THE RELATIONSHIP BETWEEN CHANGES IN SERUM LEVELS OF GROWTH HORMONE AND MOBILIZATION OF FAT DURING EXERCISE IN MAN

Abstract

Mobilization of fat into the blood, evidenced by increased plasma levels of glycerol, was detectable 5–10 min. after healthy men in the post‐absorptive state began to exercise on a bicycle ergometer at a work load of 400 Kgm./min. Plasma levels of immunoreactive human growth hormone (IRHGH) did not change until after 20 min. of exercise, rose rapidly until 60 min. and then began to fall. No detectable fall in plasma level of glucose preceded the rise in IRHGH. These results are incompatible with a major role for growth hormone in mobilization of fat during such exercise. When subjects were given nicotinic acid before exercising, fat mobilization was blocked, but the pattern of increase in levels of IRHGH was not altered during the first hr. of exercise. IRHGH levels continued to increase until exercise was stopped and then fell more slowly than in control subjects. The rise in IRHGH during exercise does not, therefore, depend upon prior mobilization of fat, but such mobilization may inhibit its continued secretion.