As winter approaches, it is harder to get vitamin D naturally through exposure to sunlight. While the “sunshine vitamin” has been linked to better outcomes from COVID-19, it is also important for other health reasons. Unfortunately, more than 40% of people in the U.S. are suffering from vitamin D deficiency.

According to Parade, the most common sign of deficiency is fatigue. “Fatigue, most commonly defined by people visiting their doctor as tiredness or a state of ‘low energy’ is very common, impacting one-third of people in the U.S.,” says Dr. Jason Prescott, an endocrinologist at NYU Langone Hospital in Brooklyn.

Prescott advises people who feel tired all the time to visit their health care professional and have a blood test called a 25-hydroxy vitamin D test. Levels of vitamin D should be between 30-80 ng/mL. Anything below 30 ng/mL is considered deficient and below 20 ng/mL is severe deficiency.

“Multiple studies have shown a relationship between low body vitamin D levels and fatigue, with some reports identifying fatigue in 75% or more of people who also have low vitamin D levels.”

Other signs to look for include:

• Physical weakness. A study published in the Western Journal of Medicine found that vitamin D deficiency can cause muscles weakness. Fortunately, this condition is reversible.

• Bone loss. Vitamin D plays an important role in calcium absorption and bone metabolism, says Healthline. Having adequate levels of vitamin D is especially important for seniors suffering from bone loss and increased risk of fractures.

• Hair loss. Stress can trigger hair loss, but low levels of vitamin D can also be the culprit, especially in women.

• Depression. Extensive research has found a link between vitamin D and depression, bipolar disorder, and other mental health issues. Canadian scientists reviewed 14 studies on the topic and found the lower the level of vitamin D, the greater the chance of depression, especially in older adults. Experts advise taking supplements or increasing your sun exposure. Foods rich in this important nutrient include fatty fish, like salmon or sardines, or fortified dairy products, according to Healthline.

Experts recommend taking vitamin D3 supplements if you can’t get enough sunlight to create vitamin D in your skin. And they warn that too much exposure can cause skin cancer, so it’s best to keep your sun exposure moderate, says Prescott.

Dr. Annette Faller, an internal medicine expert at the Mayo Clinic Health system in Wisconsin says she has been encouraging her patients to take D3 supplements since the pandemic and suggests 4,000 IU daily to combat fatigue, boost immunity, and help stave off illnesses.