Free cortisol in obesity; effect of fasting.

Plasma and urinary corticosteroids were measured in 13 obese subjects before and after high and low protein diets, and after fasting. During isocaloric high and low protein diets, urinary 17-oxogenic steroids and to a lesser extent urinary free cortisol excretion rose and fell in parallel with protein intake. Plasma unbound cortisol levels were not much changed by high or low protein intake. However, during 7 to 11 days total fasting, there was a highly significant rise in plasma unbound cortisol at 24.00. A smaller rise occurred at 09.00. The overall effect was a considerable diminution of the day-night variation of plasma unbound coritsol levels during fasting, and a rise in prevailing unbound cortisol levels and urinary free cortisol excretion. In 3 subjects tested these changes were reversed immediately by glucose re-feeding.