Beta-Carotene
β-Carotene is an organic, strongly colored red-orange pigment abundant in fungi, plants, and fruits. It is a member of the carotenes, which are terpenoids, synthesized biochemically from eight isoprene units and thus having 40 carbons. Among the carotenes, β-carotene is distinguished by having beta-rings at both ends of the molecule. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate.
In some Mucoralean fungi, β-Carotene is a precursor to the synthesis of trisporic acid.
β-Carotene is the most common form of carotene in plants. When used as a food coloring, it has the E number E160a. The structure was deduced by Karrer et al. in 1930. In nature, β-carotene is a precursor to vitamin A via the action of beta-carotene 15,15'-monooxygenase.
Isolation of β-carotene from fruits abundant in carotenoids is commonly done using column chromatography. It can also be extracted from the beta-carotene rich algae, Dunaliella salina. The separation of β-carotene from the mixture of other carotenoids is based on the polarity of a compound. β-Carotene is a non-polar compound, so it is separated with a non-polar solvent such as hexane. Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is very lipophilic.
Provitamin A activity
Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known provitamin A carotenoid. Others include α-carotene and β-cryptoxanthin. Carotenoid absorption is restricted to the duodenum of the small intestine and dependent on class B scavenger receptor membrane protein, which is also responsible for the absorption of vitamin E. One molecule of β-carotene can be cleaved by the intestinal enzyme β,β-carotene 15,15'-monooxygenase into two molecules of vitamin A.Absorption efficiency is estimated to be between 9 and 22%. The absorption and conversion of carotenoids may depend on the form of β-carotene, the intake of fats and oils at the same time, and the current stores of vitamin A and β-carotene in the body. Researchers list these factors that determine the provitamin A activity of carotenoids:
- Species of carotene
- Molecular linkage
- Amount in the meal
- Matrix properties
- Effectors
- Nutrient status
- Genetics
- Host specificity
- Interactions between factors
Symmetric and asymmetric cleavage
Conversion factors
Since 2001, the US Institute of Medicine uses retinol activity equivalents for their Dietary Reference Intakes, defined as follows:Retinol activity equivalents (RAEs)
1 µg RE = 1 µg retinol1 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
RAE takes into account carotenoids' variable absorption and conversion to vitamin A by humans better than and replaces the older retinol equivalent . RE was developed 1967 by the United Nations/World Health Organization Food and Agriculture Organization.
Another older unit of vitamin A activity is the international unit. Like retinol equivalent, the international unit does not take into account carotenoids' variable absorption and conversion to vitamin A by humans, as well as the more modern retinol activity equivalent. Unfortunately, food and supplement labels still generally use IU, but IU can be converted to the more useful retinol activity equivalent as follows:
International Units
- 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 RAE1
Dietary sources
The average daily intake of β-carotene is in the range 2–7 mg, as estimated from a pooled analysis of 500,000 women living in the US, Canada, and some European countries.
The U.S. Department of Agriculture lists these 10 foods to have the highest β-carotene content per serving.
Item | Grams per serving | Serving size | Milligrams β-carotene per serving | Milligrams β-carotene per 100 g |
Carrot juice, canned | 236 | 1 cup | 22.0 | 9.3 |
Pumpkin, canned, without salt | 245 | 1 cup | 17.0 | 6.9 |
Sweet potato, cooked, baked in skin, without salt | 146 | 1 potato | 16.8 | 11.5 |
Sweet potato, cooked, boiled, without skin | 156 | 1 potato | 14.7 | 9.4 |
Spinach, frozen, chopped or leaf, cooked, boiled, drained, without salt | 190 | 1 cup | 13.8 | 7.2 |
Carrots, cooked, boiled, drained, without salt | 156 | 1 cup | 13.0 | 8.3 |
Spinach, canned, drained solids | 214 | 1 cup | 12.6 | 5.9 |
Sweet potato, canned, vacuum pack | 255 | 1 cup | 12.2 | 4.8 |
Carrots, frozen, cooked, boiled, drained, without salt | 146 | 1 cup | 12.0 | 8.2 |
Collards, frozen, chopped, cooked, boiled, drained, without salt | 170 | 1 cup | 11.6 | 6.8 |
Side effects
Excess β-carotene is predominantly stored in the fat tissues of the body. The most common side effect of excessive β-carotene consumption is carotenodermia, a physically harmless condition that presents as a conspicuous orange skin tint arising from deposition of the carotenoid in the outermost layer of the epidermis. Adults' fat stores are often yellow from accumulated carotenoids, including β-carotene, while infants' fat stores are white. Carotenodermia is quickly reversible upon cessation of excessive intakes.Excessive intakes and vitamin A toxicity
The proportion of carotenoids absorbed decreases as dietary intake increases. Within the intestinal wall, β-carotene is partially converted into vitamin A by an enzyme, dioxygenase. This mechanism is regulated by the individual's vitamin A status. If the body has enough vitamin A, the conversion of β-carotene decreases. Therefore, β-carotene is considered a safe source of vitamin A and high intakes will not lead to hypervitaminosis A.Drug interactions
β-Carotene can interact with medication used for lowering cholesterol. Taking them together can lower the effectiveness of these medications and is considered only a moderate interaction. 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. Consuming alcohol with β-carotene can decrease its ability to convert to retinol and could possibly result in hepatotoxicity.β-Carotene and lung cancer in smokers
Chronic high doses of β-carotene supplementation increases the probability of lung cancer in smokers. The effect is specific to supplementation dose as no lung damage has been detected in those who are exposed to cigarette smoke and who ingest a physiologic dose of β-carotene, in contrast to high pharmacologic dose. Therefore, the oncology from β-carotene is based on both cigarette smoke and high daily doses of β-carotene.Increases in lung cancer may be due to the tendency of β-carotene to oxidize, and may hasten oxidation more than other food colors such as annatto. A β-carotene breakdown product suspected of causing cancer at high dose is trans-β-apo-8'-carotenal, which has been found in one study to be mutagenic and genotoxic in cell cultures which do not respond to β-carotene itself.
Additionally, supplemental β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.
Research
Medical authorities generally recommend obtaining beta-carotene from food rather than dietary supplements.Research is insufficient to determine whether a minimum level of beta-carotene consumption is necessary for human health and to identify what problems might arise from insufficient beta-carotene intake, although strict vegetarians rely on pro-vitamin A carotenoids to meet their vitamin A requirements. Use of beta-carotene to treat or prevent some diseases has been studied.