Even if you live in sunny California, chances are pretty good that you’ll still benefit from a boost of vitamin D. About 42% of Americans are deficient in vitamin D, and people over 65 are more likely to have a deficiency. 

We spoke with Dr. KC Hayes, Professor Emeritus at Brandies University and nutrition expert, to answer our most pressing questions about who needs to take a vitamin D supplement, what to look for in a supplement, and how much vitamin D you should be taking. 

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What does vitamin D do?

Vitamin D plays a few essential roles in the body. Your bones need calcium and phosphorus to stay strong and resist diseases like osteoporosis, breaks, and fractures. Vitamin D allows your body absorb to calcium. 


It also supports muscle and nerve development and plays a role in a strong immune system. Some studies suggest that vitamin D supplementation can prevent the flu and other respiratory infections. 

Vitamin D testing

To know for sure whether you’re deficient, you can ask your doctor for a vitamin D test, which requires a blood sample. Hayes points out that “When you receive your vitamin D test results, it’s important to know which unit of measurement, ng/mL or nmol/L you’re dealing with. In the US, the most common test measures nanograms per milliliter, or ng/mL.”

Doctors still debate what qualifies as a healthy level of vitamin D. However, accoring to Hayes, “Most experts agree that a blood level below 25 nmol/L is considered deficient. Levels this low are associated with bone diseases like rickets or osteomalacia.”

“Some experts argue that 25-30 nmol/L in the blood is sufficient, some say a level over 50 nmol/L is optimal for good bone health for most people, while others again advocate for 75 nmol/L or even higher,” says Hayes.