What are the main nutrient deficiencies strict vegans are at risk of and how to avoid them?
Nutrients of concern for Vegans
There are a few nutrients that may be more difficult to obtain on a vegan diet as compared to an unrestricted (omnivorous) diet.
fats EPA and DHA are long-chain omega-3 fatty acids that are synthesized from simpler fats like ALA. Non-vegans would get these nutrients by eating fish and other seafood. Human bodies have the ability to create these nutrients when the diet is replete with ALA and low in omega-6 fats. However, synthesis rates vary between individuals and among different populations. Vegans who do not supplement EPA and DHA will have lower levels of these fats, but it's not clear whether this matters for the regular health of adults. Women who are planning to become pregnant, or are currently pregnant or breastfeeding should probably take a vegan DHA supplement with advice of a doctor in order to avoid problems with brain development in the child. EPA and DHA can be safely added to a diet because they are fats that can be metabolized for energy and do not accumulate in the body.
iodine Iodine may be less available on a vegan diet than one that includes milk. Although iodine is present in plant-based foods, the use of iodine for cleaning cow teats provides extra iodine in dairy products. Surprisingly, iodine deficiency affects nearly 2 billion people around the world and is the leading cause of intellectual disability. This is why many countries add iodine to table salt to make iodized salt. Vegans, especially those in Europe where soils contain less iodine, should ensure a reliable source of iodine either by taking a multivitamin or regularly using iodized table salt.
zinc Zinc is an important mineral that may be less available on a vegan diet. Although there are many vegan foods that provide zinc, the removal of meat (especially beef) may not be fully balanced unless the diet is well planned. Males tend to have slightly higher zinc requirements because some zinc is lost through ejaculation. Zinc supplements should be taken carefully because an excess of zinc may inhibit copper absorption.
protein Protein may be a concern on a diet high in refined fats, sugars, or other junk foods. The Standard American Diet compensates low-protein foods (like oils and sugars) with high-protein foods (like meat), so if one follows the Standard American Diet but just avoids meat they may not get enough protein. However, a diet that provides sufficient food energy and incorporates a variety of foods (especially legumes, beans, and pulses) can easily meet requirements for proteins and specific amino acids.
Nutrients of General Concern
These nutrient concerns are not specific to vegans, but are good to be aware of in general.
iron Iron may be a mineral of concern for pre-menopausal women because of the amount of iron that is lost in menstruation. Even though vegan women tend to have lower iron levels, it looks like they experience iron deficiency anemia at about the same rate. Dark chocolate is a surprisingly high source of iron. Iron supplements should be taken carefully and on advice of a doctor because consuming too much iron can be harmful.
vitamin-d Vitamin D can be obtained either from sun exposure or in diet. Vitamin D deficiency is very prevalent in northern countries among all people regardless of diet. Small amounts of Vitamin D (1000 IU) can be safely added to diet, but larger amounts should only be taken on advice of a doctor because vitamin D may accumulate in the body.
When in doubt, consult your doctor, and do bloodwork!
Iron. I have never had anemia problems and iron is replete in dark leafy greens.
Vitamin A. Primarily found in meats, but also in cooked vegetables. If you are a raw vegan you are at risk for Vitamin A.
Vitamin D. You are likely at risk anyway if you live in a northern climate. Vitamin D affects both immune system and mood.
Vitamin B12. This is probably the most important. You need a supplement, probably a high-sounding dose like 1000mg / day, and B12 affects a variety of body functions.
Protein? In my experience people worry about protein too much but understandably vegans tend to be concerned about this. I have been fine just sticking to eating lots of beans and nuts often. Research deficiency symptoms and if you suspect you are deficient, work with your doctor.
Whilst the Vegan Society contains a more comprehensive list of the common deficiencies, I'm going to mention vitamin D.
Vitamin D supports your immune system and is thought to contribute to a positive mental state, though personally I think going out in the sun could just as easily have that effect regardless of its nutritional worth.
The reason it's probably not mentioned on some vegan resource sites is because it's not strictly a vegan issue, however, supplementing for it can be tricky because there are non-vegan sources of vitamin D.
Most people know that sunlight contains vitamin D, and might assume that you simply need to go outside every day to get enough, but if you live in a climate like we have here in England you can't rely on getting your vitamin D from sunlight.
In this case, with the exception of fortified foods, it's sensible to supplement to ensure you get enough, vitamin D3 is more readily absorbed by the body, just make sure it isn't made from an animal source, products such as VEG1 and those by Deva are worth a try.
Iron is available in dark leafy greens but still is a concern being less bioavailable than from meat. Iron absorbs better when eaten along with vitamin C so a balanced diet ought to prevent anemia but supplements may be considered, particularly for women as their Iron requirements are higher. Note that it is inadvisable to take Iron supplements if you don't need to, conditions like Haemochromatosis can go be asymptomatic and undiagnosed in young adults.
B12 is a major concern for vegans and strict vegetarians, as moderate intake of eggs or dairy is sufficient to prevent deficiency. It is a stored vitamin which vegans are at risk of developing long-term so monitoring B12 blood levels and supplementation may be necessary.
Incomplete proteins are not a problem on a balanced diet (rather than relying on a single staple crop such as corn), any amino acids missing in one plant food source are available in another. Protein should not be a major concern for adult vegans since all plants contain proteins, even if they are often higher in fibre and starch than fats or protein. However, protein intake may be a concern for growing children, teenagers, or active people (such as athletes) in which case nuts, beans, and legumes are good vegan protein sources. If you have nut allergies or FODMAP intolerance it may be advisable to consider an ovo-lacto vegetarian diet to ensure enough protein intake for these people.
There is also a carnitine deficiency, the symptoms of which are less subtle.
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According to research B12 is available in Tempeh, a traditional, Indonesian soy product which involves fermentation. B12 is produced by bacteria and archaea. (Good sources of B12 also include insects, which some vegetarians, but not vegans, may eat).