Insulin Inhibition of Insulin Secretion

To the Editor: The studies by Elahi et al. (May 20 issue)¹ demonstrate that insulin administration to “glucose-clamped” subjects results in suppression of endogenous insulin secretion. However, contrary to their claims regarding a short-loop feedback, they have not demonstrated the directness of the effect. For instance, circulating branched-chain amino acids can stimulate insulin secretion,² are more likely to do so in obese subjects,³ and are diminished in response to acute elevations of plasma insulin.^4,5 Might not the insulin infusions described by Elahi et al. have lowered plasma branched-chain amino acids, thus lessening beta-cell stimulation, bringing about the decreased C-peptide.