Prolactin, Growth Hormone, Thyrotropin, 3,5,3′-Triiodothyronine, and Thyroxine Responses to Exercise after Fat- and Carbohydrate-Enriched Diet*

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Abstract
The effect of 4 days of intake of a fat-enriched (76%) diet (F1 and F2 experiments) or a carbohydrate-enriched (77%) diet (CH experiment) on circulating levels of glucose, insulin, GH, PRL, and thyroid hormones during prolonged exercise was studied in seven healthy males. After each of the three diet periods, the subjects ran on a treadmill at 70% of individual maximal oxygen uptake. Atexhaustion, a 10-min rest was allowed, after which the subjects were encouraged to run while glucose (F1 and CH experiments) or glucose and insulin (F2 experiment) concentrations were restored to preexercise levels. The fat diet decreased the mean concentrations of glucose, insulin, and T3 at rest and increased the mean GH level, whereas, mean PRL, T4, and TSH levelswere not changed by diet. During exercise, the decline in glucose and insulin concentrations as well as the progressive increase in GH and PRL concentrations in plasma were enhanced by the fat diet compared to the carbohydrate diet (P < 0.02). Plasma concentrations of T4, T3, and TSH were not significantly (P > 0.05) changed during exercise, but at identical times the T3 concentration was significantly (P < 0.05) higher in CH experiments than in F experiments. Glucose or glucose and insulin infusionduring continued exercise eliminated hypoglycemic symptoms, and a further small rise in plasma PRL concentration was seen after the fat-enriched diet and glucose infusion (F1 experiment) but not during the F2 and CH experiments. In contrast, GH,TSH, T3, and T4 levels were not significantly (P > 0.05) changed in any of the experiments. These data suggest that glucopenia can be an important modulator of plasma GH and PRL during exercise. Furthermore, it is suggested that GH secretion is more sensitive to a decrease in glucose and insulin levels than PRL secretion.