Most vitamins cannot be synthesized by the body in sufficient amounts, which means they must be ingested. Some vitamins, like A, D, E and K, are fat-soluble and are stored by the body so it is not necessary to consume them every day. Others, such as vitamin C and the B-complex vitamins, are water-soluble and must be replenished on a daily basis.
Among its other functions, vitamin A contributes to the growth and development of bones, cell development, prevention of infectious diseases and improvement of vision. It also contributes to the normal functioning of the reproductive system.
Although the active form of vitamin A, retinol, is found only in animal products, this doesn’t pose any problem for a vegan diet. Plants contain more than 50 components called carotenoids, like β-Carotene, which the body can convert into vitamin A. Carotenoids also have antioxidant properties and other benefits that may reduce the risks of cancer and heart disease.
Vitamin A can be obtained from orange, red and dark green vegetables, such as carrots, sweet potatoes, pumpkin, kale, spinach, radicchio, lettuce, broccoli, Brussels sprouts, asparagus, tomatoes, peaches and cantaloupe. Cooking as well as chopping and crushing fruits and vegetables improve absorption. Including some fats like nuts, seeds or avocados can also increase absorption of carotenoids. Carrots are especially rich in vitamin A and carotenoids, so much so that the necessary daily amount is contained in a small raw carrot (30 g).
It appears that sun drying destroys vitamin A, and frying can also destroy it. Canned vegetables lose between 15% and 30% of their vitamin A, and it is also missing in fried foods.
Vitamin B2 has various functions, such as converting food into energy. It works with other vitamins in the B-complex, and it’s important for body growth and the production of red blood cells. It plays a role in tissue regeneration, and it helps preserve the cells of the nervous system. These are just some of its many functions.
No clinical deficiency of riboflavin has been found in vegans. A diet that includes a variety of vegetables, legumes and grains will meet our needs for this vitamin.1
Both sunlight and fluorescent light can destroy riboflavin. For this reason it’s recommended that foods rich in riboflavin be stored in opaque containers.
Ingestion of very low levels of vitamin B12 can cause anemia and nervous system damage. Many people who aren’t vegan have serious problems with vitamin B12 deficiency, often due to poor assimilation related to their diet. However, in the case of vegans, as indicated above, it’s a particular problem to have any type of deficiency in this vitamin, not just for health reasons as in the case of non-vegans, but also because of the bad image of the vegan diet it may give.
For many years there has been a discussion concerning vegetable sources of vitamin B12, with some people claiming that B12 can be obtained from spirulina and certain seaweeds. Research indicates that these are not reliable sources,2 but this should not be a problem for vegans, as there are foods fortified with this vitamin, as well as supplements.
There are several ways to obtain good levels of vitamin B12 on a vegan diet. The first is to consume fortified foods several times a day with amounts adding up to 4-7 mcg.3 A second way is to take a daily B12 supplement of at least 25 mcg, preferably Cyanocobalamin, the form with the most proven effectiveness. A third possibility is to take a 1,000 mcg supplement twice a week, or larger amounts. Note that when supplements are consumed daily, the doses must be much smaller.4 This is because the body can absorb only a small percentage of each dose. Drug stores and some supermarkets sell bottles of 1,000 mcg B12 tablets that are inexpensive, so this may be the easiest way to avoid B12 deficiencies.
Vitamin D aids in the absorption of calcium and phosphorus in the intestines and contributes to them being deposited in the bones and teeth. It also contributes to cellular growth and strengthens the immune system among other functions.
Recent research shows that high levels of vitamin D may aid in the prevention of fibromyalgia, rheumatoid arthritis, multiple sclerosis, upper respiratory infections, PMS, polycystic ovary disease, psoriasis, muscular weakness, back pain, diabetes, high blood pressure, cancer and asthma, as well as improving mood. Vitamin D is required for the absorption of calcium and phosphorus to such an extent that the absorption of calcium decreases by 15%, and that of phosphorus by 60%, when there is a vitamin D deficiency.5
We can often synthesize vitamin D through sun exposure, though this depends on several factors that are explained below. Our body retains vitamin D for several weeks, so we can have good levels of this vitamin even on the days without sun exposure. There are also vegan foods fortified with vitamin D.
There are various factors that affect how much sun exposure we need in order to produce adequate amounts of vitamin D. Some studies have found that someone with light skin needs about 10 to 15 minutes of sun exposure on their face and forearms without sunscreen during the part of the day when sunlight is the strongest (between about 10 AM and 2 PM) to produce adequate amounts. For someone with dark skin, it may take 20 minutes or more, and for the elderly, who have the poorest assimilation, about 30 minutes is recommended.
Those with very dark skin may require up to two hours in the sun, twice a day. Even though there is no risk of vitamin D toxicity due to excessive sun exposure (since the excess vitamin precursors and vitamin D are broken down on the skin into inactive metabolites), this level of sun exposure is impractical for many people today.6 Therefore, it’s recommended that those with very dark skin eat foods fortified with vitamin D or take a supplement.7
Many people, of all skin colors, don’t spend enough time outdoors to synthesize adequate amounts of vitamin D or live in areas with little sunlight. In such cases, it is necessary to obtain vitamin D in another form, by ingesting 800 IU (20 mcg) per day either by taking a supplement or by consuming fortified foods, such as soymilk, cereal, or juice.8
Another important consideration is that Europe, Canada, large parts of Asia and most U.S. states are above the 35th parallel north, and New Zealand and parts of Argentina and Chile are below the 35th parallel south. In these places the ability to synthesize vitamin D from sunlight during the winter and spring is greatly reduced, due to the angle of the sunlight during those months.9 In such places, special attention should be paid to meeting nutritional needs for vitamin D during those months.
For someone whose main sources of vitamin D are fortified foods or supplements, the recommended daily amount is 800 IU (20 mcg) per day for adults, and 400 IU (10 mcg) for children. Many experts recommend 1,000-2,000 IU (25-50 mcg) or more for optimal health with an upper limit of 4,000 IU (100 mcg) without medical supervision.10 Vitamin D2 (ergocalciferol) is the type of vitamin D that is always vegan, though recently a few brands have begun to sell vegan D3.
Unlike us, some animals of other species, such as cats, cannot synthesize vitamin D with exposure to sunlight. For this reason, cats must consume vitamin D through foods supplemented with it. There are various brands of vegan cat food that are supplemented with vitamin D.
Melina, V.; Craig, W. & Levin, S. (2016) “Position of the Academy of Nutrition and Dietetics: Vegetarian diets”, Journal of the Academy of Nutrition and Dietetics, 116, pp. 1970-1980 [accessed on 21 January 2017].
1 Norris, J. & Messina, V. (2011) Vegan for life, Philadelphia: Da Capo, p. 76.
2 Dagnelie, P. C.; van Staveren, W. A.; van den Berg, H. (1991) “Vitamin B-12 from algae appears not to be bioavailable”, American Journal of Clinical Nutrition, 53, pp. 695-697; Schwarz, J.; Dschietzig, T; Schwarz, J.; Dura, A.; Nelle, E.; Watanabe, F.; Wintgens, K. F.; Reich, M. & Armbruster, F. P. (2014) “The influence of a whole food vegan diet with Nori algae and wild mushrooms on selected blood parameters”, Clinical Laboratory, 60, pp. 2039-2050.
3 Bor, M. V.; von Castel-Roberts, K. M.; Kauwell, G. P. A.; Stabler, S. P.; Allen, R. H.; Maneval, D. R.; Bailey, L. B. & Nexo, E. (2010) “Daily intake of 4 to μg dietary vitamin B-12 is associated with steady concentrations of vitamin B-12–related biomarkers in a healthy young population”, The American Journal of Clinical Nutrition, 91, pp. 571-577 [accessed on 17 November 2016].
4 Hokin, B. D.; Butler, T. (1999) “Cyanocobalamin (Vitamin B-12) status in Seventh-day Adventist Ministers in Australia”, The American Journal of Clinical Nutrition, 70, suppl. 3, pp. S576-S578. Allen, L. H. (2009) “How common is vitamin B-12 deficiency”, The American Journal of Clinical Nutrition, 89, pp. S693-S696. Norris, J. & Messina, V. (2011) Vegan for life, op. cit., p. 27-32. Davis, B. & Melina, V. (2014) Becoming vegan, comprehensive ed., Summertown: Book Publishing, p. 217.
5 Holick, M. F. (2007) “Vitamin D deficiency”, The New England Journal of Medicine, 357, pp. 266-281. Schwalfenberg, G. (2007) “Not enough Vitamin D: Health consequences for Canadians”, Canadian Family Physician, 53, pp. 841-854. Urashima M.; Segawa, T.; Okazaki, M.; Kurihara, M.; Wada, Y. & Ida. H. (2010) “Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren”, The American Journal of Clinical Nutrition, 91, pp. 1255-1260.
6 Urashima M.; Segawa, T.; Okazaki, M.; Kurihara, M.; Wada, Y. & Ida. H. (2010) “Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren”, op. cit.
7 Painter, K. (2009) “Your health: Skin color matters in the vitamin D debate”, USA Today, 4/19/2009 [accessed on 5 October 2016].
8 Norris, J. & Mesina, V. (2011) Vegan for life, op. cit., p. 47-50.
9 Quesada Gómez, J.; Díaz Curiel, M.; Sosa Henríquez, M.; Malouf Sierra, J.; Nogues Solan, X.; Gómez Alonso, C.; Rodríguez Mañas, L.; Neyro Bilbao, J. L.; Cortes, X. & Delgadillo J. (2007) “Low calcium intake and insufficient serum vitamin D status in treated and non-treated posmenopausal osteoporotic women in Spain”, Journal of Bone and Mineral Metabolism, 22, p. S309.
10 Davis, B. & Melina, V. (2014) Becoming vegan, op. cit., p. 226. Norris, J. & Messina, V. (2011) Vegan for life, op. cit., p. 47.