Yesterday I recommended 4000 IU of vitamin D each day  as a good starting point for most people. Though, it’s difficult – nay, impossible – 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.
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 is probably why melanin-rich black skin was the default hue for early humans living in equatorial Africa. Among those of us who moved to cooler climates with actual seasons, lighter skin with greater sensitivity to sunlight was selected.
But damn if we aren’t a mobile people, and that mobility throws a monkey wrench into the whole thing. The whitest white guy with Celtic blood running through his veins can live in sun-drenched Southern California, just as a black woman several generations removed from Nigeria might have settled down in Norway. Each is imbued with a vitamin D3 production capability that “conflicts” with their current locale. As far as melanin content and resistance to sunlight is concerned, they’re on permanent vacation. Have they outpaced natural selection? A bit, but they can avoid serious complications by knowing the facts and acting accordingly. Dark skinned folks will need more sun – or supplements to make up for it – while lighter skinned folks need less sun to produce D:
10-45 minutes full sun (without burning) for light skin
2-2.5 hours full sun for dark skin
No food enters our body in a vacuum. It is a mistake to only think of individual nutrients or vitamins without giving thought to how they might react with the hundreds of other substances you take in on a daily basis. Vitamin D is one of the biggest. What we eat has powerful effects on vitamin D metabolism.
Magnesium – Magnesium is important for bone health because a sufficient amount enables the mobilization of active vitamin D3 – calcitriol. One study  even showed that magnesium deficiency resulted in 50% less calcitriol, increased inflammatory markers in the bone, and lowered bone mass.
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.
Meat –It has been shown that a Western style plan of omnivory, with particular emphasis on meat , dairy , and eggs , protects against rickets in the presence of severe vitamin D deficiency. Dunnigan thinks  the meat is responsible, and that vitamin D deficiency is necessary but not sufficient for rickets. You’ve also got to avoid the meat to really make the D3 deficiency manifest as something bad.
As Peter said , you can get away with vegetarianism in the tropics, where D3 is plentiful. Moving north requires meat in the diet. It’s probably true that a Primal eating strategy rich in animal protein makes complications from vitamin D3 deficiency uncommon, even without getting much sun or taking many supplements.
Grains – Grains  in the diet decrease calcium absorption, thus increasing the demand for vitamin D. Eat grains and you’d better be getting plenty of good sunlight.
Don’t eat grains .
Vitamin A – A heated debate over vitamin A and vitamin D (with a bit of K2  sprinkled in) rages on. The Vitamin D Council  boys claim A antagonizes and competes with D, while Chris Masterjohn  and the WAPF  stress balance of the two for proper function.
Rather than demonize one and exalt the other, just get sun, take supplements when you need them, and eat plenty of beta-carotene in the form of veggies, along with some preformed retinol in the form of liver , eggs , and other animal fats . Don’t take cod liver oil for the DHA/EPA ; take half a teaspoon or so for the vitamins. Stick to fish oil  for your omega-3s .
PUFA/SFA – People report increased resistance to sunburn since replacing dietary PUFA  with SFA . I’ve definitely noticed similar effects, but I was unable to bring up any literature confirming it. Anyone know more?
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.
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.
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, you can use your judgment.
Slather it on, they tell us , and we do it – usually. 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 is way down (it’s not  (PDF)).
As sunscreen usage and sales have skyrocketed, skin cancer has plummeted (the opposite has happened).
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.
In the end, it seems like 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.
(Here’s a cool hack  for figuring out how much vitamin D you might be getting based on several factors, like cloud cover, surface altitude, latitude, time of year, etc. It’s imperfect, but a good general guide that’s fun to play with.)
Keep the questions coming and stay tuned for future coverage of vitamin D. Thanks for reading, everyone.