Vitamins are essential nutrients that humans need to obtain in order to be healthy. The only vitamins that are not readily obtainable from unprocessed plant foods are Vitamin B12 and Vitamin D. All the other essential vitamins are readily obtainable from eating a range of plant foods, including vegetables, fruit, legumes, nuts, seeds and whole grains. It is important for anyone eating a herbivorous (/vegan) diet to know about vitamin B12 and vitamin D and where to get them.
Vitamin B12 is important for the nervous system and brain as well as for the formation of healthy red blood cells. It is not reliably found in natural plant foods and a deficiency can have severe consequences, including including anaemia, nerve damage, depression, psychiatric disorders and even death. Sub-optimal intake of vitamin B12 (which may not be low enough to cause an outright deficiency) can have negative health consequences. People with sub-optimal levels of vitamin B12 have been found to have higher levels of the toxic substance homocysteine in their blood, which puts them at increased risk of cardiovascular disease. It is very important to have a reliable source of vitamin B12 when eating herbivorously. Fortunately, this is easy, as Vitamin B12 can be obtained from fortified foods or supplements, which are readily available in most places..
Some foods are "fortified" with vitamin B12, meaning that it has been added during the manufacturing process. For example, some soy and rice milks and some vegetarian sausages and some yeast extract spreads have had vitamin B12 added. It is important to always check the label to be sure, as fortification varies between different brands and between different countries. (For example, in the USA vitamin B12 can be added to breakfast cereals, but this is not allowable in Australia.)
It is important to pay particular attention to meeting Vitamin B12 needs when switching to a plant-based diet.
This can be done by eating 3 serves of fortified foods every day and/or obtaining vitamin B12 from supplements. Supplementing is recommended as it could be inconvenient to eat the required aount of fortified foods every day, and in any case most of these foods are highly processed and could displace healthier, less processed foods from your diet. The following is recommended for adults:
(1) Take a daily multivitamin that contains 100 micrograms or more of Vitamin B12 OR
(2) Take a daily Vitamin B12 supplement containing at least 25 micrograms of Vitamin B12 OR
(3) Take a weekly Vitamin B12 supplement containing 2000 micrograms of Vitamin B12. (The much higher dose is needed when it is a weekly dose as less is absorbed when it is a single dose.)
It is important that infants of vegan mothers are given a vitamin B12 supplement from 6 months of age (or from birth if the mother did not supplement throughout the pregnancy). Children on vegan diets should be supplemented with vitamin B12 throughout childhood, adolescence and adulthood.
For infants and children, the following dosages of vitamin B12 are recommended: (Note: The higher dose is recommended if the vitamin B12 is in a multivitamin supplement rather than a B12-only supplement.)
Age: 6-12 months: 10-40 mcg per day | Age 1-3 years:10-40 mcg per day | Age 4-8 years: 15-50 mcg per day | Age 9-13 years: 20-75 mcg per day | Age 14+: 25- 100 mcg per day
If you are wondering where non-human herbivores get their vitamin B12 from, that is a good question. Vitamin B12 is synthesized by bacteria and can be found in soil, dirty water and faeces. Animals that are "foregut fermenters" (such as cows) can absorb the vitamin B12 that is produced in their own gut, so do not need to rely on an external source. Animals that are "hindgut fermenters" such as gorillas and humans, cannot absorb the vitamin B12 produced in their gut and therefore need an external source. Some would obtain vitamin B12 from contamination of their food with soil or accidental ingestion of grubs and insects, whereas others (such as rabbits and pigs) eat some of their own faeces and obtain vitamin B12 that way. Gorillas eat a 97% herbivorous diet but will eat termites and other insects as a source of vitamin B12.
Fortunately for humans we now have the technology and do not have to eat dirt, our faeces or insects to obtain vitamin B12. We can obtain it safely and cleanly from fortified foods and supplements. There have been some reports of vitamin B12 being found in some fermented plant foods (such as tempeh), mushrooms, some seaweeds and spirulina. However it has since been found that the amounts found are extremely variable and are mostly biologically inactive analogues, meaning that they are not useable by our bodies. These analogues could even contribute to a vitamin B12 deficiency, by competing with and blocking the absorption of biologically active "true" vitamin B12. At this stage there is no reliable known natural plant source of vitamin B12, and to attempt to rely on any of these so-called sources of vitamin B12 is endangering one's health.
Vitamin D is an important vitamin for strong bones as well as some other functions. Insufficient Vitamin D can result in weak bones, and can cause rickets in children (where bones are weak and can bend), causing deformed limbs.
Vitamin D is not found in many foods, and is not normally found in plant foods. (Some foods may have added vitamin D, such as some margarines and some soymilks - check the label.) However it is not essential to obtain vitamin D from food, as it can actually be produced by the action of sunlight on the skin. For people who live in sunny climates, adequate vitamin D can normally be obtained by spending time outdoors (a couple of hours a week) with some bare skin (without sunscreen) exposed.
However for people who don't live in a sunny climate, or who keep covered up when outdoors (eg for cultural reasons), who always use sunscreen or who rarely get outside, getting enough vitamin D from sunlight is difficult. Darker skinned people need longer exposure to sun than fair skinned people do to obtain adequate vitamin D. Breastfed babies are at risk of vitamin D deficiency if their mothers are deficient.
The recommended amount of vitamin D to supplement with if obtaining very little sunlight on bare skin is typically 5 to 10 micrograms per day. Many multivitamins contain this amount of vitamin D.
If supplementing with vitamin D, it is worth noting that there are two forms of the vitamin. Vitamin D3 is usually derived from lanolin from sheep's wool, and vitamin D2 is derived from non-animal sources. So if you want to avoid all animal-derived ingredients, choose vitamin D2 or seek out a source of vitamin D3 that has not been derived from animal sources.
Vitamin D from the sun
Aim for 10 to 30 minutes of sun exposure every day, without sunscreen.
Be very careful with the midday sun, and avoid getting sunburnt.
The more skin you expose and the lighter your complexion, the less time you need in the sun.
If in doubt, consider a vitamin D supplement.