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.

Other 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.

And 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.