Β-Carotene

Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known provitamin A carotenoid. ==Absorption, metabolism and excretion==

As part of the digestive process, food-sourced carotenoids must be separated from plant cells and incorporated into lipid-containing micelles to be bioaccessible to intestinal enterocytes. ==Conversion factors==

For counting dietary vitamin A intake, β-carotene may be converted either using the newer retinol activity equivalents (RAE) or the older international unit (IU). • 1 μg RAE = 1 μg retinol from food or supplements • 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 RAE takes into account carotenoids' variable absorption and conversion to vitamin A by humans better than and replaces the older retinol equivalent (RE) (1 μg RE = 1 μg retinol, 6 μg β-carotene, or 12 μg α-carotene or β-cryptoxanthin). International Units Another older unit of vitamin A activity is the international unit (IU). Like retinol equivalent, the international unit does not take into account carotenoid variable absorption and conversion to vitamin A by humans, as well as the more modern retinol activity equivalent. Food and 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 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. Beta-carotene is found in many foods and is sold as a dietary supplement. The U.S. Department of Agriculture lists foods high in β-carotene content: No dietary requirement Government and non-government organizations have not set a dietary requirement for β-carotene. == Side effects ==

Excess β-carotene is predominantly stored in the fat tissues of the body. Carotenosis Carotenoderma, also referred to as carotenemia, is a benign and reversible medical condition where an excess of dietary carotenoids results in orange discoloration of the outermost skin layer. Consumption of greater than 30 mg/day for a prolonged period has been confirmed as leading to carotenemia. No risk for hypervitaminosis A At the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class B, type 1 (SCARB1). Absorbed β-carotene is then either incorporated as such into chylomicrons or first converted to retinal and then retinol, bound to retinol binding protein 2, before being incorporated into chylomicrons. The conversion process consists of one molecule of β-carotene cleaved by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene, into two molecules of retinal. When plasma retinol is in the normal range the gene expression for SCARB1 and BCO1 are suppressed, creating a feedback loop that suppresses absorption and conversion. Because of these two mechanisms, high intake will not lead to hypervitaminosis A. Consuming alcohol with β-carotene can decrease its ability to convert to retinol and could possibly result in hepatotoxicity. Research on animal feeds, suggests that β-Carotene might act as an "antivitamin D" that counteracts the availability in forages of vitamin D. β-Carotene and lung cancer in smokers Chronic high doses of β-carotene supplementation increases the probability of lung cancer in smokers while its natural vitamer, retinol, increases lung cancer in smokers and nonsmokers. 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 physiological dose of β-carotene (6 mg), in contrast to high pharmacological dose (30 mg). Increases in lung cancer have been attributed to the tendency of β-carotene to oxidize, yet based on the pharmacokinetics of β-carotene absorption and transport through the intestine and the lack of specific β-carotene transporters, it is unlikely that β-carotene reaches the lung of smokers in sufficient quantities. Additional research is required to understand the link between the increased risk of cancer and all-cause mortality following β-carotene supplementation. Additionally, supplemental, high-dose β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality irrespective of smoking status. == Industrial sources ==

β-carotene is industrially made either by total synthesis (see ) or by extraction from biological sources such as vegetables, microalgae (especially Dunaliella salina), and genetically-engineered microbes. The synthetic path is low-cost and high-yield. ==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. However, a 2018 meta-analysis mostly of prospective cohort studies found that both dietary and circulating beta-carotene are associated with a lower risk of all-cause mortality. The highest circulating beta-carotene category, compared to the lowest, correlated with a 37% reduction in the risk of all-cause mortality, while the highest dietary beta-carotene intake category, compared to the lowest, was linked to an 18% decrease in the risk of all-cause mortality. Macular degeneration Age-related macular degeneration (AMD) represents the leading cause of irreversible blindness in elderly people. AMD is an oxidative stress, retinal disease that affects the macula, causing progressive loss of central vision. β-carotene content is confirmed in human retinal pigment epithelium. Reviews reported that for intervention trials using only β-carotene, there was no change to risk of developing AMD. Cancer A meta-analysis concluded that supplementation with β-carotene does not appear to decrease the risk of cancer overall, nor specific cancers including: pancreatic, colorectal, prostate, breast, melanoma, or skin cancer generally. High levels of β-carotene may increase the risk of lung cancer in current and former smokers. Results are not clear for thyroid cancer. Cataract A Cochrane review looked at supplementation of β-carotene, vitamin C, and vitamin E, independently and combined, on people to examine differences in risk of cataract, cataract extraction, progression of cataract, and slowing the loss of visual acuity. These studies found no evidence of any protective effects afforded by β-carotene supplementation on preventing and slowing age-related cataract. A second meta-analysis compiled data from studies that measured diet-derived serum beta-carotene and reported a not statistically significant 10% decrease in cataract risk. Erythropoietic protoporphyria High doses of β-carotene (up to 180 mg per day) may be used as a treatment for erythropoietic protoporphyria, a rare inherited disorder of sunlight sensitivity, without toxic effects. == See also ==