Vitamin D is an extremely important vitamin that has powerful effects on several systems throughout your body ( 1 ).

Unlike other vitamins, vitamin D functions like a hormone, and every single cell in your body has a receptor for it.

Your body makes it from cholesterol when your skin is exposed to sunlight.

It’s also found in certain foods such as fatty fish and fortified dairy products, though it’s very difficult to get enough from diet alone.

The recommended daily intake (RDI) is usually around 400–800 IU, but many experts say you should get even more than that.

Vitamin D deficiency is very common. It’s estimated that about 1 billion people worldwide have low levels of the vitamin in their blood ( 2 ).

According to a 2011 study, 41.6% of adults in the US are deficient. This number goes up to 69.2% in Hispanics and 82.1% in African-Americans ( 3 ).

Here are 7 common risk factors for vitamin D deficiency:

Having dark skin.

Being elderly.

Being overweight or obese.

Not eating much fish or dairy.

Living far from the equator where there is little sun year-round.

Always using sunscreen when going out.

Staying indoors.

People who live near the equator and get frequent sun exposure are less likely to be deficient, as their skin produces enough vitamin D to satisfy their bodies’ needs.

Most people don’t realize that they’re deficient, as symptoms are generally subtle. You may not recognize them easily, even if they’re having a significant negative effect on your quality of life.

Here are 8 signs and symptoms of vitamin D deficiency.