Altering the macronutrient ratios of meals is known to have an effect on weight loss. Evidence suggests that higher intakes of protein increases satiety and can aid in weight control. Meals with a higher intake of protein decrease subsequent energy intakes because they decrease postprandial hunger. Proteins have a larger thermic effects than carbohydrates and fats because they cannot be stored and so must be metabolised following ingestion. Proteins also contribute to gluconeogenesis and may therefore prevent falls in blood sugar. Research also suggests that proteins stimulate the release of certain gut peptides that may have an effects of satiety. Glucagon-like peptide 1 (GLP-1) and cholecystokinin both increase satiety and reduce gastric emptying and release is stimulated by protein ingestion. However, little is known about how protein ingestion may modulate the release of ghrelin, a hormone that increases appetite
To investigate the effects of a high protein meal on the hunger stimulating peptide ghrelin, researchers1 fed 15 healthy men either a high protein breakfast or a high carbohydrate breakfast. Blood samples as well as subjective measures of satiety were assessed for 3 hours following isocaloric breakfasts containing either 58.1 % protein and 14.1 % carbohydrate (high protein) or 19.3 % protein and 47.3 % carbohydrate (high carbohydrate). Gastric emptying was also assessed using an acetaminophen test. The results showed that the high protein breakfast decreased postprandial ghrelin levels more than the high carbohydrate breakfast. The high protein breakfast increased levels of glucagon and cholecystokinin, and showed a trend towards raising glucose-dependent insulinotropic peptide and GLP-1. The high protein breakfast also decreased gastric emptying rate compared to the high carbohydrate breakfast.
These results support previous finding that higher protein meals increase satiety because they stimulate release of peptides responsible for satiety and inhibit those responsible for stimulating hunger. The high protein diet decreased subsequent energy intake in the next meal by ≈ 439 kJ, but was not statistically significant. The high correlation between ghrelin (inhibited) and glucagon and GIP (stimulated) suggest that the postprandial decrease in ghrelin may be in part be brought about through release of GIP and glucagon. Interestingly, other studies that have found no decrease in ghrelin levels after protein ingestion have used meat as the source of protein. In this study a decrease was detected, but the source of protein was whey enriched dairy protein. This may suggest that different proteins have different effects on satiety. For example GIP is secreted from the gut in response to milk protein but not turkey.
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