What actually causes vitamin D deficiency?

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Nearly a quarter of Americans are estimated to have a vitamin D deficiency. Why is that?
A 2022 study of National Health and Nutrition Examination Surveys (NHANES) data estimated that about 22% of Americans have a moderate vitamin D deficiency. That number was higher in women, African Americans, people between the ages of 20 and 29 and all people in general during the winter months.
The body absorbs vitamin D through diet (both food and supplements) and from sunlight via the skin. It's essential for "bone health, immune function, muscle strength, mental health and potentially cardiovascular health," Washington, D.C.-based dietitian Caroline Thomason, R.D., previously told USA TODAY.
How can you tell if you actually need help getting more vitamin D? Here's what health experts say.
It depends on their age. The recommended dietary allowance, as dictated by the National Institutes of Health's Office of Dietary Supplements, is 10 mcg (400 IU) for infants up to 12 months, 15 mcg (600 IU) for ages 1 to 70 and 20 mcg (800 IU) for those older than 70.
For reference, a 3-ounce portion of rainbow trout contains about 16 mcg of vitamin D, while a cup of vitamin D-fortified milk contains about 3 mcg, per WebMD. Other foods high in vitamin D include fortified cereals (around 2 mcg) and orange juice (2.5 mcg).It's often difficult to get a full day's worth of vitamin D from diet alone, which is why getting outside – and sometimes adding supplements when medical professionals suggest it – is important, too.
In the spring and summer when about 25% of the body is normally exposed to the sun, it takes about 8 to 10 minutes of sun exposure to produce the recommended amount of vitamin D, according to UCLA Health. And if you need more, many over-the-counter vitamin D supplements range from 25 to 125 mcg (1,000 to 5,000 IU). You shouldn't take more than 100 mcg (4,000 IU) of vitamin D in a day unless a doctor recommends you do so to combat a deficiency, Thomason says.
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Vitamin D deficiency can result for a number of reasons, from not spending enough time outside to having certain health conditions to having a darker skin pigment, according to Yale Medicine.
"Many Americans are deficient in vitamin D due to limited sun exposure from indoor lifestyles, sunscreen use and our northern latitude," Thomason says. "Plus in Northern America, our typical diet is low in vitamin D-rich foods like fatty fish and fortified products."
Health conditions that make it difficult for bodies to absorb vitamin D include inflammatory bowel disease, celiac disease, pancreas conditions, cystic fibrosis, kidney or liver failure and certain cancers including lymphoma, per Yale. People who have had sections of the small intestine removed, or those who have had bariatric weight loss surgery may have similar issues, too.
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Potential signs of vitamin D deficiency can range from "subtle to severe," Thomason says. They can include "muscle weakness, fatigue, mood changes, frequent infections, hair loss, bone pain and slow wound healing."
If you're experiencing any of those symptoms, experts say it's best to check in with your primary care physician.
"The most reliable way to check vitamin D levels is a blood test, and consultation with a healthcare provider is recommended for personalized advice," Thomason says.
This article originally appeared on USA TODAY: What causes vitamin D deficiency? Health experts explain.