Abstract (provisional): High protein diets are increasing popularized in lay media as a promising strategy for weight loss by providing the twin benefits of improving satiety and decreasing fat mass. Some of the potential mechanisms that account for weight loss associated with high protein diets involve increased secretion of satiety hormones (GIP, GLP-1), reduced orexigenic hormone secretion (ghrelin), the increased thermic effect of food and protein-induced alterations in gluconeogenesis to improve glucose homeostasis. There are, however, also possible caveats that have to be considered when choosing to consume a high-protein diet. A high intake of branched-chain amino acids in combination with a western diet might exacerbate the development of metabolic disease. A diet high in protein can also pose a significant acid load to the kidney. Finally, when energy demand is low, excess protein can be converted to glucose (via gluconeogenesis) or ketone bodies and contribute to a positive energy balance, which is undesirable if weight loss is the goal. In this review, we will therefore explore the mechanisms whereby a high protein diet may diet may exert beneficial effects on whole body metabolism while we also want to present possible caveats associated with the consumption of a high-protein diet.
Alex’s Notes: High-protein diets must really be catching on if researchers from Yale University School of Medicine are writing reviews about them. As the title alludes, this review focuses on the nutrient-specific mechanisms of protein-induced satiety for weight loss and preservation of fat-free mass, with a conclusion looking at possible caveats. Interestingly enough, I would argue that this information isn’t restricted to “high” protein diets however; as the researchers claim that a diet is high in protein if it exceeds the RDA of 0.8 g/kg bodyweight (BW) or the habitual intake of 15-16% total calories. Both those numbers are definitely on the low side.
Regardless, an ideal weight loss strategy revolves around satisfaction that comes from satiety and preservation of lean body mass (LBM) despite a hypocaloric diet. It is well known that protein is the most satiating nutrient, and this effect is most apparent in meals with a protein content of 25-81%. Of course protein type matters as well. Whey protein is more satiating than casein or soy protein. Several factors contribute to this short-term satiety response to protein:
- Increased energy expenditure
- Increased concentrations of anorexigenic hormones and amino acids
- Altered gluconeogenesis
Diet-induced thermogenesis (DIT) is the main culprit for the increased energy expenditure. Protein is an expensive nutrient, and the DIT values are highest for protein (15-30%), followed by carbohydrates (5-10%) and fats (0-3%). In fact, during energy restriction, a high-protein diet (36%) helps preserve the decline in energy expenditure seen with dieting relative to a normal-protein diet (15%) that is high in carbohydrates or fat. There is indeed a relationship between satiety and DIT, and it may be due to the increased oxygen demand to metabolize the proteins.
The secretion of neuropeptides that induce satiety is another side-effect of high-protein diets. Glucagon-like peptide 1 (GLP-1), cholecystokinin (CCK), and peptide YY (PYY) all appear to be increased in high-protein diets, while “hunger” hormones such as ghrelin are reduced. It was suggested back in 1956 that increased serum amino acid concentrations produced feelings of satiety while decreasing concentrations induced hunger, and this is supported by preload studies demonstrating more satiating effects of protein preloads compared to isoenergetic carbohydrate and fat preloads.
Altered gluconeogenesis was an interesting factor that the researchers brought forth, and high-protein low-carbohydrate diets do report increased gluconeogenesis in order to maintain blood glucose levels. However, a recent human study found that the increased gluconeogenesis following a high-protein intake was unrelated to appetite suppression, and instead suggested that the observed increase in β-hydroxybutyrate was responsible. Ironically enough, this may explain the anecdotal suppression of hunger on ketogenic diets.
Putting it to use
High(er) protein diets are now a common prescription for dieting, especially in athletes. Care must be taken, however, to not crowd out other nutrients such as carbohydrates when performance must be maintained. On the other hand, in sedentary obese persons who don’t use carbohydrates significantly, this is a great strategy. Overall, the long-term effects of high-protein diets will depend on the population studied as well as the diet composition.
So what caveats are there? Perhaps the most interesting is the interplay between the branched-chain amino acids (BCAAs) and a high-fat diet. Metabolomics studies have suggested that a high-intake of BCAAs and aromatic amino acids such as phenylalanine and tyrosine may be associated with the development of metabolic diseases only when in combination with high-fat diets. Without getting into the biochemistry of it, the end result is a possible increased mitochondrial stress, impaired insulin action, and perturbation of glucose homeostasis.
The other caveat put forth was the possible negative effects of a high-protein diet on kidney function. While the researchers acknowledge that the concerns with bone mass have been refuted, they argue that the protein-induced acid-load to the kidneys remains. The researchers recommend a simple solution. When eating a high-protein diet, ensure consumption of acid-base buffers such as vegetables and foods high in potassium. Glutamine and baking soda are other supplemental options.
So there you have it, a brief Yale review of high-protein diets.