The postprandial glycaemic response to food is altered in diabetics. In addition, the ability of food components to alter postprandial glycaemia appears altered in diabetic state. For example, comparisons of subjects with type 2 diabetes to normal healthy subjects shows that legumes have more of an effect on postprandial glycaemia in the healthy subject. Both protein and fat can alter the metabolic responses to a meal in healthy subjects but in type 2 diabetes this is less apparent. This has lead to speculation that some individuals may be predisposed to development of type 2 diabetes because they are sensitive to carbohydrates and other food components do not beneficially alter glycaemia to as great an extent as in healthy people. Studies have compared the hormonal responses to protein, carbohydrate and fat alone and in combination in diabetic subjects and healthy individuals. Such studies have generally shown that diabetics have abnormal responses to macronutrients compared to healthy controls.
For example, in a study published in the American Journal of Clinical Nutrition in 19851, both fat and protein in the diet could significantly reduce the glycaemic response to carbohydrates in normal healthy subjects. This relates to the ability of protein and lipids to inhibit gastric emptying and slow the rate of transit through the gastrointestinal tract. However, in subjects with type 2 diabetes, the presence of fat and protein had no effect on postprandial glycaemia. In subjects with type 1 diabetes, the protein actually increased the postprandial response to the carbohydrate meal. When the authors analysed the glucose dependent insulinotropic peptide (GIP) and insulin responses to the meals, they were similar for all groups. In this respect protein acted as an insulin secretagogue in the normal and both diabetic groups. This was likely because some amino acids stimulate insulin release. When protein was accompanied by fat and carbohydrate, there was a significant increase in GIP release in all groups.
These results suggest that care should be taken when comparing healthy subjects to those in the diabetic state. It is known that serious metabolic disturbances accompany diabetes, and this likely has a significant impact on the postprandial response to macronutrients. Fat is known to inhibit gastric emptying in normal healthy people, but it is unclear why this effect is less apparent in those with type 2 diabetes. This may relates to changes in the physiology of the stomach with the development of the disease, or to hormonal factors that have as yet been unidentified. Protein can inhibit gastric emptying because detection of undigested peptides in the small intestine cause the release of cholecystokinin, which in turn inhibits the gastric emptying to ensure full protein digestion. However, again, it is unknown why this response should be altered in the diabetic state. It is of course possible that these changes were present prior to development of diabetes and that these are a contributory factor in causing the original dysfunction in blood sugar control.
