How much vitamin D comes from food, and how much vitamin D comes from sunlight? For most of the past 20 years, we were told that 90% of our vitamin D comes from skin production, given adequate full body sun exposure.
Back in 2013, Professor Robert Heaney of Creighton University wrote another masterpiece looking at this topic. He has asked a simple question. Where does our vitamin D come from? Again, we are told that 90% of it comes from skin production. Dr. Heaney and colleagues wanted to know if that was true.
Most studies show that we get around 120–180 IU of vitamin D/day from food, although some studies show it as high as 300 IU/day. We are told that skin synthesis makes up the rest. However, that assumption appears to rest on the simple conclusion that if we don’t get it in food, we must get it from sunlight.
In this study, the authors took a closer look. They used 8 studies that show a dose response curve [the amount of vitamin D needed to raise 25(OH)D]. The authors then used some sophisticated and complicated (at least complicated and sophisticated to me) mathematical formulas to calculate total vitamin D intake from food and from sunlight to reach those 25(OH)D levels.
They found the following:
The hypothesis presented in this paper is that meat contains 25(OH)D or calcidiol and that food source of 25(OH)D have not been recognized enough in the past, as the routine measurement of vitamin D3 content of food has neglected to also assessed the 25(OH)D content of food.
Professor Heaney wrote:
“We conclude that: 1) all-source, basal vitamin D inputs are approximately an order of magnitude higher than can be explained by traditional food sources; 2) cutaneous, solar input in these cohorts accounts for only 10-25% of un-supplemented input at the summer peak; and 3) the remainder must come from undocumented food sources, possibly in part as preformed 25(OH)D.”
I’d like to emphasize that this study looked at 8 studies in populations from developed countries, where people don’t get much sun exposure. Looking at traditional living equatorial populations, we know that naturally living humans get much more of their vitamin D from sunlight than from food.