Vitamin A and predisposition to its deficiency

Vitamin A (or rather beta carotene): the magic ingredient in carrots supposed to help you see in the dark. Unfortunately, carrots won’t give you super-human night-vision, but there is definite truth in the fact that vitamin A is crucial for your sight. Hence, its other name - retinol.

Vitamin A, or retinol, is a fat-soluble vitamin. It is either ingested or converted into a vitamin by your body from beta carotene (mostly) or other carotenoids (vitamin A precursors) [1]. Beta carotene is the compound that gives fruits and vegetables their yellow, orange, and red colors.

Although vitamin A is best known for its importance for your eyesight, it has many other functions. The immune system needs it for the health of the lining of the intestines and other cavities through which germs can enter [2].

In the early 1900s, vitamin A was identified to aid in the body’s fight against infections [3] and has anti-inflammatory effects [2]. Importantly, vitamin A is crucial for the growth and development of unborn babies [1]. Without it, a baby’s eyes, skeleton, and some other organs won’t develop and function properly [1].

Babies continue to be very dependent on their mothers’ vitamin A status after they are born. A baby’s primary source of vitamin A is through breastfeeding [4].

Vitamin A deficiency is quite prevalent. It may even be the most widespread vitamin deficiency. Worldwide, children are affected the most, especially those under the age of five years [4]. Because of this and the necessity of vitamin A for the proper growth and development of cells, it is of utmost importance to ensure adequate intake for children [5].

Night-blindness is one of the first and most obvious signs of a vitamin A deficiency. As the name suggests, retinol is involved in producing an essential pigment in the retina of your eyes. Very severe retinol deficiency can even lead to complete blindness [3].

Other symptoms of a vitamin A deficiency could include [3]:

• Dry skin
• Coughing
• Diarrhea
• Cloudiness of the eyes
• Dry eyes
• Gall or kidney stones
• Frequent infections, especially of the respiratory tract

The healthy blood (serum) retinol level is between 1 - 3 𝞵mol/L. To maintain a healthy level, a daily vitamin A intake of 600 - 900 𝞵g is needed [6, 7].

Vitamin A, or precursors thereof, can be found in plant and animal sources, although animal sources seem better for absorption. Dairy products, liver, and fish oils are good sources, and orange fruits and vegetables (carrots, pumpkin, papaya, mango) and spinach, kale, red palm oil, and sweet potatoes [1].

If you do not consume any fat, your body will struggle to absorb vitamin A - it needs to mix with fat to be absorbed [1, 8]. Diseases of the pancreas, liver, or intestines may also cause you to have difficulty with absorption [1].

For the conversion of carotenoids, certain enzymes are necessary. The body’s ability to convert beta-carotene into retinol is in part dependent on the activity of these enzymes. Variations in the genes that code for the enzymes impact the ultimate capacity of the body to form retinol from beta carotene [9].

Because vitamin A is a fat-soluble vitamin, excessive intake could have severe consequences [1].

The risk for excessive intake of vitamin A depends on which forms of the vitamin you mainly take in [7, 10]. If you primarily consume it in its precursor form, your body will regulate your levels and prevent excessive conversion into retinol. If your primary sources are supplements or fortified foods, however, you could more easily take in too much [7]. If so, you may suffer from vitamin A toxicity.

If vitamin A levels are more than 10% of the healthy blood levels, it is considered toxic. Acute toxicity occurs when children consume 20 times more than the recommended amounts and adults consume about 100 times the recommended amounts over a few hours to a few days [7].

Consuming high doses of vitamin A, i.e., more than 100 000 IU (30mg) daily for at least six months or more than 25 000 IU daily for more than six years, is toxic [7, 10]. These levels will differ between individuals, though. Children and the elderly are more sensitive, thus at increased risk for excess intakes [7]

If a pregnant woman takes too much of this vitamin just, her baby could be born with several possible birth defects, and a miscarriage could even be caused [1, 10].

Chronic vitamin A toxicity could lead to the following [7, 10]:

• Decreased bone mineral density leading to osteoporosis
• Altered skeletal development in children
• Adult bone abnormalities and pain
• Abnormalities or damage of the liver
• Vomiting
• Weight loss
• Fever
• Increased intracranial pressure
• Acute toxicity could lead to [10]:
• Nausea & vomiting
• Diarrhea
• Headache
• Fever
• blurred vision
• Decreased muscle coordination

Too much and too little intake of retinol can have adverse effects. Knowing your genetic predisposition to your vitamin A levels is thus essential for the purpose of knowing how much of the vitamin you need to take.

By testing your DNA, you can determine whether you should be able to maintain your vitamin A levels through the intake of beta carotene in your diet or whether you may need to add a vitamin A supplement.

References:

[1] Maia, S. B., Souza, A. S. R., Caminha, M. D. F. C., da Silva, S. L., Cruz, R. de S. B. L. C., Dos Santos, C. C., & Filho, M. B. (2019). Vitamin a and pregnancy: A narrative review. Nutrients, 11(3), 1–18. https://doi.org/10.3390/nu11030681

[2] Huang, Z., Liu, Y., Qi, G., Brand, D., & Zheng, S. (2018). Role of Vitamin A in the Immune System. Journal of Clinical Medicine, 7(9), 258. https://doi.org/10.3390/jcm7090258

[3] Sommer, A. (2008). Vitamin A deficiency and clinical disease: An historical overview. Journal of Nutrition, 138(10), 1835–1839. https://doi.org/10.1093/jn/138.10.1835

[4] Wirth, J. P., Petry, N., Tanumihardjo, S. A., Rogers, L. M., McLean, E., Greig, A., Garrett, G. S., Klemm, R. D. W., & Rohner, F. (2017). Vitamin a supplementation programs and country-level evidence of vitamin A deficiency. Nutrients, 9(3), 1–18. https://doi.org/10.3390/nu9030190

[5] Ross, A. C. (2010). Vitamin A. In J. A. Milner & D. F. Romagnolo (Eds.), Bioactive Compounds and Cancer (pp. 335–356). Humana Press. https://doi.org/10.1007/978-1-60761-627-6_16

[6] Dary, O., & Mora, J. O. (2002). Food fortification to reduce vitamin A deficiency: International Vitamin A Consultative Group recommendations. Journal of Nutrition, 132(9 SUPPL.), 2927–2933. https://doi.org/10.1093/jn/132.9.2927s

[7] Prakash, R. (2006). The acute and chronic toxic effects of vitamin A. American Journal of Clinical Nutrition, 83, 191–201. https://doi.org/10.1093/ajcn/84.1.462

[8] Borel, P., & Desmarchelier, C. (2017). Genetic variations associated with vitamin a status and vitamin A bioavailability. Nutrients, 9(3). https://doi.org/10.3390/nu9030246

[9] Leung, W. C., Hessel, S., Méplan, C., Flint, J., Oberhauser, V., Tourniaire, F., Hesketh, J. E., Lintig, J., & Lietz, G. (2009). Two common single nucleotide polymorphisms in the gene encoding β‐carotene 15,15′‐monoxygenase alter β‐carotene metabolism in female volunteers. The FASEB Journal, 23(4), 1041–1053. https://doi.org/10.1096/fj.08-121962

[10] Allen, L. H., & Haskell, M. (2002). Estimating the Potential for Vitamin A Toxicity in Women and Young Children. The Journal of Nutrition, 132(9), 2907S-2919S. https://doi.org/10.1093/jn/132.9.2907S