There’s a lot of controversy in the medical field about just what the ideal blood level of vitamin D is, with some experts saying that 20 nanograms per milliliter (ng/mL) is plenty and others arguing that the target should be as high as 70 ng/mL.

This is no insignificant question. When blood levels are sufficient, the “sunshine vitamin” reduces the risk for cardiovascular disease, osteoporosis, diabetes, Alzheimer’s disease, breast cancer and uterine fibroids…protects joints against arthritis…and maintains the immune system. However, when vitamin D blood levels are too high, there’s an increased risk for blood vessel calcification, heart rhythm abnormalities and kidney damage.

That “too much of a good thing” phenomenon could explain why, despite vitamin D’s many benefits, a host of studies have failed to show that taking vitamin D supplements helps people live longer. So researchers from Israel set out to determine just what is the right amount of vitamin D to have in your blood. What makes their findings so compelling is that they had a huge database to draw from.

LARGE SAMPLE SHOWS SMALL RANGE

The researchers had access to the health records of more than 4 million members of the country’s largest HMO—and of those members, nearly 423,000 men and women age 45 and up had had at least one test to measure their vitamin D blood levels between 2007 and 2011.

The researchers then cross-referenced those 423,000 test results against the 16,213 “events”—which they defined as a new diagnosis of acute coronary syndrome (a sudden blockage of blood flow to the heart, which can lead to heart attack) or death from any cause—that occurred during the study period. Finally, they adjusted for numerous other health risk factors (diabetes, LDL “bad” cholesterol levels, body mass index, blood pressure, smoking history, etc.) to determine the vitamin D “sweet spot”—the blood level associated with the lowest risk for acute coronary syndrome or death.

Just right: The optimal blood level of vitamin D turned out to be between 20 and 36 ng/mL. Compared with people whose blood levels fell into that range, those with levels below 10 ng/mL were 91% more likely to develop coronary problems or die from any cause during the study period…those with levels from 10 ng/mL to 19 ng/mL had a 26% higher risk…and those with levels above 36 ng/mL had a 13% higher risk.

How many people in the study fell into that ideal range of vitamin D blood levels? Only about one-third of those who had had their levels tested. Most of the rest had levels that were too low, but some had levels that were too high.

WHY TESTING IS VITAL

Because vitamin D deficiency is common and its dangers have been widely publicized, many well-meaning people pop a vitamin D pill daily, assuming they need it. However, if their blood levels are already high enough or too high, they are getting too much of a good thing—which is a bad thing.

Best bet: Your vitamin D supplement dosage should be carefully tailored to your own current blood level. Before you start taking vitamin D, have your doctor test you. If you are already taking vitamin D, ask your doctor whether you should get tested now or whether you should stop taking the supplements for a certain amount of time and then have the test done. It’s also important to repeat the test periodically, because vitamin D blood levels are likely to change based on the season, your diet and your supplement regimen. Note: When your test results come in, be sure to check the unit of measure used. Some labs (particularly non-US ones) use a unit called nanomoles per liter (nmol/L) rather than the ng/mL typically used in the US—in which case the ideal range is 50 nmol/L to 90 nmol/L.

In general, each 100 IU of vitamin D taken daily will, over several months, raise your blood level by about 1 ng/mL. If, for instance, your blood level currently is 15 ng/mL, your doctor may prescribe an initial dosage of, say, 1,000 IU per day—also taking into account factors such as your intake of vitamin D–fortified foods, your overall digestive health and your medication regimen—then when your vitamin D blood level is up to where it should be, you may be instructed to reduce your daily supplement dosage to a maintenance level.

The researchers also noted that daily supplementation with 1,600 IU to 2,400 IU of vitamin D—a dosage level that is not uncommon—might over time raise the blood levels of most of the population beyond the safe range. How so? Suppose a person started at 15 ng/mL and took a dose of 2,000 IU per day. After three months or so, he’d be at about 35 ng/mL, which is fine—but if he kept taking that same 2,000 IU per day, by the end of a year his vitamin D blood level would be dangerously high.

If testing reveals that your blood level is already too high, your doctor will probably advise you to stop taking vitamin D supplements immediately. Since multivitamins typically include 400 IU to 600 IU of vitamin D, your doctor also may tell you to stop taking your multivitamin until your blood level of vitamin D has decreased.