Too many people are earning an F on their vitamin D.

A study published in Nutrition Research found that 41.6% of American adults are deficient in it, and that number climbs even higher for certain groups — 82.1% for Black people, 69.2% for Hispanics.

Vitamin D is essential for your body to function, and not getting enough of it can cause fatigue, mood issues like depression or anxiety, and impaired immunity. Long term, you’re looking at problems with bone health, muscle function and susceptibility to chronic diseases.

So how do we fix it? Some people make the case for more SPF-free time in the sun, skin cancer risks be damned. Many doctors recommend supplements. There are food sources, but it’s difficult to get enough from them.

Now, a new company is touting the “health benefits of personalized sunlight,” saying their FDA-cleared device delivers just UVB wavelengths — not UVA, which make up 95% of the sun’s UV rays — directly to the skin.

Why we need vitamin D in the first place — and how much we should get

Vitamin D helps “the body to absorb calcium and phosphorus, both of which are essential for building and maintaining strong bones and teeth,” family medicine doctor Dr. Brynna Connor, a Healthcare Ambassador at NorthWestPharmacy.com, told The Post.

Getting enough becomes “even more important as we age, helping to ward off health issues such as osteopenia and osteoporosis, rickets and overall weakness.” Connor also points to benefits for the immune system, muscle and nerve function, and reduction of inflammation.

Most adults need 600 to 800 IU daily. If you’re hoping to get that directly from sunlight, though, it’s hard to tell how much sun you need.

“It depends on your skin tone, how much surface area is exposed, the time of day, the season, where you live, and more,” Dr. Geeta Yadav, a dermatologist and founder of FACET Dermatology told The Post.

A new way to dose sunlight

Those different needs are among the problems that Solius Labs hopes to solve with their Solius Pro, a light therapy panel that’s used for about five minutes, once a week.

The system measures the user’s skin tone before each session to dose the appropriate amount of UVB, which is what stimulates vitamin D production in our bodies.

“It provides a way to access the biological health benefits associated with sunlight while dramatically reducing unnecessary UV exposure,” explained CEO Chris Kiple.

There are two kinds of UV exposure: UVA and UVB. While both contribute to the development of skin cancer, UVA is responsible for things like premature skin aging and wrinkles, while UVB is to blame for sunburns.

UVA has nothing to do with vitamin D production, though — that’s all UVB. So by isolating UVB light in the Solius Pro, the company says it can give “controlled, consistent and personalized” UV exposure to help the body make more vitamin D while cutting down on some of the risks.

They also say they’ve got a one-up on supplements, because pills have to be digested, while light acts directly on the skin.

The device (and a lifetime membership) costs $2995 for a limited time. After, there will be a $10 monthly membership, or $100 annual charge.

Is it safe?

Besides calibrating the appropriate dosage, the device won’t work if you’re not wearing protective eyewear.

It also “continuously monitors positioning and distance,” Kiple said, and users are prompted to provide feedback on how their skin responded after each session — so future doses can be automatically adjusted.

And you can’t overuse it: Once a session is completed, the system locks out for a time.

“All of these safeguards were validated through multiple third-party human clinical studies, where Solius demonstrated safe delivery 100% of the time of personalized UVB light therapy without erythema, skin irritation or adverse events,” Kiple said.

These measures made it possible for the FDA to create an entirely new product code (SGZ) for Solius. The device is cleared by the FDA for over-the-counter use in people 22 and older.

So… do you need one?

This UVB tech isn’t actually new — but the fact that it’s being touted as a vitamin D booster (and that you can get it over the counter) is.

Solius reports that users saw a 76% increase in vitamin D on average, but that’s “only part of the story.” They also say the treatment can improve metabolism, immune function, cardio health and mood.

Dermatologist we spoke to aren’t totally sure.

“We’ve used narrowband UVB phototherapy for years to treat conditions like psoriasis and eczema, so the underlying technology is legitimate and well understood,” Yadav said. “The question is whether it’s the right tool for vitamin D, and for most healthy people, it isn’t the one I’d reach for first.”

UVB directly damages skin cell DNA, Yadav said. And when treating skin disease, it’s used under supervision, “not as a wellness tool or a vitamin D booster … This device is borrowing the credibility of medical phototherapy for a very different and far less established goal.”

Dr. Viktoryia Kazlouskaya, a dermatologist and an owner of Dermatology Circle PLLC clinic, agrees.

“For most healthy people, I don’t think a specialized UVB panel is necessary,” she said. “My broader concern is that we’re increasingly turning normal aspects of healthy living into medical devices … We should remember that UV radiation is still UV radiation, and no ultraviolet exposure is completely risk-free.”

Yadav says she’d suggest vitamin D supplements, which don’t have any risk of UVB exposure. Solius’ counterpoint is that a swallowed supplement is processed by the liver and kidneys and can be met with “malabsorption challenges.”

Connor, meanwhile, recommends getting vitamin D from whole foods: “The best sources of vitamin D from food include fatty fish, such as salmon, trout, and tuna, egg yolks, and beef liver.”

If you want to go the supplement route, consult your doctor, which may include a blood test to check vitamin D levels, she added.