Effect of short term fasting on glucose tolerance and insulin secretion: influence of the initial glucose level

To evaluate the effect of fasting on glucose tolerance (GT) and insulin secretion, a 5 h oral glucose tolerance test was performed after an overnight fast and after 3-6 days of fasting in 66 obese subjects presenting a normal (n = 22), impaired (n = 23) or diabetic (n = 21) GT. Insulin secretory capacity was assessed using two glucose-independent parameters of beta cell function. In the normal group, fasting induced a fall in basal glycemia from 84 +/- 1 to 58 +/- 2 mg/dl (P less than 0.001) and an increase in the area of glucose (+58 +/- 8%, P less than 0.001), insulin (+75 +/- 10%; P less than 0.001) and C-peptide (+58 +/- 10%; P less than 0.001) during OGTT, these responses were consistent with the emergence of insulin resistance. The insulin secretory capacity was significantly decreased. In the diabetic group, fasting was associated with an increase in insulin (+34 +/- 10%; P less than 0.005) and C-peptide (+34 +/- 8%; P less than 0.001) responses to OGTT despite a reduction in basal glycemia from 174 +/- 11 to 86 +/- 4 mg/dl (P less than 0.001) and in glucose response (-20 +/- 3%; P less than 0.001), indicating an improvement of insulin secretory capacity. In the group with impaired GT, basal glycemia decreased from 97 +/- 2 to 70 +/- 2 mg/dl (P less than 0.001) but glucose, insulin and C-peptide curves were not significantly affected by fasting.(ABSTRACT TRUNCATED AT 250 WORDS)