"It is possible to get too much vitamin D from sun and it's harder to calibrate than supplements, so I avoid it."
That's probably wrong. Again, vitamin d3 production triggered by sun exposure self-regulates--this means no more is produced once a certain level is achieved, regardless of how long you are in the sun. And the equilibrium level--10,000 IU per day--has been shown to be safe in many studies (at least if generated endogenously).
Let's start with the Wiki stub:
"Vitamin D
2 is derived from fungal and plant sources, and is not produced by the human body. Vitamin D
3 is derived from animal sources and is made in the skin when
7-dehydrocholesterol reacts with UVB
ultraviolet light at
wavelengths between 270–300 nm, with peak synthesis occurring between 295-297 nm.
[4][5] These
wavelengths are present in sunlight at sea level when the sun is more than 45° above the horizon, or when the
UV index is greater than 3.
[6] At this solar elevation, which occurs daily within the
tropics, daily during the spring and summer seasons in
temperate regions, and almost never within the
arctic circles,
adequate amounts of vitamin D3 can be made in the skin only after ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen. With longer exposure to UVB rays, an equilibrium is achieved in the skin, and the vitamin simply degrades as fast as it is generated.[1]"
...
Exposure to sunlight for extended periods of time does not cause vitamin D toxicity.
[34] T
his is because within about 20 minutes of ultraviolet exposure in light skinned individuals (3–6 times longer for pigmented skin) the concentration of vitamin D precursors produced in the skin reach an equilibrium, and any further vitamin D that is produced is degraded.[35] Maximum endogenous production with full body exposure to sunlight is 250 µg (10,000 IU) per day.[34]"
Footnotes 34 & 35 of the Wiki stub lead to studies published in the
American Journal of Clinical Nutrition 1995 & 1999; I checked, and the conclusions therein do indeed precisely track the Wiki stub:
"
Ultraviolet exposure beyond the minimal erythemal dose does not increase vitamin D production further. The ultraviolet-induced production of vitamin D precursors is counterbalanced by degradation of vitamin D and its precursors. The concentration of previtamin D in the skin reaches an equilibrium in white skin within 20 min of ultraviolet exposure (
41). Although it can take 3–6
times longer for pigmented skin to reach the equilibrium concentration
of dermal previtamin D, skin pigmentation does not affect the
amount of vitamin D that can be obtained through sunshine exposure
(
42)''
Copied from the full text of the 1999 study (Footnote 34), with additional citations to other supporting studies.
As regards this issue, the US National Institute of Health's Vitamin D Fact Sheet cites approvingly a 2001 study that itself relied on said 1995 and 1999 studies concluding that it isn't possible to achieve toxic levels of vitamin D through sun exposure.
As far as that South India study goes, its results seem pretty isolated. It is mentioned in the Wiki stub in the section dealing with Vitamin D's role in the prevention of coronary disease. I don't think those results have been duplicated elsewhere, and I don't think any other study has even suggested an increased risk of coronary disease due to sun exposure. To the contrary, other studies in the UK, etc. have shown decreasing levels of risk for heart disease as sun exposure increases, and a Harvard study found that risk of cardiovascular disease increased in vitamin D deficient people. All things considered, I find these other studies more predictive and reliable, and the South India study an anomaly with results that may be better explained by factors unique to that narrow population group extrinsic to length of sun exposure.
Edited by TianZi, 27 April 2008 - 05:42 AM.