Vitamin D, Fiat Lux, and Circadian Rhythms

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.


UV rays that reach Earth



Part 2

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).


electromagnetic spectrum


Vitamin D is a bit more picky: stronger preference for specifically UVB (290 nm).  Sunbeds provide roughly the same proportion of UVA and UVB as sunlight, but it’s much more concentrated.


shorter wavelengths pack a stronger punch


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.

EDIT: These Sperti units are a bit pricey, but provide up to 30% UVB!

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:




Part 3

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.


calories proper


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  • “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.”

    Tanning beds and hypervitaminosis D: a case report.

    ^^^ this girl started using a sunbed and her 25(OH)-Vitamin D3 levels doubled. 1,25(OH)2-Vitamin D3 didn’t budge.

  • Ultraviolet A sunbeds and vitamin D.

    This article says that the UV from some of old school sunbeds is 99% UVA, 1% UVB. No bueno.

  • Phil Thompson “For clear
    atmospheric conditions, no dermal vitamin D production occurs at 51 degrees latitude and
    higher during some periods of the year. At 70 degrees latitude, vitamin D synthesis can be
    absent for five months” – with an online calculator at

    • Wow, I was really just guessing at 40 degrees North/South.

      “during some periods of the year”

      Hopefully, sunlight is a priority during the other periods of the year…

      • Phil Thompson

        At 52.6 N (England) the calculator says no Vit D synthesis in Dec & Jan, an hour or so in Nov, 4h in Feb and 6-7 in Oct and March.

    • AnEctomorph

      Love the online calculator! Very useful! Thanks for sharing.

  • rs711

    What do you make of Masterjohn’s point at 7:37 about recommendations to go above (approximately) 32g/mL of 25(OH)D currently being dangerous/unwise given the paucity of data within those ranges?

    From his paper “Vitamin D toxicity redefined: Vitamin K & the molecular mechanism” he also states: “The hypothesis [A – D – K interaction] therefore has the potential not only to explain why vitamin D may contribute to these endpoints at given doses, but why the responses of these endpoints to a given dose or serum level of vitamin D may vary widely between different groups that may have differential intakes of vitamins A and K or differential use of coumadin derivatives.”

    PS: I’ve become a lizard.

    • My “opinions” sans viewing are: 1) we need to focus on blood levels, not dose; and 2) LIGHT-induced Vit D doesn’t reach toxicity.

      If supplementing, do like Jeanie: get tested & re-tested to find the right dose.

      P.S. Repti-Glo?! or the uber-Sperti?

      • Jack Kruse

        Taking Vitamin D in winter actually blocks the uncoupling proteins in mitochondria. The best way to get it is via the sun. So where you live determines if you can sulfate D3 or cholesterol once again showing you the environment trumps ancestral beliefs/WAP and their dietary dogma’s.

        • Joshua

          Question is: Do mitochondria function differently because it’s darker (or colder) during winter months. Vit D amounts and Cold Thermogenesis? DHA’s paramagnetic abilities in the cold vs hot temperature and Vit D resources needed? Those Hadza off the charts vit D studies might be off the charts b/c its HOT where they live. I’d love to see amounts in Northern Tribes. Maybe we need to look closer at at a ‘natural’ year long circadian supercycle (the sum of all circadian rhythms).

          • Jack Kruse

            yes they do…….because they interact with magnetoelastic waves of seasons. This is why the cell membrane of the inner mitochondrial membrane is so radically different then the one on skin cells where Vitamin D is made. Atoms are engines of creation because they absorb and radiate all types of energy. The key is their organization to take advantage of the thermodynamics of the environment they are found in. The physics of organisms can enrich biologic understanding when we marry its ability to carry information and energy to and fro. The cell membrane is a non-equilibrium structure with an enormous electrical potential gradient across it. It also has atoms in it that allow for it to act like a topologic insulator. This gradient makes it ideal to carry both information and energy to the rest of the cells in tissues.

          • Jack Kruse

            The first hurdle to understanding organisms in terms of physics is that they are highly differentiated, ordered, heterogeneous systems that transform energy at the level of individual molecules, whereas conventional physical theories apply to large ensembles of identical molecules.

          • Jack Kruse

            So what links organisms kinetically and thermodynamically? Their cell membranes are the shot answer. Cell membranes are only two molecules thick in most cases. These membranes are held together electrically. Here we can see how the electromagnetic force begins to control atoms in life. Cell membranes consist of negatively charged atoms in molecules bound together by positively charged calcium ions which act like a form of biophysical cement. Aberrant electromagnetic field cause calcium ions to efflux from atomic positions of strength. Their position allows for proper electrical and magnetic field reception for cells.

          • Jack Kruse

            Pulses carried by microwaves are particularly dangerous when you consider how topologic insulators work in cell membranes with native and non native electric and magnetic fields. This is because their very short wavelength allows the transmission of pulses with extremely rapid rise and fall times, and it is the rate of change of the field strength rather than their total energy that does most of the biological damage; it catapults vital calcium ions away from cell membranes, which in turn makes them leak more. This is how leaky gut really begins in humans. This leakage can explain the great majority of the observed adverse health effects of prolonged exposure to electromagnetic radiations.

            A simple way to explain the selective removal of calcium ions is to imagine trying to harvest ripe apples from unripe apples by shaking the tree. If you don’t shake it hard enough, no apples fall off, but if you shake it too hard, they all fall off. However, if you get the shaking or oscillations just right, only the ripe ones fall off and are ‘selectively harvested’.

            We can apply the same logic to the positive ions bound to cell membranes. Alternating voltages and Dirac masses try to drive these ions off and then back onto the membranes with each half cycle. If the voltage or magnetic field is too low powered, nothing happens. If it is too highly powered, all the ions fly off, but they are capable of return when the voltage or magnetic field reverses or returns to normal. This happens every night and day transition and as season change. Vitamin D sulfation is like and apple in our cell membrane. Same is true with sulfated cholesterol that makes up the bulk of human cell membranes

      • Morgan Pfiffner

        I’m curious if you’ve you watched his talk yet?

        “Lifeguards in the tropics can reach blood levels in the 50s and 60s naturally from sun exposure, suggesting these levels are “natural,” although lifeguards in Israel have twenty times the rate of kidney stones as the general population”

  • Here’s another interesting talk recently released by UCSD:

  • Vitamin D storage in adipose tissue ?

  • Joshua

    The question is what is the 1,25(OH)2-Vitamin D3 production with someone of impaired liver or kidney function. Or of a BMI of 30?

    • 1) gotta get tested

      2) 1,25(OH)2 is highly regulated. Blood levels don’t budge unless severely deficient. 25(OH) is a better indicator

      3) enzyme activity is rarely limiting for vitamin D synthesis, and cause/effect might be difficult to tease out… I’d bet low vit D correlates with diseases of many organs, not just those involved in vit D synthesis

  • Joshua

    Also this is an interesting read for anyone that’s interested in clouds.m 99% attenuation of UVB during overcast conditions.

  • John Lushefski

    I haven’t had the same issue as Woo with D, but I got a cold twice after a few days of using Thorne liquid k2: 2mg/day; occurrences separated by ~3 years. Once it happened with wheat germ oil too. Tinkering with fat solubles is dangerous!

    • “Tinkering with fat solubles is dangerous”

      Maybe not “life-or-death” dangerous, but definitely something to pay attention to!

  • Skin Under the Sun: When Melanin Pigment Meets Vitamin D