bright sunshine makes vitamin D

It’s a sunny day in the middle of winter.  You bask in the sunlight outside during lunch.

You’re getting your daily dose of vitamin D, right?

Maybe not.

Winter is not the best time of year to make vitamin D regardless of where you live. The sun is just not at the right angle to get UV light to the earth – and UV light is needed to synthesize it in the skin.

In the summer, vitamin D production can be high because UV light levels are high. Just a short time spent in sunshine in the middle of the day will do it. In the winter, however, UV levels from the sun are much lower (UV index is less than 3). So your levels of vitamin D synthesis may drop, even if you spend time in sunlight.

A simple test is to look at your shadow. If its the same height or shorter than you, you’re getting enough sun to make vitamin D. If its longer than you, you’re probably not. In most places in the US in the winter, you can probably guess what you’re going to see.

UV Index in January in the northern US is generally too low for vitamin D synthesisOther things influence vitamin D made by your body, including:

  • time in the sun (getting outside for longer is better)
  • amount of sunlight (those cloudy days lower the UV levels)
  • time of day (best in the middle of the day, not good at the beginning or end)
  • skin color (darker skin needs more UV light)
  • amount of skin that is exposed (your warm winter clothes will block light to your skin)

When put together, is it any wonder vitamin D status is low in the coldest months of the year?

The reality is that in many parts of the United States, even in the best conditions, the amount of vitamin D made from sunlight exposure from November to February is small—that’s why it’s been called the “vitamin D winter.” In the southern hemisphere, this is typically in June through August. The further towards the poles you go, the longer the winter lasts.  In the higher latitudes, it can run until March or as long as April in the north, or September in the south.
That spells real trouble for people who rely on sunlight for vitamin D during wintertime. Levels tend to bottom out around January or February (August in the southern hemisphere). Interestingly enough, that’s the prime time for the cold and flu. Several studies show that vitamin D is linked with immune system function and the risk of respiratory tract infection goes up when levels in the body go down.

Milk Vitamin DAnd research at the Linus Pauling Institute and Oregon State University has linked low D levels to levels of depression. Sound like the winter blues?

So keep D in mind this (and every) winter. The best idea is to get your blood levels checked—ideally by a certified laboratory—even if you do go out in the sun or eat vitamin D rich foods.

Studies suggest it’s best to have your levels above 30 ng/mL (80 nmol/L). If your levels are low, your doctor can suggest a supplementation strategy that works for you. Taking a supplement or eating food naturally high in or fortified with vitamin D (such as fish and dairy products) can help.

For people with adequate status, the Linus Pauling Institute recommends 2,000 IUs of vitamin D (from diet or supplements) each day to help maintain levels in the body and keep you in good health.  You can review the scientific evidence supporting this recommendation by visiting our page on the Micronutrient Information Center.

For more information about vitamin D and your health, check out our recent webinar.

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8 thoughts on “Can You Rely on Sunlight to Get Enough Vitamin D This Winter?

  1. You refer to Vit D as we see on retail packaging. But this can be only D2 when you really need Vit D3 that acts like a hormone on prostate cancer. I have a daily pill of 2,500IU of D3 post prostate cancer diagnosis. My ‘winter liquid sunshine’!
    Please add this as a topic so all my understand the different types of Vit D. Thanks and kindly acknowledge.

  2. Hello Alexander

    Somewhat offtopic, but given your interests, are some Vitamin C preparations better than others?
    What about natural source Vitamin C such as Camu Camu, Acerola, Bitter orange, Amla etc- are they better absorbed? Has that been researched recently?
    As I remember Pauling said at one point that there was no difference in efficacy, but test sensitivity has potentially improved over the decades.

    Also whats the latest on safe average dosage? I remember hearing recently that its around 500mg, do you concur?
    And for what reasons?

    Re the artcle – I have MS and used to have chronically low Vit D.
    There is so much contradictory info on dosage – I supplemented and my blood levels had gone up to over 100- Im trying to bring that down a bit and havent supplemented for over a year on my doctors advice, but it doesnt seem to be coming down, although getting tests from my NHS doc is a real challenge…

    Instinctively I feel the need to supplement now [Im in the UK]but am concerned about my erstwhile high levels, last tests were 6 months ago and two prior were all with high levels, and at that time [while not supplementing] there was no sign of decline in the levels…..I dont get much sun.

    Is excess levels of vit D that dont decline, a possible sign of Liver damage?
    Possibly thats a strange question 😉 but Im feeling anxious about this and to be perfectly honest I dont think my doctor has a clue, and Im not getting useful advice.

    He seems amazed my levels are so high now despite my repeatedly telling him that I suplemented with CLO and vit D rotationally. He has a waiting room full of Somali women with dark skin and head to foot dark clothes all clearly desperate for supplementation – the clinic give huge one off doses – with poor results – the mans an idiot, he doesnt listen, nor does he investigate developements.
    Thank you

    • We really have little good information about vitamin C preparations – so far, it doesn’t look like any source is better than any other. All of them make claims, but none provide evidence. Obtaining that evidence is very expensive, but would help them beat out the competition, so it unfortunate that nobody tries to do the proper research.

      The Linus Pauling Institute advocates that people take 400 mg a day – however, I think if you are taking 500 mg a day, you’re just fine. The upper limit for vitamin C is 2,000 mg per day, and even that dose has not been associated with many side effects.

      As for your vitamin D levels, it is difficult to say without further testing. Consistently high vitamin D levels may be a sign of liver or kidney disease, but without any evidence that such disease is present, I wouldn’t worry – but consult with your physician. Another test that a doctor could perform is to look at calcium levels in the blood – if those are also high the excess vitamin D might be a problem.

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