9 Signs You Might Be Vitamin D Deficient

You’re Suffering from Stress Fractures

This is the only true symptom of low vitamin D, says Adrian Gombart, Ph.D., an associate professor in the Linus Pauling Institute at Oregon State University at Corvallis who studies vitamin D extensively. Vitamin D helps the body absorb calcium (the building blocks of bones) from food.

“If you have less than 20 micrograms per milliliter of vitamin D in your blood, you don’t have enough D to transport calcium into your bones.” The result? Weaker bones, which can increase your risk of injury, including stress fractures.

You Keep Getting Sick

Vitamin D helps maintain the immune system, and low levels may set you up for more sick days. Findings from systematic review and meta-analysis in the International Journal of Environmental Research and Public Health suggest that vitamin D supplementation in those with a deficiency can lower both the risk of respiratory tract infections and their severity.

On top of that, people with low levels of vitamin D are more likely to develop pneumonia than those with higher levels, according to a meta-analysis in the journal Medicine. So if that cough keeps coming back and you can’t figure out why—you wash your hands thoroughly, get enough sleep, eat a balanced diet, and so on—it might be due to a vitamin D deficiency. Talk to your doc.

Your Hair is Thinning

Are you noticing excess hair shedding? Studies link vitamin D deficiency with hair loss, as vitamin D plays a role in the hair cycle and the signaling pathways of hair growth. Of course, there can be many reasons for hair loss, including genetics, but it may be worth checking your vitamin D levels to see if that might be a factor in the hair change and to help prevent further hair loss.

You’re Moody

Low vitamin D levels are associated with increased symptoms of depression and anxiety, according to research in Current Nutrition Reports.

The vitamin regulates the conversion of the amino acid tryptophan into serotonin—you know, the neurotransmitter that regulates mood. So if you can’t shake your sad feelings, know that those lows could be a sign of vitamin D deficiency.

You Have Muscle Pain or Weakness During Workouts

Vitamin D is needed for muscle fibers to develop and grow, which explains why a review in the journal Nutrients found that a vitamin D deficiency may affect muscle function and athletic performance. If you experience more muscle pain or muscle weakness during your regular workouts and exercise just suddenly seems harder, it may be worth a vitamin D check.

(BTW, here’s how to build muscle the *right* way, even if you’re not struggling with low vitamin D symptoms.)

You Feel Run Down

Fatigue is a common symptom of many illness and deficiencies, including low vitamin D. If your exhaustion and low energy levels are due to a lack of vitamin D, the good news is that studies show that supplementation may help resolve or improve fatigue.

You Live in a City

Low-light areas mean less exposure to vitamin D-producing sunlight (we’re looking at you, northerners). When studying residents in Buffalo, New York, nutrition researcher Peter Horvath of the University at Buffalo School of Public Health and Health Professions found that nearly half of people have insufficient amounts of vitamin D.

You’re Wishy-Washy

By now you know about vitamin D’s role in creating the all-important serotonin, right? Well, the neurotransmitter doesn’t just affect your mood; it also influences other cognitive functions, like decision making and impulse control, according to a study published in FASEB Journal. If your resolve is all over the place, this could be a symptom of low vitamin D.

You Don’t Get Enough Sun

First of all, sunscreen is not a bad thing. I repeat: sunscreen is not (!!) a bad thing. Buy it. Apply it. Reapply it. “But your body can’t create vitamin D from sun exposure when you wear sunscreen,” says Gombart. No need to choose between skin health and bone health: Gombart suggests talking to your doc about adding a supplement to your routine.