Vitamin D Deficiency

Vitamin D deficiency is among the most widespread nutritional shortfalls in the world. In the United States alone, nearly 42% of adults are deficient, with prevalence climbing to approximately 63% among Hispanic adults and 82% among African American adults. Vitamin D supports the body's ability to absorb calcium (essential for bone strength), and it plays additional roles in nervous system function, muscle performance, and immune regulation.

Deficiency develops when the body does not have enough vitamin D to carry out these functions effectively, which can lead to weakened bones, muscle discomfort, and increased susceptibility to illness. The majority of people with vitamin D deficiency experience no obvious symptoms, and because routine screening is not standard practice, many individuals remain deficient without realizing it. Certain groups face higher risk, including older adults, people with limited sun exposure, those with darker skin pigmentation, and individuals on restrictive diets.

Vitamin D deficiency results primarily from insufficient intake through diet, inadequate sun exposure, or the body's inability to properly absorb or utilize the vitamin. The skin synthesizes vitamin D when exposed to ultraviolet B radiation from sunlight, and spending 15 to 20 minutes daily in sunlight with roughly 40% of the skin surface exposed can support adequate production. However, many factors interfere with this process.

People who work indoors, wear clothing that covers most of the body, apply sunscreen consistently, have darker skin, carry excess body weight, or are older produce less vitamin D through sun exposure. Aging reduces the skin's capacity to synthesize the vitamin efficiently. Medical conditions can also increase risk, including kidney or liver disease, digestive disorders that impair fat absorption, and certain medications. Individuals who are lactose intolerant, have a milk allergy, or follow vegan or ovo-vegetarian diets may not obtain sufficient vitamin D from food sources alone. Conditions that impair the kidney's ability to convert 25(OH)D to its active form, or that reduce intestinal absorption, can also contribute to deficiency.

Routine vitamin D screening is not standard practice, but testing may be recommended for individuals with specific risk factors, symptoms, or medical conditions that increase the likelihood of deficiency. Testing is particularly beneficial for those experiencing unexplained bone pain, muscle weakness, frequent infections, or persistent fatigue. Individuals over age 65 face elevated risk because both skin production and intestinal absorption of vitamin D decline with aging; those carrying excess body weight are also at increased risk.

Other situations where testing may be warranted include a family history of osteoporosis, following a diet with limited vitamin D-rich foods, spending the majority of time indoors, or taking medications that may interfere with vitamin D metabolism. People with chronic conditions such as kidney disease, liver disorders, or digestive diseases that affect fat absorption may benefit from periodic monitoring of their vitamin D status.

Vitamin D status is assessed by measuring 25-hydroxyvitamin D [25(OH)D] in the blood. Levels below 20 ng/mL are widely considered deficient, while levels between 21 and 29 ng/mL are classified as insufficient. The goal for most individuals is to maintain levels above 30 ng/mL, though expert recommendations for optimal ranges vary: some guidelines target 20 to 40 ng/mL (50 to 100 nmol/L), while others recommend 30 to 50 ng/mL (75 to 125 nmol/L).

Low results may prompt healthcare providers to recommend increased sun exposure, dietary modifications, or supplementation. If calcium levels are also depressed, it may indicate that the deficiency has progressed to the point of disrupting mineral balance, which has implications for bone health. Levels above the normal range are typically the result of excessive supplementation and may require dosage adjustment. A healthcare provider can interpret results within the context of individual health factors and recommend an appropriate course of action.

Maintaining adequate vitamin D involves balancing safe sun exposure, dietary choices, and, when necessary, supplementation. The body produces vitamin D naturally when the skin is exposed to sunlight, though excessive sun exposure carries risks of skin damage and skin cancer, leading many people to rely on alternative sources. Foods that naturally contain vitamin D include fatty fish (salmon, mackerel, sardines), egg yolks, and liver, while many dairy products, cereals, and plant milks are fortified with the vitamin.

Regular weight-bearing exercise supports bone health and works synergistically with adequate vitamin D levels to maintain skeletal strength. For individuals at higher risk of deficiency, collaborating with a healthcare provider to develop an appropriate supplementation plan can help ensure levels stay within the target range. Once deficiency is identified and treated according to provider recommendations, most people are able to increase their intake to an adequate level, which typically resolves associated symptoms and reduces long-term health risks.