BCO1 Gene: Converting Beta-Carotene to Vitamin A

Everyone knows that carrots and sweet potatoes are great sources of vitamin A, right? Eat your carrots so that you’ll have great eyesight, like Bugs Bunny.

Well… It turns out it isn’t that straightforward for everyone. The conversion of beta-carotene, found in orange fruits and vegetables, results in the form of vitamin A (retinol) that our bodies can use.

Genetics plays a huge role in how well you convert beta-carotene into vitamin A. This article covers the research on the conversion of beta-carotene to vitamin A and how genetic variants decrease the conversion process for some people.

Vitamin A and Beta Carotene:

Vitamin A is a general term that covers several different forms of the vitamin.

  • Animal food sources mainly provide retinyl palmitate, which breaks down in the intestines into retinol. It is stored, in this form, by the body and then converted to an active form for use.
  • Carotenes are the plant forms of a precursor to vitamin A. The most common form, beta-carotene, shows up in abundance in carrots and other orange-colored foods. An enzyme in the intestine breaks down beta-carotene also forming retinol.[ref]

Interestingly, most carnivores (entirely meat-eating animals) are poor converters of beta-carotene. For example, cats cannot create any vitamin A from beta-carotene.

How does the body use vitamin A?

About 80-90% of the retinoids in the body are stored in the liver and used to maintain a steady level in the blood.[ref]

The cells in your body use retinoids in a variety of ways.

Retinol is important for:

  • stem cells
  • photoreceptors in the eye
  • epithelial cells
  • embryonic cells
  • various immune cells
  • red blood cells
  • circadian rhythm

Deficiency: A deficiency in vitamin A can cause poor night vision, worsen infectious diseases, and, when severe, cause blindness. In the immune system, retinol is involved with both the innate and adaptive immune responses.[ref]

Skin problems: Low levels of vitamin A may cause skin problems such as some types of acne and keratosis pilaris (bumps on the back of the arms).[ref]

How is beta-carotene absorbed?

Carotenoids, including beta-carotene, are produced by a number of different plants and some microorganisms (bacteria and algae). While over 600 different carotenoids are produced in nature, only about 20 have been identified in humans via their dietary intake. The primary sources of carotenoids in the diet are colorful fruits and vegetables, such as carrots and spinach.[ref]

Beta-carotene (and all the carotenoids) are fat-soluble micronutrients. They are digested in the upper part of the digestive tract and dissolve in any available fat from the meal. It forms a micelle (droplets of fat surrounding a molecule), easily absorbed in the intestines.

Processing – or how the food is prepared – also impacts the absorption of beta-carotene. A study investigating the bioavailability of beta-carotene and other carotenoids found that raw carrots had about a 2% bioavailability for beta-carotene, while carrot juice was much higher at 14%.[ref]

Within the intestines, the beta-carotene has to be taken into the intestinal cells, called enterocytes. Recent research shows that there are a couple of transporters that facilitate this process.[ref]

How does beta-carotene convert to vitamin A?

Once the beta-carotene has been digested, mixed with fats, and absorbed, it has to be converted into retinol. This conversion uses the enzyme β-carotene 15,15′-monooxygenase (BCMO1 or BCO1 gene), which converts beta-carotene into retinal. The retinal converts into retinol.[ref]

Genetic variants in the BCO1 gene cause varying amounts of the enzyme to be produced and cause a large difference in the amount of vitamin A produced from dietary beta-carotene.

There is also a feedback loop in the body where higher levels of retinoic acid will decrease the production of the BCO1 enzyme, thus decreasing the amount of beta-carotene converted to retinal.[ref]

The BCO1 enzyme is active in the intestines, liver, and mucosal epithelium (e.g. lining of the lungs). As a result, the conversion of beta-carotene to vitamin A occurs in all of those locations.[ref]

Turning orange from carrot juice?

Interestingly, up to 40% of carotenoids are not metabolized and used by the body.[ref] This can have consequences when you consume a lot of foods high in beta-carotene.

For example, excessive consumption of carrot juice causes your skin to take on an orange-ish hue called carotenemia.[ref]

A carotenoid reflection spectroscopy device (called the Veggie Meter :-) tests the changes in skin hue from carotenoid consumption.[ref]

The orangish coloration in some salmon species is another example of beta-carotene not being converted. Salmon do not convert carotenoids into vitamin A very well and thus accumulate these colorful nutrients in their flesh, depending on the carotenoid content of their diet.[ref]

Is consuming large amounts of beta-carotene healthy?

Some people consider turning orange from too much beta-carotene benign, but studies indicate excess beta-carotene may not be a good thing.

First, let me caution that the research is not definitive on this topic. Therefore, I will simply present the research and let you draw your conclusions.

Overall mortality rates are lower in people with higher levels of beta-carotene. It is indicative of links between higher fruit and vegetable consumption and lower all-cause mortality.[ref]

Increased Cancer Mortality: On the other hand, some large studies using beta-carotene supplementation to prevent cancer showed increased deaths linked to beta carotene supplementation for a couple of specific types of cancer.

One large study (18,000+ people) included smokers, former smokers, and people exposed to asbestos. Half of the study participants received a combo of beta-carotene and vitamin A, and the other half received a placebo. The results showed a 46% increase in the relative risk of lung cancer in the group receiving the beta-carotene and vitamin A supplement. Researchers ended the trial early to prevent more deaths.[ref][ref]

Another large study examined the effects of supplementing with beta-carotene, vitamin E, a combo of both, or placebo. Again, an association exists between beta-carotene supplementation and an increased risk of lung cancer, especially in smokers.[ref]

The large studies are backed up by several smaller studies with the same results: Beta-carotene (20 – 30 mg/day) increases the risk of lung cancer in smokers.[ref] Beta-carotene supplementation also increases the risk of bladder cancer a little bit.[ref]

Why would high doses of beta-carotene cause an increase in cancer risk?

Research shows that carotenoids can act as pro-oxidants at higher levels. The breakdown products from beta-carotene include aldehydes and epoxides, which impair mitochondrial function.[ref]

Similarly, animal studies show that low-dose beta-carotene supplementation is health-promoting after a heart attack, but that higher doses were not beneficial and possibly deleterious.[ref]

Animal studies also show that high doses of beta-carotene also caused lung cancer in animals exposed to cigarette smoke. The high beta-carotene actually caused an increase in enzymes that destroyed retinoic acid (the active form of vitamin A).[ref] Paradoxically, high levels of beta-carotene act as a pro-oxidant and also decrease retinoic acid.

What else does BCMO1 do?

While this article has focused on beta-carotene, the BCMO1 enzyme is involved in conversion of other carotenoids, such as lycopene.

Lycopene is a bright red carotenoid found in tomatoes, watermelon, and papayas.

The variants below that affect beta-carotene conversion may also affect the way that the body reacts to lycopene. One study on men with prostate cancer showed a trend towards BCMO1 variants impacting the benefits of dietary lycopene consumption.[ref]

Genetic variants that impact the conversion of beta-carotene:

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BCO1 (aka BCMO1) gene:

There are two well-studied genetic variants in the BCMO1 gene that help determine a person’s ability to convert beta-carotene into useful retinol for the body.

Check your genetic data for rs7501331 (23andMe v4 and v5, AncestryDNA):

  • C/C: typical
  • C/T: decreased beta-carotene conversion
  • T/T: decreased beta-carotene conversion[ref]; lower lutein (another carotenoid) levels[ref] may affect lycopene also[ref]

Members: Your genotype for rs7501331 is .

Check your genetic data for rs12934922 (23andMe v4 and v5):

  • A/A: typical
  • A/T: decreased beta-carotene conversion; increased risk of atherosclerosis with poor diet
  • T/T: decreased beta-carotene conversion[ref][ref; increased risk of atherosclerosis with poor diet[ref]  may affect lycopene also[ref]

Members: Your genotype for rs12934922 is .

Combo of the above: People with a T allele on both rs12934922 and rs7501331 have a 69% decreased conversion of beta-carotene to retinol. For people with only a single T in the rs7501331 SNP, the conversion is decreased by 32%.[ref]


Less impactful variants decrease the conversion of beta-carotene to the retinol form of vitamin A a little. These may be more important if you carry two copies of the variant or if you have a combination of all of the variants below.

Check your genetic data for rs11645428 (23andMe v4, v5, AncestryDNA):

  • G/G: lower beta-carotene conversion
  • A/G: slightly higher beta-carotene conversion
  • A/A: higher beta-carotene conversion[ref]

Members: Your genotype for rs11645428 is .

Check your genetic data for rs6420424 (23andMe v4, v5, AncestryDNA):

  • A/A:  lower beta-carotene conversion[ref]
  • A/G: slightly lower beta-carotene conversion
  • G/G: normal beta-carotene conversion

Members: Your genotype for rs6420424 is .

Check your genetic data for rs6564851 (23andMe v4, v5, AncestryDNA):

  • G/G: lower beta-carotene conversion (thus higher circulating beta-carotene)[ref]
  • G/T: somewhat lower beta-carotene conversion
  • T/T: typical conversion

Members: Your genotype for rs6564851 is .


You may be wondering – if you carry the variants that decrease the BCMO1 enzyme production is it worthwhile to eat vegetables?

The answer seems to be yes. A study that looked into the effects of the BCMO1 genetic variants on lung cancer risk and fruit and vegetable consumption found that, regardless of enzyme function, higher fruit and vegetable consumption reduced the risk of lung cancer considerably.[ref]

Even with reduced enzyme function, you still get some vitamin A from fruits and vegetables – along with all of the other nutritional benefits.

Test your vitamin A levels:

If you have symptoms of vitamin A deficiency (poor night vision, lowered immunity, skin problems, keratosis pilaris, etc.), check your vitamin A levels with a blood test. You could get this through your doctor or order it online. (UltaLab Tests – Vitamin A Retinol – $63)

What should you eat?

If you are looking for ideas and recipes for retinol-rich foods, check out this article: Foods rich in retinol and recipe ideas

Vegan and Vegetarian Diets:

If you eat a vegan or vegetarian diet, your body’s sole source of vitamin A is from converting beta-carotene. Suppose you don’t process beta-carotene into retinol very well, and you don’t eat large amounts of beta-carotene. In that case, you may want to increase your vegetables that are high in beta-carotene.

  • Increasing beta-carotene consumption may help your vitamin A status – up to a point. Research shows that “in humans, beta-carotene conversion to vitamin A decreases as the dietary dose increases.”[ref]
  • Another option is to consider supplementing with a retinol form of Vitamin A. Some supplement forms of retinyl palmitate are derived from soy oil and are considered vegan.[source]

You may be wondering if the overconsumption of dietary beta-carotene would have the same detrimental cancer risk-increasing effects as the supplementation studies. It’s a good question:

  • First, consider that the studies were multi-year studies in smokers, so the results may not apply if you are a non-smoker.
  • Additionally, the cancer studies were using 20 – 30 mg/day of supplemental beta-carotene. The absorption of beta-carotene from foods will be lower than from a supplement.
  • Even if you eat 20 mg of beta-carotene per day – every day – you are still probably not absorbing nearly as much from food as you would from a 20mg supplement pill.

How much beta-carotene is in carrot juice?

According to the Nutridesk website, one cup of carrot juice contains 22mg of beta-carotene. The site also claims, for an average person, the conversion of only 1/12th of that beta-carotene into retinol gives 1.8 mg of retinol per cup of carrot juice. If you are a poor converter of beta-carotene, you will get less than 1.8 mg/ cup of pure carrot juice.

Caution for high-dose vitamin A:

Vitamin A is a fat-soluble vitamin that can build up in the body, so you don’t want to go overboard with it. (Get a blood test to determine your level.)

The upper daily recommended limit for vitamin A is 3000 μg/day (3 mg/day). Most supplements list the amount of vitamin A in IU (international units). One IU is equivalent to 0.3 μg (.0003 mg) of vitamin A. Therefore, 8000 IU would equal 2.4 mg of retinol.

Pregnancy warning: High doses of vitamin A can cause birth defects. If you plan to get pregnant, talk with your doctor before taking vitamin A supplements.

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About the Author:
Debbie Moon is the founder of Genetic Lifehacks. Fascinated by the connections between genes, diet, and health, her goal is to help you understand how to apply genetics to your diet and lifestyle decisions. Debbie has a BS in engineering and an MSc in biological sciences from Clemson University. Debbie combines an engineering mindset with a biological systems approach to help you understand how genetic differences impact your optimal health.