Obesity and Underweight Both Linked to Infection Risk in Chronic Kidney Disease

Individuals with chronic kidney disease (CKD) face a heightened risk of infection, and new research highlights a surprising connection: both obesity and being underweight can significantly increase that risk. A study published in the Clinical Journal of the American Society of Nephrology in December 2025, analyzed data from nearly 5,500 participants in the Chronic Renal Insufficiency Cohort Study (CRIC) and found that both extremes of the body mass index (BMI) spectrum were associated with a greater likelihood of hospitalization due to infection.

The CRIC Study: A Deep Dive into CKD and Infection

The CRIC study, a long-term observational study, included 5,499 participants with CKD. Researchers, led by Yefei Yu, MPH, examined the relationship between various measures of obesity and body composition and the time to first hospitalization for infection. BMI categories included underweight (<18.5 kg/m²), normal weight (18.5–24.9 kg/m²), overweight (25.0–29.9 kg/m²), and three classes of obesity (class 1: 30.0–34.9 kg/m², class 2: 35.0–39.9 kg/m², and class 3: ≥40.0 kg/m²). Body composition was assessed using bioelectrical impedance to measure phase angle (body cell mass) and vector length (tissue hydration status).

Key Findings: A Dual Risk

Over a median follow-up period of 4.7 years, 2,913 participants were hospitalized with an infection. The study revealed:

• Class 3 Obesity: Individuals with class 3 obesity (BMI ≥40.0 kg/m²) had a 35% increased risk of all-cause infection compared to those with a normal weight (adjusted hazard ratio [aHR], 1.35; 95% confidence interval [CI], 1.16-1.57).
• Underweight: Participants who were underweight (<18.5 kg/m²) faced an even higher risk, with a 76% increase in infection risk compared to those with a normal weight (aHR, 1.76; 95% CI, 1.06-2.93).
• Body Composition Matters: Lower quartiles of phase angle and vector length were as well associated with increased infection risk (aHR, 1.39; 95% CI, 1.22-1.57 and aHR, 1.17; 95% CI, 1.02-1.33, respectively).
• Skin Infections: The link between obesity and infection was particularly strong for skin and soft tissue infections (aHR, 1.98; 95% CI, 1.54-2.54).

Why Does This Matter?

These findings underscore the complex relationship between body composition, immune function, and infection risk in individuals with CKD. CKD itself impairs immune function, making patients more susceptible to infections. This study suggests that both extremes of weight – obesity and being underweight – can further compromise the immune system and increase vulnerability.

Looking Ahead

The researchers emphasize the require for further investigation into why obesity is strongly associated with skin infections in this population. Understanding these mechanisms could lead to targeted interventions to reduce infection risk in individuals with CKD. Maintaining a healthy weight and optimal body composition may be crucial for improving immune function and overall health in this vulnerable patient group.