“Vitamin D has been oversold.”
You’ve probably heard about many studies linking low levels of vitamin D with higher risks of such wide-ranging problems as cancer, heart attacks, infections, Alzheimer’s disease, autoimmune conditions, depression, and obesity. When his friends cite these reports, Dr. Rosen points out that the studies don’t show cause and effect. “These associations can be really misleading,” he says. “Without more trials, we just don’t have a good sense of how vitamin D supplements help.” The one exception: Vitamin D intake has conclusively been shown to promote strong bones. You need vitamin D to absorb calcium—that’s why the two are often packaged together. But too much vitamin D (above the safe upper limit of 4,000 IU for adults) can lead to nausea, vomiting, constipation, and weakness. And because vitamin D boosts calcium absorption, amounts above the upper limit can increase risks from both nutrients.
While blood levels of vitamin D are widely reported to be abysmally low, the latest research shows that most people have enough for healthy bones (above 20 ng/ml), making supplements less needed. Since intake from food sources, such as fortified milk, fatty fish, eggs, and mushrooms, is subpar, most of us get surprisingly meaningful amounts from the sun, which triggers D production in the skin.
“Even ten minutes of casual exposure on the hands and feet seems to make a difference,” Dr. Rosen says.
So should you take a supplement?
Most adults probably don’t need one. But after age 70, when your bones need more vitamin D, your body begins to make less of it from the sun, so Dr. Rosen recommends taking an extra 400 IU a day. Supplements may also be worthwhile for teens (who are in their prime bone-building years), people with disorders such as Crohn’s or celiac disease (which can affect D absorption), and obese people (whose excess body fat may stockpile the nutrient, so less circulates in blood).
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