The relationship between dietary carbohydrates and insulin is well researched. The quantity and type of carbohydrate food ingested affect the rate at which the food is digested and enters the blood circulation. Insulin is required for the uptake of glucose from the blood to the cells and the amount of insulin released is generally related to the quantity of glucose that enters circulation as well as the rate. Carbohydrate foods are designated a glycaemic rating based on their effect on blood glucose levels or an insulin rating based on their ability to stimulate insulin, known as the glycaemic or insulinaemic index respectively. Insulin resistance is a condition that is characterised by hyperglycaemia and hyperinsulinaemia which can result in abdominal weight gain, inflammation as well as increased risk factors for diabetes and cardiovascular disease.
Evaluating the glycaemic and insulinaemic response of foods is important because it is known that some foods do not display responses on the glycaemic or insulinaemic index that might be expected. For example, researchers1 investigated some common food sources of protein on postprandial glucose, insulin, amino acids, glucose-dependent insulinotropic polypeptide (GIP) and glucagons-like peptide (GLP-1) blood levels in 12 healthy volunteers. Subjects were fed either a bread test meal or reconstituted milk powder, cod, cheese, whey protein or wheat gluten, all with an equivalent level of lactose to the milk. The results showed that milk proteins caused a large increase in the release of insulin comparable to that seen by cod, and wheat protein, but did not show the same increase in blood glucose levels caused by the white bread reference (-62% and -57% for milk and whey respectively).
This is interesting because it shows that milk proteins have a disconnect between insulin release and blood glucose levels not seen with carbohydrate foods. This might suggest that it is not the lactose component of milk that is responsible for insulinotropic effects, but the protein component. For whey protein the insulinotropic effect was even greater than milk, whey protein being very low in lactose. Because milk is composed of both casein and whey, these results suggest that it is the whey protein fraction of milk that is mainly responsible for the pronounced insulinotropic effect. Indeed, the whey protein caused an even higher insulin rise that milk. There was also a correlation between the rate that amino acids appeared in the blood and the insulin response seen. This effect was strongest for the branched chain amino acids which are found in high concentrations in whey protein.
Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are both known to be released in response to meals and are thought o act to enhance insulin release. Some evidence suggests that protein can cause the release of GIP. The release of GLP-1 did not differ significantly between the different treatments, but the plasma GIP levels were significantly higher after the whey protein meal compared to the other test meals and the bread reference. This may suggest that whey protein stimulates the release of GIP, which in turn is responsible for the increase in plasma insulin levels. However, the milk protein did not cause an increase in GIP compared to the white bread reference but did have a pronounced insulinotropic effect. Therefore it is unclear as to the contribution of GIP in the insulinotropic effect seen with whey protein.
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