Trivalent chromium is an essential element in humans and other mammals. Dietary chromium is absorbed from food and then must be converted to its active form, the glucose tolerance factor (GTF) chromium. The active form of chromium is required for the correct function of the insulin receptor, and low intakes of chromium are thought to be a cause of insulin insensitivity. Exercise is associated with improved insulin sensitivity, and this is thought to occur by an increased uptake of chromium into skeletal muscle, and other organs. However, exercise increases the excretion of chromium, and it has been reported that following one hour of weight training, urinary chromium excretion increases significantly. High intakes of carbohydrate necessitate higher intakes of chromium, because once insulin has bound to the insulin receptor chromium complex, the chromium is excreted from the cell. High intakes of refined carbohydrates, refined of their chromium content, may therefore increase the risk of insulin resistance.
The present understanding of the functions of chromium on mammalian physiology has been pieced together using humans and animal studies. Animal studies are useful because they allow the measurement of body tissues through sacrificing the animals. For example, in one study1, researchers investigated the effects of exercise on the chromium concentrations of insulin sensitive tissues in rats. Analysis of the tissues of the rats using flameless atomic absorption spectrophotometry showed that those rats exposed to exercise throughout their lives had increased tissue levels of chromium. Because exercise stimulates appetite and these rats would have eaten more than normal rats, this could have accounted for the increased tissue chromium levels. To control for this the researchers fed another group of rats the same amount of food, but their tissue chromium levels decreased. A control group of rats who were sedentary and had a normal ad libitum diet showed decreases in tissue chromium levels with age.
These results suggest that exercise might improve insulin sensitivity because a training effect that causes an increased tissue concentration of chromium may exist. The results also confirm that ageing and a sedentary lifestyle decrease tissue levels of chromium. Therefore exercise is associated with improved insulin sensitivity, while ageing and sedentary lifestyles are associated with decreased insulin sensitivity. However, there is one important caveat to these results, as the diets of the rats had adequate chromium. Trying to increase tissue levels of chromium through exercise while eating a chromium deficient diet would likely not be possible. This partially explains the inability of exercise to cause weight loss in the obese. Without dietary improvements that increase chromium intake, exercise is not able to improve tissue concentrations of chromium and therefore no increases in insulin sensitivity are seen. Eating a high quality diet with adequate chromium is therefore likely a necessity if weight loss is a goal.
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