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. For most members of the Vitamin D Council, who usually sunbathe or take 5,000 IU/day, the following paper is of academic interest only. It only applies to those who live the normal indoor life and do not adequately supplement, which is more than 90% of the population.
Professor Robert Heaney of Creighton University has written another masterpiece looking at where the vitamin D of 90% of the population comes from. He has asked a simple question. Where does their vitamin D come from? We were told that 90% of it comes from skin production. Dr. Heaney and colleagues wanted to know if that was true.
Most studies show that the normal population of modern day indoor, non-supplementing humans get around 120–180 IU of vitamin D/day from food and supplements, although some studies show it as high as 300 IU/day. What we have always been told is that skin synthesis makes up the rest. As I said above, it is often repeated that skin synthesis accounts for 90% of the vitamin D of those who are chronically deficient. 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 mathematical formulas to calculate total vitamin D intake for those who do not sunbathe or take 5,000 IU/day from both food and from sunlight to reach those 25(OH)D levels.
They found the following for the vast majority of modern day humans:
- The average total input of vitamin D in these 8 studies is around 2,200 IU per day, resulting in mean 25(OH)D levels of 20-25 ng/ml.
- Solar synthesis is not playing a very large role in actual vitamin D status in many contemporary first-world populations. The largest relative contribution of sun exposure in any of the 8 studies was only 25% and the largest absolute contribution at the summer peak was 485 IU/day, much lower than we believed before.
- The average amount of vitamin D from sunlight is around 320-480 IU/day.
- Food inputs of cholecalciferol only contributed about 120 -160 IU/day.
- Thus the total intake of vitamin D from food and sunlight is around a total of 400–600 IU/day.
- This leaves an unaccounted input gap of around 1,600 IU/day. Where does it come from?
The hypothesis presented in this paper is that meat contains 25(OH)D 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 does not assess 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’d likely find that people get much more of their vitamin D from sunlight than food.