Nutrient Issues
B-12, B-6, Red/White Blood cells
(1982) Dong & Scott record that 92% of vegans, 64% of lactovegetarians, 47% of lacto-ovovegetarians, and 20% of semivegetarians are deficient in vitamin-B12. Their blood counts are similar to non-vegetarians, but their blood quality is worse on average. Most had mean corpuscular volume just above 100 femtolitres, indicating slight macrocytosis.
http://ajcn.nutrition.org/content/78/1/131.long
(2003) Herrmann et al. show that omnivores, lactovegetarians or lactoovovegetarians (LV-LOV), and vegans have differing B12 levels.
11% of omnivores were found to have low holotranscobalamin II, compared to 77% of LV-LOV and 92% of vegans. 5% of omnivores have elevated methylmalonic acid levels, compared to 68% of LV-LOV, and 83% of vegans. Hyperhomocysteinemia was founded in 16% of omnivores, 38% of LV-LOV, and 67% of vegans.
In all, the vast majority of both diet-restricting groups were B12 deficient with significant and noticeable side-effects.
B12 deficiency can lead to weakness, numbness, neuropathy, and heart disease.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3875920/
(2013) Gröber, Sisters & Schmidt show that B12 deficiency expresses itself by "a wide variety of neurological manifestations such as paraesthesias, skin numbness, coordination disorders and reduced nerve conduction velocity," and, "progressive brain atrophy," alongside, "an increased risk of dementia, notably Alzheimer’s disease."
ttps://www.ncbi.nlm.nih.gov/pubmed/26502280
(2015) Schüpbach et al. show that 58% of vegetarians tend to be vitamin-B6 deficient, 34% are niacin deficient, and 47% of vegans are zinc deficient.
https://www.ncbi.nlm.nih.gov/pubmed/1797957
(1991) Vudhivai et al. conclude that 29% of vegetarians are deficient in vitamin-B6 and should be classified as a vulnerable group for vitamin-B6 deficiency. A quarter are deficient in vitamin-B2 as well.
https://www.ncbi.nlm.nih.gov/pubmed/7852684
(1995) Janelle & Barr show that vegetarians have lower intake of vitamin-B2.
(2016) Richter et al. show that vegans tend to be deficient in protein, especially indispensable amino acids, long-chain n-3 fatty acids, other vitamins (riboflavin, vitamin D) and minerals (calcium, iron, iodine, zinc and selenium), and do not recommend a vegan diet for pregnant or lactating women, or infants, children, or adolescents.
They recommend that if someone wishes to pursue a vegan diet that they permanently supplement for key nutrients like B12. A significant portion of them are deficient in as diverse a set of nutrients as B2 to B6 to B3 to B12 and so on.