Does combining my carbohydrates with protein or fat dilute the glycemic index of the carbohydrates and lower the effective blood sugar rise?

Ingestion of protein and of fat (especially monounsaturated fat) along with carb-rich foods often does indeed blunt the post-meal rise in blood glucose, effectively lowering the glycemic index of the meal. This appears to reflect increased production of certain hormones, produced by cells in the intestinal tract, that act on the pancreatic beta cells to boost the insulin response to absorbed glucose. These hormones can also slow the emptying of the stomach into the upper intestine, possibly decreasing the effective glycemic index of food still remaining in the stomach. These hormones also promote satiety — which helps to explain why meals containing protein or fat tend to be more satisfying than meals of pure carbohydrate. The downside of this mechanism is that, whereas it helps to restrain the post-meal rise in glucose, it accomplishes this mainly by boosting insulin secretion — which rather misses the point if your goal in CC dieting is to minimize your daily insulin levels. Nonetheless, moderating postmeal rises in glucose is inherently beneficial for diabetics, and may also aid in diabetes prevention.

Dietary protein and fat — especially monounsaturated fat — can influence post-meal glucose rise by acting on special cells in the intestinal lining to promote the production of the hormone glucagon-like peptide-1 (GLP-1), which modulates the function of the pancreatic islet and of the stomach. The islet cells are the pancreatic cells that secrete the hormones insulin and glucagon. GLP-1 amplifies the response of islet beta cells to an increase in glucose, thereby boosting insulin secretion; simultaneously, it suppresses glucagon production by the islet alpha cells. Insulin of course blunts the blood sugar rise, and the suppression of glucagon inhibits release of glucose by the liver; both of these effects therefore serve to moderate blood glucose levels after a meal. Meanwhile, GLP-1 acts on the stomach to slow gastric emptying, which may delay or protract the digestion of a meal. And GLP-1 also acts to promote satiety — which helps to explain why meals containing some protein or monounsaturated fat tend to be more satisfying than meals that are pure carb. The release of GLP-1 provoked by fat or protein in a meal therefore tends to decrease the effective glycemic index of a meal — but at the cost of boosting the insulin response. This may be a good trade-off if you are diabetic and your chief priority is to keep your blood sugar in control to avoid diabetic complications, but it is a very mixed blessing if you are following a CC diet to try to keep your daylong insulin levels low. Note that the low glycemic index of milk and other carb-rich dairy products reflects the fact that they have a very high insulin index — they provoke a massive rise in insulin secretion that prevents post-meal glucose levels from rising very high. Some dietary experts who promote low-glycemic-index eating for weight control also caution against combining protein-rich foods with carb-rich foods in the same meal, to avoid a protein-mediated potentiation of carb-evoked insulin secretion.

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