Limited exposure to sunlight and impaired intestinal absorption are the main causes of vitamin D3 deficiency, according to a Mayo Clinic Proceedings article on PubMed Central. Those whose diets lack foods with vitamin D3 or supplements are also at risk of deficiency, explains WebMD.
Individuals who have darker skin or who live in higher latitudes are more likely to suffer from vitamin D3 deficiency, notes Healthline. Melanin makes it harder for the skin to absorb vitamin D3, while people in higher latitudes have limited exposure to the sun. Digestive diseases such as celiac disease and Crohn's disease also lead to vitamin D3 deficiency because they make it difficult for the body to absorb this vitamin.