10 Things You Didn't Know About Vitamin D cover

10 Things You Didn't Know About Vitamin D


Vitamin D plays a role in every system in the body, affecting your skin, brain health, bones, blood sugar, cholesterol, hormone balance, joint health, risk for cancer, and likelihood of developing autoimmune disease.
And that’s just the short list. But this post is not going into the myriad of benefits of vitamin D. I will instead getting you up to speed on some little known facts about this vitamin (and correct some common myths), to ensure you don’t become (or stay) vitamin D deficient.

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10 Things You Didn't Know About Vitamin D

Sunshine Vitamin

If you’ve picked up a newspaper or magazine in the past 10 years, you’ve read about vitamin D.

Maybe you’ve heard it called the “sunshine vitamin” or read that deficiency in vitamin D is linked to heart disease or cancer.

Both of those are true.

You’ve probably also heard that “15 minutes of morning sun” is enough to meet your vitamin D needs or that “3 servings of dairy” will do the trick.

Those statements are far from the truth (and kind of make me want to strangle magazine editors).

Soaking Up D?

Soaking Up D?

Vitamin D Researcher

I had the great fortune of studying vitamin D in a graduate-level Vitamin Metabolism class taught by, get this, a vitamin D researcher.

She was, by far, the best professor I had and sparked my interest in this vitamin before my nutrition career even began.

I remain fascinated (so much so that I’ve presented a crazily detailed lecture on the role of vitamin D in pregnancy at two gestational diabetes conferences).

I share this so you can understand why I care so much about this vitamin! I’ve spent hundreds of hours pouring over the research and realized that it might be a good time to fill you in on the deets.

Why Should You Care About Vitamin D?

Vitamin D plays a role in every system in the body, affecting your skin, brain health, bones, blood sugar, cholesterol, hormone balance, joint health, risk for cancer, and likelihood of developing autoimmune disease.

And that’s just the short list. But this post is not going into the myriad of benefits of vitamin D. I will instead getting you up to speed on some little known facts about this vitamin (and correct some common myths), to ensure you don’t become (or stay) vitamin D deficient.

Need To Know

I’ll be covering things like:

• How much vitamin D do you actually make from sun exposure?

• Where do we get most of our vitamin D? (This will surprise you)

• How much vitamin D do you need per day? (It’s waaay different than previously thought)

• Does where you live affect your ability to produce vitamin D?

• What type of vitamin D supplements are best? (One is 87% more potent at raising vitamin D levels… and it happens to be the easiest type to find!)

Vitamin D

Vitamin D

1. It’s a Hormone, Not a Vitamin

Vitamin D is unique in that it’s not used in the body like most vitamins. It’s actually better classified as a hormone.

Its primary role in the body is to maintain normal blood levels of calcium and phosphorus, but it has far-reaching effects across body systems, from bones, cardiovascular, endocrine, neural function, immune health, and a whole lot more. Vitamin D is so crucial that it actually regulates the activity of over 200 genes, which is many fold more than any other vitamin.



2. You Don’t Get Much From Dairy

If I asked you to do a quick word association exercise with vitamin D, two words would probably pop into your head: 1) sunshine and 2) dairy.

The second one is thanks to some very clever marketing tactics from the dairy industry. But dairy is not always a great source of vitamin D. That’s because animals, just like humans, make vitamin D from sunlight exposure. Most dairy cows in the US are not outside in pasture, grazing on grass, but raised indoors.

2. You Don’t Get Much From Dairy

Secondly, vitamin D is fat-soluble and most Americans (sadly) opt for fat-free or low-fat dairy products.

That means most of the naturally-occurring vitamin D is gone. Although most milk is fortified with vitamin D, that’s less common in other dairy products, like yogurt and cheese. Whatever vitamin D is added to that non-fat or low-fat dairy probably won’t be absorbed very well (it’s called a fat-soluble vitamin for a reason).

So, if you’re expecting to get much vitamin D from dairy, opt for products made with milk from grass-fed, pastured-raised animals and always buy full-fat.

(Side note: If you suspect dairy products don’t agree with you and want to figure out which types of dairy products might be easier on your tummy, read this. Of course, it’s perfectly fine to ditch dairy altogether. Don’t worry – you probably won’t be deficient in vitamin D because of it. )

Not Much D

Not Much D

3. Sun (& Supplements) are Primary Sources

Not food.

Even if you are a milk-a-holic (not recommended, by the way), you’d be silly to assume you’re getting enough vitamin D from your diet. That’s because our primary source of this vitamin is sun exposure or supplementation, not our diet (unless you’re guzzling cod liver oil).

In fact, sun exposure accounts for ~90% of vitamin D in the body in individuals who do not take supplements. (American Journal of Clinical Nutrition, 2004)

3. Sun (& Supplements) are Primary Sources

That means, if you’re not in the sun much (and believe me, that’s most people as I’ll explain below), you’ll need a supplement to meet your needs.

And the measly amount in in your multivitamin probably isn’t enough (I’ll get into that later, too).

Just to put this into perspective. The amount of vitamin D in a glass of milk is ~100IU. If you have pale skin and lay outside in a bathing suit (termed modest sun exposure in the research, meaning skin was not burned, but lightly pink 24 hours later), you’ll make anywhere from 15,000-20,000 IU of vitamin D. (Journal of Investigative Medicine, 2011)

Let that sink in for a second.

Soaking Up D

Soaking Up D

4. It’s the Only Vitamin You Make From the Sun

One of the coolest things about vitamin D is how it’s made. I’m kind of obsessed with this whole process because there are so many factors that can mess it up.

But, let’s talk about it first as if it’s a well-oiled machine. Vitamin D is made in your skin when you are exposed to UVB light at a very specific wavelength. The UVB rays interact with a precursor in your skin, 7-dehydrocholesterol, turning it into vitamin D3. From there, vitamin D3 is transported to different places in the body where it gets “activated,” meaning it’s ready for use by the body.

Now that you know the basics, let me explain more cool (or not-so-cool) things about this process.

5. Skin Tone Affects Vitamin D Production

Those of you lucky folks who tan easily or naturally have beautiful dark skin are at a slight disadvantage when it comes to vitamin D.

That’s because you have higher concentrations of the skin pigment, melanin, which blocks the sun’s UVB rays. That means it takes more sun exposure for you to produce adequate amounts of vitamin D. In fact, people with dark skin may require 3-6x more time in the sun to produce the same amount of vitamin D as a very pale person. (The Lancet, 1982)

There’s some very interesting theories suggesting that a gene controlling skin pigmentation mutated over 10,000 years ago after humans migrated to Europe, which allowed those humans living in sun-deprived environments to produce enough vitamin D. (Science, 2005)

Crazy, right?

Harder Work For D

Harder Work For D

6. You Can’t Make it in the Winter

(in many areas)

This brings me to the next commonly overlooked challenge with this nutrient. Remember the sun rays that give us vitamin D? Well, they are shining year-round if you live in the tropics, but the further you get from the equator, the less of those rays you get. And because the Earth rotates on a tilted axis, those rays are virtually non-existent in winter months at far northern or southern latitudes.

Of course, you’re pretty unlikely to lay out in a bikini mid-winter, but one unlucky graduate student did just that in Boston (what we do in the name of science, right?) in winter and produced ZERO vitamin D. Now you might be thinking “But Boston is pretty far north!” and it is (42° north), however reduced vitamin D production is seen in latitudes as low as 33° north.

6. You Can’t Make it in the Winter

That corresponds to anything above Long Beach, CA, Phoenix, AZ, and Atlanta, GA, meaning that at least two thirds of the US is at a latitude where vitamin D production from the sun during winter is nearly impossible.

The farther north you go, the more months of the year you’re not able to get those rays. (#Alaskaproblems)

Above and below latitudes of approximately 33°, vitamin D3 synthesis in the skin is very low or absent during most of the winter.” – The Journal of Clinical Endocrinology & Metabolism, 2011

This explains why native populations that live in far northern latitudes traditionally ate so much marine animal fat, which is rich in vitamin D (such as seal, whale, and organ meats, like cod liver).

No D North Of Here

No D North Of Here

7. Sunscreen Blocks 95% of Vitamin D Production

But, let’s say it’s summer and you DO get out in the sun regularly. You should be fine, right? Not so fast.

If you’re wearing sunscreen or covering up with protective clothing, you’re not making much vitamin D either. Sunscreen with an SPF of 30 pretty effectively blocks UVB rays, reducing vitamin D production by 95%. (The Journal of Clinical Endocrinology & Metabolism, 2011)

Obviously, sunburns are harmful, so if you have pale skin or burn easily, it’s ideal to get a little sun exposure on bare skin and then lather or cover up for the remainder of your time outside (or simply avoid long periods of time in the sun).

(Sidenote: Opt for mineral-based sunscreens as the chemical ones are highly toxic. Consider wearing a hat to protect the delicate skin on your face.)

D With Friends

D With Friends

8. You Make Most Vitamin D Mid-Day

You might have heard that you should avoid mid-day sun exposure, since that’s when the sun emits more damaging rays.

However, those “damaging” rays are the ones that your skin uses to make vitamin D. UVB rays are at their peak between roughly 10am to 2pm, so if you’ve been taking an early morning walk to get your vitamin D, you might want to rethink it (or spend some additional time outdoors mid-day).

Failure to understand the nature of human exposure to sunlight has led to misguided advice concerning the sun exposure necessary for an adequate vitamin D status.” – Photodermatology, Photoimmunology & Photomedicine, 2010

9. Deficiency is Super Common

Now you can understand why deficiency in vitamin D is so common! As one of the world’s leading vitamin D researchers puts it, “Vitamin D deficiency is now recognized as a pandemic.” – Michael Holick

Deficiency is widespread and more prevalent at northern latitudes. Your best bet is to get your vitamin D levels checked by your doctor via a test called “25-hydroxy vitamin D.” Typical labs suggest vitamin D levels around 30 ng/ml, but many researchers suggest optimal vitamin D levels are above 50 ng/ml.

If you are deficient, it may take a few months of supplementation to normalize your levels. And since vitamin D levels tend to vary with the seasons, it’s wise to get your vitamin D levels checked periodically and possibly supplement more in the winter if your levels dip.

9. Deficiency is Super Common

Pregnant women are especially at risk for vitamin D deficiency.

One study in South Carolina (latitude 32 degrees) found high rates of deficiency, especially in women of color, despite living in a latitude where year-round vitamin D production from the sun is possible. In that study, vitamin D deficiency was seen in 67% of Caucasian women, 81% of Hispanic women, and 98% of African American women. I talked about the crucial role of vitamin D in pregnancy (and more) in this prenatal nutrition interview and extensively in Chapter 6 of my book, Real Food for Gestational Diabetes.

Special Needs

Special Needs

10. Current Vitamin D Recommendations are Way Too Low

In fact, a recent review of the standard vitamin D recommendations found that the Institute of Medicine (IOM) underestimated vitamin D requirements by a factor of ten. Not a typo. A FACTOR OF 10!

So if you’re relying on a multivitamin that gives you “100% of the daily value of vitamin D,” there’s a pretty good chance you’re not getting enough. The current recommended intake is set at a paltry 600 IU for adults, yet Robert Heaney, a well-known vitamin D researcher and medical doctor is speaking out.

We call for the NAS-IOM and all public health authorities concerned with transmitting accurate nutritional information to the public to designate, as the RDA, a value of approximately 7,000 IU/day from all sources.” – Robert Heaney, MD

What Does This Mean for You?

Odds are you’re deficient in vitamin D, just like the rest of us. I highly recommend monitoring your vitamin D levels to see if that’s really the case.

If you are, in fact, deficient, always opt for vitamin D3 when choosing a supplement, which is more effective at maintaining blood levels of the vitamin than vitamin D2.

D3 is approximately 87% more potent in raising and maintaining serum 25(OH)D concentrations and produces 2- to 3-fold greater storage of vitamin D than does equimolar D2… Given its greater potency and lower cost, D3 should be the preferred treatment option when correcting vitamin D deficiency.” – The Journal of Clinical Endocrinology & Metabolism, 2011

D For Life

D For Life

Read Your Labels

Unfortunately, many prescription vitamin D preparations use D2 instead of D3 (mind boggling), so read your labels carefully.

Vitamin D3 is both inexpensive and easy to find over the counter (and it’s identical to the form of vitamin D you make from the sun). I like this one. Vitamin D supplements should be taken with a meal that contains fat to be absorbed properly (remember, it’s fat-soluble!).


There are a variety of nutrients that work synergistically with vitamin D, including vitamin A, vitamin K2 (both of which are found in animal fats, like the fat from grass-fed beef), magnesium, iron, calcium, zinc and boron.

So, don’t expect vitamin D supplements to be a cure-all. A well-rounded real food diet that’s rich in micronutrients is essential.

Are you sick of science yet? I know I could use a break!

But before you go, I’d love to hear from you.

Did you learn something new about vitamin D today?

Which of the 10 benefits listed above did you find most interesting?

Until next week,


Originally posted to Pilates Nutritionist