Body Mass Index and Its Implications in Urologic Practice

Obesity rates in the United States have steadily increased in the past decade. As the body mass index (BMI) of patients rises, unique challenges face the urologist. The influence of obesity in urologic oncology, erectile dysfunction, stone disease, and BPH has the potential to change not only the outcomes of these larger patients, but also the clinician’s approach to treatments.

The connection between BMI and urologic cancers is an elusive association. No definitive correlation has been made between BMI and the cancer outcomes of prostatectomy, nephrectomy or cystectomy. Motamedinia et al described the relationship between BMI and radical prostatectomy in a 2007 AUA abstract. The study retrospectively viewed 943 prostatectomy patients, 17.6% were obese. BMI was not an independent predictor of outcome when adjusting for stage, grade and PSA, and patients did not have a higher risk of biochemical progression. Prior studies have documented a higher surgical margin positivity rate in obese males as well as worse outcomes in urinary continence. Serum PSA levels have been recently described by Freedland et al to be inversely correlated with increasing BMI. They have proposed that a dilutional effect of increasing plasma volume may actually be the driving force behind this association.

Donat et al also found no correlation with BMI and cancer specific survival in 859 patients who underwent nephrectomy at MSKCC. BMI was not an independent predictor of overall survival between the normal, overweight and obese cohorts. Obese patients did have a higher incidence of conventional cell histology (37%) compared to normal weight patients (24%). Hafron et al investigated 288 patients with a median BMI of 26.98 who underwent cystectomy. In patients with organ-confined bladder cancer, there was no survival difference between normal and obese patients (p=.44).

In terms of cancer incidence BMI does not correlate with incidence of prostate or bladder cancer however has been demonstrated to be directly correlated with incidence of renal cell carcinoma.

Although the current research shows the insignificance of BMI in outcome of most urologic cancers, the influence of BMI in other urologic aspects, such as erectile dysfunction (ED), is more tangible. Kratzik et al evaluated how age, BMI and testosterone affected ED in 675 men. Both age and BMI independently lowered the score on the International Index of Erectile Function (IIEF-5), which measures ED, in a multivariate model. Obese men were 1.952 times more likely to have ED than normal men. Testosterone was not an independent predictor of ED. However, lower testosterone did correlate with a worse score on the IIEF-5, and did predict ED in a univariate model.

Stone disease can also be positively correlated to BMI. Taylor et al associated higher BMI with increased stone formation in both men and women. In a large cohort of 241,623 patients from the Health Professionals Follow-up Study and the Nurses Health Study, 4877 patients had stone disease. Stone formation was more likely to be found in obese patients than normal weight patients (p=0.001). The study suggests that perhaps hyperinsulinemic overweight patients secrete more calcium, or that obese patients produce a more concentrated uric acid, a risk factor for calcium oxalate stones.