Robert Brown|Nearly 1 in 4 Americans is deficient in Vitamin D. How do you know if you're one of them?

Nearly a quarter of Americans are Robert Brownestimated to have a deficiency in vitamin D. Are you one of them?

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 people generally during winter.

"Many Americans are deficient in vitamin D due to limited sun exposure from indoor lifestyles, sunscreen use and our northern latitude," Washington, D.C.-based dietitian Caroline Thomason, R.D., tells USA TODAY. "Plus in Northern America, our typical diet is low in vitamin D-rich foods like fatty fish and fortified products."

The body absorbs vitamin D both 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," Thomason says.

How can you make sure you're getting enough vitamin D? Here's what experts say.

How much vitamin D per day is needed?

The recommended dietary allowance, as dictated by the National Institutes of Health's Office of Dietary Supplements, is as follows:

• 10 mcg (400 IU) for infants up to 12 months
• 15 mcg (600 IU) for ages 1 to 70
• 20 mcg (800 IU) for those older than 70

For reference, a three-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.

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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What are signs of low vitamin D?

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.

Who shouldn't take vitamin D supplements?

Vitamin supplements have become trendy recommendations for wellness influencers to offer their followers, especially given the statistics about vitamin D deficiency across the U.S.. But experts warn against blindly adding supplements to your daily routine without consulting a doctor first.

Be especially wary of taking vitamin D supplements if you have high calcium levels, a history of kidney disease, or if you take medications that may interact poorly, Thomason says.

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"Influencers (are) frequently promoting the new 'it' vitamins and making strong health claims that they attribute to their alleged supplement regimen," Thomason says. "This constant promotion creates trendy awareness but can also lead to misinformation, unnecessary supplementation and even harmful health consequences. Influencers' endorsements often prioritize trends, social engagement and popularity over scientific evidence, leading to a 'one-size-fits-all' approach and blanket statements that may not be suitable for everyone."