The main reasons for low levels of vitamin D are:

• Lack of vitamin D in the diet, often in conjunction with inadequate sun exposure
• Inability to absorb vitamin D from the intestines
• Inability to process vitamin D due to kidney or liver disease

Inadequate intake

Infants, children, and elderly adults are at risk for low vitamin D levels because of inadequate vitamin D intake. Human breast milk contains low levels of vitamin D and most infant formulas do not contain adequate vitamin D. Elderly adults often do not consume enough vitamin D rich foods, and even when they do, absorption may be limited.

Inadequate sun exposure

Parents of infants and children are often advised to keep their child out of the sun, which reduces vitamin D synthesis from the skin. Exposure to the sun is not recommended as a source of vitamin D for infants and children due to the potential long-term risks of skin cancer.

Adults who have limited sun exposure are also at increased risk of vitamin D deficiency, especially if their skin is dark. In addition, reduced amounts of vitamin D are made in the skin and stored in the body as we age. This is especially true in the winter months in some northern areas, such as Boston, Massachusetts and Edmonton, Alberta, where the skin virtually ceases to produce vitamin D between October and April. In the summer months, the use of sunscreen limits vitamin D synthesis.

Diseases or surgery that affect fat absorption

Certain diseases affect the body’s ability to absorb adequate amounts of vitamin D through the intestinal tract. Examples of these include celiac disease, Crohn’s disease, and cystic fibrosis.

Surgery that removes or bypasses portions of the stomach or intestines can also lead to low vitamin D levels. An example of this type of surgery is gastric bypass.

Kidney and liver disease

The liver and kidney have important enzymes that change vitamin D from the sun or food to the biologically active form of vitamin D. People with chronic kidney and liver disease are at increased risk of low active vitamin D levels because they lack these enzymes.

Less common causes of vitamin D deficiency include familial diseases that impair the enzymes in the liver or kidney that create the biologically active form of the vitamin. This results in inadequate amounts of active vitamin D.