A deficiency in any form of vitamin D can produce symptoms including bone pain, muscle weakness and cognitive decline in elderly individuals, explains WebMD. Low levels of vitamin D also place patients at an increased risk of cardiovascular disease-related death, cancer and severe asthma in the case of children. Vitamin D3, or cholecalciferol, is one major form of vitamin D, with vitamin D2, or ergocalciferol, being the other most-prominent source, notes Siemens.
Plants produce vitamin D2, while the human body synthesizes vitamin D3 when sunlight comes into contact with the skin, states Siemens. Vitamin-fortified foods, such as milk and breakfast cereals, can contain either form of the vitamin, and prescription-grade vitamin D is usually in the form of D2 in North America, as of 2015. However, there is some scientific evidence suggesting vitamin D3 is more-efficient at maintaining adequate blood levels because the binding protein that carries the vitamin to the liver is better able to bind to vitamin D3.
A vitamin-D deficiency can result due to inadequate dietary intake or low levels of sun exposure, with individuals who consume vegan diets or avoid dairy products due to lactose intolerance being especially at risk, notes the National Institutes of Health. It can also result from medical problems, such as osteomalacia and rickets.