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[Beta]-Carotene and other provitamin carotenoids are the major sources of vitamin A for most vegetarians. Carotenoids are also believed to be fat-soluble antioxidants. Therefore, [Beta]-carotene metabolism and its conversion to vitamin A are of importance to vegetarians. Carotenoid metabolism appears to be highly variable in healthy, well-fed individuals. We hypothesized that vitamin A status, body fat stores, and oxygen consumption might influence serum carotenoid metabolism and concentrations. We measured serum carotenoid and vitamin A concentrations in 23 women participating in 3 metabolic research unit studies. Most of these women were fed low-carotenoid diets supplemented with [Beta]-carotene and mixed carotenoid capsules for [is greater than] 100 d. Activities, exercise, diet and medications were controlled. We compared serum carotenoid concentrations to dietary intakes of carotenoids, vitamin A status (estimated by stable isotope dilution), body composition, and oxygen consumption. Carotenoid concentrations appeared to decrease asymptotically in all women during carotenoid depletion, then increased in all women during carotenoid supplementation. However, carotenoid concentrations were highly variable. Serum vitamin A concentrations were unchanged. Carotenoid concentration changes were negatively correlated to vitamin A status (r [approximately equals] -0.7) in one study, but this influence appeared to be relatively small. Body composition and oxygen consumption had little or no influence on serum carotenoid concentrations. The major influence on carotenoid concentrations and metabolism remains unknown but is probably genetic.

Changes in blood levels of seven carotenoids were measured in 30 men for 11 days after ingesting a dose of 12 or 30 milligrams of pure carotene or 270 grams of carrots. (Carotene and carotenoids are the yellow to red pigments found in many vegetables). Peak levels of beta carotene occurred between one and two days after the dose. A large intake of broccoli (600 grams) or tomato juice (180 grams) did not change blood carotenoid levels. It was concluded that (1) normal people vary by as much as fourfold in how efficiently they absorb carotenoids from food, (2) peak blood levels of carotenoids occur one to two days after a dose of pure beta-carotene, (3) a large single intake of carrots produces a small increase in beta-carotene but single intakes of broccoli or tomato juice do not change carotenoid levels, and (4) a dose of pure beta-carotene produces a larger change in blood carotenoid levels than does a dose of carrots containing the equivalent amount.