We have recently developed a unique stable isotope tracer method for studying the absorption, metabolism, and biokinetics of physiological oral doses of beta-carotene and its primary metabolites directly in human subjects. The approach is based on use of per-labeled 13C-beta-carotene and a new generation of high precision gas chromatography-combustion-high precision gas isotope ratio mass spectrometer. This technique typically achieves 24 orders of magnitude more sensitivity and precision than stable isotope techniques relying on lower isotopic enrichments and conventional quadrapole mass spectrometry. 13C-beta-carotene, greater than 8% 13C, has been biosynthesized in significant quantity and purified by crystallization and HPLC. In preliminary studies with a doses as low as 0.5 mg, we have followed 13C-beta-carotene absorption, intestinal conversion to 13C-retinyl esters and 13C-retinol, and the subsequent plasma kinetics of all three 13C-analytes in the absence of changes in total analyte concentration in plasma. At present we are the only laboratory capable of performing such studies in humans at non-perturbing doses resembling those of typical daily intake. We now propose to use this technique to examine several fundamental aspects of the metabolism of beta-carotene in humans. A series of single-dose biokinetic studies in healthy adults are proposed to (a) fully describe and model the fundamental phenomena of beta-carotene absorption, metabolism and distribution; (b) determine the degree of within- and between-person variability in these parameters; (c) determine the effect of beta-carotene dose (1 mg v. 46 mg) on extent of absorption and intestinal metabolism; (d) determine the effect of prior dietary intake of beta-carotene (deprivation or supplementation) on the metabolism of a subsequent dose of 13C-beta-carotene; and (e) compare the absorption and metabolism of 9-cis-beta-carotene to that of the all-trans isomer. A model intestinal mixed lipid micelle system will be used to probe the effects of bile salts, fatty acid unsaturation, cholesterol, phytosterol, and alpha-tocopherol on the capacity of mixed micelles to incorporate beta-carotene. We will then determine if factors shown capable of altering micellar beta-carotene incorporation in vitro also alter 13C-beta-carotene absorption or metabolism in vivo in human subjects. Finally, a rat model will be used to initiate investigation of the effect of iron deficiency on beta-carotene metabolism, in light of intriguing preliminary data showing such an effect.
| You, C S; Parker, R S; Goodman, K J et al. (1996) Evidence of cis-trans isomerization of 9-cis-beta-carotene during absorption in humans. Am J Clin Nutr 64:177-83 |
| Swanson, J E; Wang, Y Y; Goodman, K J et al. (1996) Experimental approaches to the study of beta-carotene metabolism: potential of a 13C tracer approach to modeling beta-carotene kinetics in humans. Adv Food Nutr Res 40:55-79 |
