1. The concentration of amino acids in whole blood was measured before and during standard 5 h oral glucose tolerance testing in six male patients with myotonic dystrophy and five normal males. The plasma levels of insulin and glucose were also determined.
2. From 90 to 240 min after glucose ingestion there was a striking decline in venous alanine concentration in the patients with myotonic dystrophy in contrast to a slight rise in alanine in the normal group.
3. The patients displayed normal glucose tolerance, and there was a sustained fall in the venous concentration of the insulin-sensitive amino acids comparable with that seen in the normal controls. However, the patients showed a threefold increase of plasma insulin after glucose.
4. These data indicate an abnormal regulation of alanine in myotonic dystrophy which may be the result of an alteration in muscle synthesis of this amino acid. This defect in alanine synthesis may be due to a decreased availability of intracellular pyruvate caused by the insulin resistance that exists in these patients.