Vegan diets are often promoted as being the healthiest and most nutritionally dense diets that we can consume.
However, according to the latest scientific research, vegan diets are actually prone to causing multiple basic essential nutrient deficiencies such as Vitamin B12.
In my opinion, the most serious and fundamental issue with vegan diets is that they don’t supply all the basic essential nutrients that we need to stay healthy without resorting to supplements.
An example of basic essential nutrients that the vegan diet does not reliably provide in any quantity would be Vitamin B12 and Vitamin D.
Proponents of the vegan diet and nutritionists often use the term of a “well-planned” vegan diet.
A “well-planned” vegan diet basically refers to a strict vegetarian plant-based diet that is required to include a number of expensive vegan synthetic supplements and fortified foods, in order to substitute for the fact that the vegan diet does not reliably provide intake of basic nutrients such as Vitamin B12, Vitamin D, Iodine, Iron, Calcium, Zinc, microalgaes for omega-3(DHA), Co-Enzyme Q10 and many others.
This is really just another way of saying that the vegan diet is not a nutritionally complete diet plan without a number of synthetic supplements or fortified synthetic foods. Relying on synthetic nutrition to obtain full intake of certain nutrients such as Vitamin B12, is not what I would consider to be a healthy or optimal diet.
Any healthy diet should be perfectly capable of naturally supplying all the basic essential nutrients that we need to stay healthy, which as we know a vegan diet cannot achieve without having to resort to including a number of synthetic supplements.
In this article, we will explore the most common nutritional deficiencies, which the scientific research has confirmed that vegans are potentially at risk of developing.
1. Vitamin B12 Deficiency
Scientific research has now consistently confirmed that vegetarians and in particular strict vegans have the lowest levels of Vitamin B12, the highest levels of cardiovascular and neuro-toxic homocysteine and the highest rates of Vitamin B12 deficiency, out of all the diet groups(omnivore and lacto-ovo-vegetarian).
The reason why vegans are so at risk of developing Vitamin B12 deficiency is because strict vegan diets do not reliably provide any dietary source of Vitamin B12 and thus have a very low cobalamin intake.
A study from 2003 concluded:
“Vegan subjects and, to a lesser degree, subjects in the lacto-vegetarian and lacto-ovo vegetarian group had metabolic features indicating vitamin B-12 deficiency that led to a substantial increase in total homocysteine concentrations. Vitamin B-12 status should be monitored in vegetarians.
Health aspects of vegetarianism should be considered in the light of possible damaging effects arising from vitamin B-12 deficiency and hyperhomocysteinemia.” 
Another study review from 2014 published in the European Journal Of Clinical Nutrition, which assessed the prevalence of vitamin b12 deficiency among vegetarian subjects concluded:
“Individuals following vegetarian diets are at risk for developing vitamin B12 deficiency owing to suboptimal intake.
Higher deficiency prevalence was reported in vegans than in other vegetarians. Thus, with few exceptions, the reviewed studies documented relatively high deficiency prevalence among vegetarians.
Vegans who do not ingest vitamin B12 supplements were found to be at especially high risk. Vegetarians, especially vegans, should give strong consideration to the use of vitamin B12 supplements to ensure adequate vitamin B12 intake.” 
Common symptoms of Vitamin B12 deficiency and pernicious anemia include weakness, fatigue, rapid heartbeat, shortness of breath, pins and needles in the extremities, pale skin and much more.
Vitamin B12 deficiency can cause a variety of very serious problems if left untreated in the long-term including irreparable nervous system damage, spinal cord degeneration and even raising the risk of cardiovascular disease.
A serious consequence of severe Vitamin B12 deficiency is the potential to develop a condition known as hyperhomocysteinemia.
Elevated levels of the non-protein amino acid homocysteine have been proven to increase an individual’s risk of developing cardiovascular disease and also play a role in the etiology of neurodegenerative diseases such as Parkinson’s and Alzheimer’s.
Multiple studies have now confirmed that many vegans suffer from hyperhomocysteinemia, which is the direct result of severe Vitamin B12 deficiency.
A study review from 2009 concluded:
“Overall, the studies we reviewed showed reduced mean vitamin B-12 status and elevated mean homocysteine concentrations in vegetarians, particularly among vegans.” 
2. Vitamin D Deficiency
Most of the common dietary sources of Vitamin D are of animal based origin such as dairy, seafood, eggs and liver for example.
As a result, vegans tend to consume no dietary Vitamin D intake, which if you reside somewhere with inadequate sunlight, you may well be at risk of developing severe Vitamin D deficiency.
Vitamin D is required for healthy cell division, bone health, immune system function, calcium metabolism and many other functions in the body.
Vitamin D deficiency has been linked to the development of a number of serious health problems including cardiovascular disease, auto-immune disorders, depression, and osteoporosis.
“In the EPIC-Oxford study, vegans had the lowest mean intake of vitamin D (0.88 μg/d), a value one-fourth the mean intake of omnivores.” 
It’s worth mentioning that the oily fish sardines are an excellent and rare dietary source of Vitamin D3. One can of sardines provides around 50% of the RDA of Vitamin D3.
Sardines also supply many other nutrients which are commonly low on a vegan diet such as Vitamin B12,
Vitamin D, Pre-Formed Omega-3s(EPA/DHA), Bio-available Calcium, Selenium, Iodine, Zinc, RNA/DNA, Protein, and Co-Enzyme Q10.
3. Omega-3 DHA Deficiency
Alpha-linolenic acid(ALA) must first convert into EPA(eicosapentaenoic acid) and finally to DHA(docosahexaenoic acid), which are the long-chain forms of Omega-3 fatty acids required for brain and cardiovascular health.
There are relatively few studies currently on vegans and DHA status, the research which is there has mixed findings. However, personally I would still consider Omega-3 DHA to be potential nutrient of concern for vegans, especially those who are following extremely low fat and/or excessive Omega-6/low Omega-3 fatty acid diets.
One study from 2009 on DHA status in vegetarians found:
“Docosahexaenoic acid (22:6n-3; DHA) is absent from vegan diets and present in limited amounts in vegetarian diets.
The proportions of DHA in plasma, blood cells, breast milk, and tissues are substantially lower in vegans and vegetarians compared with omnivores.
Preformed DHA in the diet of omnivores explains the relatively higher proportion of this fatty acid in blood and tissue lipids compared with vegetarians.” 
Scientific research has found that adult conversion of ALA to DHA to be unreliable and severely restricted.
A study from 1998 published in the International Journal For Vitamin & Nutrition Research concluded:
“A diet including 2-3 portions of fatty fish per week, which corresponds to the intake of 1.25 g EPA (20:5n-3) + DHA (22:6n-3) per day, has been officially recommended on the basis of epidemiological findings showing a beneficial role of these n-3 long-chain PUFA in the prevention of cardiovascular and inflammatory diseases.
The parent fatty acid ALA (18:3n-3), found in vegetable oils such as flaxseed or rapeseed oil, is used by the human organism partly as a source of energy, partly as a precursor of the metabolites, but the degree of conversion appears to be unreliable and restricted.
More specifically, most studies in humans have shown that whereas a certain, though restricted, conversion of high doses of ALA to EPA occurs, conversion to DHA is severely restricted.
The use of ALA labelled with radioisotopes suggested that with a background diet high in saturated fat conversion to long-chain metabolites is approximately 6% for EPA and 3.8% for DHA.
With a diet rich in n-6 PUFA, conversion is reduced by a whopping 40 to 50%. It is thus reasonable to observe an n-6/n-3 PUFA ratio not exceeding 4-6.
These findings indicate that future attention will have to focus on the adequate provision of DHA which can reliably be achieved only with the supply of the preformed long-chain metabolite.” 
Whilst I’m not necessarily suggesting that it’s impossible to maintain a healthy DHA status as a vegan, as said previously, I would urge on the side of caution with this nutrient considering the above evidence.
4. Iodine Deficiency
Other than seaweed, most of the common dietary sources of Iodine are of animal food origin such as eggs, seafood, and dairy for example.
Iodine is a very important nutrient for thyroid gland health and is involved in the synthesis of thyroid hormones.
Iodine deficiency can cause a wide range of symptoms such as goiter, hypothyroidism, cold hands and feet,
thinning hair, weight gain and depression.
A study from 2003 assessing iodine deficiency in vegetarians and vegans found:
“One fourth of the vegetarians and 80% of the vegans suffer from iodine deficiency (iodine excretion value below 100 microg/l) compared to 9% in the persons on a mixed nutrition.
The results show that under conditions of alternative nutrition(vegetarian, vegan etc), there is a higher prevalence of iodine deficiency, which might be a consequence of exclusive or prevailing consumption of food of plant origin, no intake of fish and other sea products, as well as reduced iodine intake in the form of sea salt.” 
The best dietary source of bioavailable iodine, in my opinion is sea vegetables such as kelp, nori, dulse and wakame for example. A small amount of kelp weekly will meet your iodine needs easily.
Sea vegetables are also one of the best dietary sources of trace and ultra-trace elements, which are often deficient in the diet when only consuming land plant-based foods.
5. Copper & Zinc Deficiency Imbalances
The reason why vegan diets cause copper and zinc imbalances is because strict plant-based diets tend to be extremely rich in total copper, whilst often being relatively low in balancing zinc by comparison.
Whilst it is not impossible to achieve 100% of the RDA for zinc eating a varied vegan diet rich in foods such as beans, legumes, nuts, seeds, tofu and other whole foods. However even if you are achieving 100% of the RDA for zinc, you will likely be consuming around 2-3+ times the amount of copper in the process.
Most plant food sources which are rich in zinc, tend to also be excessively high in copper. This can make achieving copper/zinc balance notoriously difficult for individuals following a vegan diet. Most of the common zinc rich dietary sources such as lean meat, eggs and seafood tend to be of animal food origin.
The ideal ratio of Zinc to Copper is thought to be 8:1 in zincs favor. Strict plant-based vegan diets can often almost reverse this optimal ratio in copper’s favor.
There are many fad vegan diets such as fully raw, Doug Graham’s 80/10/10 and raw till 4 which eliminate or severely restrict the few zinc rich plant-food sources that there are such as beans, legumes, soy/tofu, nuts, and seeds. These overly restrictive vegan diets tend to be even more prone to causing zinc deficiency and copper imbalances, due to the low intake of dietary zinc.
Copper and zinc metabolism imbalances can have a devastating impact on health and can cause weak immune system, adrenal fatigue, thyroid problems, anxiety/panic disorder, autism spectrum disorders, connective tissue problems, hormonal imbalances/PMS, mental health disorders and much more.
We are beginning to see more scientific research on the role of copper and zinc metabolism imbalances in many health conditions such as PMS and even Autism Spectrum Disorders.
6. Iron Deficiency
Vegetarians and in particular vegans are at an increased risk of developing low levels of the mineral iron, although it is not known from the research, whether this results in a greater incidence of iron-deficiency anemia or not.
There are two types of iron in the human diet, one of which is heme iron, found predominately in animal foods and the other is non-heme iron, which is found in plant foods.
Scientific research has found that non-heme iron is very poorly absorbed compared to heme-iron, which is likely the reason why vegetarians often have lower iron stores than omnivores. High intakes of phytic acid rich foods such as grains and nuts, may also lower iron and zinc absorption.
Anecdotally, women in particular often complain of an inability to maintain a healthy iron status as a vegan and usually need to resort to synthetic iron supplements. Women are more prone to iron deficiency than men because they lose iron during menstruation.
However, there are tips to increase the absorption and utilization of non-heme plant-based iron. One tip is to pair iron-rich plant foods such as dark leafy greens, with foods rich in Vitamin C such as bell peppers for example. Vitamin C has shown in numerous studies to increase the absorption of non-heme iron.
7. Miscellaneous Vegan Diet Nutrient Deficiencies
Some other nutrients commonly found to be low in vegans include calcium, selenium, co-enzyme q10 and conditionally essential amino acids such as taurine and carnitine.
Vegan diets are often low in sulfur bonded amino acids such as cysteine and taurine, which are important nutrients for healthy detoxification, especially for the removal of toxic heavy metals such as mercury.
One study assessing plasma and urine taurine levels in vegans found:
“Inadequate intake of zinc; and negligible intake of taurine. Prolonged absence of dietary taurine intake causes decreased plasma taurine and severely restricted urinary taurine output.” 
Taurine isn’t the only amino acid that has the potential to be deficient on a vegan diet, some others include cysteine, carnitine, and methionine.
Vegan diets also provide no dietary cholesterol, which is often thought to be a good thing. However I don’t agree with this, cholesterol also happens to be one of the most misunderstood nutrients of all time.
Cholesterol is actually one of the basic building blocks of all hormones in the body including both adrenal(cortisol, adrenaline) and steroid sex hormones such as testosterone. It is not surprising, that many vegans suffer from chronic fatigue, burnout, adrenal gland issues and low libido.
 Vitamin B-12 status, particularly holotranscobalamin II and methylmalonic acid concentrations, and hyperhomocysteinemia in vegetarians.
 The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature.
 Vitamin B-12 and homocysteine status among vegetarians: a global perspective.
 Health effects of vegan diets.
 DHA status of vegetarians.
 Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)?
 Iodine deficiency in vegetarians and vegans.
 Plasma and urine taurine levels in vegans.
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.