Last week, I outlined the importance of vitamin D for healthy bodies and how recommendations for adequacy published in 2011 were, in retrospect, far too extreme. To understand why vitamin D assumed such a huge role in the health of the nation — vitamin D tests are still the fifth-most-common Medicare-funded lab tests — it's worthwhile to compare "observational studies" with "randomized trials." Observational studies simply note a correlation. Say you notice that people with lighters in their pockets are more prone to get lung cancer than those without lighters. It doesn't say that lighters cause lung cancer, it simply notes the correlation without citing the probable connection, i.e. smoking. (This example is from Christopher Labos delightfully skeptical book, Does Coffee Cause Cancer? which takes on the vitamin D controversy, among others.)

About a decade ago, a slew of observational studies found that low vitamin D levels were associated with increased risks of cancer, cardiovascular disease, diabetes and falls. However, when researchers followed through with randomized trials that involved tens of thousands of participants (the gold standard for medicine), there was no correlation: Those taking placebos showed no differences from those taking vitamin D supplements.

This seems to be a classic case of "correlation isn't causation." Instead of assuming that low vitamin D levels caused cancer, cardiovascular disease, etc., researchers looked for something else that might cause both the diseases and the low vitamin D levels. How about something as simple as, people taking vitamin D supplements are particularly health-conscious and motivated to take care of their bodies through exercise, good diet and enough sleep? Or: If you're sick, you don't get outside much, so you're not getting your vitamin D from sunshine? And so on.

The associate director of the Bone Density Center at Massachusetts General Hospital, Dr. Joel Finkelstein, with his colleagues, published a study of over 2,000 perimenopausal women over a 10-year period. They found that for these women, who are prone to a higher bone fracture risk than most people (because menopause reduces the body's available vitamin D for bone strengthening), blood vitamin D levels less than 20 nanograms/milliliter were indeed associated with a slightly increased risk of fractures, and supplementation was warranted. However, and in contrast with the Endocrine Society's 2011 recommendations, they stated, "For the vast majority of healthy individuals, levels much lower, 15, maybe 10, are probably perfectly fine."

And now, according to a June 3, 2024, press release, the Endocrine Society has (finally) adopted new guidelines: "Healthy adults under the age of 75 are unlikely to benefit from taking more than the daily intake of vitamin D recommended by the Institutes of Medicine (IOM) and do not require testing for vitamin D levels ... For children, pregnant people, adults older than 75 years and adults with high-risk prediabetes, the guideline recommends vitamin D higher than the IOM recommended daily allowance." (Which, assuming minimal sun exposure, is 400 IU of vitamin D daily; "those 71 years and older may need up to 800 IU daily.")

According to Finkelstein's study, in addition to perimenopausal women, people in some other groups who are at risk for a true deficiency should have their vitamin D levels checked and possibly take supplements. They include: dark-skinned people who don't get outdoors much; people with anorexia nervosa; those who have had gastric bypass surgeries; people who suffer from malabsorption syndromes; those diagnosed with osteopenia or osteoporosis; and pregnant and lactating women.

Which leaves most of us. We're doing fine and don't need to take vitamin D supplements. All this information is readily available online or in magazines and books, so please don't take my word for it. Your doctor knows best.

Barry Evans (he/him, [email protected]) is not a physician. EMT certified yes, doctor no.