The colors of many fruits and vegetables are due to a class of compounds known as carotenoids. Over 600 carotenoids have been identified in nature. Humans are unique in that they can assimilate carot-enoids from the foods that they eat whereas many other animals do not. Thus, carotenoids are an important class of phytochemicals. Phytochemicals are compounds derived from plants that may or may not have nutritional value. While many carotenoids circulate in humans, the most commonly studied ones are ft-carotene, a-carotene, ft-cryptoxanthin, lycopene, lutein, and zeaxanthin (Figure 1). The nutritional significance of carotenoids is that some are used by the body to make vitamin A. Indeed, approximately 50 carotenoids can be converted by the body into vitamin A and are known as provitamin A carotenoids. The three most abundant provitamin A carotenoids in foods are ft-carotene, a-carotene, and ft-cryptoxanthin. Provitamin A carot-enoids, especially ft-carotene, provide less than one-half of the vitamin A supply in North America but provide more than one-half in Africa and Asia.
Dietary recommendations for the intake of specific carotenoids have not been established due to lack of an adequate evidence base. To date, carotenoids are not considered essential nutrients. Dietary recommendations for vitamin A exist: 900 retinol activity equivalents (RAE) for men and 700 RAE for women. An
RAE is equivalent to 1 mg of retinol. The recommendations for infants and children are less and range from 300 to 600 RAE depending on age. Consumers need to eat sufficient amounts of carotenoid-rich fruits and vegetables to meet their daily vitamin A requirement, and to achieve optimal dietary carotenoid intake to lower the risk of certain chronic diseases. In 2001, the Institute of Medicine revised the amount of caro-tenoids needed to provide vitamin A from foods as being approximately 12 mg of ft-carotene or 24 mg of other provitamin A carotenoids to yield 1 RAE. Currently, high-dose pharmacological supplementation with carotenoids is not advised. Despite this, a tolerable upper intake level, the maximum daily amount of a nutrient that appears to be safe, has not been established for any individual carotenoid; however, supplemental ft-carotene at 20mgday-1 or more is contraindicated for use in current heavy smokers by the European Commission.
Because many factors affect the assimilation of carotenoids from foods (Figure 2), conversion factors need to be considered. This is especially important when most sources of vitamin A are from provitamin A carotenoids in the population. Bio-availability of preformed vitamin A, i.e., retinol and retinyl esters, is not a major concern because 80-95% of them are absorbed. However, foods that are high in preformed retinol (liver, eggs, and fortified milk) are not necessarily consumed by everybody. When discussing carotenoids from food, four terms need to be defined (see Table 1):
• bioaccessibility refers to how much carotenoid can be extracted from the food and is available for absorption;
• bioavailability is how much carotenoid is absorbed from the food and is available for physiological function;
• bioconversion relates to the provitamin A carot-enoids and is defined as the amount of retinol that is formed from absorbed provitamin A carot-enoids; and
• bioefficacy encompasses all of the biological processing of provitamin A carotenoids and is the amount of retinol formed from the amount of carotenoid contained in the food.
The study of carotenoid bioefficacy from foods is important in international health as the most frequently consumed sources of vitamin A are fruit and vegetables. A 100% bioefficacy means that 1 mmol of dietary ft-carotene provides 2 mmol of reti-nol in the body; however, 100% bioefficacy does not actually occur in the process of digestion and carotenoid uptake by the body.
^-Carotene a-Carotene a-Carotene
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