My vitamin-d is 4 ng/ml. the reference range is more than 30 ng/ml. what are the causes of vitamin-d deficiency? is low vitamin-d related to cancer?

Can take Vit-D supp.: 19y/o Male w low vit D 4ng/ml (30-100) asks What are the causes of VitD deficiency? Is low Vit D related to cancer? Causes many: Diet esp Vegies only diet. One needs Animal base foods like Fish/Fish Oil/Egg yolk/cheese/Beef liver. Poor Digestive Absorption eg Chron's dis. Obesity; cos Fat cells absorb the Vit D from one's blood. Lack of Sunlight exposure.One Can take Vit D supplement Yes cancer re