The changes in the circulating insulin levels to various combinations of macronutrients is well studied and extensively reported in the literature. However, less attention has been paid to the effects of dietary components on circulating glucagon levels. As plasma glucose concentrations rise, circulating levels of insulin rise. This relates to the function of insulin which is to stimulate the transport of glucose to cells. Glucagon is the antagonistic hormone to insulin, and its effects are to release glucose to the circulation in the fasting period between meals, via stimulation of gluconeogenesis and glycogenolysis. Elevations in blood glucose therefore not surprisingly depress circulating levels of glucagon. As glucose levels fall, glucagon levels rise, with peak glucagon release occurring when hypoglycaemia occurs. Dietary protein and amino acids stimulates the release of glucagon and certain amino acids also cause a significant release of insulin. In contrast the ingestion of dietary fat has little effects on either glucagon or insulin.
Studies investigating the effects of mixed meals on glucagon release have been performed. In one such study1, researchers administered standard high fat, high protein or high carbohydrate mixed meals to healthy subjects. The mixed meals were consumed 3 times over a 12 hour period with 4 hours separating each meal. The changes to the plasma amino acids and the glucagon levels were then measured. The high fat and high carbohydrate means typically contained about 1 gram of protein per kg of body weight, whereas the high protein diet typically contained about 4 grams per kilogram of body weight. As expected the rise in the plasma amino acid concentration of the subjects receiving the high protein mixed meal was greater than for either the high fat or high carbohydrate mixed meal. However, the rise in the plasma amino acid concentration was proportional to the amount of protein in the diet. Amino acid concentrations rose quickly following consumption of the meals and stayed elevated throughout the day.
With regards to glucagon, the high fat mixed meals only caused modest elevations in circulating levels. As would be expected the high carbohydrate mixed meals caused depression of circulating levels of glucagon. In contrast, the consumption of the high protein mixed meals significantly elevated circulating levels of glucagon. The data in this study showed that the protein mixed meals caused a 2.6-fold increase in glucagon compared to the carbohydrate mixed meals. The rise in the plasma amino acid concentration following ingestion of protein causes the stimulation of both glucagon and insulin. The insulin stimulates the uptake of the amino acids to skeletal muscle and stimulates protein synthesis. The glucagon stimulates the uptake of the amino acids to the liver and the activation of the gluconeogenic pathway. As carbohydrates negate the requirement for gluconeogenesis, they depress the release of glucagon. Protein digestion can take many hours, which explains the long duration of glucagon elevation.
RdB
