You know what I mean: how long do improved vitamin D levels last? If I had a good summer of sun exposure without burning will this help me through the winter? What if I have three good summers in a row and then go for 12 months with no sun exposure and no supplementation? Do levels change from day to day? If I go on a business trip and don't get to go out in the sun, does it make sense to think "uh-oh, I better supplement, or I'm going to get sick"?
This is the question that's on my mind as the sun heads off to her southern retreat. Put generally: what is the duration of vitamin D in the body?
If you look at this graph you see that in the UK vitamin D status declines from September through to March.
It is however no good thinking that the levels humans attain now through sunlight are in any way comparable to those that would have been attained even 50yrs ago. http://rstb.royalsocietypublishing.org/content/365/1554/2835.full Ozone concentrations [O3] in the industrialized countries of the Northern Hemisphere have been rising at between 1 and 2 per cent per year (Chameides et al. 1994). The surface [O3] has now reached a global mean of approximately 50 ppb (8 h summer seasonal average; Fiscus et al. 2005). Nearly a quarter of the Earth's surface is at risk of experiencing concentrations in excess of 60 ppb during mid-summer. Over the same period 25(OH)D levels have declined at the same rate.because ozone in the atmosphere blocks UVB and prevents it reaching ground level. That is why people in URBAN environments or downwind of towns/industry always have lower vitamin D status than their rural counterparts at the same latitude. At no point in the year to UK adults actually have sufficient vitamin d to maximise absorption of calcium from diet. To allow human breast milk to become a vitamin d replete food for human babies requires a 25(OH)D around 60ng/ml 150nmol/l. Double the UK peak level and more than 4 times the Winter low point. Humans only begin to store D3 when 25(OH)D is above 40ng/ml and it's only around 60ng/ml that the store is sufficient to see us through the winter safely. There is no possibility that Vitamin D stored this summer would last more than a year. It's true a seasonal variation in status would have occurred but the natural status of people living more exposed outdoor lives would have allowed D3 storage so the drop in status would not have led to insufficiency or deficiency status. Dropping from 100ng/ml down to 75ng/ml or from 75ng/ml down to 50ng/ml still leaves people with reserves of D3. Compare that with UK adults, at best at 30ng/ml dropping to below 15ng/ml. It's no wonder we are vulnerable to colds and flu in winter. There is a viable alternative to having a flu jab every winter.
I've read before that a summer's tan will supply the body through winter, but I'm not sure it applies at my more northern latitude (Chicago). I wish I could dig up the reference, but I can't think where to begin googling.
Just to play it safe, I'm probably going to visit a tanning salon throughout the fall/winter.
I can't remember where I read it - and a quick google didn't help - but I am sure that the body stores Vit D for up to 3 months. Assuming, of course, that you have plenty extra. D3 is cheap in pill form.
Probably depends on many factors, like latitude and how long summer and winter are, how often you were out and how much upkeep exposure you get in the winter, and how much melanin in your skin, i.e. darker folks need more exposure. http://dermnetnz.org/systemic/vitamin-d.html says 30-60 days, and gives some general vitamin D references.
well some of the affects that happened to lots of people from lack of exposure to sunlight/d that happened in winter are things like colds/sickness, lethargy, depression, SAD (seasonal affective disorder, not standard American diet for once etc), weight gain, which all seem to be linked to lack of sunlight/D.. I know when I was at school in Rochester Ny and didn't see the sun for months in college it completely wrecked me for awhile.
so some effects should be noticeable, also I believe your body can only store up to a week or 2 worth of d I'm not sure when it would run out, but as I'm a night dwelling computer programing I take it year round now.
Modern humans evolved in Africa, but we've been living in Europe and North America for almost 50,000 years, very far from the equator. Native Americans living in higher latitudes were much darker than their European relatives, yet we can assume they weren't deficient in Vitamin D, otherwise selective evolution would have produced lighter skin tones. Some think that light skin came about only with the advent of agriculture (12,000 years ago), as food sources in the hunter-gatherer diet provided supplemental Vitamin D even in higher latitudes.
I think it's very safe to assume that people living in higher latitudes (and their descendents) are adapted to a seasonal rise and fall in serum D levels. There were no supplements in 50,000 B.C., and while hunter-gatherers' food sources would have provided much higher levels of vitamin D than a modern diet, it couldn't have been more than a few thousand IUs daily. Vitamin D is fat-soluble and hunter-gatherers could have been producing upwards of 20,000-40,000 IU from the sun daily for 5-6 months out of the year, so it would make sense that we can build up stores of vitamin D to access during the "vitamin D winter."
I'm lucky in that I'm able to get an hour or two of full-body (including torso) sun exposure year round (if I want to; I usually put a shirt on in December). I live around 39 degrees latitude so I might experience a vitamin D winter for 2-4 months, but I'm of European descent and I think the vitamin D I produce during the spring, summer and fall is enough to hold me over. I take FCLO during those months as well, but that's only a couple thousand IUs - again, I'm (hopefully) relying on vitamin D stores.
If you work indoors and aren't able to sunbathe on your lunch break, you're in a tough spot. Food supplements don't have very high levels of D, but supplemental D is also a bit iffy. Every day we find out that some vitamin's dangerous to supplement with (but fine if you get it naturally or in food). Maybe move to Africa?