Vitamin D synthesis is TEAMWORK!
Skin: 7-dehydrocholesterol + UVB = previtamin D3.
Liver: Previtamin D3 –> 25(OH)-Vitamin D3.
Kidney: 25(OH)-Vitamin D3 –> 1,25(OH)2-Vitamin D3 if you need it or 24,25(OH)2-Vitamin D3 if you don’t.
N.B. one of the major regulatory pathways occurs in skin: if you’re getting a lot of sunlight, then skin darkens to block this step. Supplemental and dietary Vitamin D3 bypass this… but the dietary Vitamin D supply rarely produces toxicity because it’s not very abundant. In other words, sunlight Vitamin D never reaches toxic levels. Supps could (rare, but possible).
Disclaimer: I’m not against Vit D supps, but prefer sunlight whenever possible.
The other major regulatory step is in the kidney. Production of 1,25(OH)2-Vitamin D3 is tightly regulated — so blood levels don’t decline until your very deficient… so 25(OH)-Vitamin D3 is a better indicator of skin production and dietary intake.
Disclaimer #2: this post is not about any of the pleiotropic effects of Vitamin D or D supps, which range in value from worthless to helpful to possibly harmful.
For some of that stuff, see:
Vitamin D: still a scam, still immunosuppressive… by Jane Plain
The Vitamin D seminar by Ivor Cummins
Vitamin D 101 by Kris Gunnars
The Vitamin D Debacle with Ivor Cummins and Sam Feltham
all of the above are more pro-D supps than me, which I find perfectly OK.
The ultraviolet light from the sun that reaches the Earth is approximately 95% UVA (320-400 nm) and 5% UVB (280-320 nm). About 10% of the sunlight that reaches Earth is ultraviolet. Vitamin D and skin tanning are results of ultraviolet light.
Melanogenesis (skin tanning) is not very picky – it responds to UV light in general, with a modest preference to slightly shorter wavelengths (320-280 nm).
Even UVB-rich reptile bulbs don’t provide much more UVB % than sunlight or sunbeds – their UV spectrum is around 5% UVB… although the reptile bulbs are much weaker than sunbeds, which is OK for pets because they spend much more time basking in it than humans do in sunlight or sunbeds.
On a bright sunny day, you can make 1000 IU Vitamin D per minute. PER MINUTE! Get nekkid in a sunbed and this is enhanced due to: 1) more skin exposure; and 2) more concentrated UV (see below). Concentrated UV, as in a sunbed, can be damaging and cause skin aging… so if this is your source of Vitamin D, please be mindful of duration.
The light-entrainable oscillator, which governs circadian rhythms in the brain, is set off by blue light… and similar to Vitamin D, melanopsin (in your eyes) it’s more picky than melanogenesis (in your skin), but prefers a longer wavelength, ~480 nm. However, bear in mind that both sunlight and sunbeds deliver a considerable amount of this light.
1) get your Vitamin D levels checked.
2) if deemed inadequate (or suboptimal):
2a) go for a walk around lunchtime (best UVB % at high noon)
2b) if you live in an extreme latitude or it’s just too cold outside, and a sunbed is the only option, keep it brief – just a few minutes, a few times per month (see below) could do it (and get your Vitamin D levels checked – you want to keep sunbed time to a minimum to prevent damage and premature skin aging). Also, don’t do it at night because this type of light includes some blue and can disrupt circadian rhythms.
If you opt for sunbeds, get out before skin gets red. You obviously won’t know this until it’s too late, so note the duration for next time: for example, if skin is red the day after 5 minutes in a sunbed, try 2-3 minutes next time. This is for Vitamin D, not getting a tan. Also, I don’t generally recommend sunbeds, but they work:
Twice weekly sunbed exposure was roughly equivalent to 2000 IU/d in this study! And thrice weekly was better than 1000 IU/d in this study. The difference between these two studies likely lies in sunbed duration & baseline skin tone, although both clearly improve Vitamin D status. Point being, if you start getting a tan, you’re beyond the minimal duration for Vitamin D synthesis. Also, this is assuming you get some dietary D.
Unfortunately, sitting indoors by a window on a cold but otherwise sunny day won’t be very effective because standard window glass blocks out most UVB. Seafood is a great source of dietary Vitamin D, but this shouldn’t be your only source of Vitamin D. Ie, get some sun — the benefit of sunlight extends beyond Vitamin D.
Good news: there is evidence that you can “stock up” on Vitamin D in summer; some can be stored in adipose… so depending on where you live, you may not need to worry about Vitamin D in winter.
Average Vitamin D levels: about 40-80 ng/mL. The upper end of this range is likely optimal. But not too much higher… Vitamin D toxicity is not pleasant: vomiting, irritability, constipation, weakness, immune dysfunction, and worse… but this doesn’t happen with sunlight Vitamin D (and is rare with dietary D). Vitamin D synthesis shuts down when levels get high; the only way around this is supps: no bueno. Take ’em if absolutely necessary, but at least get Vit D levels checked, like Jeanie:
@CaloriesProper *nods* I checked mine every quarter until I found a good level, now yearly. 8k in winter, 4-6k in summer keeps me normal.
— Jeanie WitcraftShiau (@jwitcraft) January 7, 2015
Bronzers: possibly harmful, because they may function, in part, like sunblock.
Melanotan is a drug which induces melanogenesis (skin tanning): probably harmful because skin exposed to ultraviolet light responds with: 1) Vitamin D synthesis; and 2) tanning to prevent damage. Darker skin is less efficient at Vitamin D synthesis. Melanotan: 1) skips the Vitamin D synthesis step; and 2) darker skin further suppresses Vitamin D synthesis.
On the bright side (which really isn’t very bright), bronzers and Melanotan won’t interfere with the positive impact of sunlight on circadian rhythms.
None of this applies to those living near the equator, or maybe about 40 degrees South to 40 degrees North latitude? …I’m guessing here, and skin tone matters, too. People living in these areas probably get enough sunlight without trying most of the time.