Does sunlight fix vitamin D deficiency?

Yes, when UVB is available where you live. Midday sun with arms and legs out, three or four times a week, raises 25(OH)D over two to three months, the same timescale as a supplement. From late autumn to early spring above about 37 degrees north there is no UVB to work with, so sunlight cannot fix a deficiency that season.

Can you get enough vitamin D without the sun?

Yes. Oral D3 raises 25(OH)D reliably, and fatty fish, egg yolks and liver carry some. What you do not get without the sun is the rest of what skin does in light: nitric oxide from UVA, beta-endorphin and alpha-MSH from the POMC cascade, and the sulfated form of vitamin D.

Are vitamin D and D3 the same?

D3, cholecalciferol, is the form your skin makes and the form in most supplements. D2, ergocalciferol, comes from fungi and fortified foods and is prescribed at high doses. Both raise 25(OH)D, but D3 raises it further per IU and holds the level longer (Tripkovic 2012).

Can you get too much vitamin D from the sun?

No. Once previtamin D3 in the skin reaches about one minimal erythemal dose, further UVB converts it into lumisterol and tachysterol, which are inert. Sun toxicity does not occur. Toxicity from pills does, typically at blood levels above 150 ng/mL after months of very high doses.

Do you actually track how much sun you're getting, or do you just supplement and hope for the best?

Track it, then test. The 25(OH)D result is the only number that settles the question. Sunlight is Life is a free app that estimates the IU from each sun session using your skin type, the exposed area and the UVB reaching your coordinates, so the summer half of the ledger is not a guess.

Is Sunlight Better Than Vitamin D Supplements?

Peter CowanSeptember 16, 20265 min read

Sunlight, when UVB is available. Skin in midday sun makes vitamin D3 and a sulfated form of D, and releases nitric oxide and beta-endorphin. A capsule delivers one molecule down one pathway. Take the capsule when the sun cannot do the job: winter above 37 degrees north, dark skin at high latitude, or 25(OH)D below 30 ng/mL.

The capsule contains cholecalciferol, the same D3 your skin makes, so the blood test cannot tell the two routes apart at the 25-hydroxyvitamin D level. That is where the equivalence ends. UVB strikes 7-dehydrocholesterol in the epidermis, opens a ring in the molecule, and body heat finishes the job over the next day. The D3 enters the blood bound to vitamin D binding protein and stays in circulation longer than swallowed D3, which arrives packaged in chylomicrons and clears faster (Haddad 1993).

Sunlight makes more than cholecalciferol

The same exposure that makes D3 runs at least three other pathways. UVA, 315 to 400 nanometers, photolyzes nitrate and nitrite stored in the skin and releases nitric oxide, which dilates arteries and lowers blood pressure for hours (Liu 2014, Opländer 2009). UVB drives keratinocytes to cleave pro-opiomelanocortin into beta-endorphin, ACTH and alpha-MSH; beta-endorphin is the reason sun on skin feels the way it does (Fell 2014). And the skin makes cholecalciferol sulfate, a water-soluble form that oral D3 does not become to the same extent. What the sulfated form does is still being worked out.

A supplement does none of this. It raises 25(OH)D, which is worth doing when the level is low, and stops there. That is the case made under "A Note on Supplementation" in how much sun you need for vitamin D: vitamin D was part of a system, and the pill is one part.

The skin route has a ceiling and the pill does not

Skin synthesis self-limits. Once previtamin D3 in the epidermis reaches roughly one minimal erythemal dose, the dose that just reddens your skin, further UVB converts it to lumisterol and tachysterol instead, both inert (Holick 2007). A full-body exposure at one MED yields about 10,000 to 25,000 IU and then plateaus, however long you stay out.

Pills have no such brake. Blood levels above 150 ng/mL, usually after months at 40,000 IU a day or more, produce hypercalcemia: nausea, kidney stones, confusion. The Endocrine Society sets the adult upper limit at 10,000 IU a day (Holick 2011). If you have sarcoidosis or another granulomatous disease, your tissues activate vitamin D on their own and high-dose supplementation can push calcium up at far lower intakes; test before you dose.

The ceiling is also the argument for timing over quantity. The skin is built to be dosed in short midday windows, then left alone. Staying out past one MED buys a burn, not more vitamin D.

When a supplement is the right call

Winter at latitude. Above roughly 37 degrees north (San Francisco, Denver, Washington DC and everything north), the sun never clears about 30 degrees of elevation for two to five months, and no UVB reaches the ground. Your shadow is always longer than you are. Supplement from late autumn until midday shadows shorten again.
Dark skin at high latitude. Fitzpatrick types V and VI need roughly four to six times the exposure of types I and II for the same D3, which a British or Canadian summer rarely supplies. This is why deficiency rates run highest in people with dark skin living far from the equator.
A low test result. A 25(OH)D below 30 ng/mL in summer means your habits are not producing enough; below 20 ng/mL is deficiency by any definition. Sunlight will lift it, but a capsule lifts it faster while you change the habits.
Age. Epidermal 7-dehydrocholesterol falls with age; by your 70s the skin holds about half what it did at 20 (MacLaughlin and Holick 1985). The same twenty minutes makes half the D3.
Malabsorption, reversed. With celiac disease, Crohn's disease or after bariatric surgery, oral D3 is poorly absorbed. Here the skin route is the reliable one.

If you take a photosensitizing medication such as doxycycline, amiodarone or hydrochlorothiazide, the sun-time figures in this pillar do not apply to you, and the supplement route deserves more weight.

How to dose if you supplement

Test first. A 25(OH)D blood test costs about the price of a bottle of capsules. The research converges on 40 to 60 ng/mL as the range where bone, immune and metabolic markers look best, and in my experience people feel better toward the top of it.

Then work the arithmetic. At steady state, each 1,000 IU a day of D3 raises 25(OH)D by roughly 7 ng/mL in an average adult, less in heavier people because vitamin D is stored in fat (Heaney 2003). Someone at 22 ng/mL who wants 45 ng/mL needs about 3,000 IU a day, plus whatever the sun contributes. The Endocrine Society maintenance range of 1,500 to 2,000 IU a day is for holding a level, not for climbing to one (Holick 2011). Steady state takes about three months, so retest then, not sooner.

Choose D3 over D2, which raises the level less per IU and holds it less well (Tripkovic 2012). Take it with a meal that contains fat.

What to do

Between mid-spring and early autumn, get midday sun with arms and legs out, 10 to 15 minutes for fair skin and 45 to 60 for dark skin, three or four times a week. The best time of day is the two hours either side of solar noon.
Test 25(OH)D at the end of summer and again in late winter. The gap between the two tells you how much your winter supplement needs to cover.
Above 37 degrees north, supplement from November to March with D3 at 1,000 to 3,000 IU a day, scaled to the test.
Keep the morning and evening light regardless of the season. That light does no vitamin D work, but it does the circadian and nitric oxide work no pill can.

Estimate the IU from your next sun session →

The through line

The question sets up a contest that biology does not hold. Sunlight is a signal that runs several systems at once, and vitamin D is one readout of it. A capsule is a repair for the readout when the signal is missing. Use the sun when it is there, the pill when it is not, and let a blood test rather than a season decide which is which.

Common questions

Does sunlight fix vitamin D deficiency?: Yes, when UVB is available where you live. Midday sun with arms and legs out, three or four times a week, raises 25(OH)D over two to three months, the same timescale as a supplement. From late autumn to early spring above about 37 degrees north there is no UVB to work with, so sunlight cannot fix a deficiency that season.
Can you get enough vitamin D without the sun?: Yes. Oral D3 raises 25(OH)D reliably, and fatty fish, egg yolks and liver carry some. What you do not get without the sun is the rest of what skin does in light: nitric oxide from UVA, beta-endorphin and alpha-MSH from the POMC cascade, and the sulfated form of vitamin D.
Are vitamin D and D3 the same?: D3, cholecalciferol, is the form your skin makes and the form in most supplements. D2, ergocalciferol, comes from fungi and fortified foods and is prescribed at high doses. Both raise 25(OH)D, but D3 raises it further per IU and holds the level longer (Tripkovic 2012).
Can you get too much vitamin D from the sun?: No. Once previtamin D3 in the skin reaches about one minimal erythemal dose, further UVB converts it into lumisterol and tachysterol, which are inert. Sun toxicity does not occur. Toxicity from pills does, typically at blood levels above 150 ng/mL after months of very high doses.
Do you actually track how much sun you're getting, or do you just supplement and hope for the best?: Track it, then test. The 25(OH)D result is the only number that settles the question. Sunlight is Life is a free app that estimates the IU from each sun session using your skin type, the exposed area and the UVB reaching your coordinates, so the summer half of the ledger is not a guess.