|Clifford, Andrew - UC DAVIS, NUTR. DEPT.|
Submitted to: International Vitamin A Consultative Group
Publication Type: Proceedings
Publication Acceptance Date: November 17, 2004
Publication Date: November 17, 2004
Citation: Burri, B.J., Clifford, A.J. Human carotenoid metabolism assessed with radioisotope techniques. International Vitamin A Consultative Group. Technical Abstract: Recently we have used radioisotopes as tracers to assess B-carotene absorption and metabolism in healthy human adults. We use Accelerator Mass Spectrometry (AMS) that measures 14C/12C ratios to parts per quadrillion (10-15) in milligram-sized samples to quantify the absorption and metabolism of small oral doses of 14C-B-carotene in humans. There are two advantages of radioisotope over the more common stable isotope assessment methods. First, it is much easier to discover, identify, and quantitate carotenoid metabolites, such as retinyl esters. Second, lower concentrations of B-carotene can be fed and traced. When B-carotene is absorbed into the blood,it first appears in 5.5 h and peaks 16 to 24 h after the oral dose. The concentration profile for labeled retinyl esters is different than for retinol, and shows two rapid maxima at 4 and 6 h. The amount of B-carotene absorbed can be quite variable, but part of the variability previously reported is due to the fact that retinyl esters formed from B-carotene appear in serum at the same time as carotene itself appears, and these retinyl esters were very difficult to identify and measure prior to the advent of radioisotope tracers. Most carotenoids are excreted in the feces but small amounts appear in urine. Patterns of 14-C in plasma from an oral dose of 14C- B-carotene provided to the same subject in a bolus dose with oil, and without oil, differ. In the absence of fat, the first retinyl ester peak is very small, while the second is unaffected. The appearance of the 14C-B-carotene plasma peak is also much smaller in the absence of fat.