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 beta-carotene in orange fruits and vegetables has to be converted into the form of vitamin A (retinol) that our bodies can use.
Genetics plays a huge role in how well you convert the carotenes into retinol. Some people are great at absorbing and converting the carrots in their diet into the retinol form. Others of us carry genetic variants that significantly impair that conversion.
Background information on Vitamin A:
Vitamin A is a general term that covers several different forms of the vitamin.
Animal food sources mainly provide retinyl palmitate, which is broken down in the intestines to retinol. In this form, it is stored 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 is beta-carotene which is found in abundance in carrots and other orange-colored foods. Beta-carotene is broken down by an enzyme in the intestines to also form retinol.[source]
Interestingly, most carnivores are poor converters of beta-carotene, and cats cannot create any vitamin A from beta-carotene.
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 including in:
- stem cells,
- photoreceptors in the eye,
- epithelial cells,
- embryonic cells,
- various immune cells,
- red blood cells
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.[study] Vitamin A can also help with skin problems such as some types of acne and hyperkeratosis.[study]
I had keratosis pilaris (bumpy skin on the back of my arms) since childhood and never knew what caused it. After learning that I don’t convert beta-carotene very well, I starting taking a retinyl palmitate supplement. A couple of months later, my arms were smooth for the first time in several decades. (To be totally honest here, I didn’t realize the connection until I stopped taking the vitamin A supplement and the bumps came back a month later. I now judge my vitamin A status by the smoothness of my arms. )
Genetics of Beta-carotene Conversion:
Beta-carotene is converted by the enzyme β-carotene 15,15′-monooxygenase (BCMO1 gene) into retinol. It is then used by the body in the same way as preformed vitamin A from animal products is used or stored.
There are two gene variations in the BCMO1 gene that help determine a person’s ability to convert beta-carotene into the retinol a body uses. A study in 2008 shows that the SNP’s rs12934922 and rs7501331 control a person’s conversion rate of beta-carotene into retinol.
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%.
Check your genetic data for rs7501331 (23andMe v.4 and v.5, AncestryDNA):
- C/C: normal
- C/T: decreased beta-carotene conversion
- T/T: decreased beta-carotene conversion
Check your genetic data for rs12934922 (23andMe v.4 and v.5):
- A/A: normal
- A/T: decreased beta-carotene conversion
- T/T: decreased beta-carotene conversion
Three other variants that are found near the BCMO1 gene have also been shown in a small study to affect the rate of conversion by about 50%
Check your genetic data for rs11645428 (23andMe v4, v5, AncestryDNA):
- G/G: lower (normal?) beta-carotene conversion
- A/G: higher beta-carotene conversion
- A/A: higher beta-carotene conversion[ref]
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
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: normal conversion
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 it into retinol very well, you may want to increase your vegetables that are high in beta-carotene or consider supplementing with a retinol form Vitamin A.
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]
A word of caution:
Vitamin A is a fat-soluble vitamin that can build up in the body, so you don’t want to go overboard with it. You can 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.
So how much beta-carotene is in carrot juice?
According to the Nutridesk website, one cup of carrot juice contains 22mg of beta-carotene. That site also claims that for an average person, 1/12th of that beta-carotene is converted into retinol thus giving 1.8 mg of retinol per cup of carrot juice. If you are a poor converter of beta-carotene, you could be getting more like 0.5mg from a cup of pure carrot juice.
Retinol forms of vitamin A:
Carlson’s Vitamin A on Amazon.com is an inexpensive source of the retinol form of vitamin A. There are other good brands as well — just be sure to read the labels to know what you are getting.
Beef liver is also an excellent source of vitamin A. A three-ounce serving of liver packs a big punch with about 15,000 IU of vitamin A.
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] So be sure to get out in the sunshine for vitamin D, and if you are getting your vitamin A levels checked, you may want to also get vitamin D tested at the same time.
Other articles you might enjoy:
- Lactose Intolerance Genes: How much milk should you drink?
- Shining Genetic Light on Your Vitamin D Levels
- Vitamin C and Your Genes