Vitamin D: Do you need to supplement?

I’m not a fan of daylight savings time, it feels like our winter days are just so short. The sun’s still not up when many of us leave in the morning, and it’s often dark by the time we get home.  We can easily go weeks without truly getting direct sunlight.  
And since the sun is our most significant source of vitamin D, our body’s production of D can drop significantly during these winter months. 
We’ve known for years that vitamin D is essential for optimal bone health.  But we’re also learning that it may help prevent and treat conditions like diabetes, high blood pressure, and multiple sclerosis.  Optimal vitamin D levels can enhance athletic performance and might even protect against certain types of cancer.  Yet three out of four Americans aren’t getting enough vitamin D, and most don’t even know it.  
So who’s at risk? For starters, people who get very little sun exposure.  This includes people with jobs and hobbies that keep them inside, as well as those who live in the northern part of the country.  And the darker your skin, the higher your risk of deficiency, since the pigment melanin reduces the skin’s ability to produce vitamin D.
Being overweight also puts you at risk, since body fat stores more of the fat-soluble vitamin D, and doesn’t release as much into your bloodstream.
People over fifty, as well as infants, also have cause for concern.  Over the age of 50, our skin can’t make vitamin D as efficiently, and the American Academy of Pediatrics says that all breastfed infants need to be supplemented with 400 International Units (IU) of vitamin D daily.
How can you know if you’re getting enough?
The only way to be certain is to ask your doctor to check your levels of vitamin D, specifically 25-hydroxy-vitamin D, also referred to as 25(OH)D.  But there is an ongoing debate over just what blood levels of vitamin D are optimal.  
The National Institutes of Health says that blood levels of 20 ng/mL is sufficient, though other experts recommend higher levels, suggesting a minimum of 30 ng/mL, with ideal blood levels around 50 ng/mL.  
So how do we get our daily dose of D?
The sun is our best source of vitamin D by far, and the general consensus is that most people need about 10 to 15 minutes in ‘direct’ sunlight (at a time of day when your shadow is shorter that you are).  Even if you spend much more time in the sun, our skin regulates its production of vitamin D, and it won’t make too much.
The American Academy of Dermatology says that we should use sunscreen any time we’re out in the sun, but using an SPF 8 or greater appears to block the vitamin D-producing rays.    

And no matter how bright the sun, the UVB radiation doesn’t penetrate glass, so even if you’re getting most of your sun exposure through a window or while driving in your car, you won’t be getting your dose of vitamin D.

If you’re not getting enough strong sunlight, or you’re at high risk because of your skin color, weight, or age, you’ll want to take a look at how much vitamin D you’re getting through food and supplements. 

The Recommended Dietary Allowance (RDA) is 600 IU vitamin D daily for adults, though many experts say we need at least 800 to 1,000 IU daily. It’s nearly impossible to get this much vitamin D from through diet alone, since only a few foods are naturally good sources of vitamin D.

A single tablespoon of cod liver oil provides 1360 IU vitamin D (but how many of us do that?), but you’ll only get 100 IU of vitamin D per ounce of salmon or cup of fortified milk.

Supplements can help you get your required dose of vitamin D.  Look for supplements that contain vitamin D3, which is more effective than vitamin D2 in raising blood levels of vitamin D.   

And though it’s possible to consume too much vitamin D, it’s not likely.  The upper limit for adults is currently 2,000 IU, but recent research supports a safe upper limit as high as 10,000 IU daily.

Since vitamin D requirements can vary widely depending on where you live, your skin type, and how much time you spend outside, a blood test is the only way to know for sure that you’re getting enough.

If it turns out that you are vitamin D deficient, you and your physician can work together to figure out what combination of supplements, diet, and safe sun exposure are best for you.  Re-check your levels after three months to ensure that your personal plan is working.

Molly Kimball, RD, CSSD is a registered dietitian + nutrition journalist in New Orleans, and founder of Ochsner Eat Fit nonprofit restaurant initiative. Tune in to her podcast, FUELED | Wellness + Nutrition and follow her on Facebook, Instagram and Twitter at @MollyKimballRD. See more of Molly’s articles + TV segments at www.mollykimball.com.