We're in the middle of Winter here in the Northern hemisphere, and that means more hours spent indoors and fewer daylight hours. A major nutritional concern of getting less natural sunlight is vitamin D deficiency.

Vitamin D is a fat-soluble nutrient that our bodies can synthesize with exposure to sunlight, specifically UVB rays. It’s difficult to get enough vitamin D through food alone as it’s naturally available in very few foods. Sunscreen usage has also steadily increased over time.

As a result, vitamin D deficiency is now an international public health problem that affects a billion people around the world. Learn more about getting tested and how to prevent this common deficiency that typically presents few symptoms, if any.

The dangers of deficiency

Vitamin D works in conjunction with calcium, magnesium and vitamin K2 to promote bone health. It helps your body absorb minerals like calcium and helps your bones grow and form. 

Deficiency can lead to thin, brittle bones, which presents a higher risk for fractures and osteoporosis. In children, vitamin D helps prevents rickets, a softening of bones that leads to weakness, pain and delayed development. 

Vitamin D plays other roles in the body, including regulation of cell growth, immune function and reducing inflammation. Low levels of vitamin D are linked to depression and other mood disorders. 

At-risk groups

Vitamin D deficiency impacts some people more than others. Most dietary sources of vitamin D come from fortified milk products or fatty fish, so people who avoid dairy and fish are more susceptible. Because vitamin D is fat-soluble, people who have trouble absorbing fat due to digestive conditions are also vulnerable.

Older adults have a higher deficiency risk because as we age, our bodies can’t make vitamin D as efficiently. Young people who spend too much time indoors are also at risk. 

Breastfed infants are at significant risk for developing rickets without vitamin D supplementation through droplets, according to the American Academy of Pediatrics. They recommend 400 IU daily starting shortly after birth. 

How much is enough?

The best way to check if you are deficient is to ask your doctor for a blood test to check serum concentration of 25-hydroxyvitamin D, or 25(OH)D. The Institute of Medicine says that a concentration of under 30 nmol/L is considered deficient. Deficiency usually presents none or very subtle symptoms that are easy to miss, so it’s important to get tested.

A simple way to combat deficiency is to spend about 15 minutes in the sun every day, but this is not always possible. Many people work long hours indoors or live in a place with cloudy weather. This is where vitamin D supplements come in. But how much is enough?

The IOM recommends no more than 4,000 IU of vitamin D daily. For most people, a daily dose of 1,000 IU is both safe and effective! Too much vitamin D can lead to kidney stones and trigger calcium deposits in blood vessels, which puts you at risk for blood clots. 

Experts like Dr. JoAnn Manson, professor of medicine at Harvard Medical School, warn against super-dosing vitamin D without medical supervision. She oversees a multiyear large-scale vitamin D study known as VITAL, which will yield more insights when it concludes later this year.