Fitness predicts survival in kidney disease regardless of body weight
In 13,521 veterans with kidney disease, the fittest had about half the mortality risk of the least fit, whatever their BMI. Higher BMI also tracked with lower mortality.
Summary
Doctors have long noticed that people with chronic kidney disease who carry extra weight often outlive thinner patients, a pattern called the obesity paradox. This study of 13,521 US veterans with kidney disease asked how physical fitness fits in. Everyone had completed a treadmill stress test and was followed for about 7.4 years. Heavier patients did live longer than normal-weight patients, with 29% lower mortality risk in the obese group. But fitness was the stronger signal. The fittest quartile had 51% lower mortality than the least fit, at every body size. Normal-weight patients with poor fitness fared worst, with 55% dying during follow-up. The study is observational and almost entirely male, so it cannot prove that training saves lives, but it suggests fitness testing deserves a place in kidney care.
Detailed Summary
Chronic kidney disease (CKD) affects roughly 37 million US adults, and a puzzling pattern has followed it for decades: patients with a higher body mass index (BMI) often outlive leaner ones, the so-called obesity paradox. Whether this reflects true protection or a statistical artifact matters for the weight advice patients receive, and few studies have examined where cardiorespiratory fitness (CRF) fits in. This study tested whether fitness, a well-established longevity biomarker, explains or modifies the paradox in kidney disease.
Researchers analyzed 13,521 veterans with CKD from the Exercise Testing and Health Outcomes Study. All had completed a clinically indicated maximal treadmill test (Bruce protocol) between 1999 and 2020. The cohort was 97.4% men (13,165 men and 356 women), averaged 63.4 years of age, and was followed for a mean of 7.4 years through November 2021. Fitness, measured in metabolic equivalents (METs), was ranked into quartiles within four age bands so age would not drive the results. BMI was grouped as normal, overweight, or obese.
During follow-up, 5,144 participants (38.0%) died, roughly 51.3 deaths per 1,000 person-years. Compared with normal-BMI patients, mortality risk was 18% lower in overweight patients (HR 0.82, 95% CI 0.70-0.95, P=0.007) and 29% lower in obese patients (HR 0.71, 95% CI 0.61-0.82, P<0.001). Crude mortality was 44.7% in the normal-BMI group versus 36.9% in overweight and 37.4% in obese patients. The paradox was clearly present, consistent with prior findings in dialysis and heart failure.
Fitness showed a stronger, graded relationship. Versus the least-fit quartile, low fitness carried a nonsignificant 10% lower risk (HR 0.90, P=0.305), moderate fitness 39% lower (HR 0.61, 95% CI 0.50-0.74), and the most-fit quartile 51% lower (HR 0.49, 95% CI 0.40-0.60; both P<0.001). Crude mortality fell from 47.5% in the least fit to 27.6% in the most fit. Across all 12 BMI-fitness combinations, obese and most-fit patients fared best (HR 0.36, 95% CI 0.25-0.51; 25.7% mortality). Normal-weight, least-fit patients fared worst (55.3% mortality). The BMI-by-fitness interaction was not significant (P=0.41).
The authors caution that the paradox may be partly or wholly bias. Depletion of susceptibles, collider bias from conditioning on CKD, and reverse causation from disease-related weight loss could all create it, and BMI cannot separate fat from muscle. They excluded BMI below 18.5 and the first six months of follow-up, and sensitivity analyses gave consistent results. Their key message is that fitness was associated with survival independently of BMI, and that unfit patients of normal weight may be a vulnerable group that BMI-based assessment overlooks. The findings fit current KDIGO guidance of 150 minutes per week of aerobic activity plus strength training.
Important caveats apply. The design is observational, with one baseline measurement of BMI and fitness. Patients were almost all male veterans referred for clinically indicated exercise testing, which limits generalizability to women and to CKD patients who cannot be tested. Fitter patients may also simply be healthier at baseline. The study cannot show that raising fitness lowers mortality, so randomized trials are needed.
Key Findings
- Over a mean 7.4 years, 5,144 of 13,521 veterans with CKD (38.0%) died, about 51.3 deaths per 1,000 person-years
- Obese patients had 29% lower mortality than normal-BMI patients (HR 0.71, 95% CI 0.61-0.82, P<0.001); overweight patients had 18% lower (HR 0.82, P=0.007)
- Most-fit quartile had 51% lower mortality than least fit (HR 0.49, 95% CI 0.40-0.60, P<0.001); moderate fit had 39% lower (HR 0.61)
- Crude mortality fell from 47.5% in the least-fit quartile to 27.6% in the most-fit quartile
- Obese and most-fit patients had the lowest risk (HR 0.36, 95% CI 0.25-0.51; 25.7% mortality), a 64% reduction versus the reference
- Normal-BMI, least-fit patients had the highest mortality at 55.3%, a vulnerable group easily missed by BMI alone
- No significant BMI-by-fitness interaction (P=0.41); results held when early deaths and post-baseline cardiovascular events were included
Methodology
Retrospective cohort study of 13,521 veterans with CKD (97.4% men) drawn from the Exercise Testing and Health Outcomes Study, who completed maximal Bruce-protocol treadmill testing between 1999 and 2020, with mortality follow-up through November 2021. BMI was categorized as normal, overweight, or obese. CRF was ranked into quartiles within four age strata (<50, 50-59, 60-69, ≥70 years). Cox models adjusted for age, sex, race, diabetes, hypertension, and baseline cardiovascular disease were used, with a formal BMI-by-fitness interaction test, Kaplan-Meier curves, and two prespecified sensitivity analyses. Patients with BMI <18.5 or under 6 months of follow-up were excluded in the main analysis.
Study Limitations
The cohort was 97.4% male veterans referred for clinically indicated exercise testing, limiting generalizability to women, nonveterans, and CKD patients unable to be tested. BMI and fitness were measured once and BMI cannot distinguish fat from lean mass. The observational design leaves room for the biases the authors discuss (depletion of susceptibles, collider bias, reverse causation) and precludes causal inference.
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