The dietary macronutrients are protein, carbohydrate and lipids. As well as providing substances required for metabolic regulation, these macronutrients have differing and interesting effects on gut function. In fact the basis of many weight loss diets is through manipulation of the ratio of dietary macronutrients in order to produce beneficial physiological responses in the gut and during absorption. These changes can then favourable influence downstream metabolic regulation, and this can cause weight loss effects. One aspect of protein and lipids that have been researched with regard possible weight loss effect are their ability to regulate the glycaemic effects of foods, which may have indirect appetite regulatory benefits, as well as direct control of appetite at the level of the gut. In terms of the former, protein and lipids may delay the release of carbohydrate from the stomach and this decrease gastric motility. In terms of the latter, protein and lipids have direct effects on the release of hormones that affect gut motility in a load-dependent manner.
Researchers have assessed the effects of protein and lipids on gastric function. Much research in this area has focussed on the ability of protein and lipids to stimulate gastrointestinal hormone release, to affect gut motility, as well as to modulate insulin and blood glucose plasma levels postprandially. For example, in one study1, researchers assessed the effect of intraduodenal protein and lipid infusions alone and in combination, on lean healthy individuals. The infusions were all protein, all lipid, a 1:2 lipid:protein infusion, a 1:2 protein:lipid infusion or a control with no protein or lipids. The results showed that all infusions suppressed stomach antral and duodenal pressures, but increased pyloric pressures, suggesting that stomach emptying and food transit had been inhibited. Both the pure lipid and pure protein loads also significantly reduced the appetite of the subjects but none of the combinations of protein and lipids had this effect. Therefore protein and lipids may influence the ability of food to move through the gut.
The researchers also assessed the ability the the protein and lipid infusions to affect postprandial insulin and glucose levels. Protein was able to stimulate both insulin and glucagon release in a dose dependent manner, which may be expected as certain amino acids are known to be insulinogenic and glucagonogenic in nature. However lipids had no effect on either insulin or glucagon release. Despite the effects on insulin and glucagon release, normoglycaemia was maintained, suggesting that protein ingestion may not affect blood sugar levels. Lipids stimulated the release of both cholecystokinin and glucagon-like peptide-1 (GLP-1) and caused constriction of the pyloric sphincter but the reason for these effects may have been the nausea they induced. Cholecystokinin is known to slow gastric emptying by feedback mechanisms, and protein has been shown previously to stimulate its release. Protein can also stimulate release of GLP-1, which when in the circulation stimulates the release of insulin before being rapidly degraded.
Dr Robert Barrington’s Nutritional Recommendation: Higher protein diets are known to have weight loss effects. This may relate to the ability of protein to slow gastric emptying, favourably influence postprandial glycaemia and also possibly due to the extra energy costs of using protein as a store of energy. On the whole higher protein diets appear to have beneficial effects on appetite. The benefits of lipids on appetite and body weight are more controversial. This picture is complicated by the fact that fats are such a diverse group of compounds. However, generally it is accepted that healthy fats such as the essential fatty acids and the monounsaturated fats are beneficial to appetite regulation and may aid weight loss efforts.
RdB
