The ABCs of vitamin D: What you need to know

Food sources: Vitamin D is found naturally in fatty fish like salmon, tuna and mackerel (a tablespoon of cod liver oil packs a walloping 1360 IU) and in fortified milk, breakfast cereal, infant formula and other foods like orange juice and yogurt.   

Dietary supplements: If you plan on taking a supplement, recent research points to D3 as a better choice.

Recommended daily allowance: The Institute of Medicine’s RDA is 600IU for those between 1 and 70 years old and 800IU for those over 70. The safe upper intake level for adults and children 8 and older is 4,000 IU per day. These recommendations are based on an assumption of minimal sun exposure. They’re also based solely on vitamin D’s role in bone health.  

Extra dosing. If you are low on D, your doctor will help you determine the proper amount you should take to become replete. Do not double dose in an effort to achieve greater health benefits. Too much vitamin D via supplements can cause toxic effects. Excessive sun exposure does not lead to vitamin D toxicity, but it does increase the risk of skin cancer.

Medication interactions: Certain types of medication can reduce the absorption of vitamin D. Talk with your doctor.

Sunshine. Most people meet at least some of their D needs through sun exposure. Cloud cover and shade can reduce UVB radiation, which the skin uses to synthesize D; glass blocks UVB completely so don’t expect to get your D by standing indoors at the window. Recommendations vary but 5-30 minutes of sun exposure to bare skin without sun block twice a week should lead to sufficient D synthesis. Mornings and afternoons are preferable to mid-day, when the sun is stronger. All that said, UV radiation is known to cause skin cancers, so limiting exposure is a prudent choice. Those with a history of — or high risk for — skin cancer should supplement other ways.

High risk for low D. Older adults are at risk because they spend more time indoors and aging skin can’t synthesize D as efficiently. African Americans and others with high melanin levels are also at risk as are infants who’ve been breastfed only, people who live in cloudy climes and people who are obese. These groups in particular may want to have their levels checked.

Vitamin D test. Vitamin D levels can be checked via a simple blood test.

Information provided by the National Institutes of Health, the National Cancer Institute and Fred Hutch researchers.

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