How much Vitamin D does the body produce if you're out for a whole summer day?

Question

Imagine that a person is outdoors the whole day in the summer. They're tanned enough that they don't get sunburn. How many IU (International Units) of bioavailable Vitamin D do they produce in a way?

To provide a more specific scenario: Consider an average summer day in Berlin, where an average-sized male wears pants, but nothing above the waistline. The amount of tan is the amount that the average inhabitant of Berlin develops if he's constantly out during the summer in the same outfit.

I'd also be happy to hear other specific examples.

Answer 1

Vitamin D is produced in the skin by UV irradiation of 7-dehydrocholesterol. But vitamin D in the skin is also broken down by UV irradiation and this leads to a maximum equilibrium level of vitamin D in the skin of the order of 25,000 IU (different sources will give different figures of this maximum value, The vitamin D council says it's between 10,000 and 25,000 IU).

But this does not mean that you can get 25,000 IU per day from the Sun, because it takes a few days for the vitamin D in your skin to be transported to the liver where it gets converted to calcidiol. The maximum daily dose of vitamin D you can get from the Sun is estimated to be about 10,000 IU/day, see e.g. here. So, if you expose your body to UV radiation from the Sun for the first time since last summer, then you can produce 25,000 IU in your skin, but by the next day only 10,000 IU will has been taken away from there, this means that with 15,000 IU still left in the skin you can only add 10,000 IU until you're back at the maximum of 25,000 IU.

This also means that to get a high dose of vitamin D, you don't need to expose your skin to the Sun every day. If we pretend that lower amounts of vitamin D in the skin don't affect the speed at which it is removed, then the body would be able to extract 20,000 IU in two days from the 25,000 IU, the amount of vitamin D left under your skin is then 5000 IU, exposing your skin to the Sun will add back that 20,000 IU. In reality, with lower amounts of vitamin D in the skin you'll extract less from it per unit time, so you won't get 10,000 IU/day if you go into the Sun every other day, but it's not going to be a factor of two lower as one could naively expect.

Answer 2

It's difficult to say, as the tan itself will reduce the level you are making. It acts as a barrier. You also have to account for showering. If you shower right after sun exposure, you will reduce some of your D3 because it is synthesized at skin level & then absorbed. It is possible for a human to make 50,000iu in a day. There are always factors in play, such as complexion, tanning, and global location that will impact exactly how much a given person makes. There have been studies where this is attempted to be measured, but even in situations where they exposed 2 people to the same thing, they had one person with 20ng higher blood values than the other, and that is significantly higher.

Personally I live in a Northern climate so I just supplement year round. If you take D3 (not D2) it is all coming from natural sources & will operate within your body just as effectively as sunshine. I also supplement my children from birth. I can't swear it works, but I can say we have never needed a sick visit to a doctor other than one time for one child for excessive vomiting, which I have never thought D3 could prevent. We do not experience typical childhood illness at all & we are not particularly careful other than to aim for a very high nG value on bloodwork because I happen to personally believe it matters for immune system functioning. So what we take is well in excess of what is generally recommended, but it's all completely approved through our Drs & all have been extremely happy that we are taking the levels we are taking. I can also say I am the only parent I know of multiple children who completely skipped early infant illnesses. I worked too, so they weren't home all day with me, but none of my children ran so much as a fever or runny nose before age 2. They didn't inherit that. Their parents both were the typically sick children that you anticipate all kids will be, lots of coughs, colds, general stuff.

Here is some info on the various nG levels that were found for various people under the same situations/exposures. https://www.vitamindcouncil.org/how-do-we-know-how-much-vitamin-d-you-make/

Here is a lot of information on how D3 impacts immune functions. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2678245/

Sorry I cannot be more accurate to determine precisely what a given person would make in a given situation. I do not think bodies work like that though. It seems on the studies I have come across, similarly sized people of the same complexion & same exposure still manage to synthesize it differently enough to be notable. It's relatively easy & inexpensive to get your blood tested for D3, at least where I live, so if you are truly curious about your own production, you can certain request testing. I think all people should be checked at least yearly. It's such an important nutrient. It behaves much more like a hormone than a vitamin & there is a receptor for D3 on nearly every tissue & cell type in your body. It impacts all things, from immune health to brain development & function, to heart health. So I think on the whole, we are missing the mark medically when we so rarely test it & recommend such low supplementation while living in an era where people see less sunshine that likely ever before.