High BMI Raises Kidney Disease Risk

High BMI Raises Kidney Disease Risk

Debra Blair, MPH, RD, CSR
April 10 2008

Risk is greater for overweight individuals than for those of normal weight, meta-analysis shows

THE POSITIVE association between obesity and kidney disease (KD) is strongly supported by results of a recent meta-analysis by Wang et al (Kidney Int. 2008;73:19-33) that demonstrate a significantly increased risk for people with BMI greater than the normal range of 18.5-25 kg/m2. Along with CKD, the study also examined the occurrence of renal calculi and kidney cancer. According to the authors, 24.2% and 33.9% of KD cases among American men and women, respectively, could be prevented if overweight and obesity were eliminated.

The meta-analysis looked at more than 40 epidemiologic studies published from 1980 to 2006, with a focus on 18 prospective cohorts for reliability of findings, ability to determine causality, and generalizability of results. Of significance, persons categorized as overweight (BMI greater than 25 but less than 30 kg/m2) had a 40% increased risk of kidney disease compared with normal-range BMI, whereas obesity (BMI over 30 kg/m2) was associated with an 83% higher risk. Among individuals with a BMI greater than 30 kg/m2, women had a 92% greater risk of kidney disease compared with a 49% increased risk for men, possibly because of differences in body composition (percent fat vs. muscle). The researchers acknowledge that more studies are needed to determine etiologic factors.

Paradoxically, higher BMI seems to confer survival benefit to patients with stage 5 CKD on dialysis. This reverse epidemiology was evident in a study by Glanton et al (Ann Epidemiol. 2003;13:136-143), which examined U.S. Renal Data System data regarding short-term survival in 151,027 obese new-start dialysis patients. After controlling for demographics and comorbidities, results from this retrospective cohort indicate that obesity is independently associated with reduced all-cause mortality. These results confirmed findings of an earlier study by Port et al (J Am Soc Nephrol. 2002;13:1061-1066), which followed survival data of 45,967 incident dialysis patients. The study demonstrated that small body size is an independent risk factor for mortality (42% higher in low vs. high BMI).

Although higher BMI may confer survival benefit to dialysis patients, obesity is often an obstacle to renal transplantation. A new study by Segev et al (J Am Soc Nephrol. 2008;19:349-355) looks at the association between BMI and waiting time for a kidney transplant, examining 132,353 candidates listed with the United Network for Organ Sharing (UNOS) from 1995 to 2006. Based on their findings, the authors noted that “likelihood of receiving a transplant decreased and likelihood of being bypassed increased significantly for higher BMI categories, even after adjustment for all factors relevant to the allocation system, factors possibly influencing access to health care, and factors that could influence provider risk-benefit decisions.” Whether higher BMI should contraindicate renal transplantation is a matter of debate because a previous study by Glanton et al (Kidney Int. 2003;63:647-653) of 7,521 dialysis patients with a BMI above 30 kg/m2 enrolled on the renal transplant waiting list from 1995 to 2000 found a lower risk of mortality in obese transplanted compared with those remaining on the list (though this benefit did not accrue to those with a BMI above 41 kg/m2).

So, when should weight control be addressed? Current information indicates potential benefit for patients in early stages of CKD and for those desiring renal transplantation. Weight control is difficult at best for the average individual, and particularly challenging in CKD. Adequate calorie intake to minimize muscle catabolism and prevent malnutrition is important, especially in stages 3 and 4 CKD when protein intake may be restricted. For stage 5 CKD patients the additional dietary and fluid limitations required by hemodialysis or the extra calories from peritoneal dialysate complicate efforts to lose weight. The American Dietetic Association's “Adult Weight Management Evidence-Based Nutrition Practice Guideline” (www.adaevidencelibrary.com) provides information that may assist in advising CKD patients.

Strategies associated with successful weight control outcomes include:

• Physical Activity: “...at least 30 minutes or more of moderate intensity physical activity on most, and preferably, all days of the week, unless medically contraindicated. Physical activity contributes to weight loss, may decrease abdominal fat, and may help with maintenance of weight loss.”

• Comprehensive Weight Management Program: “Weight loss and weight maintenance therapy should be based on a comprehensive weight management program including diet, physical activity, and behavior therapy. The combination therapy is more successful than using any one intervention alone.”

• Realistic Weight Goals: “…to reduce body weight at an optimal rate of 1-2 lbs per week for the first 6 months and to achieve an initial weight loss goal of up to 10% from baseline. These goals are realistic, achievable, and sustainable.”

• Multiple Behavior Therapy Strategies: “A comprehensive weight management program should make maximum use of multiple strategies for behavior therapy (e.g., self monitoring, stress management, stimulus control, problem solving, contingency management, cognitive restructuring, and social support).”

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