Among 13,521 US veterans with chronic kidney disease followed for 7.4 years, 38% died. Higher BMI tracked with lower mortality (overweight HR 0.82; obese HR 0.71 versus normal weight), the familiar obesity paradox. Yet age-adjusted cardiorespiratory fitness from treadmill testing showed a stronger, graded association: moderate fitness carried HR 0.61 and top-quartile fitness HR 0.49 versus the least fit. Obese veterans in the fittest group had a 64% lower mortality risk (HR 0.36) than normal-weight, least-fit peers.
The key lesson is that fitness is a far better target than weight. The obesity paradox likely reflects reverse causation and confounding: illness-driven weight loss, malnutrition, inflammation, and muscle wasting make thin CKD patients look unhealthy, while BMI cannot separate lean mass from fat. Fitness sidesteps that problem because it integrates cardiac, vascular, and muscular capacity. The result echoes Kokkinos's earlier Veterans Affairs work showing the same fitness gradient in cardiovascular disease and in the general population. Caveats matter: the cohort is 97.4% male, fitness and BMI were measured once, and people able to complete a maximal treadmill test are healthier than the typical CKD patient, so selection bias is likely. Observational data cannot show that raising fitness lowers death risk. Still, the effect size is large and consistent with trial evidence that exercise improves function in CKD. This is confirmatory rather than paradigm-shifting, but it supports prescribing exercise over weight-loss advice alone for people with kidney disease.