It’s often stated that the human body can make 10,000 to 25,000 IU of vitamin D, maybe more, in a single session of full body sun exposure. This might make you wonder, how do we know that?
As many know, we measure your vitamin D “stores” by measuring 25(OH)D in your blood. When you input vitamin D, it gets sent to your liver to turn it into 25(OH)D, so the rest of your body can put it to use.
What many don’t know, is that you can actually measure serum vitamin D in the blood, essentially a measure of vitamin D before the liver has converted it to 25(OH)D. After a good day’s worth of sun exposure (or sunbed) or after a large oral dose of vitamin D, you’d expect serum vitamin D to spike briefly, as the body needs some time to transport this serum vitamin D to the liver and start metabolizing the vitamin D into 25(OH)D.
Thus, we can start to compare, how much serum vitamin D is in your blood immediately after sun exposure and how much serum vitamin D is in your blood immediately after you take a certain amount of supplement?
Before we take a look at the research, remember that we measure UVB doses in units of J/m². For skin type II (white skin, usually burns), a dose of 200 J/m² will produce a slight pinkness to the skin. So to give you some orientation before we talk about other doses, remember that 200 J/m² is about equal to 15 to 30 minutes of solar noon sun exposure in the summer.
Now, let’s start by looking at some UVB studies and how much serum vitamin D was made when subjects were exposed to lamps:
Now, let’s look at the studies that look at some supplementation studies and how much serum vitamin D was found:
When researchers gather all this data, they can start to reasonably estimate and project how much vitamin D you make from sun exposure based on the subsequent serum vitamin D levels. When we subject ourselves to full body sun exposure, enough to induce a slight pinkness, we probably make between 10,000 to 25,000 IU of vitamin D.
If we put this concept into a graph, it would look something like this:
As shown in the Clemens et al study in 1982, the human body can make even more when getting enough sun exposure to get a sunburn. After a dose of 540 J/ m² (about equivalent to an hour and a half of intense sun exposure), one participant with fair skin achieved serum vitamin D levels of 61 ng/ml, more than double what an oral intake of 25,000 IU achieved (29.6 ng/ml). Given this, it’s not unreasonable to extrapolate that the human body can make up to 50,000 IU of vitamin D after a day of burn-inducing sun exposure.
Please note that this does not give justification to supplement daily with intakes in this range. If you got daily sun exposure over a long period of time, your skin would usually develop a tan, shielding the skin from much UVB and slowly decreasing the amount of vitamin D you could make per day. For this reason, it appears that oral intake of 5,000 IU/day is about equal to what your body is capable of making with year-round sun exposure, though every person’s requirements vary a little.