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Chromium potentiates the action of insulin in vivo and in vitro (Mertz, 1969, 1993; Mertz et al., 1961). Schwarz and Mertz (1959) identified chromium as the element that restored glucose tolerance in rats. Impaired glucose tolerance of malnourished infants responded to an oral dose of chromium chloride (Hopkins and Majaj, 1967; Hopkins et al., 1968); subsequently, benefits of chromium chloride were reported in a patient receiving total parenteral nutrition (TPN) (Jeejeebhoy et al., 1977).

A number of studies have demonstrated beneficial effects of chromium on circulating glucose, insulin, and lipids in a variety of human subjects and animal species; however, not all reports of supplementation are positive (Anderson, 1997; Anderson et al., 1991) (for reviews see Anderson, 1997; Mertz, 1993; Offenbacher et al., 1997; Stoecker, 1996). Progress in the field has been limited by lack of a simple, widely accepted method for identification of subjects who are chromium depleted, and thus who would be expected to respond to chromium supplementation, and by the difficulty in producing chromium deficiency in animals.

Recent work by Davis and Vincent (1997a, 1997b) and Vincent (1999) suggests that a low molecular weight chromium-binding substance (LMWCr) may amplify insulin receptor tyrosine kinase activity in response to insulin. It is proposed that the inactive form of the insulin receptor (IR) is converted to the active form by binding insulin, which stimulates the movement of chromium from the blood into the insulin-dependent cells and results in the binding of apoLMWCr to chromium (Figure 6-1). The holoLMWCr then binds to the insulin receptor activating the tyrosine kinase. The ability of LMWCr to activate insulin receptor tyrosine kinase depends on its chromium content. When insulin concentration drops, the holoLMWCr is possibly released from the cell to terminate its effects.

Physiology of Absorption, Metabolism, and Excretion

Absorption estimates for chromium III, based on metabolic balance studies or on urinary excretion from physiological intakes, range from 0.4 to 2.5 percent (Anderson and Kozlovsky, 1985; Anderson et al., 1983, 1991, 1993a; Bunker et al., 1984; Doisy et al., 1971; Offenbacher et al., 1986).

Most chromium compounds are soluble at the pH of the stomach, but less soluble hydroxides may form as pH is increased (Mertz,

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