Gastrointestinal tract uranium

After inhalation exposure, the primary route of excretion is in the feces following ciliary clearance from the lungs to the gastrointestinal tract (Wrenn et al. 1981). Fecal excretion may account for as much as 97% of total excretion (Fisher et al. 1983). Higher levels of thorium-230 were excreted in the feces by active crushermen (uranium mill workers exposed to uranium ore dust in the crusher building) compared to retired workers or controls (Fisher et al. 1983). Levels of thorium-230 in the urine were comparable to those of retired workers, and the levels in both were significantly greater than controls. 

Uranium ingested from food and water ingestion ranges from 1-5 pg uranium in uncontaminated regions to 13-18 pg uranium in uranium mining areas (ATSDR, 1999). The absorption of uranium across the gastrointestinal tract is related to the solubility of the compound and generally increases with increased solubility. Only a small fraction is... 

Absorption of uranium through the entire gastrointestinal tract of the rat. Int. J. Radiat. Biol. 81 473-82. 

Lang, S., Raunemaa, T. (1991). Behavior of neutron-activated uranium dioxide dust particles in the gastrointestinal tract of the rat. Radial Res. 126 273-9. 

Evidence from several animal studies showed that the amount of uranium absorbed from the gastrointestinal tract was about 1% (Harrison and Strather 1981 Larsen et al. 1984 LaTouche et al. 1987 Maynard et al. 1953 Sullivan 1980a). A range of gastrointestinal absorption rates of 0.038-0.078% has been estimated by others based on data from a 2-year study in which rats were fed diets containing... 

Ingested uranium is excreted mostly in the feces urinary excretion is generally low. The biological halftimes of soluble uranium compounds (uranium hexafluoride, uranyl fluoride, uranium tetrachloride, uranyl nitrate hexahydrate) are estimated in days or weeks those of the less soluble compounds (uranium tetrafluoride, uranium dioxide, triuranium octaoxide) are estimated in years. No information is currently available on the excretion of dermally absorbed uranium. Transdermally absorbed uranium is expected to behave identically to uranium compounds absorbed through the lungs and the gastrointestinal tract. 

Evidence also suggests that the toxicity of uranium varies according to the route of exposure and to its compounds. This finding may be partly attributable to the relatively low gastrointestinal absorption of uranium compounds. Only <0.1-6% of even the more soluble uranium compounds are absorbed in the gastrointestinal tract. On the basis of the toxicity of different uranium salts in animals, it was concluded that the relatively more water-soluble salts (uranyl nitrate hexahydrate, uranyl fluoride, uranium pentachloride) were primarily renal and systemic toxicants. The less water-soluble compounds (uranium trioxide, sodium diuranate, ammonium diuranate) were of moderate-to-low toxicity, while the insoluble compounds (uranium tetrafluoride, uranium dioxide, uranium peroxide, triuranium octaoxide) were... 

The value for the fractional absorption of ingested uranium (fj) was adjusted from the adult value of 0.02 (2%) to a value of 0.04 (4%) for children under the age of 1 year. This adjustment was made based on animal data (Sullivan et al. 1980b Sullivan and Gorham 1982) and information on postnatal changes in the human gastrointestinal tract. For ages over 1 year, the adult value for fractional absorption was used. 

In contrast, the chemical toxicity of uranium is more important than its radiological hazard. In body fluids, uranium is present as soluble U(VI) species and is rapidly eliminated from the body (60% within 24 h Goyer and Clarkson (2001)). It is rapidly absorbed from the gastrointestinal tract and moves quickly through the body. The uranyl carbonate complex in plasma is filtered out by the kidney glomerulus, the bicarbonate is reabsorbed by the proximule tubules, and the liberated uranyl ion is concentrated in the tubular cells. This produces systemic toxicity in the form of acute renal damage and renal failure. 

The solubility of the uranium compound is an important consideration in determining adsorption and distribution as toxicity is related to uptake efficieney of the gastrointestinal (GI) tract. Generally, absorption is inereased with the... 

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