tissues are highly variable and likely depend on a number of factors such as food sources, efficiency of absorption, amount of fat in the diet, and so forth (Table 8-1).
The serum concentration of carotenoids after a single dose peaks at 24 to 48 hours post dose (Johnson and Russell, 1992). The earliest postprandial serum appearance of carotenoids is in the chylomicron fraction. It has been proposed that the increase in carotenoids in the triglyceride-rich lipoprotein fraction (primarily chylomicrons) be used for quantitating carotenoid absorption (van Vliet et al., 1995). This would provide a more direct measure of absorption because total serum carotenoid content is not an exclusive measure of newly absorbed carotenoids.
Data from the Third National Health and Nutrition Examination Survey (NHANES III) demonstrate the variability of normal serum carotenoid concentrations (Appendix Table F-4, Table F-5, Table F-6, Table F-7, through Table F-8). This variability is attributed to a variety of life-style and physiological factors. In a recent population-based study, Brady et al. (1996) reported that lower serum concentrations of α-carotene, β-carotene, β-cryptoxanthin, lutein, and zeaxanthin, but not lycopene, were generally associated with male gender, smoking, younger age, lower non-HDL cholesterol, greater ethanol consumption, and higher body mass index.
The delivery of carotenoids to extrahepatic tissue is accomplished through the interaction of lipoprotein particles with receptors and the degradation of lipoproteins by extrahepatic enzymes such as lipoprotein lipase. Carotenoids are present in a number of human tissues including adipose, liver, kidney, and adrenal, but adipose tissue and liver appear to be the main storage sites (Parker, 1996). However, based on a wet tissue weight, the liver, adrenal gland, and testes contain the highest per-gram concentrations (Stahl et al., 1992). Similar to what is reported in serum, β-carotene, lutein, and lycopene are the main tissue carotenoids, although α-carotene, β-cryptoxanthin, and zeaxanthin are also present (Boileau et al., 1999). In contrast to serum profiles, 9-cis-β-carotene is consistently present in storage tissues. In both serum and tissue storage, lycopene cis-isomers constitute greater than 50 percent of the total lycopene present (Clinton et al., 1996; Stahl et al., 1992).
If adequate retinol is provided in the diet, there are no known clinical effects of consuming diets low in carotenes over the short term. One study of premenopausal women consuming low-carotene