immediate metabolic demands of maintaining skeletal muscle mass through protein synthesis, rather than to body fat storage. The way that protein is metabolised also produces a slightly higher thermic effect of food when compared to either carbohydrate or fat. Although these differences are small, over the course of a few years, they can make a significant difference to body weight changes. However, as well as these mechanisms, protein may also cause decreases in appetite, and this may contribute significantly to the overall weight loss effects of protein.
The effects of a mixture of amino acids on the appetite of healthy subjects has been assessed in comparison to a placebo1. The subjects were given a preload of amino acids (3 g phenylalanine, 2 g valine, 2 g methionine, 1 g tryptophan) 30 min before a meal, and the energy intake of the subjects observed. This was repeated on five occasions in a random allocation between placebo and amino acid treatment. The results of the study showed that the energy intake of the subjects was 10 % lower on average when the amino acid preload was taken in comparison to the placebo. However, when the subjects were grouped into categories dependent on their deviation from ideal body weight, the reduction in energy intake was 22.5 % in those who were over 5 % above their ideal body weight. This compared to the 1.75 % decrease in energy intake for those who were within 5 % of their ideal body weight. Therefore protein taken 30 minutes before food may reduce energy intake at a subsequent meal, especially in overweight individuals.
Dr Robert Barrington’s Nutritional Recommendation: High protein diets cause weight loss. A large part of this effect may be the satiety effects that come from protein foods. Protein is able to induce satiety for two possible reasons. Firstly, protein in the stomach is the determining factor in the digestion rate through the stomach. The presence of undigested protein in the small intestine causes the release of cholecystokinin from the I cells of the duodenum, and this inhibits the emptying of the stomach by contraction of the pyloric sphincter. In this way protein remains in the stomach until it is fully digested and this has a satiety effect. Secondly, the protein hypothesis leverage states that humans (and other organisms) have a minimum requirement for protein. Any failure to fulfill that requirement stimulates appetite. If the foods ingested are low in protein, a higher energy intake results. Eating high protein foods satisfies the requirement for a lower energy cost and therefore does not cause concomitant weight gain.
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