Winter is coming, and unfortunately, we're not talking about "Game of Thrones." We're talking about shorter days, less sunlight and the associated risks of developing a vitamin D deficiency.

Dubbed "the sunshine vitamin," vitamin D sounds sort of adorable. Like maybe it gives you cute freckles or a boost of cheer; but it's actually a pretty intense nutrient that aids with the absorption of calcium and support of the musculoskeletal system. Some studies indicate that vitamin D can also help with the immune system, mood regulation, blood pressure, and even fending off diabetes and certain cancers.

But before you jump on Amazon to order a bottle of vitamin D supplements, let's consider the facts, starting with the great big ball of light in the sky: the sun.

A Little Bit Of Sun Can Go A Long Way

“Vitamin D is produced in the body when UV light from the sun hits the skin and triggers vitamin D synthesis," says Dr. Stefani Kappel, a dermatologist.

Dr. Zhaoping Li, MD, PhD, director, center for human nutrition and chief of the division of clinical nutrition at UCLA notes that if you're wear sunblock, you're also shielding yourself from vitamin D.

"Sunscreen blocks out UV rays, so your cells are not activated to vitamin D," says Li.

And sunscreen is highly recommended, even in the winter, and not just because the prospect of skin cancer is worse than that of a vitamin D deficiency, but also because the amount of UV rays you need to promote vitamin D is fairly small. In other words, a little bit of sunshine can go a long way.

“We get enough UV light entering the skin on a daily basis just walking around outside for five minutes a day," says Kappel, adding that while most of us are pretty good about wearing sunscreen on our face, "the rest of our body is often without sunscreen and [thus] getting stimulation of vitamin D synthesis."

There's More To Vitamin D Than Sunshine

While it is true that people living in regions with long, dark winters are more prone to developing vitamin D deficiencies, the sun doesn’t guarantee you the vitamin D you need.

Dr. Li for instance lives in sunny Los Angeles and was shocked to find out that she had a fairly severe vitamin D deficiency. Another Los Angeleno, Kay Wilson, says she was "diagnosed with vampire low vitamin D deficiency earlier this year," a surprise given her balmy locale.

Both Li and Wilson wear sunscreen, but their deficiencies could point to another scientific fact: not everybody processes sun in the same way.

Some studies indicate that vitamin D can also help with the immune system, mood regulation, blood pressure, and even fending off diabetes and certain cancers.

"Generally places with less sunlight have an increased association of vitamin D deficiency, but you can be out it the sun and still have a deficiency; it's just a very individual thing," says Dr. Brian Feldman, endocrinologist at Stanford Health Care. "And for some people, sunlight may not be enough."

Feldman adds that nowadays, people simply spend less time in the sun that they did "back when we ran around hunting and gathering," and so rain or shine, one optimal way to get vitamin D is from your diet.

Vitamin D-rich foods include fortified milk, mushrooms, spinach, and fish. Dr. Li points out that red meat is also rich in vitamin D, but adds that it's also regrettably rich in cholesterol, so it may not be the best way to go. And even with these foods, it’s hard to know if you’re getting enough of it.

What A Vitamin D Deficiency Can Do

It's also difficult to know if you even have a deficiency, as generally subpar amounts of vitamin D don’t have clear symptoms (though they might include low mood, weakened immunity and fatigue). It’s more about the potentially debilitating conditions to which a deficiency can contribute.

“The [health issues] that are definite are related to calcium, as without sufficient vitamin D the body cannot absorb calcium, ” says Li, citing osteoporosis as one of the most major related outcomes of a serious deficiency. “[Depressed] mood and increased allergies are also possible results, but since other factors can contribute to these, they’re not as clearly defined.”

The medical community is still learning just how impactful a vitamin D deficiency can be, and what it can lead to — but evidence is mounting.

“Some of the strongest data comes out of the oncology world, [suggesting] that a deficiency can be linked to breast cancer and colon cancer,” says Feldman. “Some studies show that people with a deficiency may be at risk not only for developing these types of cancers, but that they may also have a poorer prognosis with them.”

Studies also point to vitamin D deficiencies possibly enhancing the risk of cardiovascular disease, inhibiting pancreatic function, and increasing risk of hyperparathyroidism and neurological and psychiatric disorders, with tons of other research in the works.

Li adds that the science that insufficient vitamin D can cause problems with the immune system is gaining ground, as there is ample evidence that “when pregnant women take vitamin D, their offspring has a decreased risk of developing asthma.”

Want to Know if You're Vitamin D Deficient? Get A Blood Test

The only way to be certain that you have a vitamin D deficiency is through a blood test.

Often this test is included in a basic panel you'd get during an annual physical, and is "relatively cheap," Feldman says. But even if you do have to shell out some dough, Feldman puts it into perspective, reasoning: "If you are going to commit to taking a hormone, you should have the data."

Wait up — vitamin D is not a vitamin? It’s…a hormone? Yup! Both Li and Feldman find the misnomer confusing, to say the least.

"A very important myth around vitamin D is in its name: it’s not a vitamin, it’s a steroid hormone, so it has a lot of different functions in the body," says Feldman.

It’s an important distinction, especially when you’re reaching for that bottle of supplements.

“I don’t at all object to vitamin D supplements, but I think people should be cautious,” stresses Feldman. “Deciding you need more of a hormone is not a trivial thing.”

A very important myth around vitamin D is in its name: it’s not a vitamin, it’s a steroid hormone, so it has a lot of different functions in the body.

Feldman recommends that if patients do discover a deficiency from a blood test, they get guidance from their physician about exactly how much of a supplement to take. If they have a severe deficiency, they may need to start off with an “industrial strength” 10,000 IUs, and then adjust accordingly upon follow up blood tests.

Generally, Feldman doesn’t recommend adding a supplement unless you know you have a deficiency. Dr. Li has a less conservative view, recommending that the general population (this of course excludes people with certain medical conditions) take a vitamin D supplement of 1000 IUs a day every day, all year round, adding that the odds that you’re getting too much vitamin D are beyond slim.

“Vitamin D has very low if ever associated toxicity,” says Li. “I recommend taking it — especially if you’re indoors most of the day, and if you’re not eating a lot of vitamin D-rich foods.”

Of course, you can overdo any supplement, so if you are going to venture out on your own, start with a low dose. But really, a blood test is your best bet.

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