International Journal of Sport Nutrition, 1992, 2, 111-122
©1992 Human Kinetics. This excerpt is approved by the publisher.
This material may not be copied or reprinted.
Robert G. Lefavi, Richard A. Anderson, Robert E. Keith, G. Dennis Wilson, James L. McMillan, and Michael H. Stone
Soon after Curran (17) suggested that chromium ions might have a biological function, it was noted that glucose tolerance was impaired in rats fed a chromium-poor diet (53). Subsequently, Schwarz and Mertz (72) postulated the existence of an unrecognized dietary factor that contains chromium and is necessary for maintaining normal glucose tolerance in rats. This dietary agent was termed glucose tolerance factor (GTF) and trivalent chromium was later identified as its active ingredient (73).
Since then, further research on this necessary role of biologically active chromium has led to its general acceptance as an essential trace element in humans. Although chromium's essentiality has recently been challenged (36), a criticism of this opinion clarified the manner in which chromium functions as an essential nutrient (3). Furthermore, research continues to identify chromium as the biologically active component of a substance with GTF properties (87).
The biological function of GTF appears to be related to its organic structure. Mertz (48) has specified markers of true GTF activity and other researchers have attempted to identify the exact structure of this compound. GTF has been hypothesized (22) to be a low molecular weight compound, and nicotinic acid, glycine, glutamic acid, and cysteine have been associated as the GTF components associated with chromium (57,77). Consistent with these findings, further research has suggested that a nicotinic acid - chromium structure is essential in the GTF complex (61,62,78), although the exact structure of GTF (2) and the exact location of its synthesis is still unknown.
GTF has been shown to potentiate the effects of insulin at target tissues (48,56,66), and other research has documented a more direct chromium-insulin relation. For instance, it appears that the presence of chromium is required for the potentiation of insulin's actions (49), perhaps explaining why chromium-deficient rats were less responsive to insulin than chromium-supplemented rats (45,65). Additionally, fasting serum chromium levels were found to correlate with fasting serum insulin levels (30,41), further suggesting that chromium and insulin may be closely involved in a similar biological function.
A mechanism of action, whereby chromium acts as a ternary complex with insulin and the insulin receptor to facilitate a disulfide exchange at the target cell membrane, has been proposed (15,32). Combined, these observations lead to two conclusions. First, chromium may play a necessary role in the normal physiological system by acting as a "cofactor" for insulin, amplifying its actions (46,67,75). Second, it logically follows that the efficiency of insulin's effects - glucose, amino acid, and fatty acid flux into the cell with subsequent glycogen, protein, and triglyceride synthesis (16,29,76) - would be dependant upon the maintenance of adequate chromium stores.
Investigators have found that biologically active chromium concentrations vary in mammalian tissues. Although it has been demonstrated that the liver contains a substance with GTF properties (12,55), the kidneys, spleen, and testes retain more chromium than other organs (34). However, the concentration of chromium in chromium stores may be altered by exercise since trained rats had higher chromium levels in heart and kidney tissue compared to controls (81). This apparent training adaptation may occur due to the need for efficient pre-and post-exercise insulin function, or it may result from a large amount of glucose being processed during exercise. These studies indicate that biologically active chromium may occupy a special physiological pool that is mobilized when needed.
Given the possibility of a suboptimal chromium status in regularly exercised individuals, it appears that either a high-chromium diet or chromium supplementation may be useful in athletes. Clearly, encouraging an athlete to eat a high-chromium diet (unprocessed instead of processed foods, a minimal consumption of sucrose-laden foods, etc.) would also serve to establish good eating habits and should be the first choice of any sport nutritionist.
However, there are few available foods that are also good choices for athletes and exceptionally high in chromium. Thus, athletes may need to eat much more food to ensure adequate chromium intake, often a predicament for those trying to restrict calories. In many cases the supplementation of a chromium compound may be the most productive way to protect endogenous chromium stores while avoiding the addition of unwanted calories.
The restoration of depleted chromium stores would promote optimal insulin function and may play a positive role in exercise preparation, performance, and recovery. When these issues are coupled with reports demonstrating the biological activity (46,51,61,79,80) and very low toxicity (51) of inorganic and GTF-like chromium compounds, the concept of chromium supplementation in athletes mat be more palatable to the most conservative sport nutritionist. Supplements that are most likely to assist in restoring chromium pools are inorganic chromium complexes such as chromium chloride, and organic forms, both natural (Brewer's yeast) and synthetic (GTF-like compounds).
One should be reasonable when counseling athletes on chromium supplementation and the potential anabolic nature of supplemental chromium. Even in an extreme case whereby supplementation of biologically active chromium restored depleted chromium pools, large, short-term muscle mass increases would not be expected. Rather, marginal body weight increases, occurring over a relatively long period of time, would be more consistent with the anabolic nature of enhanced insulin function..
That is, increasing insulin efficiency may improve anabolism within muscle cells. However, a great deal of time, perhaps many months, would be necessary for intracellular protein synthesis to be sufficiently stimulated such that lean body mass differences would differ significantly between treatment and placebo groups, particularly when dealing with normal, healthy athletes. Moreover, since insulin assists in fat storage, decreases in percentage of body fat may not be detectable for a longer period of time, if at all, depending on caloric intake.
Furthermore, preliminary body mass data from our recent chromium supplementation study using male athletes as subjects (40) support the concept that possible anabolic increases are likely to be moderate and insignificant when the supplementation period is brief. In this investigation we have found no significant increases in body composition parameters in weight-lifters following 8 weeks of supplementation with a chromium-nicotinic acid (GTF-like) compound. In short, anabolic steroid-like muscle mass increases do not appear to be consistent with effects normally seen when chromium in any form is added to the diet of a healthy subject, regardless of the training regimen employed.