[paulrhjtimmers]

If this were an observational study, I'd completely agree. In that case, it could be that people who eat better diets and do exercise happen to have lower iron levels, but it is the diet and exercise that makes them live longer, not the iron.

Our study actually used a different method! We found that the genes that raise iron levels in the blood also have an effect on how long and healthy we live. Since you don't get to choose the genes you are born with, this method is more robust to confounding. It's not perfect by any means, but it does provide reasonably robust evidence for a causal role of iron rather than just a correlation. Still, we would like to verify this in follow-up experiments, possibly using animal models.