Metal ions enter the body by one of three main routes: via the gut wall, by inhalation, and through wounds. For most individuals, the important route of aluminum uptake is through the gut wall. This is true even though the bioavailability of aluminum may be higher by other routes.
Like most polyvalent metal ions, most ingested aluminum passes through the intestinal tract without being absorbed. An estimate of the daily aluminum uptake by the body from all sources, based on an estimate of average daily intake and the level of
Concentration (ugg 1) Food excreted aluminum, indicated an absorbed fraction of 0.001.
More precise measurements of absorption have been made using the isotope 26Al. The first reported study utilizing 26Al indicated that in the presence of excess citrate, 1% of the metal was absorbed. This result was considered consistent with the ability of citrate to complex metal ions, holding them in solution at physiological pH values, but unrepresentative because of the large amount of citrate employed. Moreover, the study estimated aluminum uptake from the results of single blood analyses, which may not provide a true measure of uptake. Later studies employing this method have indicated lower uptake values, typically 0.0005. They have also shown that some subpopulations, including those suffering from Alzheimer's disease and some 'normal' individuals, absorb more aluminum than average members of the population and that the coingestion of silicic acid inhibits aluminum absorption by a factor of approximately 3.
Complete balance studies using the 26Al tracer have been undertaken to determine bioavailability. These showed that the fractional uptake of aluminum following administration as a citrate solution was 0.005 and following its intake as hydroxide was 0.0001. The coadministration of citrate with aluminum hydroxide enhanced aluminum uptake by a factor of approximately 10. A later study measured the bioavailability of aluminum in drinking water, and a fractional uptake of 0.002 was determined. It follows that at a maximum concentration of aluminum in drinking water of 200 mg^1, this source will normally account for approximately 6% of total (non-medical) aluminum uptake.
In addition to the concentration of citrate in ingested food, other factors have been shown to affect the bioavailability of aluminum: age—metal uptake in milk-fed infants is higher than average and uptake may also be greater in the elderly; gut contents reduce metal bioavailability; silicic acid binds strongly to aluminum, reducing its bioavailability; and local gut conditions affect the ability of the gut wall to sequester aluminum, changing the time available for its uptake. Overall, results suggest that aluminum bioavailability varies between approximately 0.01 and 0.0001.
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