The association of FGF-23 concentrations with obesity and insulin resistance is unclear [51,52,53,54], possibly because some studies reporting positive associations with BMI [51,55] measured total FGF-23 (full-length and C-terminal fragments) instead of measuring intact FGF-23 as we have done (Supplementary Data Text). Here, FGF23 is linked to obesity due to melanocortin 4 receptor deficiency.