1. Seven patients with acromegaly and nine controls were studied before, during and after 30 min of moderate steady exercise on a bicycle ergometer. Venous blood samples were taken for estimation of growth hormone (HGH), immunoreactive insulin (IRI), pyruvate, lactate, glucose, free fatty acids (FFA), glycerol and ketone bodies.

2. Exercise caused a rise in HGH concentrations in the acromegalic patients, despite their pituitary tumour, and also in control subjects. Concentrations of IRI fell during exercise in control subjects, but rose in the acromegalic patients.

3. Concentrations of glycerol, FFA and ketone bodies rose rapidly to a maximum during exercise in the acromegalics and appeared to be suppressed before the end of exercise: there was no increase in the concentration of ketone bodies after exercise. In control subjects there was a gradual increase in glycerol and FFA concentrations towards the end of exercise, but no change in ketone bodies occurred until the post-exercise period, when ketone-body concentrations rose.

4. We conclude that exercise causes remarkable differences in metabolite concentrations in the blood of acromegalic patients compared with controls, with the concentration of fat metabolites reaching a maximum, then decreasing during the period of constant exercise. There was also elevation, instead of the normal fall, of plasma IRI activity and it is suggested that the decreased concentration of fat metabolites occurred because of the change of insulin concentration. It is further suggested that in acromegaly insulin retains its effect on re-esterification of fat in spite of resistance to its effects on carbohydrate metabolism.

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