Kuang-Chung Shih, Low-Tone Ho
This review addresses the effects of growth hormone on diurnal insulin sensitivity in normal, patients with type 2 diabetes mellitus (T2DM) and animal models described in our previous studies. Results confirmed the presence of diurnal insulin sensitivity, or greater insulin sensitivity in the morning, in normal subjects. The exact cause of this circadian rhythm in plasma glucose levels in healthy subjects has not been established, and nocturnal surges in growth hormone secretion remain a possible explanation. Results showed that growth hormone is an important factor controlling the diurnal variation of glucose tolerance and insulin sensitivity in rats. Data from our previous study provided direct evidence that the role of growth hormone in regulating insulin sensitivity in healthy subjects may be related to changes in the metabolic clearance rate of insulin and the metabolism of non-esterified fatty acids. A different circadian rhythm for plasma glucose appears to be present in patients with T2DM with nadirs in the early evening and peaks in the early morning, indicating greater insulin sensitivity in the evening. As in normal subjects, the exact cause of these circadian rhythms in the plasma glucose levels of patients with T2DM provided direct evidence that the reduction in insulin sensitivity may be due to the nocturnal surge of growth hormone in the early morning hours.