An RDA is technically the amount of a nutrient every member of a population should ingest to ensure that 97.5% of its members would meet a specified criterion of nutritional adequacy. For vitamin D, the IOM panel determined that the criterion for adequacy was a serum concentration of a particular vitamin D derivative (25-hydroxyvitamin D) of 20 ng/mL or higher, and that for adults up to age 70, 600 IU of vitamin D per day was the RDA.

Both of those figures provoked immediate and unprecedented dissent from a diverse group of nutritional scientists, but the disagreement centered mostly around the IOM panel’s reading and interpretation of the evidence, rather than its calculation of the RDA. The Edmonton statisticians took the dissent a step further, showing that the actual calculation was itself wrong. Here’s what seems to have happened.

Not everyone gets the same response to a given intake of any particular nutrient. Some individuals require more than others to reach the specified target, and while the average response to a certain dose of vitamin D may be above the target level, a substantial fraction of a population can still be below it. The RDA will always be higher than the average requirement, and for some nutrients, substantially so. As a consequence, ensuring that every member of a population receives the RDA guarantees that 97.5% of that population will be getting at least enough, while some will be getting more than they actually need.

The IOM panel identified a number of published studies showing the 25-hydroxyvitamin D response to various vitamin D doses. They plotted the average response in each of those studies against dose, thereby generating what is termed a dose response curve. This is a way to estimate how much of a response would be predicted for any given vitamin D intake. But, to make a long story short, because it used average responses, that curve tells us nothing about the intake requirement for the individual members of a population, and particularly those whose response to a given dose falls in the bottom 2.5 percentile. The IOM panel surely knew that the average intake required to meet or exceed 20 ng/mL was not the same as the RDA, as it would be inadequate for all those with below average responses (about half the population). So, to catch the “weak” responders, they calculated the 95% probability range around their dose response curve, designating as the RDA the point where the bottom end of that probability range exceeded 20 ng/mL. While this might seem to have been the right approach, it was not. The panel appears to have overlooked the fact that the 95% probability range for their curve is for the average values that would be expected from similar studies at any particular dose. The dispersion of averages of several studies is much more narrow than dispersion of individual values within a study around its own average. And it’s the 2.5th percentile individual values from those studies, not the study averages, that should have been used to create the relevant dose response curve.

It’s this latter approach that the Canadian statisticians used. They took precisely the same studies as the IOM had used and demonstrated that the requirement to ensure that 97.5% of the population would have a value of at least 20 ng/mL, was 8,895 IU per day. Recall that the IOM figure was less than 1/10 that, i.e. 600 IU per day up to age 70 (and 800 IU per day thereafter). When my colleagues and I analyzed the large GrassrootsHealth dataset, we calculated a value closer to 7,000 IU per day, still a full order of magnitude higher than the estimate of the IOM, and not substantially different from the estimate of Veugelers and Ekwaru.

Nutrition Doesn’t Know What “Normal” Is

Thermostats, Feedback, and Adaptation

Living On the Plateau

Defining Normal – Origins and Resiliency

Taking Recommendations to Task

About Dr. Robert P. Heaney

Dr. Heaney was a full time professor at Creighton University who also donated his time and energy, starting in 2012 until his passing in 2016, as Research Director at GrassrootsHealth. In this capacity Dr. Heaney consulted on studies, methodologies, and how to best change public health direction. Dr. Heaney provided nearly 50 years of advancements in our understanding of bone biology, osteoporosis, and human calcium and vitamin D physiology. He is the author of three books and has published over 400 original papers, chapters, monographs, and reviews in scientific and educational fields. At the same time, he has engaged nutritional policy issues and has helped redefine the context for estimating nutrient requirements. Dr. Heaney was presented a lifetime achievement award in the US House of Representatives on November 10, 2015 (Watch the video here). He was an inspiration to researchers everywhere – his intellect, dedication, integrity, and caring was unsurpassed.

Read more about Dr. Heaney and a few of his accomplishments here.

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