NO VITAMIN A IN CARROTS

Carrot is a food item which is recommended as a good source to obtain vitamin A, if not as its primary source. The misconception that vitamin A and beta-carotene are the same can also be triggered by the food industry, which uses “vitamin A” on the packaging of plant foods instead of “pro-vitamin A carotenoids” or “beta-carotene”. And far more misleading information is circulating about vitamin A (which we will discuss in several other articles), causing that consumption of its true sources is neglected.

True vitamin A (retinol) is only found in animal foods, the best sources include fish oil (mainly cod liver oil), liver, butterfat, high-fat milk and dairy from grass-fed animals, and pastured-raised egg yolks [1]. Carrots, not even any other vegetables and fruits, do not contain any vitamin A at all, but a potential precursor to vitamin A, pro-vitamin A carotenoids.

Pro-vitamin A carotenoids are carotenoids which can be converted to vitamin A (retinol) in a human body, β-carotene is converted into retinol most efficiently of all of them. The Food and Nutrition Board estimated the conversion factor to 12:1 by weight [2] which means that you need 12 molecules of β-carotene to get 1 molecule of vitamin A. The rate of this converison is still the subject of research, the original value was 4:1 following by value 6:1 [3]. The current conversion factor cannot be applied uniformly to all foods, a recent study has shown almost an 8-fold difference in β-carotene conversion factors from various plant foods [4].

The main sources of pro-vitamin A carotenoids include green leafy and yellow-colored vegetables and orange-colored fruit (e.g.carrots, broccoli, cantaloupe, and squash) [5, 6, 2]. Carotenoids are also accumulated in the fat of animals and then we can find them in animal products like butterfat, egg yolks, and salmon skin [7].

Is it good to rely on the conversion of β-carotene to vitamin A?

In fact, the efficiency of conversion of  β-carotene to vitamin A is highly variable, and people with  non-functioning gallbladder or its surgical removal [8, 9], low thyroid function [10], or inadequate stomach acidity [11] might not make the conversion at all.

Unfortunately, the list of factors limiting β-carotene bioavailability and conversion is long also for healthy population. 

DIETARY FAT SHOULD BE CONSUMED TOGETHER WITH BETA-CAROTENE

The bioavailability of pro-vitamin A carotenoids in raw vegetables has been estimated to be 5–10 %, while it may be as high as 50 % when dissolved in fat (kind of fat is  currently also discussed) [12]. The conversion ratio for β-carotene in oil is estimated at 2 𝜇g, means 2 𝜇g of beta carotene you need for 1 molecule of vitamin A [2]. Why does a dietary fat help?

Pro-vitamin A carotenoids are fat-soluble nutrients, which means dietary fat is absolutely necessary for pro-vitamin A carotenoids absorption in the digestive tract [12]. This should be in mind especially for people who are afraid of fat and are on a very low fat diet. The richest source of pro-vitamin A carotenoids are red palm oil and oil of the buriti palm tree because of the carotenoids dissolved in the fat [13].

Food processing is another critical factor which can significantly affect carotenoid bioaccessibility from food. Juicing can increase bioaccessibility of beta-carotene from almost 3 % from raw carrots to 14 % from carrot juice. Cooking, like steaming, can improve bioavailability of beta-carotene, on the contrary excessive heating (e.g. boiling) can result in their degradation [14, 15].

High intake of dietary fibre can also decrease the absorption of carotenoids [16], and excessive alcohol intake may inhibit the conversion of β-carotene to retinol [17].

If you want to outsmart problematic bioavailability of beta-carotene through supplementation, it’s useless. Cleavage products of beta-carotene can block vitamin A activity and xcessive doses of beta-carotene may make your skin yellow [18, 19].

And on top of that, the genetics play a governing role in the conversion. Humans have different abilities to convert β-carotene to vitamin A. They differ in the variation in the gene that produces the β-carotene conversion enzyme, BCMO1. The activity of the enzyme can be as low as 10 percent or less, on the other hand, up to 90 percent in some cases. Those with a poorly functioning version of the BCMO1 gene are in a higher risk of vitamin A deficiency, therefore pro-vitamin A carotenoids might not be a good vitamin A source for them [4, 20, 21].

Why make it complicated when it can be easy?

As you could see: numerous factors conspire pro-vitamin-A carotenoids as being a good source of vitamin A. Therefore, it may be better to include dietary sources of true vitamin A into your diet than to rely on pro-vitamin A carotenoids only and to hope for good genetic variation. 

Listed below are approximate levels of vitamin A in common foods per 100 grams[7]. These amounts can vary according to different animals feeding, the season (in the case of butter) etc. Recommended daily intake (RDA) is 650 mcg (2 167 IU) for female and 750 mcg (2 497 IU) for male [20].

Raw state (100 g) Vitamin A
Fish oil, cod liver oil 218 029 IU = 65 408 mcg
Cod liver 100 000 IU = 30 000 mcg
Duck liver 39 908 IU =  11 732 mcg
Beef liver 16 898 IU = 4 968 mcg
Chicken liver 11 078 IU = 296 mcg
Butterfat (from grass-fed cows) 3 571 IU = 1 020 mcg
Egg yolk (free-range) 1 442 IU = 381 mcg
Milk high in fat from grass-fed cown 140 IU = 40 mcg

RDA for vitamin A could be achieved with only a half teaspoon of cod liver oil, approx 20 g beef or chicken liver, 74 g of butter or with 4 eggs.

And what is vitamin A good for? – You can read in the next article.

 

SOURCES

[1] Moore T. Vitamin A. Amsterdam: Elsevier, 1957.

[2] Institute of Medicine, Dietary Reference Intakes for Vitamin A,
Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron,
Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc,
National Academy Press,Washington, DC, USA, 2002.

[3] National Research Council. Food and Nutrition Board Recommended dietary allowances. Washington, DC: National Academy Press, 1989.

[4] Tang G. Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. Am J Clin Nutr. 2010;91(5):1468S-1473S. 

[5] Ross CA. Vitamin A. In: Coates PM, Betz JM, Blackman MR, et al., eds. Encyclopedia of Dietary Supplements. 2nd ed. London and New York: Informa Healthcare; 2010:778-91.

[6] Solomons NW. Vitamin A. In: Bowman B, Russell R, eds. Present Knowledge in Nutrition. 9th ed. Washington, DC: International Life Sciences Institute; 2006:157-83.

[7] US Department of Agriculture, Agricultural Research Service, Nutrient Data Laboratory. USDA National Nutrient Database for Standard Reference, Release 28, May 2016.

[8] Hofmann AF. The function of bile salts in fat absorption. The solvent properties of dilute micellar solutions of conjugated bile salts. Biochem J. 1963;89:57–68.

[9] Tyssandier V, Lyan B, Borel P. Main factors governing the transfer of carotenoids from emulsion lipid droplets to micelles. Biochim Biophys Acta. 2001;1533(3):285–92.

[10] Bralley J.A., L. R. (2008). Laboratory Evaluations for Integrative and Functional Medicine, Metametrix Institute.

[11] Tyssandier, V., Reboul, E., Dumas, J. F., Bouteloup-Demange, C., Armand, M., Marcand, J., Sallas, M., & Borel, P. (2003). Processing of vegetable-borne carotenoids in the human stomach and duodenum. Am J Physiol Gastrointest Liver Physiol, 284, G913- 923.

[12] D. M. Deming and J. W. Erdman Jr., “Mammalian carotenoid
absorption and metabolism,” Pure and Applied Chemistry, vol.
71, no. 12, pp. 2213–2223, 1999.

[13] Rains-Mariath JG, Cavalcanti Lima MC. Pacheco Santos
LM. Vitamin A activity of buriti (Mauritia vinifera Mart.)
and its effectiveness in the treatment and prevention of
xerophthalmia. Am J Clin Nutr 1989;49:84953.

[14] J. M. Gaziano, E. J. Johnson, R. M. Russell et al., “Discrimination
in absorption or transport of beta-carotene isomers after oral
supplementation with either all-trans- or 9-cis-beta-carotene,”
The American Journal of Clinical Nutrition, vol. 61, no. 6, pp.
1248–1252, 1995.

[15] D. M. Deming, S. R. Teixeira, and J. W. Erdman Jr., “All-trans
𝛽-carotene appears to be more bioavailable than 9-cis or 13-cis
𝛽-carotene in gerbils given single oral doses of each isomer,”
Journal of Nutrition, vol. 132, no. 9, pp. 2700–2708, 2002.

[16] Goltz S.R., Ferruzzi M.G. (2013) Carotenoid Bioavailability: Influence of Dietary Lipid and Fiber. In: Tanumihardjo S. (eds) Carotenoids and Human Health. Nutrition and Health. Humana Press, Totowa, NJ. 

[17] Leo MA, Lieber CS. Alcohol, vitamin A, and β-carotene: adverse interactions, including hepatotoxicity and carcinogenicity. Am J Clin Nutr. 1999;69(6):1071-1085. 

[18] N.V.Dimitrov, C.Meyer,D. E.Ullrey et al., “Bioavailabilityof 𝛽-
carotene in humans,”TheAmerican Journal of ClinicalNutrition,
vol. 48, no. 2, pp. 298–304, 1988.

[19] Novotny JA, Harrison DJ, Pawlosky R, Flanagan VP, Harrison EH, Kurilich AC. Beta-carotene conversion to vitamin A decreases as the dietary dose increases in humans. J Nutr. 2010;140(5):915-918.

[20] Lindqvist A, Sharvill J, Sharvill DE, Andersson S. Loss-of-function mutation in carotenoid 15,15′-monooxygenase identified in a patient with hypercarotenemia and hypovitaminosis A. J Nutr. 2007 Nov;137(11):2346-50.

[21] Lietz G, Oxley A, Leung W, Hesketh J. Single nucleotide polymorphisms upstream from the β-carotene 15,15′-monoxygenase gene influence provitamin A conversion efficiency in female volunteers. J Nutr. 2012 Jan;142(1):161S-5S.

[22] EUROPEAN FOOD SAFETY AUTHORITY. Scientific Opinion on Dietary Reference Values for vitamin A. EFSA Journal 2015;13(3):4028.

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