Vitamin D! Do you have enough? The numbers have been squishy. At one point, it seemed likely that everyone in the U.S. was suffering from a “deficiency.” But what did that mean, exactly, outside of the non-FDA-regulated claims of vitamin D retailers? A new study says that vitamin D deficiency for people over 65 is 20 nanograms/milliliter of 25(OH)D in your blood. Fall below that for too long, and bad things tend to happen.

University of Washington researchers went looking for a bright statistical line to see how levels of vitamin D circulating in the blood correlated with the risk of a major medical event. “Major” in this case meaning: heart attack, hip fracture, cancer diagnosis, or death. (We can all agree that death is as major an event as it gets.)

Their sample was drawn from a study of 1,621 Caucasian adults that was intended to ferret out risk factors for (and progressive stages of) heart disease in people age 65 and over. They had an eleven-year timeline of 25-hydroxy-vitamin D levels–in short, 25(OH)D–to look at. (One glaring limitation here is that the original study included no one of color, and while the jury is still out, there’s some concern that melanin-protected skin can cut down on vitamin D production, just as SPF 8 does.)

Today, their findings appeared in the Annals of Internal Medicine: “Serum 25-Hydroxyvitamin D Concentration and Risk for Major Clinical Disease Events in a Community-Based Population of Older Adults.”

Luckily UW News is there to put results into English for you: “[T]he researchers concluded that the risk of these disease events rose when the concentration of 25(OH)D fell below 20 ng/milliliter or 50 nmol/liter.”

The first thing to note is that as Dr. Ian de Boer, assistant professor of medicine in the Division of Nephrology, points out, this is mostly in line with the threshold level suggested by the Institute of Medicine. Here’s the NIH on IOM recommendations: “Practically all people are sufficient at levels ≥50 nmol/L (≥20 ng/mL); the committee stated that 50 nmol/L is the serum 25(OH)D level that covers the needs of 97.5% of the population.”

Daily supplements of 600 t0 800 IU of vitamin D3 should do the trick, if you’re not getting much vitamin D from anywhere else. (The Institute of Medicine, by the way, does not recommend exceeding 4,000 IU per day of vitamin D3 for adults.) A separate study on vitamin D dose response in post-menopausal women found that “response was curvilinear and tended to plateau at approximately 112 nmol/L in patients receiving more than 3200 IU/d of vitamin D3. The RDA of vitamin D3 to achieve a 25 (OH)Dlevel greater than 50 nmol/L was 800 IU/d.”

The second thing suggested by the study is that vitamin D levels are best understood in terms seasonal snapshots: Typically, you will top off the tank in summer, thanks to exposure to the sun. If it’s not replenished, that vitamin D bank balance gets drawn down all through next spring, so that you may be lower on vitamin D in March than October, even though the amount of sun is about the same. You might think you’d increase supplements most in winter and taper off in spring, but this study indicates otherwise.