Veganism
Vegans must make sure their diet includes enough  vitamin B12, because it does not occur reliably in plant foods. Vitamin B12 deficiency can have serious bad effects on the person's health. These might include anaemia and neurodegenerative diseases. California-based dietitian nutritionist Ashley Lytwyn said “Nutrients like B12 can be tough to get with a vegan diet, and the iron in vegetables isn’t as bioavailable as iron in animal proteins.” “Even if he ate enough iron-rich vegetables, the body can’t always absorb what it needs.”  Vegan societies recommend that vegans either eat foods with added B12 or take a B12 supplement. Tempeh, seaweed, spirulina, organic produce, and intestinal bacteria do not have enough B12 for a vegan diet.  Vitamin D deficiency is possible in the absence of dairy products (which are normally fortified with vitamin D). Vitamin D deficiency can lead to rickets in children and osteomalacia in adults. In rickets, bones do not mineralize normally, causing growth retardation and skeletal deformities. It can be prevented by supplements, and by direct injections.