Doctors have recently found that the majority of those who live in cool climates- whether omnivore, vegetarian or vegan - are vitamin D deficient. All this is in spite of the fact that some fish contain vitamin D and milk, cheese, and ice cream are fortified with Vitamin D for those who follow an omnivore or vegetarian diet. For vegans rules are the same. People who do not get adequate sun exposure of Vitamin D fortified foods are at the risk of developing Vitamin D deficiency unless they supplement.

Vitamin D deficiency classically causes rickets and osteomalacia (thin bones) and is more common in children who are vitamin D deficient. Prolonged breastfeeding is a cause of rickets in some children who don't get enough vitamin D in their breast milk. Excessive use of sunscreens can cause vitamin D deficiency. In adults, vitamin D deficiency can cause bone weakness and boney pain.

Those who are especially encouraged to supplement with vegan vitamin D include those greater than age 50 that are at increased risk of developing vitamin D deficiency. People with limited sun exposure should also consider supplementation, especially if they are homebound and live in Northern Latitudes. People with dark skin don't get enough Vitamin D in their system. Those with fart malasorption need to be supplemented with vitamin D and other fat soluble vitamins. Those who are obese and have a BMI greater than 30 often are vitamin D deficient and should supplement with vitamin D.

Vitamin D is used to support those with osteoporosis, those with fragile bones and an increased risk of bone fractures. Insufficient vitamin D can contribute to osteoporosis by reducing calcium absorption.