Influence of insulin on beta-endorphin plasma levels in obese and normal weight subjects

Objective: To establish the possible role of hyperinsulinemia in the elevation of plasma β-endorphin (β-EP) levels observed in obese patients after an oral glucose load. Design: Oral glucose tolerance test (OGTT) and euglycemic-hyperinsulinemic clamp. Subjects: Two groups of six (age: 22-39 y, BMI: 30-48 kg/m2) and eight obese men (age: 18-37 y, BMI: 35-45 kg/m2), respectively, and five normal weight healthy men (age: 22-30 y, BMI 22-23 kg/m2). Measurements: Glucose, insulin and β-EP levels at baseline and every 30 min until 180 min during the OGTT; glucose, insulin, C-peptide and β-EP concentrations at baseline and in steady state condition (i.e. during the last 30 min of insulin infusion) in the euglycemic-hyperinsulinemic clamp studies. Results: In the six obese patients undergoing the OGTT a significant elevation of β-EP plasma levels was observed between 60 and 90 min after glucose ingestion. In the clamp studies no significant differences in β-EP plasma levels, blood glucose and serum insulin were observed between obese and normal weight subjects both at baseline and at steady state. A markedly diminished insulin sensitivity along with a lower inhibition of C-peptide during insulin infusion was observed in obese patients compared to control subjects. Conclusion: A rise in serum insulin levels unaccompanied by a concomitant increase in blood glucose concentration is unable to elicit a β-EP response in obese patients.