As far as nutrients go, vitamin D is one of the most widely useful—but oft-overlooked—things we put in our bodies. For instance, “there are more than 200 known functions of vitamin D in the body,” says osteopathic physician Ryan Greene, DO. Sure, it’s great for building your bone strength, but it’s also necessary for so many other reasons. This is precisely why you don’t want to have a vitamin D deficiency.

Neil Paulvin, DO, a New York-based longevity and regenerative medicine doctor, echoes this sentiment, saying that “vitamin D is extremely integrated into our bodies and helps it in a lot of different ways,” says Dr. Paulvin.

Below, we get into what vitamin D is, the symptoms of vitamin D deficiency, and how to provide your body with enough vitamin D for optimal function.

What is vitamin D?

Vitamin D is a fat-soluble vitamin and hormone that includes vitamins D1, D2, and D3. “It’s important to note that vitamin D behaves more like a hormone, which is a chemical made by the body that stimulates and controls functions of other parts of the body,” says internal medicine physician Neha Sangwan, MD, who is also the founder of the private practice, Intuitive Intelligence.

As Dr. Paulvin mentioned, vitamin D plays an integral role (or several) in the human body. “It helps build our immune system and is needed to help regulate our hormones,” says Dr. Paulvin. “It also helps support our bones, decreases inflammation, and regulates cholesterol formation,” he adds.

How can you tell if you have a vitamin D deficiency?

According to Dr. Sangwan, the best way to determine whether or not you have a vitamin D deficiency is to visit your primary care doctor and ask for a test called the 25-hydroxy vitamin D test. This test will tell you how much vitamin D is in your body.

Good levels of vitamin D should be anywhere between 50 to 75 nanograms per deciliter, says Dr. Sangwan. Since you’ll be getting this test at your doctor’s office, they’ll read you the results and explain whether or not your vitamin D levels are adequate.

If you’re unable to see a doctor, however, there are also a handful of symptoms that can indicate you’re deficient in vitamin D.

What are the symptoms of vitamin D deficiency?

1. Brain fog

Have you ever felt like your brain just… isn’t working? That’s what the medical community refers to as brain fog. A vitamin D deficiency can lead to this. In fact, a 2020 study found that having low vitamin D levels was associated with cognitive impairment and dementia.

2. Joint pain

Joint pain is largely due to inflammation. Because vitamin D has anti-inflammatory properties, a lack of vitamin D can cause more inflammation. This, in turn, may lead to joint pain.

3. Depression

Dr. Sangwan and Dr. Greene agree that a lack of vitamin D may also cause depression. “Low vitamin D affects serotonin synthesis in the brain, which can lead to depression and seasonal effectiveness disorder,” says Dr. Sangwan.