Function of Growth Hormone in Diet

Growth hormone is a major anabolic peptide hormone secreted from the anterior pituitary that is involved in many anabolic functions (e.g., increases muscle and skeletal growth) and metabolic regulation (e.g., increases protein synthesis, lipolysis, and glucose conservation). Dietary nutrients may indirectly modulate the resting and exercise-induced responses of growth hormone by altering the plasma concentrations of different energy substrates (Le., glucose, free fatty acids, and ketone bodies). Acute ingestion of a liquid supplement rich in fat did not affect resting values of growth hormone, but attenuated the exercise-induced increase com­pared with a high-carbohydrate supplement. Prolonged intake of a diet higher in fat results in a much different response. Merimee et al. examined the 24-hour growth hormone response to diets rich in either fat (75%), carbohydrate (80%), or protein (70%) consumed for 10 to 12 days. Although the fat and protein-rich diets had no apparent effect, the carbohydrate-rich diet clearly suppressed growth hormone concentrations in men. A similar reduction in growth hormone was detected in men who consumed diets that contained 65% of energy from carbohydrate compared with diets that were lower in carbohydrate but similar in total calories. Thus, the absolute fat content may not be as important as the absolute amount of carbohydrate. Others have also reported that resting and exercise-induced growth hormone concentrations are elevated when preceded by a high­fat diet. Exercise-induced increases in growth hormone are also higher after fasting but decreased when energy intake is moderately restricted. Exercise-induced elevations in growth hormone caused by a high-fat diet or fasting are accompanied by a more rapid decline in plasma insulin and glucose concentrations during exercise, suggesting that glucose-sensitive receptors may modulate the growth hormone response to exercise. Glucose infusion at the end of exercise does not attenuate the greater growth hormone response observed after a fat-rich diet, suggesting that other substrates (Le., glycogen, ketones, fatty acids) or hormones (insulin, catecholamines) may also be involved in the regulation of growth hormone secretion. Elevated levels of free fatty acids have also been shown to inhibit growth hormone secretion, however, this effect appears to be minimal when blood glucose concentrations are simultaneously decreased.