Iron supplements, ferritin levels, and VO2max gains in athletes

Red blood cells in the style of a hand-drawn sketch

There’s a new study out this month on iron supplements for athletes with low ferritin. The study, published by Anja Neža Šmid and colleagues at the University of Ljubljana in Slovenia, is a meta-analysis, meaning it pooled data from several different randomized trials that took athletes, assigned them to either a placebo group or an iron supplement group, then measured the difference in ferritin levels between groups after the study’s conclusion.

The results of this meta-analysis are some of the strongest evidence that I’ve seen that support the notion that athletes with ferritin levels circa 20 ng/mL will benefit from an iron supplement. However, I wanted to write up this post because that’s actually not how the authors of the study interpret their own results!

Here’s what Smid et al. say in the abstract:

Increase in serum ferritin concentration after [oral iron supplementation] was evident in subjects with initial pre-supplementation serum ferritin concentration ≤12 µg/l [ ng/mL], while only minimal, if any effect, was observed in subjects with higher pre-supplementation serum ferritin concentration.

Šmid et al 2024

How can I justify the benefits of iron supplementation at 20 ng/mL, while the authors cite a cut-off value of barely half that level? Well, it all comes down to the way you analyze the data.

Here’s the bottom line up front: when treating ferritin level as a continuous number—which it is—the study’s data clearly show that runners with ferritin levels of around 20 ng/mL or less will experience significant increases in ferritin and gains in VO2max when taking an iron supplement over the course of 6–8 weeks.

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