Carotenoids are plant compounds that give fruit and vegetables their yellow, orange and red colours. Carotenoids are bioavialable in humans and show protective effects against diseases such as a cancer and cardiovascular disease. Although many carotenoids exist in nature, researchers often uses β-carotene in studies because it is one of the most frequently consumed carotenoid in Western diets. However, the widespread use of synthetic forms of β-carotene in research means that care must be taken when extrapolating results to free-living individuals ingesting complex mixtures of many carotenoids. That being said, feeding studies using β-carotene can give useful information regarding the absorption, metabolism, storage and excretion of carotenoids and help to elucidate the mechanisms by which carotenoids may be beneficial to health. These beneficial effects are likely due to the carotenoid itself, as although conversion to vitamin A is possible, adequate plasma retinoids blocks this process.
Researchers1 have assessed the relationship between intake of β-carotene and plasma and concentrations of carotenoids and retinoids in 12 healthy vegetarians and non-vegetarian subjects. The results showed that β-carotene intakes were significantly higher in vegetarians than non-vegetarian subjects and this was reflected in higher plasma concentrations. However, the increased plasma concentrations of β-carotene did not translate into higher retinol, retinyl esters or retinoic acid concentrations as there was no significant difference between the vegetarian and non-vegetarian plasma retinoid concentrations. In the same study, administration of a 90mg synthetic β-carotene supplement or a placebo to 15 health female subjects over 60 years of age for 3 weeks resulted in a significant increase in β-carotene plasma concentrations in the treatment but not the control group. However plasma retinoid concentrations were not significantly different from baseline in either group.
These results suggest that β-carotene intake does not affect plasma retinoid concentrations in certain individuals. This supports other studies that show that adequate vitamin A intake inhibits the conversion of β-carotene to retinoids. This would suggest that an effective way to increase plasma levels of β-carotene would be to ingest adequate levels of vitamin A, thus sparing the carotenoids with potential vitamin A activity and thus increasing the carotenoid pool. To support this viewpoint, the same authors also measured the adipose tissue of healthy men after supplementation with a 120mg β-carotene supplement or a placebo. The treatment group had increases in β-carotene concentrations in adipose tissue at day 5 and 10, but by day 10 in the control group there was still no increase in adipose tissue β-carotene. There was also no change to retinoid concentrations in adipose tissue in either group.
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