Oh

Lutein

Figure 1 The structures of the most common carotenoids found in the human body. Three of them, ^-carotene, a-carotene and ß-cryptoxanthin, can be used by the body to make vitamin A. All carotenoids are antioxidants found in fruits and vegetables.

Once in the body, carotenoids can act as potent antioxidants, which are substances that neutralize free radicals formed from the natural metabolic processes of cells. Free radicals damage tissues and cells through oxidative processes. While free radical formation is a natural process in the body, environmental factors such as smoking and pollution can increase free radical load and thus disease risk. Carotenoids may counter these influences by functioning as an antioxidant and quenching oxygen-containing free radicals. In high- and low-density lipoproteins and cell membranes, carotenoids may also regenerate the antioxidant form of vitamin E as well as protect vitamin E from oxidation.

At the whole-body level, some population studies have indicated that certain carotenoids from either dietary intake or blood concentration data are associated with better immune response, lower rates of age-related macular degeneration (AMD) and cataract, as well as lower risk for certain cancers and cardiovascular disease. fi-Carotene may increase immunological functions by enhancing lymphocyte proliferation independent of its provitamin A functions. The associations between specific carotenoids

Food

Bioaccessibility = ß-Cfreed/ß-Ctotal

Bioavailability = ß-Cabsorbed/ß-Ctotal

Bioconversion = Retinal /ß-Ca

Bioavailability = ß-Cabsorbed/ß-Ctotal

Bioefficacy = Retinal /fi-Ctota|

Figure 2 A schematic outlining the path of fi-carotene (fi-C) as it moves out from the food into the intestinal wall. The definition of terms associated with understanding fi-carotene release, absorption, and conversion to retinol are illustrated: bioaccessibility, bioavailability, bionconversion, and bioefficacy. (Reproduced with permission from Tanumihardjo SA (2002) Factors influencing the conversion of carotenoids to retinol: Bioavailability to bioconversion to bioefficacy. International Journal of Vitamin and Nutrition Research 72: 40-45.)

and decreased risk of various diseases are summarized in Table 2.

Blood levels of specific carotenoids are often used as biomarkers of fruit and vegetable intake to strengthen or replace dietary intake data. A wide variation in analytical methods exists and standardization between laboratories does not routinely occur. Nonetheless, higher blood concentrations have been favorably correlated with certain disease states. For example, vitamin A and carotenoid concentrations in serum were measured in middle-aged women who later developed breast cancer. Median concentrations of fi-carotene, lycopene, lutein, and total carotenoids were significantly lower in women with breast cancer compared with case-control women who had not developed breast cancer. In contrast, vitamin A concentrations were either not different or showed a mixed response between cohorts, suggesting that carotenoids may be protective against breast cancer. Furthermore, the Nurses' Health Study, which included a cohort of over 83 000 women, also showed a significant inverse association between dietary fi-carotene intake and breast cancer risk. This was especially strong for premenopausal women with a family history of breast cancer or high alcohol consumption. However, other prospective studies have had mixed results.

Table 1 Terms that are associated with the fi-carotene vitamin A value of foods and subsequent utilization as retinol

Term

Definition

100%

Bioaccessibility

fi-Carotene freed

1 mmol freed

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