Everyone knows that carrots and sweet potatoes are great sources of vitamin A, right?
Well… it turns out it isn’t that straightforward for everyone. The conversion of beta-carotene, found in orange fruits and vegetables, results in a 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 to 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. [source]
Interestingly, most carnivores (entirely meat-eating animals) are poor converters of beta-carotene, and 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]
Your body then used the 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
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]
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]
Absorption of beta-carotene:
Carotenoids, including beta-carotene, are produced by a number of different plants and some microorganisms (bacteria and algae). While there are over 600 different carotenoids produced in nature, only about 20 have been identified in humans via their dietary intake. The main 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. This forms a micelle (a droplet of fats surrounding a molecule), which can easily be absorbed in the intestines.
Processing – or how the food is prepared – also impacts the absorption of beta-carotene. A study that investigated 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]
Conversion of beta-carotene 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 is then converted 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 some interesting 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) actually tests the changes in skin hue from carotenoid consumption.[ref]
The second example of carotenoids not being metabolized is in the red coloration in some salmon species. They do not convert the carotenoids into vitamin A very well and thus accumulate these colorful nutrients in their flesh, depending on the carotenoid content of their diet. [ref]
Can you get too much of a good thing? Turning a bit orange from too much beta-carotene might generally be considered a benign condition, but studies indicate excess beta-carotene may not be a good thing.
First, the research is not definitive on this topic, therefore I will present it and let you draw your own conclusions.
On the one hand – overall mortality rates are lower in people with higher levels of beta-carotene. This is indicative of links between higher fruit and vegetable consumption and lower all-cause mortality. [ref]
On the other hand, there have been some large studies that used beta-carotene supplementation for the prevention of cancer that showed a different result – increased deaths due to beta carotene supplementation for a couple of types of cancer.
One large study (18,000+ people) included smokers, former smokers, and people exposed to asbestos. Half of the study participants were given a combo of beta-carotene and vitamin A, and the other half received a placebo. It showed a 46% increase in the relative risk of lung cancer in the group receiving the beta-carotene and vitamin A supplement, and the trial ended early. [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]
Other studies also showed the same results, with beta-carotene (20 – 30 mg/day) increasing the risk of lung cancer in smokers.[ref] Beta-carotene supplementation also increases the risk of bladder cancer a little bit.[ref]
What is going on here? 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 dose 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] So paradoxically, high levels of beta-carotene act as a pro-oxidant and also decrease retinoic acid.
Genetic variants that impact the conversion of beta-carotene:
BCO1 (or 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 v.4 and v.5, AncestryDNA):
- C/C: typical
- C/T: decreased beta-carotene conversion
- T/T: decreased beta-carotene conversion[ref]; lower leutin (another carotenoid) levels [ref]
Members: Your genotype for rs7501331 is —.
Check your genetic data for rs12934922 (23andMe v.4 and v.5):
- A/A: typical
- A/T: decreased beta-carotene conversion
- T/T: decreased beta-carotene conversion [ref] [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]
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.
Testing vitamin A levels:
If you have symptoms of vitamin A deficiency (poor night vision, lowered immunity, skin problems, keratosis pillaris, etc), possibly 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)
Vegan and Vegetarian Diets:
If you eat a vegan or vegetarian diet, your body’s source of vitamin A is through converting beta-carotene. If you don’t process beta-carotene into retinol very well and you don’t eat large amounts of beta-carotene, 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. There is research showing 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. They can derive some forms of supplemental retinyl palmitate from soy oil and 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. First, keep in mind the studies were multi-year studies in smokers, so the results may not apply to non-smokers. Also, those studies were using 20 – 30 mg/day of supplemental beta-carotene. The absorption of beta-carotene from foods is going to be lower than from a supplement due to the need for digestion, mixing with fat, and then transport into the intestinal cells. So 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.
So 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 would get less than 1.8 mg/ cup of pure carrot juice.
Fat + beta carotene for absorption:
Beta-carotene is hydrophobic and needs fat to be absorbed in the intestines. Adding a little fat to your beta-carotene rich food should help a little with absorption.[ref]
Retinol forms of vitamin A:
Beef liver is an excellent source of vitamin A. A three-ounce serving of liver packs a big punch with about 15,000 IU of vitamin A.
Caution for high-dose vitamin A:
A word of caution on high-dose vitamin A (retinyl palmitate) supplements:
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 of 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) for vitamin A. So 8000 IU would equal 2.4 mg retinol.
High doses of vitamin A can cause birth defects, so if you are planning to get pregnant, talk with your doctor before taking vitamin A supplements.
Include Vitamin D!:
There are a few studies that show that too much vitamin A without enough vitamin D can be a risk factor for osteoporosis.[ref] Make sunshine exposure for your vitamin D levels a priority. Also, if you check your vitamin A levels, make sure you test your vitamin D at the same time.
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