From the presence of vitamin D receptors in our cells and vitamin D factories in our epidermis, it’s pretty clear that having adequate vitamin D is an essential component of being a healthy, thriving Homo sapiens. Anything our body invests precious resources to make is obviously pretty important.
Vitamin D plays a vital role in calcium metabolism, immunity, and gene expression. Deficiencies in this nutrient (which I argue should really have been classified as a hormone) are associated with brittle and misshapen bones, joint pain, hyperparathyroidism, autoimmune disease, severe COVID-19,1 and even some cancers. And yet, vitamin D insufficiency and deficiency are alarmingly common in children and adults of all ages. This has led many health experts to assert that everyone should be supplementing with vitamin D as a standard public health practice.
The questions then arise: What’s the best way to get enough vitamin D? Can’t you get enough from food? What are the benefits of supplementing, and when is it necessary? And if you’re going to supplement, what’s the best way?
How Much Vitamin D Do You Need Per Day, and How Do You Get It?
The RDA for vitamin D is 400 IU (10 mcg) per day for babies up to one year old, 600 IU (15 mcg) for kids and adults (both men and women) until age 70, and 800 IU (20 mcg) thereafter. However, this is just the minimum amount needed to ward off deficiency, not the amount needed for optimum health. And anyway, most people don’t get anywhere near this amount.
There are three ways to get vitamin D: from the sun, from your diet, and from supplements. Each has its own benefits, but it can be difficult to calculate how much vitamin D you’re actually getting on a daily, weekly, or monthly basis. Besides the fact that you can’t really measure how much you’re synthesizing from sunlight, people differ in their ability to absorb vitamin D from food and supplements. The only way to know if you’re getting the right amount is to get your vitamin D levels tested.
Ask your doctor for a blood test measuring serum 25(OH)D. As for what constitutes “good” or “optimal” levels, that’s a contentious question. The Institute of Medicine guidelines say anything over 20 ng/mL (equivalent to 50 nmol/L) is fine, while the Endocrine Society and the American Association of Clinical Endocrinologists say over 30 ng/mL (75 nmol/L) is adequate. Again, I consider these the bare minimum. Personally, I shoot for 50 to 60 ng/mL (or over 125 nmol/L).
Let’s look at how you get there:
Sun: Nature’s Source of Vitamin D
We humans are basically hairless, upright apes with hefty D3 requirements. Before oral supplements appeared, dietary vitamin D was not a very reliable source. We had to get it from the sun’s UVB radiation. I prefer getting my D3 from sunlight still, simply because it’s enjoyable to spend time in the sun, and it’s an effortless way to get something that’s critical to my health.
I don’t think it’s necessarily better than getting it through supplements, though, in terms of warding off health issues. Nature’s way is quite often better, or at least more congruent with how we’re “intended” to work, but that doesn’t always preclude the effectiveness of modern methods.
Pros and cons of getting vitamin D from the sun
The main benefit of getting vitamin D through sunlight is that you can top off your stores more efficiently and, in some ways, more safely than you can via food or supplements. Generally speaking, spending 10 to 15 minutes outside each day exposing large patches of skin (think arms, legs, torso) to midday sun—the best time for optimal UVB rays—will net fair-skinned individuals all the vitamin D they need.
However, many factors affect how much vitamin D a person actually gets from sunlight.2 Besides the obvious—how much time you spend outdoors, whether you routinely use clothing or sunscreen to shield your skin—latitude, altitude, and weather all impact UVB. You’ll need more time in the sun if you live far from the equator or when it’s cloudy or overcast. Similarly, darker-skinned individuals, older folks, and people with high levels of body fat need more sunlight to synthesize the same level of vitamin D.3 There are apps now that will monitor your sun exposure and estimate your vitamin D bank, but I’m not sure how accurate they are, frankly. (Let me know in the comments if you have a favorite).
On the flip side, you won’t get vitamin D toxicity from the sun. Sun exposure is self-regulating. Most of the time, you quite naturally opt out before it gets too intense. You get hot, and you seek shade or you cover up. Of course, it’s possible to spend too much time in the sun and end up a blistered, painful mess, but this won’t lead to excessive vitamin D in your system. Once you have produced enough vitamin D through sun exposure, those same UVB rays will begin to prevent excess vitamin D production by degrading existing cholecalciferol (vitamin D3). Ain’t homeostasis grand?
Food: Nature’s Other (Less Powerful, Less Reliable) Source of Vitamin D
Of the three, diet is the poorest source of vitamin D. Certain foods get touted as vitamin D powerhouses: wild salmon, trout, mackerel, herring, cod liver oil, eggs. However, most people don’t eat enough of these foods to hit the RDA, and they still deliver far less vitamin D that spending your lunch hour basking outdoors. Vegans and vegetarians are going to have a harder time getting vitamin D from their diet. Mushrooms and fortified non-dairy “milks” are the best non-animal-based sources, and they’re mediocre at best.
Obviously avoid fortified grain-based foods like breakfast cereals, which, besides the obvious, reduce calcium absorption. The body preferentially uses available D for calcium regulation, so when you eat grains and other junk, you’re increasing your D requirements. It’s a good thing cereals and whatnot are usually fortified with meager amounts of vitamin D, just enough to avoid rickets and keep blood calcium relatively under control.
On the plus side, the foods that are highest in vitamin D also come packaged with other nutrients like magnesium and calcium that work synergistically with D in the body. Eat these, but don’t rely on them to increase your 25(OH)D levels much.
Supplements: Science’s Solution
I agree with the health experts who assert that most people could benefit from vitamin D supplementation, especially if they live an indoor-dominant lifestyle (as most people do, much to their detriment).
A good general rule is 4000 IU per day supplemented. That’s what I take during winter time and when I go a few days trapped indoors and without sun. You may need higher doses if you have gut issues that impair your ability to absorb nutrients or fat (since vitamin D is fat soluble) or if you’re correcting a known deficiency. In these cases, talk to your doctor about how high you can go and for how long. Given that we can make around 10,000 IU in well under an hour from sunlight, supplementation up to that level is generally well tolerated.
Rather than just supplementing willy-nilly, it’s wise to get your 25(OH)D levels tested before you start supplementing, take the 4000 IU daily if you’re below where you want to be, and get them tested again after two months to make sure you’re on the right track. If you’ve already hit that 50+ ng/mL mark, supplementing probably won’t yield any additional benefits. Don’t be foolish here. You could conceivably keep popping capsule after capsule and end up with toxic blood levels. It’s difficult to do, but it’s technically possible to take too much vitamin D.
Does oral vitamin D differ qualitatively from solar D?
Let’s examine the pathways and see what happens when you produce or swallow vitamin D.
When UVB reacts with the 7-Dehydrocholesterol in your skin, cholecalciferol (D3) is produced and ends up in your liver, where it’s hydroxylated into calcidiol, also known as 25-hydroxyvitamin D or 25(OH)D.
When you swallow cholecalciferol (either in food or supplement), it ends up in your liver, where it’s hydroxylated into calcidiol, also known as 25-hydroxyvitamin D or 25(OH)D.
From that point onward, it’s all the same. It’s all calcidiol, ready for storage in fat tissue or dispersal to the kidneys for conversion into calcitriol, also known as 1,25(OH)D. Calcitriol is the active hormonal form whose primary role is to regulate blood calcium levels. If there’s enough calcidiol left in the tank after the calcium duties, it gets sent to other tissues in the body to be converted into more calcitriol to fulfill even more roles. That’s where all the immune system/cancer cell/heart health/insulin sensitivity benefits begin to kick in, and it explains why having plenty of vitamin D available—beyond what’s only required to avoid rickets and monitor blood calcium—is so beneficial.
So Sun or Supplement? What Should You Do?
Sun definitely, supplement probably.
Take stock of your living situation. If the sun is regularly available where you live, go that route whenever possible. It’s free, it’s safe, it’s easy, and it’s enjoyable. Avoid burning, of course, and you’ll be good. I find it inconceivable that the amount of sun exposure necessary to produce 10,000 IU of D3 will also kill you and give you skin cancer, but still practice safe sun exposure.
As with anything, ease into the sun. The pasty, mostly interior set will want to treat full sunbathing like learning to sprint barefoot after a lifetime of dress shoes—they’re coming from a position of weakness, of sheltered living. Learn your limits, and throw on a shirt or find some shade before you burn.
We can’t all bask in the midday sun (if such a thing even exists in our neck of the woods). For those of us unable to run shirtless and shoeless through a sun kissed meadow from noon to 12:45, our option is oral intake. In all seriousness, take a good D3 supplement if you can’t get real sunlight. As long as you don’t go overboard on the dosage, you’re good to go. If it’s not in an oil-based capsule, just take it with a bit of fatty food (not a stretch for a Primal eater).
And do eat those vitamin-D-rich foods, not just for the D but because they are some of the most nutrient-dense foods on the planet, period.
Lastly, don’t wait until you have symptoms of a vitamin D deficiency to measure your levels and start shoring them up. Be proactive. Symptoms of low vitamin D are non-specific—muscle, bone, and joint pain, fatigue, low mood, hair loss—and easy to write off as normal (although they are far from it) or blame on something else. Your doctor shouldn’t balk at you requesting a routine test, and it’s a simple step you can take that pays big dividends.
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.