Factors That Affect How Much Vitamin D You Get From the Sun

In the past, I recommended 4000 IU of vitamin D each day as a good starting point for most people. Though, it’s difficult to provide a perfect, universal prescription for vitamin D3 intake. People, and their lifestyle behaviors and environmental conditions are just too different. It’s like with diet. Everyone does well with the basic building blocks, stuff like meat, fat, vegetables, fruit, and nuts, but the optimal ratios are going to differ for individuals based on genetics, dietary history, activity level, and glucose tolerance. Everyone needs vitamin D, but multiple confounding factors must be taken into consideration to determine the right dosage. To start with? Yes, 4k is a good starting point. From there, though, things get considerably more complicated – as they always do.

Now, I don’t want to overcomplicate things, however. The same basic advice holds: get unfiltered sunlight, avoid burning, and take supplements when sunlight is unavailable. But I do want you to be aware of certain factors – environmental, climatic, dietary, genetic, etc. – that may affect vitamin D3 production, requirements, and availability when exposing yourself to sun.

Skin Color

A person’s skin color affects his or her ability to create D3 in the skin. The darker you are, the longer you must remain exposed for optimal D3 production. Dark skin also protects you from the sun—which means you can stay in the sun for a longer duration to get the vitamin D you need. Another reason for the darker skin of even European hunter-gatherers was that the vitamin D in the animal food-rich diet was extremely potent. There’s something about animal-borne vitamin D that Among those of us who moved to cooler climates with actual seasons, lighter skin with greater sensitivity to sunlight was selected.

Controlled studies find that when white and black people with similar vitamin D levels are exposed to the same dose of UV-B radiation, the white people make far more vitamin D.1

Before the Industrial Revolution, human mobility was severely limited. It took real effort to travel far distances, whereas nowadays we often find ourselves in totally different solar environments than our skin color “expects.” Consider a white man with Irish blood living in sun-drenched Southern California, or the black woman woman several generations removed from West Africa living in Norway. Each is imbued with a vitamin D3 production capability that “conflicts” with their current locale. The white man has to watch his sun intake, while the black woman has to increase hers (or supplement if it’s too scarce).

10-45 minutes full sun (without burning) for light skin
2-2.5 hours full sun for dark skin


What we eat has powerful effects on vitamin D metabolism.

Magnesium: Magnesium enables the mobilization of active vitamin D3—calcitriol. In other words, magnesium makes vitamin D work better, so we need “less” of it from the sun. One study even showed that magnesium deficiency resulted in 50% less calcitriol, increased inflammatory markers in the bone, and lowered bone mass.2

A Primal Blueprint diet high/sufficient in magnesium may reduce your need of D3 supplements. If you’re getting plenty of magnesium through leafy greens, nuts, and supplements, you may want to stick to D3 from sunlight or in smaller doses closer to 4,000 IU. If you’re not getting enough magnesium, you won’t make as much vitamin D from sun exposure.

Seed oils: People report increased resistance to sunburn since ditching seed oils and eating more fatty fish and animal fat. The more resistant you are to sunburn, the longer you can remain in the sun to obtain vitamin D.

UV Index

The UV index isn’t a property of the environment. It’s a measurement that takes into account all of the basic environmental factors—strength of the sun, position of the sun, cloud cover, latitude, altitude, ozone levels, seasons, and time of day that affect UV radiation exposure, which is a good proxy for vitamin D production potential.

If the UV index is 3 or higher, you will make vitamin D from sun exposure. If 2 or below, you will not.


Remember how they used to warn us about CFCs depleting the ozone layer and letting in too much UV light? That was probably a fairly valid warning, seeing as how full spectrum UV in naturally occurring ratios is what we should be getting. Any kind of imbalance deserves wariness. Strangely enough, now the real issue is insufficient UVB caused by too much ozone. One study found that “urban tropospheric ozone”—a benign-sounding name for manmade air pollution—was to blame for widespread vitamin D deficiency among physically active urban Belgian women who spent plenty of time outdoors in the summer. Rural women doing the same activities at the same time of year were mostly free of deficiency.3

In a rare stroke of irony tinged with good news, researchers are even now estimating that the benefits of excess solar radiation due to ozone depletion outweigh the downsides.4

Air pollution in general is strongly and consistently linked to lower vitamin D levels, probably due to airborne particles reducing UV exposure.5

To sum up? Urban sunlight probably isn’t as potent as rural. If you’re primarily an urban dweller who’s unable to get his or her D3 levels to budge despite dedicating time to sunlight, maybe get outside city limits on a regular basis. Go for a hike, have a picnic.


Clouds are another confounding factor for vitamin D, but in a very convoluted way. On a completely overcast day with heavy clouds blanketing the sky, 70-90% of the UVB is blocked, which translates to greatly reduced potential for natural D production. Partly cloudy skies, however, have a different effect. The UVB can actually reflect off of the denser clouds and increase in intensity, effectively scattering itself all over the place.

The same thing can happen with UVA, too, but the effect is pretty meager.

Full cloud coverage blocks most of it out. Spotty coverage blocks direct UVB, but may increase effective UVB. As long as you’re aware that UV is still present on semi-cloudy days and that you’ll still make vitamin D, you can use your judgment.


Slather it on, they tell us. Sure, it blocks vitamin D production, but at least it protects us against skin cancer. But does it really protect us? I mean, our proclivity to follow the experts’ advice on sunblock and sun exposure has proven highly effective in the fight against melanoma, right?

Melanoma incidence has skyrocketed.6

As sunscreen usage and sales have skyrocketed, skin cancer has risen—and vitamin D levels have dropped.

What’s the deal? Most of the popular sunscreens on the market only offer meaningful protection against UVB rays. I get that. As much as I love a nice dose of vitamin D, too much UVB exposure can lead to sunburns and skin damage. But in letting through most of the UVA while blocking out UVB, they are doing users a massive disservice. The UVA ends up being a huge problem because people are staying out longer without burning. They figure they’re safe, but it’s a false sense of security: isolated UVA exposure (along with decreased D) seems to increase the chance of developing melanoma, the really dangerous kind of skin cancer.7 It’s far better to get balanced UV—both A and B.

In the end, getting full spectrum UV is essential for obtaining D3 and protecting yourself. In this case, what works best is what’s most natural—full spectrum sunlight without burning.

Keep the questions coming and stay tuned for future coverage of vitamin D. Thanks for reading, everyone.

TAGS:  prevention

About the Author

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.

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