Carotene

Carotene (Beta-Carotene and Related Carotenoids): A Comprehensive Reference

1. Identity: Chemical Names, Structure, and Classification

Carotenes are yellow-orange pigments classified as hydrocarbons — more specifically as terpenoids — with the molecular formula C₄₀H₅₆. The term "carotene" refers collectively to several related isomers, of which beta-carotene (β-carotene) is by far the most biologically and commercially significant. Alpha-carotene (α-carotene) and beta-carotene (β-carotene) are the two primary isomers found in plants; other carotene isomers found in plants are gamma-, delta-, epsilon-, and zeta-carotene (γ, δ, ε, and ζ-carotene).

Beta-carotene consists of 8 isoprene units forming an extended conjugated polyene chain with 11 conjugated double bonds. Two identical β-ionone rings flank each end of the molecule — this symmetry distinguishes it from alpha-carotene, which has one β-ionone and one ε-ionone ring.

Beta-carotene belongs to a family of natural chemicals known as carotenoids. Widely found in plants, carotenoids (along with another group of chemicals, bioflavonoids) give color to fruits, vegetables, and other plants.

The molecular formula C₄₀H₅₆ was first established analytically in the early twentieth century. Not until 1907 was the empirical formula of beta-carotene, C₄₀H₅₆, established by Willstätter and Mieg. The structure was elucidated by Karrer in 1930–31. This was the first time that the structure of any vitamin or provitamin had been established, and Karrer received a Nobel prize for his work.

Due to its unique structure and cleavage efficacy, β-carotene is the most efficient provitamin A carotenoid. Beta-carotene is a naturally occurring compound classified as a carotenoid, known for its ability to convert into vitamin A in the body, making it an essential dietary supplement for nutritional support.

2. Natural Sources

β-Carotene is the most common form of carotene in plants and can be found in yellow, orange, and green leafy vegetables and fruits. It is an important dietary resource and a precursor of vitamin A in humans.

Plants, fungi, and photosynthetic bacteria synthesize carotenes, while animals must obtain them as a dietary nutrient. Among plant food sources, beta-carotene is primarily sourced from brightly colored fruits and vegetables, such as carrots, sweet potatoes, and spinach.

The most important commercial biological source is the microalga Dunaliella salina. According to Browitzka's report (1998), the halophilic green flagellate, Dunaliella salina, is the best natural source of the carotenoid β-carotene. D. salina can accumulate a very high concentration — up to 14% of cell dry weight — of beta-carotene under stress conditions of high light, high salinity, high temperature and nutrient deprivation like nitrogen starvation.

Red palm oil, a traditional African food, contains high provitamin A, but its substitution by imported cooking oils has reduced this dietary source in many homes.

3. Common Forms and Preparations

Beta-carotene is commercially available in several forms:

• Synthetic (all-trans) form: The usual chemical form of β-carotene in applications is the all-trans configuration, which is the isomer found in root vegetables and in the synthetic material produced by the principal manufacturing technologies. The first total syntheses of beta-carotene were achieved in 1950, and Roche started producing it commercially in 1954.
• Natural algae-derived form: Beta-carotene is a fat-soluble pigment found in plants (notably carrots and many colorful vegetables and fruits) and in the sea alga Dunaliella salina and D. bardawil. Natural beta-carotene supplements are derived primarily from D. salina.
• Mixed carotenoid complexes: Some products contain a natural carotene complex extracted from concentrates including Dunaliella salina algae, palm fruit oil, and marigold flowers. These carotenoid complexes provide a broad carotenoid spectrum with alpha-, beta-, gamma-, zeta-, and cis-beta-carotene, and also contain lycopene and cis-lycopene, lutein, alpha-cryptoxanthin, beta-cryptoxanthin, and zeaxanthin.
• Microbiological fermentation: On the industrial scale, β-carotene is also obtained through cultivation of the mold Blakeslea trispora and the alga Dunaliella salina.
• Crystalline form: The final industrial product is either crystalline β-carotene (purity >96%) or it is formulated as a 30% micronized preparation.

Supplement dosage forms include softgel capsules (dissolved in oil), tablets, and powder-based formulations. For β-carotene to be bioavailable, various steps are required, including release from the food matrix, incorporation into physiological mixed micelles, uptake by intestinal epithelial cells, and packaging into chylomicrons that are secreted into the lymphatic system. The crystalline form of carotene has been found to decrease both the bioaccessibility and bioavailability of carotenes.

4. Historical and Traditional Use

Carotene's history as a defined chemical entity spans less than two centuries, but carotene-rich foods have been embedded in human food traditions across the world for millennia.

4.1 Pre-scientific Dietary Traditions

Beta-carotene-rich foods have been consumed throughout human history for nutritional purposes. Ancient civilizations including Egyptians, Greeks, and Romans cultivated carrots and other orange vegetables. Traditional Chinese medicine used carrot-family plants for eye health. Ayurvedic medicine utilized beta-carotene-rich foods like papaya and mango for skin health and immunity. Indigenous peoples worldwide recognized that orange and yellow vegetables supported vision and overall vitality, though they did not understand the underlying biochemistry.

In sub-Saharan Africa, red palm oil — a traditional African food — contains high provitamin A and was historically a primary dietary source of beta-carotene in regions where it was consumed as a staple cooking oil.

4.2 Scientific History

The earliest studies on carotenoids date back to the beginning of the 19th century. Beta-carotene was first isolated by Wackenroder in 1831. Steenbock suggested in 1919 that there could be a relationship between beta-carotene and vitamin A. The concept of provitamins — molecules which are converted into vitamins by the body — was entirely new, and proved to have great significance scientifically and commercially.

Various studies were carried out throughout the 1970s–80s to determine beta-carotene's suitability for use in food and its activity in the body. In the early 1980s it was suggested that beta-carotene might be useful in preventing cancer, and it was found to be an antioxidant. This suggestion triggered a wave of major clinical trials in the 1990s.

5. Key Constituents and Mechanisms of Action

5.1 Provitamin A Activity

Once taken up by peripheral tissue cells, the major usage of absorbed β-carotene is as a precursor to retinal via symmetric cleavage by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene. A lesser amount is metabolized by the mitochondrial enzyme beta-carotene 9',10'-dioxygenase, which is encoded by the BCO2 gene. The products of this asymmetric cleavage are two beta-ionone molecules and rosafluene.

Recommended Dietary Allowances (RDAs) for vitamin A are given as retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids, all of which are converted by the body into retinol. One mcg RAE is equivalent to 1 mcg retinol, 2 mcg supplemental beta-carotene, 12 mcg dietary beta-carotene, or 24 mcg dietary alpha-carotene or beta-cryptoxanthin.

Beta-carotene is a particularly important carotenoid from a nutritional standpoint because the body easily transforms it into vitamin A. While vitamin A supplements themselves can be toxic when taken to excess, it is believed (although not proven) that the body will make only as much vitamin A out of beta-carotene as it needs.

The human body primarily uses β-carotene in the production of vitamin A, an essential vitamin crucial for the eye to respond to light and proper functioning of the conjunctival membranes and cornea. Vitamin A is also essential for immune function, cellular differentiation, proper functioning of epithelial barriers in eyes and periocular glands as well as the gastrointestinal, genitourinary, and respiratory tracts.

5.2 Antioxidant Mechanisms

As an antioxidant, β-carotene quenches singlet molecular oxygen and scavenges reactive oxygen species, especially peroxyl radicals. Singlet oxygen quenching is likely to be restricted to the skin as the only light-exposed tissue that contains higher levels of β-carotene; other carotenoids demonstrate similar activity. Upon radical scavenging, β-carotene decomposes and cannot be regenerated. Thus, it is suggested that the major function of β-carotene in human nutrition is that of a provitamin A.

5.3 Additional Proposed Mechanisms

The pigment offers several therapeutic effects in the context of acting as a provitamin A: antioxidants neutralizing ROS, regulating connexin expression, thus improving communications through gap junctions, activating macrophages, and triggering an immune response.

Carotenoid-derived metabolites have been found to participate in gene alterations by interactions with nuclear and retinoic acid receptors.

5.4 Pro-oxidant Behavior Under Certain Conditions

The antioxidant/pro-oxidant balance of beta-carotene is context-dependent. Under hypoxic conditions and in the presence of a tumor initiator such as cigarette smoke, high doses of β-carotene have a prooxidant effect via lung cancer promotion; however, available data do not suggest β-carotene alone is a tumor initiator in lung tissue. Hypotheses concerning the beta-carotene/tobacco smoke interaction include alterations of retinoid metabolism and signaling pathways and interaction with CYP enzymes and pro-oxidation/DNA oxidation.

6. Bioavailability

The body may absorb up to 75% to 100% of retinol but, in most cases, only 10% to 30% of beta-carotene from foods.

Absorption of beta-carotene is affected by dietary fat concentration. Individuals placed on a high-fat diet showed significant increases in plasma beta-carotene compared with those placed on a low-fat diet. There is no difference between naturally occurring or chemically synthesized β-carotene with regard to basic metabolic function, though formulation significantly influences bioavailability. The crystalline form of carotene has been found to decrease both the bioaccessibility and bioavailability of carotenes.

Significant differences in β-carotene absorption between raw carrot and carrot juice have been observed, with plasma β-carotene levels peaking at 1.5 hours in both groups; the raw carrot group reached 3.74 μg/mL while the carrot juice group peaked at 8.72 μg/mL.

7. Scientific Evidence by Area of Use

7.1 Vitamin A Deficiency

The most clearly established role of beta-carotene is as a precursor for vitamin A, particularly in populations where deficiency is prevalent. According to the World Health Organization (WHO), millions of children in developing countries suffer from vitamin A deficiency. The degree of conversion of consumed provitamin A carotenes to retinol is the essential measure to which vitamin A deficiency, common in children, can be treated. This is because vitamin A deficiency is indirectly related to poor immunity of the body, night blindness, and other conditions.

The evidence supporting beta-carotene as a vitamin A source for deficient populations is strong and considered scientifically settled. The NIH ODS formally recognizes that one mcg RAE is equivalent to 2 mcg supplemental beta-carotene or 12 mcg dietary beta-carotene.

7.2 Cancer Prevention — Lung Cancer (High-Risk Populations)

This is the area where clinical evidence is strongest and most consequential. Early observational data appeared to suggest a protective association. A number of epidemiological studies reported associations of beta-carotene plasma levels or intake with decreased lung cancer risk. However, intervention studies in smokers have unexpectedly reported increased lung tumor rates after high, long-term beta-carotene supplementation.

Three large-scale clinical trials tested the effects of supplemental beta-carotene on the risk for chronic diseases such as cancer. The populations involved were Finnish male heavy smokers (the Alpha Tocopherol Beta Carotene [ATBC] trial), male asbestos workers and male and female heavy smokers (Beta-Carotene and Retinol Efficacy Trial [CARET]), and U.S. male physicians, 11% of whom were current smokers (Physicians' Health Study).

All three trials concluded that beta-carotene provided no protection against lung cancer; however, quite unexpectedly, two of the trials found a higher risk for lung cancer for those subjects given beta-carotene compared with those that were not.

Specifically, the ATBC Study reported an 18% excess in cumulative lung cancer incidence and an 8% excess in overall mortality in the β-carotene arm of the trial, whereas the CARET study showed 28% more lung cancer cases and a 17% increase in overall mortality in the active intervention group.

In the CARET trial specifically, the intervention of β-carotene (30 mg) and retinyl palmitate was halted after an observed increase in lung cancer incidence and total mortality in the supplemented group. This trial studied more than 18,000 men and women, of whom 388 developed lung cancer, with a 28% increase in lung cancer incidence in participants who received the β-carotene–retinyl palmitate combination daily for an average of 4 years compared with those who received placebo. Increased total mortality (17%) was also observed in the supplemented group.

Data from the Physicians' Health Study, in which only a small percentage of subjects were at high risk of lung cancer, showed that, after 12 years of supplementation with β-carotene, there was no significant effect on CVD or lung cancer.

The ATBC study, the CARET study, the Antioxidant Polyp Prevention trial, and the E3N study provide evidence that the adverse effects of beta-carotene supplementation are correlated with the smoking status of the study participants. In contrast, the Physicians' Health Study, the Linxian trial, and a pooled analysis of 7 epidemiological cohort studies have not supported this evidence.

Evidence strength: Very strong RCT evidence shows harm (increased lung cancer risk) from high-dose supplement use in smokers and asbestos-exposed individuals. Evidence for cancer prevention in the general, non-smoking population is neutral (no significant benefit or harm demonstrated in major trials).

7.3 Cardiovascular Disease

In those individuals in the ATBC trial who received β-carotene supplements, there was an increased incidence of lung cancer, and more deaths from hemorrhagic stroke and ischemic heart disease. The 6-year posttrial assessment data suggested that those individuals who had received β-carotene were at increased risk of first-time nonfatal myocardial infarction.

After a mean follow-up period of 4 years in the CARET trial, those individuals receiving β-carotene and vitamin A were at increased risk for cardiovascular mortality. Around that same time, data from the Physicians' Health Study showed that, after 12 years of supplementation with β-carotene, there was no significant effect on CVD or lung cancer.

Beta-carotene supplementation may increase cardiovascular risk (e.g., coronary artery disease, cardiovascular mortality), especially in current smokers.

Evidence strength: RCT data from ATBC and CARET show increased cardiovascular events and mortality in high-risk (smoking/asbestos-exposed) populations at high doses. In the general population (Physicians' Health Study), no significant cardiovascular effect — positive or negative — was demonstrated.

7.4 Age-Related Macular Degeneration (AMD)

Beta-carotene has been studied as part of antioxidant combination therapy for AMD. Taking beta-carotene by mouth along with vitamin C, vitamin E, and zinc appears to help prevent vision loss and worsening of AMD in people with more severe AMD. This combination might reduce the progression of AMD to a more advanced state in people at low risk, but results are conflicting. Taking beta-carotene plus antioxidants but without zinc does not appear to improve advanced AMD.

The AREDS (Age-Related Eye Disease Study) evaluated a formula including beta-carotene (15 mg) alongside 500 mg vitamin C, 400 IU vitamin E, and zinc. However, in the subsequent AREDS2 trial, current smokers or those who had quit smoking less than a year before enrollment were excluded from receiving beta-carotene. Despite this precaution, lung cancers were observed in 2% of participants who took an AREDS formulation with beta-carotene, compared with 0.9% of participants who took AREDS without beta-carotene. Across both groups, about 91% of participants who developed lung cancer were former smokers.

There is conflicting evidence about whether taking beta-carotene as part of the diet helps reduce the risk of developing AMD.

Evidence strength: Moderate — beta-carotene as part of the original AREDS formula showed benefit for slowing AMD progression in those with moderate-to-severe AMD, but AREDS2 findings led to reformulation replacing beta-carotene with lutein and zeaxanthin due to lung cancer risk in former smokers.

7.5 Erythropoietic Protoporphyria (EPP)

This is among the best-supported specific clinical indications for beta-carotene supplementation. A key question addresses whether antioxidant processes attributable to β-carotene are important in humans. Studies with patients suffering from erythropoietic protoporphyria support the idea that β-carotene quenches excited molecules. Due to a genetic defect, nonphysiologically high levels of the photosensitizer protoporphyrin IX circulate in the organism, generating singlet oxygen in light-exposed tissues. Symptoms are ameliorated by supplementation with high doses of β-carotene.

An early controlled clinical report from the New England Journal of Medicine (1970) demonstrated this effect: three patients with erythropoietic protoporphyria were treated with an oral preparation of beta-carotene. After treatment, two of the patients were able to tolerate five to eight hours of sun without experiencing photosensitivity, and the third could tolerate sun exposure with less discomfort. When the patients were carotenemic, they also reacted less intensely to artificial light.

Individuals with erythropoietic protoporphyria have been shown to have lower serum β-carotene levels and may require dietary supplementation to mitigate symptoms.

Evidence strength: Moderate-to-good for EPP specifically. Beta-carotene supplementation is a recognized intervention for this rare disorder, though the mechanism — whether purely singlet oxygen quenching or also via immune or other pathways — is not fully resolved.

7.6 Skin Photoprotection (General Population)

β-Carotene has been prescribed and used against photosensitivity in erythropoietic protoporphyria, but its beneficial potential in normal skin remains uncertain. After approximately 10–12 weeks of dietary intervention, a decrease in the sensitivity toward UV-induced erythema was observed in volunteers and a number of experimental studies have indicated protective effects of β-carotene against acute and chronic manifestations of skin photodamage, but there is a lack of controlled clinical studies demonstrating its beneficial effects.

β-Carotene and lycopene (0–8 µM) have been observed to decrease skin redness and damage following UV exposure, acting as soothing agents under intense sunlight.

Evidence strength: Preliminary and mixed. Benefits are established for EPP patients; evidence in the general population remains insufficient for confident clinical recommendations.

7.7 Cognitive Function

The Women's Antioxidant Cardiovascular Study was a trial of vitamin E (402 mg every other day), β-carotene (50 mg every other day), and vitamin C (500 mg daily) for the secondary prevention of cardiovascular disease. From 1995–1996, women aged 40 and older with CVD or three or more coronary risk factors were randomized. From 1998–1999, a cognitive function substudy was initiated among 2,824 participants aged 65 and older, with cognition assessed by telephone four times over 5.4 years.

Antioxidant supplementation did not slow cognitive change among women with preexisting cardiovascular disease or cardiovascular disease risk factors.

However, data from the Physicians' Health Study II produced a different picture in a different population: among 4,052 continuing participants from the Physicians' Health Study (mean treatment duration, 18 years), the mean global cognitive score was significantly higher in the beta-carotene group than in the placebo group (mean difference in z scores, 0.047 standard units; P = .03). On verbal memory, men receiving long-term beta-carotene supplementation also performed significantly better than the placebo group. The conclusion was that short-term beta-carotene supplementation had no impact on cognitive performance, but long-term supplementation may provide cognitive benefits.

Among 4,052 healthy male physicians treated with 50 mg on alternate days for 18 years, men had significantly better performance compared with men on placebo, while no association was observed among 1,904 men treated for a short duration (1 year). This raises the possibility that either duration matters or the effect of β-carotene on cognition differs among those with cardiovascular disease.

Evidence strength: Preliminary and inconsistent across populations. Long-term supplementation in healthy men showed a modest signal; no benefit in women with cardiovascular disease. Overall, evidence is insufficient to draw firm conclusions.

7.8 Immune Function

Vitamin A status modulates both innate and adaptive immunity by regulating cytokine and antibody production, migration and maturation of B and T cells, T cell gut homing, and intestinal dendritic cell function, which prompted recommendations to use vitamin A supplements as vaccine adjuvants in developing countries in order to boost immune system responses in vitamin A-deficient children, although their efficacy is controversial.

In older men, β-carotene supplementation increased the activity of natural killer cells (Hughes et al., 1997). There is also a contrary association between pigment levels in serum and systemic inflammation markers like neutrophil-to-lymphocyte ratio and resistance to insulin as well as dysfunctional beta cells.

Evidence strength: Primarily derived from vitamin A mechanistic research; direct causal evidence for beta-carotene supplementation improving immune outcomes in healthy, replete individuals is limited and mainly observational or from small studies.

8. Body Systems Associated with Carotene

• Visual system: Via conversion to vitamin A (retinal), beta-carotene is essential for phototransduction and the maintenance of corneal and conjunctival integrity.
• Integumentary (skin): Antioxidant quenching of singlet oxygen in skin; clinical use in EPP; investigated for UV photoprotection.
• Immune system: Vitamin A produced from beta-carotene is critical for differentiation and function of lymphocytes, natural killer cells, and epithelial barrier defense.
• Cardiovascular system: Subject of major RCTs; supplementation associated with harm in high-risk (smoking) populations; no benefit demonstrated in general population.
• Pulmonary system: Directly implicated in the ATBC and CARET trials; high-dose supplementation in smokers associated with increased lung cancer incidence.
• Central nervous system: Investigated in cognitive studies with long-term supplementation; evidence preliminary.
• Reproductive system: Vitamin A derived from beta-carotene is required for normal reproduction; deficiency is associated with adverse pregnancy outcomes.

9. Dosage Forms and Reported Dosages

No Recommended Dietary Allowance (RDA) has been established specifically for beta-carotene as a supplement. The Food and Nutrition Board of the National Academy of Medicine has not set safe upper limits for beta-carotene intake. Beta-carotene has no Tolerable Upper Intake Level (UL) because high amounts do not cause the same problems as excessive preformed vitamin A.

Dosages used in major clinical studies include:

• The ATBC Study was a randomized, double-blind intervention trial; 29,133 male smokers aged 50–69 years were enrolled and used a dosage of 20 mg/day of beta-carotene.
• CARET used an intervention dose of β-carotene 30 mg combined with retinyl palmitate 25,000 IU.
• The Women's Antioxidant Cardiovascular Study used β-carotene 50 mg every other day.
• In the Physicians' Health Study II cognitive sub-study, the dose was 50 mg on alternate days; among long-term participants (mean 18 years), this was associated with modestly better global cognitive scores.
• For erythropoietic protoporphyria, 180 mg per day has been used. If this dose is not effective, it can be increased to 300 mg per day.
• For AMD, 15 mg of beta-carotene plus 500 mg vitamin C and 400 IU vitamin E, with or without 80 mg zinc oxide, has been used daily (the original AREDS formula).
• Pharmacological doses of beta-carotene (45 and 90 mg) have been used in intermittent schedules (5–6 day intervals) in bioavailability studies without altering steady-state plasma levels.
• Daily administration of 15 or 45 mg beta-carotene resulted in significant increases in plasma beta-carotene levels.
• Intake of more than 30 mg of beta-carotene daily may lead to hypercarotenemia, characterized by yellowish coloration of the skin (including soles of feet and palms of hands). This is harmless and reversible.

10. Safety Considerations and Drug Interactions

10.1 Smoking and High-Risk Populations

This represents the most clinically significant safety issue with beta-carotene supplementation. Beta-carotene supplements do not reduce cancer risk. They may increase the risk of cancer in current and former smokers and in workers exposed to asbestos.

The latest EFSA (2024) review advises smokers not to consume β-carotene supplements but states the general population can use these supplements specifically to meet daily vitamin A requirements.

Even in AREDS2, where current smokers were excluded: lung cancers were observed in 2% of participants who took an AREDS formulation with beta-carotene, compared with 0.9% of participants who took AREDS without beta-carotene. Across both groups, about 91% of participants who developed lung cancer were former smokers. This indicates that former smokers also face elevated risk.

10.2 Carotenodermia

Excessive beta-carotene ingestion may cause reversible carotenodermia (yellowish skin discoloration); carotenodermia usually disappears when beta-carotene is reduced or discontinued. Yellowing of the skin occasionally occurred during daily dosing with 45 mg beta-carotene without evidence of toxicity.

10.3 Cardiovascular Risk in Smokers

In the ATBC trial, those who received β-carotene supplements had an increased incidence of lung cancer and more deaths from hemorrhagic stroke and ischemic heart disease. Post-trial assessment data suggested those who had received β-carotene were at increased risk of first-time nonfatal myocardial infarction.

10.4 Drug Interactions

Cholestyramine (Questran), colestipol (Colestid), and orlistat (Xenical, Alli) are medications associated with reduced beta-carotene absorption. Some preparations of beta-carotene contain peanut oil, which is relevant for individuals with peanut allergies. The NIH ODS notes that numerous organizations have long maintained consensus that dietary β-carotene is a safe source of vitamin A as a nutrient and as a food additive for coloring purposes.

10.5 No Tolerable Upper Intake Level for the General Population

Although there is an established tolerable upper intake level (UL) set for preformed vitamin A, there is no UL set for provitamin A carotenoids like beta-carotene. This is because beta-carotene and other carotenoids are unlikely to cause health issues even when consumed at high doses. However, beta-carotene supplements — unlike beta-carotene-rich foods — have different effects on health and may lead to negative effects.

10.6 Comparison of Dietary vs. Supplement Forms

Observational studies have linked high dietary intake of carotenoids to lower disease incidence, but intervention trials with beta-carotene supplements have generally failed to demonstrate similar protective effects. Moreover, long-term high-dose supplementation of beta-carotene may even pose risks, including a potential increase in heart disease and certain cancers, particularly among smokers. All the significant positive evidence for beta-carotene applies to beta-carotene from food sources, not supplements.

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