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FIGURE 6-1 Proposed mechanism for the activation of insulin receptor by LMWCr in response to insulin. LMWCr = low molecular weight chromium-binding substance, I = insulin, IR = insulin receptor. Adapted from Vincent (1999).

1969). The environment of the gastrointestinal tract and ligands provided by foods and supplements are important for mineral absorption (Clydesdale, 1988). Several dietary factors that affect chromium absorption will be discussed in the bioavailability section of this chapter.

In humans consuming approximately 10 μg/day of chromium, about 2 percent was excreted in urine, but only 0.5 percent was excreted when intakes approached 40 μg/day (Anderson and Kozlovsky, 1985). These data suggest regulation of chromium absorption in these intake ranges.

A number of studies have reported increased urinary excretion of chromium with aerobic exercise (Anderson et al., 1982, 1984, 1988b). A recent study using 53Cr demonstrated that acute and chronic resistive exercise may increase chromium absorption as determined by the increased urinary excretion of the 53Cr isotope (Rubin et al., 1998). Further studies will be needed to clarify how much of the observed beneficial effects of exercise on glucose and insulin metabolism may be due to improved chromium absorption.

Chromium competes for one of the binding sites on transferrin (Harris, 1977). In rats fed physiological levels of 51CrCl3, more than



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