Beta-Carotene

Beta-carotene is a strongly colored red-orange pigment belonging to the carotenoids, a family of plant compounds built from isoprene units; with its 40-carbon structure it is classified as a tetraterpenoid ^[3]. It is the most prominent provitamin A carotenoid, so called because the body can cleave it to form vitamin A ^[4]. Beta-carotene is abundant in foods such as carrots, sweet potatoes, pumpkin, and dark leafy greens like kale and spinach, where in green leaves its orange color is hidden by chlorophyll ^[3].

In the intestine, beta-carotene is converted to vitamin A (retinol) by the enzyme beta-carotene 15,15'-dioxygenase, with one molecule capable of yielding two molecules of vitamin A ^[4]. The body regulates this conversion, slowing it when vitamin A levels are already adequate, which is why eating large amounts of beta-carotene does not cause the vitamin A toxicity that pre-formed retinol can ^[4]. As a carotenoid, beta-carotene also has antioxidant properties, and it is widely used as a natural orange food coloring and as an ingredient in nutritional supplements ^[3].

Beta-carotene is a landmark example of a nutrient that behaves differently as a supplement than as part of food. Although diets rich in carotenoid-containing fruits and vegetables are linked to lower cancer risk, two large randomized trials in the 1990s testing high-dose beta-carotene supplements found the opposite in high-risk groups: in a trial of male smokers, those taking beta-carotene had a higher incidence of lung cancer and higher overall mortality than those who did not ^[1]. A later systematic review and meta-analysis confirmed that beta-carotene supplementation at high doses increased the risk of lung and stomach cancer specifically in smokers and asbestos-exposed workers, concluding that beta-carotene should not be recommended for cancer prevention ^[2]. This finding also reshaped eye-disease research: when the widely used AREDS antioxidant formula for age-related macular degeneration was updated, beta-carotene was replaced with lutein and zeaxanthin because of the increased lung cancer risk seen in former smokers ^[3][4].

Beta-carotene is available in multivitamins and standalone supplements and as a food additive, and modest amounts from food or ordinary supplementation are considered safe for the general population ^[3]. The clearest visible effect of high intake is carotenodermia, a harmless, reversible orange-yellow tint of the skin that fades when intake is reduced ^[3]. The important safety caveat is the one drawn from the trials: high-dose beta-carotene supplements are discouraged in smokers and others at high risk of lung cancer, even though beta-carotene obtained from a normal diet carries no such concern ^[1][2].

From food it's very safe; excess intake simply tints the skin orange (carotenodermia), which is harmless and reversible. The serious caveat is that high-dose isolated beta-carotene supplements increased lung cancer risk in smokers and asbestos-exposed people in large trials, so smokers should avoid supplemental beta-carotene. It doesn't cause vitamin A toxicity the way preformed retinol can, because conversion is regulated.