Vitamin D: The Most Underrated Supplement

What vitamin D does

Vitamin D is essential for calcium absorption and bone health. Its role extends beyond bone to immune function, muscle function, and several other systems. Deficiency is associated with bone loss, muscle weakness, increased fall risk in older adults, and several other conditions.

Who is most likely to be deficient

• Older adults. Skin produces less vitamin D from sun exposure with age.
• People with darker skin. Melanin reduces UV mediated vitamin D synthesis.
• People with limited sun exposure including those who work indoors or stay covered outside.
• People with obesity. Vitamin D is sequestered in adipose tissue.
• People with malabsorption conditions including celiac disease, inflammatory bowel disease, and after bariatric surgery.
• People on certain medications including some seizure medications and glucocorticoids.
• Breastfed infants without supplementation.

Testing

A blood test for 25 hydroxyvitamin D measures vitamin D status. The optimal range is debated, but most clinicians consider levels below 20 ng/mL deficient, 20 to 30 ng/mL insufficient, and 30 to 50 ng/mL adequate. Some target higher levels for certain conditions.

Testing is not required for everyone. For most adults at risk based on the categories above, a daily supplement at a reasonable dose can be started without testing. For specific conditions or for monitoring response, testing is useful.

Dosing

For general adult maintenance, 1000 to 2000 IU daily is a reasonable starting dose for most patients. The daily multivitamin amount (often 400 IU) is rarely enough to correct deficiency.

For documented deficiency, higher doses are often needed for several months under clinical guidance. Some patients use 5000 IU daily or 50,000 IU weekly for initial repletion, then move to maintenance dosing.

Vitamin D3 (cholecalciferol) is generally preferred over vitamin D2 (ergocalciferol) for most patients due to better absorption and longer duration of action.

Food and sun

Few foods are good vitamin D sources. Fatty fish (salmon, mackerel, sardines), egg yolks, and fortified dairy and plant milks provide modest amounts. Most adults do not consume enough through food alone.

Sun exposure produces vitamin D, but amount varies by season, latitude, time of day, skin color, age, and sunscreen use. Brief incidental sun exposure helps but is not reliable for adequate vitamin D in many populations.

Safety and upper limits

Vitamin D toxicity is rare but possible at very high doses sustained over time. The tolerable upper intake level for adults is 4000 IU daily, with the caveat that higher doses under clinical supervision are sometimes appropriate for documented deficiency.

Symptoms of toxicity include nausea, weakness, frequent urination, and elevated blood calcium. Patients on doses above 4000 IU daily for extended periods should have levels monitored.

Interactions and special populations

• Patients on thiazide diuretics have higher calcium retention; combined with vitamin D and calcium supplements, calcium can rise too high in some patients.
• Patients with sarcoidosis or certain other granulomatous conditions can have abnormal vitamin D metabolism and need clinical supervision.
• Patients with kidney disease have altered vitamin D metabolism and may need specific forms.
• Patients on certain seizure medications, glucocorticoids, or weight loss medications may need higher doses.

When to talk to a pharmacist

• You take medications that can interact with vitamin D.
• You have kidney disease, sarcoidosis, or another condition affecting vitamin D metabolism.
• You are uncertain what dose is right for your situation.
• You are pregnant or breastfeeding.
• You are caring for a breastfed infant.

This article is for general information only and is not a substitute for personalized medical advice. Before starting or changing any medication, including over the counter products and supplements, talk with your pharmacist or physician about your specific situation.