1. To quantify the degree of whole body insulin resistance in patients with myotonic dystrophy and to determine if these same patients display signs of a whole body decrease in the action of insulin on amino acid uptake and glucose disposal, three separate 120 min studies employing the euglycaemic insulin clamp technique (20, 80 and 200 m-units min−1 m−2) were performed on five ambulatory patients with myotonic dystrophy. The results were compared with findings obtained in identical studies in 21 normal volunteers.

2. Myotonic dystrophy patients showed a slower, less marked decline in the serum concentration of insulin sensitive amino acids (threonine, valine, leucine, isoleucine, tyrosine, phenylalanine) during all three insulin infusions compared with normals. The greatest difference occurred at the low physiological elevations of insulin produced by the 20 m-units min−1 m−2 infusion.

3. Alanine levels fell significantly below baseline in patients with myotonic dystrophy after 60 and 120 min of insulin infusion with all three rates of insulin infusion. Normal subjects had only a minimal, insignificant decline in arterialized alanine concentrations during the three different insulin infusions.

4. Creatinine adjusted rates of whole body glucose disposal were 30–40% lower in the myotonic dystrophy group at all three doses of insulin compared with the normals. This demonstrates that their insulin resistance was not due simply to a reduction in muscle mass.

5. The overall pattern of findings in these studies of patients with myotonic dystrophy indicates that there is a whole body derangement in the regulation of circulating amino acid levels by insulin as well as a marked decrease in the action of this hormone in stimulating glucose uptake by target tissues.

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