Insulin regulation of leptin synthesis and secretion in humans: the model of myotonic dystrophy

OBJECTIVE: Myotonic dystrophy (MyD) is a systemic disorder in which insulin resistance is well recognized. In the present study we have characterized plasma leptin levels in patients with MyD and in age, sex and body mass index (BMI) matched controls and assessed the influence of leptin on the clinical manifestations of MyD. DESIGN AND PATIENTS: Body composition, plasma leptin, fasting and post-oral glucose tolerance test insulin, IGF-I and IGFBP3 were studied in 34 MyD patients and 33 controls. MEASUREMENTS: Body composition was measured using a bioelectrical impedance analyzer, and circulating levels of insulin, leptin, IGF-I, IGFBP3 were measured by IRMA or RIA. Insulin sensitivity was modelled according to a homeostasis model assessment (HOMA) computer-solved model. RESULTS: Percentage body fat was higher in patients than in controls (25.6 +/- 2.28% vs 18.8 +/- 1.53%, P = 0.013). Insulin levels, both fasting and after oral glucose were higher in patients than in controls, and insulin sensitivity was lower in patients than in controls. Serum leptin was higher in patients than in controls (20.98 +/- 3.11 micrograms/l vs 10.4 +/- 1.31 micrograms/l, P = 0.004), and higher in women than in men, both in patients and in controls. In patients, leptin levels were correlated with age, BMI, fasting insulin, insulin area under curve and lower insulin sensitivity, whereas leptin levels were not correlated with body fat or other parameters of body composition. In controls, leptin levels were correlated with BMI and body fat. The results were evaluated using logistic regression models for each of the 2 populations. In the model of MyD, insulin resistance and age correctly identified higher leptin levels in relation to controls out of 87.88% of patients, and in the model of controls male sex with a negative correlation and BMI correctly identified their leptin levels out of 84.33% cases. CONCLUSIONS: These findings show that MyD provides a different model of leptin regulation in humans, and suggest that in MyD patients there are correlations between leptin and insulin resistance and age, irrespective of body fat. In contrast, leptin levels in controls, correlate with sex and BMI. The data on leptin in this population of patients can not be related aetiologically to the muscle disease itself.