Obesity, Metabolic Syndrome, and Stones

Abstract

Obesity (body mass index (BMI) above 30 kg/m²) has become an epidemic condition that affects 10–27% of men and up to 38% of women in European countries. In the United States, more than 5% of the adult population are considered morbidly obese (BMI of 40 kg/m² or more). Worldwide, more than 300 million people are estimated to be obese. According to recent epidemiologic studies, greater BMI, greater weight, larger waist circumference, and heavy weight gain are independently associated with increased risk for renal stone formation. This appears to be related to two distinct metabolic conditions: (1) Abdominal obesity in the context of the so-called metabolic syndrome predisposes to insulin resistance, which at the renal level appears to cause reduced urinary ammonium excretion and low urine pH. The consequence is an increased risk for uric acid stone formation. (2) Bariatric surgery, more and more popular as the only intervention that facilitates significant weight loss in morbidly obese people, has been shown to increase the risk for calcium oxalate nephrolithiasis. The underlying pathophysiologic mechanisms may be enteric hyperoxaluria due to fat malabsorption or decreased intestinal colonization with oxalate-degrading bacteria.