β-Carotene is a strongly coloured red-orange pigment abundant in fungi, plants, and fruits.
It contributes to the orange color of many different fruits and vegetables.
β-Carotene is a member of the carotenes, which are biochemically synthesized from eight isoprene units and thus having 40 carbons.
β-carotene is distinguished have beta-rings at both ends of the molecule.
It is the most common form of carotene in plants.
β-carotene is a precursor to vitamin A via the action of beta-carotene 15,15′-monooxygenase.
β-Carotene is a non-polar compound, highly conjugated, deeply colored, and lipophilic.
Plant carotenoids are the primary dietary source of provitamin A worldwide.
β-carotene as the best-known provitamin A carotenoid.
Carotenoid absorption is restricted to the duodenum of the small intestine.
Carotenoid absorption is dependent on class B scavenger receptor (SR-B1) membrane protein, which is also responsible for the absorption of vitamin E (α-tocopherol).
The conversion of carotenoids may depend on the form of β-carotene, cooked vs. raw vegetables, supplement, and the intake of concomitant fats and oils, and the current stores of vitamin A and β-carotene in the body.
β-carotene is cleaved either symmetrically or asymmetrically.
Symmetric cleavage gives two equivalent retinal molecules and each retinal molecule further reacts to give retinol (vitamin A) and retinoic acid.
β-Carotene is also cleaved into two asymmetric products.
Asymmetric cleavage reduces the level of retinoic acid significantly.
Supplement labels still generally use IU, but IU can be converted to the more useful retinol activity equivalent as follows:
1 µg RAE = 3.33 IU retinol
1 IU retinol = 0.3 μg RAE
1 IU β-carotene from supplements = 0.3 μg RAE
1 IU β-carotene from food = 0.05 μg RAE
1 IU α-carotene or β-cryptoxanthin from food = 0.025 μg RAE
Beta-carotene is found in many foods and is in dietary supplements.
Retinol activity equivalents (RAEs)
1 µg RE = 1 µg retinol
1 µg RAE = 2 µg all-trans-β-carotene from supplements
1 µg RAE = 12 µg of all-trans-β-carotene from food
1 µg RAE = 24 µg α-carotene or β-cryptoxanthin from food
A β-carotene molecule can be cleaved by the intestinal enzyme β,β-carotene 15,15′-monooxygenase into two molecules of vitamin A.
β-carotene absorption is estimated to be between 9 and 22%.
Blood level is a marker of fruit and vegetable intake.
Observational studies indicate high intake or elevated blood levels associated with reduced risk of lung cancer.
Three randomized studies of dietary supplementation produced no benefit or harm in the incidence of lung cancer in the Physicians Health Study (Hennekens)
Can block some carcinogenic processes and inhibit tumor growth in laboratory studies.
In the Alpha-Tocopherol, Beta-Carotene Cancer Prevention study increased the risk of lung cancer among smokers, and increased lung cancer among smokers and asbestos workers in the Beta-Carotene and Retinol Efficacy Trial (CARET).
Two large trials found that beta carotene supplementation increased the incidence of lung cancers as well as cardiovascular and all cause mortality in high risk smoking populations.
Palm oil is a rich source, as are yellow and orange fruits, such as cantaloupe, mangoes, pumpkin, and papayas, and orange root vegetables such as carrots and sweet potatoes.
The color of β-carotene is masked by chlorophyll in green leaf vegetables: spinach, kale, sweet potato leaves, and sweet gourd leaves.
Palm oil has the highest content of β-carotene of any known plant sources, 10 times higher than carrots.
The average daily intake of β-carotene is in the range 2–7 mg.
The U.S. Department of Agriculture lists these 10 foods to have the highest β-carotene content per serving.
Carrot juice, canned
Pumpkin, canned
Sweet potato
Spinach
Carrots
Collards
Excess β-carotene is predominantly stored in the the body’s adipose tissue.
Excessive β-carotene consumption is carotenodermia.
Carotenodermia is a harmless condition that presents as a conspicuous orange skin tint that arises from deposition of the carotenoid in the outermost layer of the epidermis: quickly reversible upon cessation of excessive intakes.
Adults’ fat stores are yellow from accumulated carotenoids, that include
β-carotene.
The amount of carotenoids absorbed decreases as dietary intake increases.
Within the intestinal wall mucosa, β-carotene is partially converted into vitamin A or retinol by an enzyme, dioxygenase.
The conversion process is
regulated by the individual’s vitamin A status, with enough vitamin A, the conversion of β-carotene decreases.
β-carotene is considered a safe source of vitamin A as high intakes will not lead to hypervitaminosis A.
It can interact with medication used for lowering cholesterol.
β-Carotene should not be taken with orlistat, a weight-loss medication, as orlistat can reduce the absorption of β-carotene by as much as 30%.
Bile acid sequestrants and proton-pump inhibitors can also decrease absorption of β-carotene.
Alcohol consumption with β-carotene can decrease its ability to convert to retinol.
Chronic high doses of β-carotene intake increases the risk of lung cancer in smokers.
Lung cancer increases may be related to the tendency of beta carotene to oxidize more than other food colors.
A β-carotene breakdown product suspected of causing cancer at high dose is trans-β-apo-8′-carotenal.
Supplemental β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.
Obtaining beta-carotene from food rather than dietary supplements, is recommended.
It has not been determined with a minimum level of beta carotene consumption is necessary for health.
Systemic reviews concluded that supplementation with β-carotene does not appear to decrease the risk of cancer overall.
High levels of β-carotene may increase the risk of lung cancer in current and former smokers: beta-carotene is unstable in cigarette smoke-exposed lungs where it forms oxidized metabolites that can induce carcinogen-bioactivating enzymes.
A study showed natural beta-carotene appeared to reduce premalignant gastric lesions.
There is no evidence of any protective effects of β-carotene supplementation on preventing and slowing age-related cataract.