Standard breakfast test: an alternative to glucagon testing for C-peptide reserve?

We measured C-peptide after glucagon and breakfast tests to compare the effectiveness of both tests in evaluating residual beta cell function in normal and diabetic subjects. A significantly higher C-peptide response was elicited after standard breakfast in patients with insulin-dependent diabetes mellitus of less than two years' evolution, ranging from 0.12 +/- 0.07 to 0.83 +/- 0.18 ng/ml (P less than 0.05). In nonobese noninsulin-dependent diabetes mellitus the response ranged from 0.86 +/- 0.02 to 1.89 +/- 0.48 ng/ml (P less than 0.0025); in obese NIDDM from 1.02 +/- 0.37 to 1.55 +/- 0.46 ng/ml (P less than 0.05), and in normal subjects from 0.77 +/- 0.23 to 2.11 +/- 1.22 ng/ml (P less than 0.0025). We conclude that the standard breakfast test is a useful and practical approach to the study of residual beta cell function.