The incretin effect does not differ in trained and untrained, young, healthy men

Area of Science:

• Endocrinology
• Metabolic Physiology
• Exercise Science

Background:

• Trained individuals exhibit lower peripheral insulin concentrations compared to untrained individuals post-glucose administration.
• The incretin effect, mediated by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), enhances glucose-induced insulin secretion.
• Adaptations in beta-cell function contribute to altered insulin secretion in trained states.

Purpose of the Study:

• To investigate differences in GLP-1 and GIP release between trained and untrained humans.
• To determine if the incretin effect varies between trained and untrained individuals following oral and intravenous glucose administration.
• To elucidate the mechanisms behind lower insulin concentrations in trained individuals.

Main Methods:

• A 3.5-hour oral glucose tolerance test (OGTT) was conducted in trained and untrained healthy young men.
• An isoglycaemic intravenous glucose infusion was performed to match OGTT glucose levels.
• Blood samples were analyzed for insulin, C-peptide, GIP, and GLP-1; insulin secretion rate and clearance were calculated.

Main Results:

• Plasma GIP and GLP-1 concentrations, incretin effect, and insulin clearance did not differ between trained and untrained groups.
• Trained subjects showed lower plasma glucose, insulin, and C-peptide concentrations compared to untrained subjects.
• The insulin secretion rate was also lower in trained individuals during both glucose administration tests.

Conclusions:

• The reduced plasma insulin concentrations in trained individuals are not attributable to differences in incretin effect or insulin clearance.
• Lower beta-cell sensitivity to glucose in trained individuals likely contributes to reduced insulin secretion.
• Enhanced glucose uptake in skeletal muscle is a key factor in the lower systemic insulin levels observed in trained individuals.