What can cause low vit d levels other than lack of sunlight and bad nutrition?

Several causes: Some people do not absorb enough vit d from intestines because of bowel abnormality called malabsorption(celiac disease and cystic fibrosis). The major cause is lack of sunlight which assists in vit d production. Because of all the protection of skin against sunlight to prevent skin cancer, people have become too protected. Vit d supplements are available, but 15 min sunlight exposure /day is good.