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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