hello and thank you for asking HCM, I understand your concern. The most common reasons are if you don t consume enough vitamin D per food (for example if you are on a strict vegetarian diet because most of the natural sources are animal-based, including fish and fish oils, egg yolks, cheese, fortified milk, and beef liver). Limitted exposure to sun can also lead to this problem. Problems with kidneys can also lead to this. Certain medical problems, including Crohn s disease, cystic fibrosis, and celiac disease, can affect your intestine s ability to absorb vitamin D from the food you eat. I hope this answer was helpful to you and that you found a possible reasons.