As far as the health of human beings is concerned, here are a number of studies showing the typical health problems of a vegan and why they usually quit the diet:
Vegans are deficit in b12:
http://jamanetwork.com/…/jamaneurology/fullarticle/784788
https://www.ncbi.nlm.nih.gov/pubmed/16219987
High fiber diets reduce serum half life of vitamin D3:
https://www.ncbi.nlm.nih.gov/pubmed/6299329
Vegans have weaker bones due to lower calcium intake and vitamin D3 levels:
http://jamanetwork.com/…/jamainterna…/fullarticle/486478
https://www.ncbi.nlm.nih.gov/pubmed/21092700
Vegans have a worse memory compared to non vegans due to creatine deficiency in vegans:
https://www.ncbi.nlm.nih.gov/pubmed/21118604
https://www.ncbi.nlm.nih.gov/pubmed/14561278
Vegans have less gains compared to non vegans:
http://m.ajcn.nutrition.org/content/70/6/1032.full
Vegans are deficient in omega threes:
https://www.ncbi.nlm.nih.gov/pubmed/16087975
https://www.ncbi.nlm.nih.gov/pubmed/16188209
https://www.ncbi.nlm.nih.gov/pubmed/12323090
https://www.ncbi.nlm.nih.gov/pubmed/12323085
Vegans are deficit in carnitine:
https://www.ncbi.nlm.nih.gov/pubmed/21753065
https://www.ncbi.nlm.nih.gov/pubmed/2756917
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1628441/
https://www.ncbi.nlm.nih.gov/pubmed/11043928
Vegans are deficient in taurine:
https://www.ncbi.nlm.nih.gov/pubmed/3354491
Vegans are deficient in iodine:
https://www.ncbi.nlm.nih.gov/pubmed/12748410
https://www.ncbi.nlm.nih.gov/pubmed/21613354
Vegans are deficient in Coenzyme Q10:
https://www.ncbi.nlm.nih.gov/pubmed/16873950
Vegans are deficient in iron due to the fact that iron from plant sources is less bioavailable than iron from meat sources:
https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
https://www.ncbi.nlm.nih.gov/pubmed/11269606
Vegans are deficient in vitamin A:
https://www.ncbi.nlm.nih.gov/pubmed/19103647
http://m.jn.nutrition.org/content/137/11/2346.full
http://healthybabycode.com/why-you-cant-get-vitamin-a…
vegetables (studies linked in the article)
https://www.sciencedaily.com/rel…/2009/11/091118072051.htm
http://researchnews.osu.edu/archive/betacarotene.htm
http://ajcn.nutrition.org/content/71/6/1545.full
http://www.fasebj.org/content/23/4/1041.full
http://www.thehealthyhomeeconomist.com/beta-carotene…
http://empoweredsustenance.com/true-vitamin-a-foods
https://www.westonaprice.org/…/abcs-of…/vitamin-a-vagary
https://www.westonaprice.org/…/abcs-of…/vitamin-a-saga
https://philmaffetone.com/vitamin-a-and-the-beta-carotene…
Calcium in Rats
https://www.ncbi.nlm.nih.gov/pubmed/3183773
Magnesium and Oxalates
https://www.ncbi.nlm.nih.gov/pubmed/15035687
Vegans have a lower sperm count than non vegans:
https://www.ncbi.nlm.nih.gov/pubmed/35465
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3257705/
Vegans have lower testosterone than non vegans:
https://www.ncbi.nlm.nih.gov/pubmed/1435181
http://m.ajcn.nutrition.org/content/42/1/127.abstract
https://www.ncbi.nlm.nih.gov/pubmed/159772
http://m.jap.physiology.org/content/82/1/49
Veganism causes loss of libido and erectile dysfunction:
https://www.ncbi.nlm.nih.gov/pubmed/21353476
Children who are raised on strict vegan diets do not grow normally:
https://www.ncbi.nlm.nih.gov/pubmed/4067152
https://www.ncbi.nlm.nih.gov/pubmed/8005079
Children develop rickets after prolonged periods of strict vegetarian diets:
http://www.ncbi.nlm.nih.gov/…/pdf/canmedaj01383-0052.pdf
“There are some links between vegetarians and lower birthweight and earlier labour”
https://www.ncbi.nlm.nih.gov/pubmed/7788369
Effects of vitamin B12 and folate deficiency on brain development in children:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3137939/
“Particular attention should be paid to adequate protein intake and sources of essential fatty acids, iron, zinc, calcium, and vitamins B12 and D. Supplementation may be required in cases of strict vegetarian diets with no intake of any animal products.”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912628/
These next five are case studies:
Cerebral atrophy in a vitamin B12-deficient infant of a vegetarian mother:
https://www.ncbi.nlm.nih.gov/pubmed/25076673
Severe megaloblastic anemia in child breast fed by a vegetarian mother:
https://www.ncbi.nlm.nih.gov/pubmed/8442642
Consequences of exclusive breast-feeding in vegan mother newborn - case report:
https://www.ncbi.nlm.nih.gov/pubmed/19748244
Nutritional vitamin B12 deficiency in a breast-fed infant of a vegan-diet mother:
https://www.ncbi.nlm.nih.gov/pubmed/3948463
“We report the case of a 7 month-old girl that presented with acute anemia, generalized muscular hypotonia and failure to thrive. Laboratory evaluation revealed cobalamin deficiency, due to a vegan diet of the mother.”
https://www.ncbi.nlm.nih.gov/pubmed/18293883
Most recent studies using more sensitive techniques for detecting B12 deficiency have found that 68% of vegetarians and 83% of vegans are B12 deficient, compared to just 5% of omnivores. https://www.ncbi.nlm.nih.gov/pubmed/12816782
https://www.ncbi.nlm.nih.gov/pubmed/10966896
https://www.ncbi.nlm.nih.gov/pubmed/10552882
On paper, calcium intake is similar in vegetarians and omnivores (probably because both eat dairy products), but is much lower in vegans, who are often deficient.
https://www.ncbi.nlm.nih.gov/pubmed/21139125
http://ajcn.nutrition.org/content/70/3/543s.full
Vegetarians and omnivores have similar levels of serum iron, but levels of ferritin—the long-term storage form of iron—are lower in vegetarians than in omnivores.
https://www.ncbi.nlm.nih.gov/pubmed/24871479
Fruits and Vegetables
https://www.ncbi.nlm.nih.gov/pubmed/12064344
This is significant, because ferritin depletion is the first stage of iron deficiency. Moreover, although vegetarians often have similar iron intakes to omnivores on paper, it is more common for vegetarians (and particularly vegans) to be iron deficient. For example, this study of 75 vegan women in Germany found that 40% of them were iron deficient, despite average iron intakes that were above the recommended daily allowance.
https://www.ncbi.nlm.nih.gov/pubmed/14988640
http://ajcn.nutrition.org/content/78/3/633S.long
Many plant foods that contain zinc also contain phytate, which inhibits zinc absorption. Vegetarian diets tend to reduce zinc absorption by about 35% compared with omnivorous diet.
http://ajcn.nutrition.org/content/78/3/633S.long
Thus, even when the diet meets or exceeds the RDA for zinc, deficiency may still occur. One study suggested that vegetarians may require up to 50% more zinc than omnivores for this reason.
http://ajcn.nutrition.org/content/78/3/633S.long
The Naive Vegetarian
http://www.second-opinions.co.uk/vegetarian.html…
Soy decreases your testosterone
https://www.ncbi.nlm.nih.gov/pubmed/15735098
https://www.ncbi.nlm.nih.gov/labs/articles/10798211/