How Is Body Mass Index Protective in Maintenance Dialysis?

BODY MASS INDEX (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight, and obesity in adults. According to many researchers, as indicated by BMI, obese dialysis patients exhibit improved survival. In contrast, malnutrition is an important predictor of morbidity and mortality. However, it should be remembered that BMI does not differentiate muscle mass from adipose tissue. Moreover, in the general population, obesity is associated with metabolic and cardiovascular complications. Thus, the relationship between BMI, obesity, survival, and dialysis deserves clarification. A careful analytical consideration must be performed, because BMI has poor specificity for excess adiposity, and does not characterize excessive visceral obesity, which is more consistently associated with adverse effects on metabolism, dyslipidemia, and insulin resistance.

The limits of BMI analysis in relation to survival involve the comparison of short-term survival in hemodialysis patients with long-term survival in the general population, whereas obesity is associated with increased mortality only after an average follow-up of 12 years. There is also a progressive decline in lean body mass in dialysis patients, indicating chronic ongoing catabolism associated with alterations in muscle function involved in the increased mortality and hospitalization. Thus, the question becomes one of whether the protective effect conferred by a high BMI is limited to those patients with normal or high muscle mass.

Muscle loss can also occur in obese patients. Moreover, visceral adipose tissue confers an increased cardiovascular risk, whereas subcutaneous fat does not, and thus fat distribution may potentially be the clue to explain mortality or survival in CKD patients. In dialysis patients, it appears to be of importance to report BMI with more information on body composition, including muscle mass and fat distribution.

Further studies are needed to examine the effects of lean body mass, and not only of fat mass or BMI, on outcomes such as hospitalization or death. What definitely remains, however, is the strong predictive power of mortality for low BMIs (<23) in maintenance dialysis patients.

Understanding the mechanisms of a potential “reverse paradox” of BMI in dialyzed patients is necessary before a nutritional intervention to gain weight may be considered.