Carotenoids are a class of more than 750 naturally occurring pigments synthesized by plants, algae, and photosynthetic bacteria.
These richly colored molecules are the sources of the yellow, orange, and red colors of many plants. Fruits and vegetables provide most of the 40 to 50 carotenoids found in the human diet. α-Carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene are the most common dietary carotenoids. α-Carotene, β-carotene and β-cryptoxanthin are provitamin A carotenoids, meaning they can be converted by the body to retinol. Lutein, zeaxanthin, and lycopene are nonprovitamin A carotenoids because they cannot be converted to retinol
For dietary carotenoids to be absorbed intestinally, they must be released from the food matrix and incorporated into mixed michelles (mixtures of bile salts and several types of lipids). Food processing and cooking help release carotenoids embedded in their food matrix and increase intestinal absorption. Moreover, carotenoid absorption requires the presence of fat in a meal. As little as 3 to 5 g of fat in a meal appears sufficient to ensure carotenoid absorption, although the minimum amount of dietary fat required may be different for each carotenoid. The type of fat, the presence of soluble fiber, and the type and amount of carotenoids in the food also appear to influence the rate and extent of carotenoid absorption. Because they do not need to be released from the plant matrix, carotenoid supplements (in oil) are more efficiently absorbed than carotenoids in food. Although carotenoids were initially thought to be absorbed within the cells that line the intestine only by passive diffusion, recent investigations identified the apical membrane transporters, Scavenger Receptor-class B type I (SR-BI) and Cluster Determinant 36 (CD36), suggesting active uptake of carotenoids as well.
- Carotenoids are yellow, orange, and red pigments synthesized by plants. The most common carotenoids in North American diets are α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene.
- Provitamin A carotenoids, α-carotene, β-carotene, and β-cryptoxanthin, can be converted by the body to retinol (vitamin A). In contrast, no vitamin A activity can be derived from lutein, zeaxanthin, and lycopene.
- Dietary lutein and zeaxanthin are selectively taken up into the macula of the eye, where they absorb up to 90% of blue light and help maintain optimal visual function.
- At present, it is unclear whether the biological effects of carotenoids in humans are related to their antioxidant activity and/or other non-antioxidant activities.
- Although the results of observational studies suggest that diets high in fruits and vegetables are associated with reduced risks of cardiovascular disease and some cancers, high-dose β-carotene supplements did not reduce the risk of cardiovascular disease or cancer in large randomized controlled trials.
- Two randomized controlled trials found that high-dose β-carotene supplements increased the risk of lung cancer in smokers and former asbestos workers.
- Recent meta-analyses of observational studies reported an inverse association between blood lycopene concentration and risk of developing prostate cancer. To date, most small-scale intervention studies found little-to-no benefit of lycopene supplements in reducing incidence or severity of prostate cancer in high-risk patients.
- Observational studies have suggested that diets rich in lutein and zeaxanthin may help slow the development of age-related macular degeneration (AMD). Randomized controlled trials found that lutein and zeaxanthin supplements could improve visual acuity and slow the progression to advanced AMD in subjects with AMD.
- Evidence is lacking to suggest a role for lutein and zeaxanthin in the management of other eye conditions, including cataracts, diabetic retinopathy, and retinopathy of maturity.
- Carotenoids are best absorbed with fat in a meal. Chopping, puréeing, and cooking carotenoid-containing vegetables in oil generally increase the bioavailability of the carotenoids they contain.