Insulin resistance, secretion, and metabolism in users of oral contraceptives.

Studies of insulin employing the oral glucose tolerance test demonstrate marked differences between the effects of different oral contraceptives, but provide little insight into the underlying disturbances. We investigated the metabolic basis of these disturbances by computer modelling of iv glucose tolerance test glucose, insulin, and C-peptide concentration profiles. Insulin resistance, secretion, and metabolism were evaluated in 296 oral contraceptive users and 95 nonusers. Four estrogen/progestin combinations, with similar estrogen but differing progestin contents, and 1 progestin-only formulation were studied. Effects on iv glucose tolerance test glucose, insulin, and C-peptide concentrations varied according to progestin content, with levonorgestrel-containing combinations having the greatest effect, followed by desogestrel and norethindrone. However, these formulations increased insulin resistance to a similar extent. The progestin-only formulation did not affect insulin resistance. Levonorgestrel combinations increased second phase pancreatic insulin secretion by 60-90%, but did not affect the insulin half-life. The desogestrel combination increased the insulin half-life by 28%, but did not affect insulin secretion. The effects of different combined oral contraceptives on glucose tolerance test glucose, insulin, and C-peptide concentration profiles appears to be due to a combination of estrogen-induced insulin resistance and progestin-associated changes in insulin half-life.