Vitamin D is one of those nutrients that touches practically every inch of your body, from muscles and bones to nerves and the immune system. If you want to run strong, keeping an eye on your intake is essential.

As a runner, adequate vitamin D lowers your risk for stress fractures and bone breakdown, per a research review in Nutrients. “The main function of vitamin D is to help calcium and phosphorus maintain bone health,” explains Dr Alex McDonald, a family medicine physician who specialises in sports medicine.

But vitamin D plays an important role in other processes, too. While technically a fat-soluble vitamin, vitamin D also acts as a hormone, “meaning it sends signals for reactions to occur and influences other hormones,” says Kelly Jones, board-certified sports dietitian who works with athletes and active families. In fact, your muscles need vitamin D to move, your nerves to carry messages between your brain and body, and your immune system to fight off invading bacteria and viruses, per the National Institutes of Health (NIH).

Unfortunately, many people struggle to snag enough of the “sunshine vitamin” (so named because you can obtain it through sun exposure) — runners included.

“Runners may think that because they spend a lot of time outside, they’re getting [the vitamin D] they need from the sun,” Jones says. “However, we’ve seen in research studies on those who exercise outside, and I’ve seen first-hand in my outdoor sport athletes who have regular blood work done, that many will still have vitamin D levels that are inadequate or even clinically deficient.”

The only way to know if you’re getting enough vitamin D is to visit your doctor for a blood test. But you can also watch for these symptoms of vitamin D deficiency, and use the following strategies to ensure you’re meeting your needs.

What are the symptoms of vitamin D deficiency?
It can be challenging to tell if you’re deficient in vitamin D because the symptoms are often non-specific or nonexistent, McDonald says. When symptoms do appear, they can include, per McDonald:

• Fatigue or feeling run-down
• Muscle pain and weakness or muscle cramps
• Tingling hands or feet
• Mood changes, like depression
• Frequent illness

Because vitamin D is involved in muscle movement and helping nerves carry messages throughout the body, a shortage of this nutrient can slow down these processes, leading to fatigue, muscle weakness, and tingling. The connection between vitamin D and mood isn't well understood, but research suggests that vitamin D helps the brain release substances needed for nerve cells to grow and travel. These substances, including brain-derived neurotrophic factor (BDNF), have been linked to lower risk of depression.

In more severe cases, vitamin D deficiency causes bone disease — rickets in children and osteomalacia in adults, Jones says. Both diseases cause the bones to soften and weaken due to inadequate vitamin D, which helps your bones absorb calcium and phosphorus from food.

Bone stress fractures can be a sign that low vitamin D levels are impacting bone health, per a review published in March 2021 in the journal Medicina (Kaunas). “If someone’s gone far enough to develop stress fractures, they should get a comprehensive metabolic work-up to make sure there’s nothing going on with calcium and vitamin D,” McDonald says.

How much vitamin D do you need?
The recommended daily allowance of vitamin D for all adults is 15 micrograms (mcg) or 600 international units (IU), according to the NIH. Meanwhile, the Endocrine Society recommends 1,500 to 2,000 IU/day for individuals not getting adequate sun exposure to keep concentrations in the sufficient range.

However, work with your doctor or a registered dietitian to find the appropriate dose for you.

Who should be most concerned about vitamin D levels?
Considering that sunlight is one of the significant sources of vitamin D, anyone who spends a lot of time indoors, wears sunscreen or clothing that covers a lot of skin, or lives in an area with air pollution or in a colder climate further from the equator has a greater risk for vitamin D deficiency than those with more sun exposure, Jones says. In addition, people with darker skin don’t absorb sunlight as well as people with lighter skin, which can influence vitamin D production.

Older adults are also at higher risk because the skin’s ability to make vitamin D declines with age, notes the NIH.

As a runner, you may think you’re safe from vitamin D deficiency, especially if you often run outdoors and prioritise a balanced diet. But research shows that vitamin D deficiency is common in endurance athletes.

One reason athletes tend to be deficient is that they tend to use up their vitamin D stores faster than non-athletes. “Because of the ongoing stress from running and training, there’s a lot of turnover of bone absorption and reconstruction, as well as muscle breakdown and reconstruction, so there’s a higher metabolic turnover and need to maintain adequate vitamin D levels,” McDonald explains.

Certain medical conditions can make it difficult to maintain vitamin D stores. For example, gastrointestinal (GI) diseases, such as inflammatory bowel disease and celiac disease, can limit how much vitamin D you’re able to absorb from food, McDonald says. Medications such as laxatives, steroids, statins for lowering cholesterol, and phenobarbital for seizure prevention can also lower vitamin D levels.

What are the best sources of vitamin D?
*Vitamin D numbers per serving size, according to the NIH

• 30 grams cooked salmon: 14.2 mcg or 570 IU
• 1 cup fortified milk (2% fat): 2.9 mcg or 120 IU
• 1 tbsp cod liver oil: 34 mcg or 1,360 IU
• 1 serving fortified breakfast cereal: 2 mcg or 80 IU
• 1 large scrambled egg: 1.1 mcg or 44 IU
• ½ cup sliced white mushrooms: 9.2 mcg or 366 IU
• 30 grams cooked rainbow trout: 16.2 mcg or 645 IU

Should you consider taking a vitamin D supplement?
Typically, it’s best to get nutrients from whole foods instead of taking dietary supplements. Vitamin D may be the exception.

“Very few foods are considered a good source of vitamin D,” Jones says. Fatty fish like salmon and trout are among the best sources, while egg yolks, mushrooms, cheese, beef liver, and fortified breakfast cereals and dairy products provide small amounts. It can be challenging to get enough vitamin D from such a short list of foods.

Moreover, many people struggle to get vitamin D from sunlight, especially those who live in colder climates further from the equator, have darker skin, or live in areas with air pollution. And, given that ultraviolet radiation from sunlight can cause skin cancer, it can be risky to spend prolonged periods outdoors without sunscreen, notes the NIH.

Vitamin D supplements are critical to maintain healthy levels of this essential nutrient, Jones says.

Still, McDonald recommends getting your vitamin D levels checked by your doctor before taking a supplement. This way, you can get a clear picture of how much vitamin D you need to maintain adequate levels, which can help prevent over-supplementation. “Because vitamin D is fat-soluble, you can actually build up too much, and your body has trouble getting rid of it,” McDonald explains.

While uncommon, too much vitamin D can cause nausea, vomiting, muscle weakness, pain, dehydration, and kidney stones, per the NIH.

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