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Under winter’s sunless skies, candles and crackling fires can brighten our day.  But a candle’s warm glow cannot replace the sun’s ultraviolet rays, which help our bodies manufacture vitamin D – the “sunshine vitamin” – a crucial nutrient for bone health.

As spring struggles to gain footing and the weather varies almost daily, vitamin D can give a boost to winter-weary, summer-ready bones. When the sun finally does peek out, its ultraviolet rays help our bodies manufacture vitamin D – a crucial nutrient for bone health.

Low vitamin D levels in infants and children can cause bone softening, known as rickets, which results in problems such as delayed tooth formation and severely bowed legs. In adults, vitamin D deficiency causes osteomalacia – bone softening accompanied by muscle weakness and body pains. Regular doses of sunshine can help prevent these problems because our skin makes, or “synthesizes,” vitamin D when exposed to the sun’s ultraviolet-B (UVB) rays.

“Our skin was designed to make lots of vitamin D and serve our body’s needs,” says Craig Langman, M.D., a pediatric nephrologist (kidney specialist) at Northwestern’s Lurie Children’s Hospital of Chicago.

Kidneys relate to vitamin D and bone thinning because they release several hormones into the body including vitamin D, which is a naturally occurring hormone and not a true vitamin. After vitamin D is ingested or synthesized, it is processed by the liver and kidneys, then released into the bloodstream, where it helps absorb the calcium needed for bone strengthening.  

Sunlight is the natural way to get vitamin D, and the amount needed for synthesis varies by skin color. “If you’re fair skinned,” explains Dr. Langman, “about 15 minutes of exposure on your arms and legs three times a week before applying sunscreen is enough to make a week’s worth of vitamin D.” Darker skinned people tend to need 30-45 minutes three times a week, because pigment – which helps protect against sunburn – naturally blocks UVB rays.

The winter sun’s UVB rays are too weak to produce adequate vitamin D in people living above 37 degrees north latitude – an area of North America that includes Illinois. To compensate, poeple should meet their daily requirement through fortified foods and supplements (look for vitamin D3 or “cholecalciferol” on the label).  

Since the 1930s, after researchers made the connection between low vitamin D and rickets, the vitamin has been added to milk and some cereals. Fortified milk and cereal together can provide around 200 IU (international units) of vitamin D – about 30-50% of the daily requirement, according to the Institute of Medicine’s Food and Nutrition Board (FNB). The FNB recommends 400-600 IU per day up to age 70 (400 IU for children under 1) and 800 IU for people over 70. Other dietary sources of vitamin D include eggs with their yolks, fortified foods such as yogurt, granola bars and juices, as well as cod liver oil and fatty fish such as salmon, tuna, herring, and mackerel.

About 80% of African Americans are misdiagnosed with low vitamin D, according to a study led by Ravi Thadhani, M.D., a nephrologist at Massachusetts General Hospital. Dr. Thadhani and his team noticed that African Americans with lower levels (compared to Caucasians) of 25-hydroxyvitamin D and a binding protein actually had higher bone density, indicating there was no deficiency. When the team measured the active or “bioavailable” form of vitamin D instead, the levels looked normal and consistent with study participants’ bone health. Gene variations, the study concluded, account for the difference in vitamin D levels between blacks and whites. “The main finding of the study is that we need to rethink the definition of vitamin D deficiency,”says Dr. Thadhani. “Treating different populations as one big whole doesn’t necessarily serve people well.”

Some research shows beneficial links between vitamin D and health issues like diabetes and hypertension, but the vitamin’s role in preventing or treating non-skeletal diseases is still inconclusive, according to JoAnn Manson, M.D. of Harvard Medical School. Dr. Manson is leading a study to determine whether daily dietary supplements of vitamin D or omega-3 fatty acids reduce cancer, heart disease, and stroke risk. “We’re at a crossroads with vitamin D research,” says Dr. Manson, who co-directs the VITamin D and OmegA-3 TriaL (nicknamed “VITAL”) at Brigham and Women’s Hospital in Boston, “because we really don’t yet know if there’s a cause-and-effect relationship between vitamin D status and outcomes such as heart disease and cancer.”

While waiting for the return of warm sunny days, it might be tempting to boost vitamin D synthesis in the rays of specialized ultraviolet lamps. Dr. Langman advises caution when using such lamps, or tanning beds, as a substitute for sunlight in vitamin D production. “There is no evidence that it’s safe or effective to use those lights,” he says, “and you might be exposing yourself to harmful skin radiation.”