This is a topic that I think is quite complicated; there are many unknowns and I don't know what the answer is. My gut feeling is if you are getting enough sun, then supplementation shouldn't be necessary. However...
I have been trying to learn more about vitamin D, and the more I read, the more confusing it is because very little is actually known.
I live in Florida and get full sun on a regular basis all year round. I am far enough south that the angle of the sun allows me to produce vitamin D the entire year. Yet, I technically have vitamin D deficiency (29). How is this possible? So I've been on a wild goose chase trying to understand vitamin D.
For example:
"The characteristics of 1,25(OH)2D are those of a hormone, and consequently vitamin D is a prohormone rather than a true vitamin. The structure of 1,25(OH)2D is similar to that of other steroid hormones.
As long as sunlight exposure is adequate, 1,25(OH)2D can be produced by the body without the requirement for ingestion in the diet.
Precisely defining vitamin D deficiency or insufficiency on the basis of 25(OH)D values is still a matter of much debate. A useful but rather simplistic classification of vitamin D status is shown in the
Table. A cutoff value of 30 ng/mL is sometimes used for optimal vitamin status. On the basis of measured concentrations of 25(OH)D,
many patients are given a diagnosis of vitamin D deficiency or insufficiency when most have no evidence of disease."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3012634/
Then I came across this study:
Low vitamin D status despite abundant sun exposure.
Binkley N1,
Novotny R,
Krueger D,
Kawahara T,
Daida YG,
Lensmeyer G,
Hollis BW,
Drezner MK.
Author information
Abstract
CONTEXT:
Lack of sun exposure is widely accepted as the primary cause of epidemic low vitamin D status worldwide. However, some individuals with seemingly adequate UV exposure have been reported to have low serum 25-hydroxyvitamin D [25(OH)D] concentration, results that might have been confounded by imprecision of the assays used.
OBJECTIVE:
The aim was to document the 25(OH)D status of healthy individuals with habitually high sun exposure.
SETTING:
This study was conducted in a convenience sample of adults in Honolulu, Hawaii (latitude 21 degrees ).
PARTICIPANTS:
The study population consisted of 93 adults (30 women and 63 men) with a mean (sem) age and body mass index of 24.0 yr (0.7) and 23.6 kg/m(2) (0.4), respectively. Their self-reported sun exposure was 28.9 (1.5) h/wk, yielding a calculated sun exposure index of 11.1 (0.7).
MAIN OUTCOME MEASURES:
Serum 25(OH)D concentration was measured using a precise HPLC assay. Low vitamin D status was defined as a circulating 25(OH)D concentration less than 30 ng/ml.
RESULTS:
Mean serum 25(OH)D concentration was 31.6 ng/ml. Using a cutpoint of 30 ng/ml,
51% of this population had low vitamin D status. The highest 25(OH)D concentration was 62 ng/ml.
CONCLUSIONS:
These data suggest that variable responsiveness to UVB radiation is evident among individuals, causing some to have low vitamin D status despite abundant sun exposure. In addition, because the maximal 25(OH)D concentration produced by natural UV exposure appears to be approximately 60 ng/ml, it seems prudent to use this value as an upper limit when prescribing vitamin D supplementation.
https://www.ncbi.nlm.nih.gov/pubmed/17426097
This leaves me with LOTS of questions:
1) If adequate sun exposure is all that is required to produce sufficient vitamin D, how is it possible that a majority of people who have more than enough sun exposure have "vitamin D deficiency"? Perhaps the levels used by labs are actually not valid. In fact, there is no definitive answer as to what adequate vitamin D levels are.
2) Is it possible for different people to have different optimal vitamin D levels based on their genetics? From what I've read, this seems very likely, yet there is only one range applied to all people across the board.
3) How safe is it to supplement vitamin D longterm? This is never addressed by doctors, which I find very concerning. Vitamin D is not a vitamin; the characteristics of 1,25(OH)2D are those of a hormone, and consequently vitamin D is a prohormone rather than a true vitamin. The structure of 1,25(OH)2D is similar to that of other steroid hormones.
4) As long as there is no physical evidence of Vitamin D deficiency, what is the benefit of raising it? I've taken vitamin D in the past and never noticed any improvement. I know I'm not alone in this. If someone feels better from supplementing with vitamin D, then that's great and they should continue. But if there's no improvement, why continue?
5) There is a built-in shut-off valve that prevents people from producing more than a certain amount of vitamin D from sunlight exposure after a certain amount of time. Clearly, there is a reason for this. I don't know if this feature is to keep overall levels from rising above a certain level, or to prevent the level from rising too much too quickly. How safe is it to ignore the body's threshold and subject it to far more vitamin D in the form of supplementation than it determined was appropriate?
I'm not a medical expert, and it might be that there are good answers to all of my questions. But I have yet to find a doctor who is interesting - or willing - to discuss any of them with me.