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.
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