By Caitlin Rothermel
Scientists are in an ongoing discussion as to whether our recommended vitamin D intake is too low, and whether this is due – at least in part – to a poor mathematical decision.
Some important background. In the United States and Canada, the daily recommended vitamin D intake for adults up to age 70 years is 600 international units (IU). After age 70, this increases to 800 IU. These levels were established in a 2011 report prepared by the Institute of Medicine.
The IOM also said that a blood level showing at least 20 ng/mL of 25(OH)D, a circulating marker of vitamin D, was enough to support bone health for nearly all healthy people. Bone health was the focus because the evidence for vitamin D’s effects in other areas was limited or inconsistent. The IOM estimated that 600 IU vitamin D per day would be enough for 97.5% of healthy people to reach this target.
This decision was based on a statistical analysis of many studies, each of which reported average blood levels associated with different daily intakes. The IOM combined these study-level findings to estimate how much vitamin D an average person would need to achieve 20 ng/mL of 25(OH)D.
Side note: Some people advocate for higher target 25(OH)D blood levels, up to 40 to 60 ng/mL. That’s a worthwhile discussion, but for this story, it’s best to focus on 20 ng/mL.
Was the IOM’s approach the best way to conduct the research? When it comes to internal vitamin D processing, biology shows there is no such a thing as an “average person.” A group of people taking the same daily amount will show a range of measured 25(OH)D blood levels, and this will be the case for personalized reasons that include overall health, age, race, body composition, and the ability to absorb this hormone.
That is where the statistical problem comes in. When researchers combine average results from multiple studies, those averages don’t fully capture what can be substantial differences between individuals within studies. Some people may reach the desired blood level with a small amount of vitamin D, while others will need much more.
In 2014, a group of researchers re-analyzed the same studies using a statistical approach that considered not only the average study results, but also the differences, or variation, in achieved blood levels among people taking the same amount of vitamin D. With this approach, they estimated that 8,895 daily IU of vitamin D was needed for 97.5% of healthy individuals to reach the 20 ng/mL blood level.
That was almost 15 times higher. It seemed like an informed middle ground should be possible.
In 2015, another group of researchers examined the relationship between vitamin D supplement intake and 25(OH)D blood levels, this time using data from thousands of volunteers who participated in a vitamin D testing program. This study estimated that approximately 7,000 IU of vitamin D daily, including at least 3,875 IU from supplements, was needed for 97.5% of healthy individuals to reach the 20 ng/mL blood level.
In 2017, the National Academies convened a panel of experts to review the controversy. The panel identified and even acknowledged the mathematical errors in the original IOM report. But they didn’t conduct a new data analysis, and they didn’t consider any newer data, like the 2015 study above.
Instead, the panel concluded that correcting the statistical errors was unlikely to have changed the ultimate 600 IU recommendation. How come? The original guidance was based on a body of evidence concerning bone health and 25(OH)D blood levels, rather than the disputed calculations alone.
Put another way, we can feel pretty confident that the 20 ng/mL blood level is sufficient to support calcium absorption, bone mineralization, and overall bone health. But an important question remains unanswered: How would the estimated daily requirement have changed if the original analysis had been corrected to account for individual differences in response to vitamin D supplementation?
Research is ongoing to try and pinpoint what benefits vitamin D may offer beyond the bones. So far, the findings have been mixed. A major clinical trial studying heart disease and cancer prevention didn’t show benefits with 2000 IU of daily vitamin D. Two noteworthy exceptions have been a small (3%) reduction in the risk of developing type 2 diabetes among people with prediabetes, and a 22% reduced occurrence of autoimmune disease.
Meanwhile, medical bodies have largely maintained the original 600 IU recommendations, citing a lack of evidence that greater vitamin D intake improves health.
But researchers have also started to approach these still-open questions from different directions. Rather than just focusing on 25(OH)D blood levels, they are investigating how vitamin D functions within cells, how it affects gene activity, what other biologic measurements might reflect its activity, and why people respond differently to supplementation.
Ultimately, this research could lead to more personalized recommendations. But for now – and as generally seems to be the case – it’s largely up to each of us to decide what to do with the information we have.
