What you need to know about vitamin D

Updated May 30, 2025

Confused about whether or not you’re getting enough vitamin D? You’re not alone.

Vitamin D research is hot these days. Vitamin D deficiency has been associated with everything from depression to multiple sclerosis and poor bone health. Because of that, many people take vitamin D supplements to make sure they getting enough, especially in the winter. Still, association does not mean causation — it’s not actually clear whether supplements are helpful. In fact, recent studies seem to bring up more questions than answers.  

We took a look at the evidence to get down to the bottom of it. Here’s what you need to know about vitamin D.

Vitamin D supports bone growth and regulates calcium, phosphate, and parathyroid hormone levels. Vitamin D can be obtained through diet and supplements. One of the things that makes vitamin D unique, though, is that it’s also produced by your skin cells when sunlight hits your exposed skin — one of the reasons why people often worry about deficiencies in winter more than summer.

Vitamin D deficiency is most strongly associated with weak bones. Extreme vitamin D deficiency causes soft bones and skeletal deformities in children (more commonly known as rickets), and can cause weak bones in adults (known as osteomalacia). Osteoporosis (low bone mass and structurally weak bones) is related to low levels of calcium and vitamin D.

Though not as well established, low vitamin D levels have also been associated with immune function and falls and frailty in the elderly, as well as chronic diseases including colon cancer, heart disease, and diabetes (remember, though, that association does not equal causation!).

Some groups of people are at higher risk of vitamin D deficiency.

• People with diabetes. Vitamin D deficiency is related to insulin resistance, and may impact the body's ability to produce insulin.
• People with obesity. Adipose (fat) tissue can store vitamin D, decreasing the amount that's available for the body to use.
• People with liver or kidney disease. Your liver and kidney play a role in processing vitamin D, so less vitamin D may be less effective in your body if these organs don't function optimally.
• People with certain gastrointestinal conditions or gastric bypass. Gastrointestinal conditions (like inflammatory bowel disease or celiac disease) and gastric bypass procedures can reduce your ability to absorb the vitamin.  
• People with osteoporosis: Osteoporosis is a sign of possible vitamin D deficiency.
• People taking certain medications. Medications like steroids, antifungal medications, antiviral medications, and some anti-seizure medications can affect Vitamin D levels.
• Insufficient sun exposure: Individuals who have minimal outdoor exposure or those who live at higher latitudes may be at higher risk for vitamin D deficiency in the winter months - in the US, this means latitudes above Seattle, Chicago, and New York City.
• Older adults: Skin does not produce vitamin D as efficiently as we age, and older adults often spend more time indoors, limiting their sun exposure.

The two ways to naturally increase your vitamin D levels are by eating certain foods and through sun exposure.

The best sources of vitamin D are fatty fish like salmon, tuna, and mackerel. Other foods with lesser amounts include beef liver, cheese, egg yolks, and some mushrooms. In the United States, vitamin D is also added to most milk, many cereals, yogurt, margarine, and orange juice.

If you’d prefer to get your vitamin D through sun exposure, it’s important to be make sure you’re balancing getting the right amount of vitamin D and staying healthy in the sun. Fortunately, it doesn’t take a lot of time to get adequate exposure — just 5 to 30 minutes between 10 a.m. and 3 p.m. can be enough. Remember that exposing your skin to direct sunlight also puts you at increased risk of developing skin cancers, so make sure you wear sunscreen when going outside.

At One Medical, we don't recommend routine screening for everyone, but it may make sense to get checked if you have a condition that makes vitamin D deficiency more likely.

So, what can you do? Talk to your provider at your regular visits about your health and any symptoms you may be having, and you can decide together whether checking your vitamin D levels is right for you.