A deficiency in vitamin D can result from inadequate exposure to sunlight, inefficient production in the skin, insufficient dietary intake, and health conditions such as gastrointestinal disorders, renal diseases, and liver diseases. Understanding the causes, risk factors, diagnosis, and treatment options is the first step toward correcting this widespread deficiency.
What Causes Vitamin D Deficiency?
Darker Skin: Melanin absorbs UV-B radiation and reduces the skin’s capacity to produce vitamin D3 by 95–99%. Dark-skinned individuals require at least three to five times longer sun exposure to produce the same amount of vitamin D as someone with lighter skin. African-Americans have an average serum 25(OH)D level of 16 ng/mL, compared to 26 ng/mL in white Americans.
Weight: Being overweight or obese significantly increases the risk of vitamin D deficiency. A review of 23 studies showed that obese subjects had 35% higher rates of deficiency compared to normal weight subjects. One study found that 24 hours after similar sun exposure, there was 57% less vitamin D in the blood of obese subjects compared to non-obese subjects—due to differences in the release of vitamin D from skin into the circulation.
Limited Sun Exposure: Sunscreen with SPF 30 decreases vitamin D synthesis in the skin by more than 95%. Most adults work indoors and expose only 10–15% of their bodies to UV light for short periods, which is insufficient to meet daily Vitamin D needs. Even when outdoors, total vitamin D production is affected by the season, time of day, ozone levels, latitude, and cloud cover.
Malabsorption: People with fat malabsorption syndromes such as Crohn’s disease or celiac disease, and those who have had bariatric surgery, are often unable to absorb enough fat-soluble Vitamin D.
Age: The body’s ability to synthesize vitamin D from sunlight decreases with age—by as much as 25% in adults over 70. In children, studies have found that as body weight increases, vitamin D levels decrease correspondingly.
Medications: Antifungal medications, anticonvulsants, glucocorticoids, and AIDS/HIV medications can enhance the breakdown of vitamin D and lead to low levels.
Who Is at Risk?
The Endocrine Society recommends screening and treatment for individuals at risk, including older adults with a history of falls or non-traumatic fractures, obese children and adults (BMI above 30), African-American and Hispanic children and adults, pregnant and lactating women, and people with musculoskeletal diseases, chronic kidney disease, hepatic failure, or malabsorption syndromes.
How Is Vitamin D Deficiency Diagnosed?
The preferred blood test is 25-hydroxyvitamin D (written as 25(OH)D)—the most accurate measure of current vitamin D status, reflecting intake from diet, supplements, and sunlight. The other test, 1,25(OH)₂D, is not a reliable measure due to its short half-life and influence by other hormones.
IOM Guidelines: Deficient: below 12 ng/mL | Insufficient: 12–20 ng/mL | Sufficient: above 20 ng/mL
Endocrine Society Guidelines: Deficient: below 20 ng/mL | Insufficient: 21–29 ng/mL | Sufficient: 30–60 ng/mL | Ideal: 40–60 ng/mL | Considered safe: below 100 ng/mL
The largest meta-analysis covering published studies from 1966 to 2013 found that the lowest mortality risk is seen when 25(OH)D levels are above 40 ng/mL—suggesting that IOM guidelines may be too conservative.
Treatment for Vitamin D Deficiency
Vitamin D3 is the preferred supplement form, as it raises blood levels more effectively than vitamin D2. Supplements should always be taken with a meal containing fat—studies show this increases absorption by an average of 32%, and taking supplements with the largest meal of the day raised blood levels by an average of 56.7% over three months.
The Endocrine Society recommends the following treatment protocols:
Children (1–18 years): 2,000 IU/day of vitamin D3 for at least six weeks, or 50,000 IU once a week for at least six weeks, followed by maintenance therapy of 600–1,000 IU/day.
Adults: 50,000 IU of vitamin D3 once a week for eight weeks, or 6,000 IU daily, to achieve a blood level above 30 ng/mL—followed by maintenance therapy of 1,500–2,000 IU/day.
Obese patients or those with malabsorption: Higher doses of 6,000–10,000 IU/day are
