Metabolic responses during modified fasting and refeeding. The role of sympathetic nervous system activity and thyroid hormones.

Thyroid hormones, sympatho-adrenomedullary activity and metabolic responses have been studied in eight healthy obese subjects during modified fasting (0.8 MJ, 200 kcal/d) for 14 d, followed by hypocaloric-carbohydrate refeeding (2.4 MJ, 600 kcal/d) for 3 d. In the week preceding modified fasting, the subjects received a 7.2 MJ (1800 kcal) diet daily containing 20 mmol sodium, in order to distinguish the effects of salt depletion from those of energy restriction. Sodium (20 mmol/d) and potassium (80 mmol/d) were kept constant throughout the study. During modified fasting a low T3-high rT3 state developed; concomitantly a marked decrease in sympatho-adrenomedullary activity (assessed by 24-h urinary excretion of catecholamines) was observed. Upon refeeding plasma T3 increased and plasma rT3 decreased, but not completely to pre-fast values. Upon refeeding urinary catecholamine excretion markedly increased. A strong inverse relationship was found between the marked decrement of both thermogenic hormones and the catabolism of protein. No correlation between these parameters was found during brief refeeding. The observed decline of both thermogenic hormones may represent an adaptation to protect tissues from the catabolic effect of these substances during severe energy deficiency.