Kidney stone disease, also known as urolithiasis, is a common disease with cases on a rise around the world. In India, the cases are found more in the stone belt region, which comprises of Maharashtra, Gujarat, Rajasthan, Punjab, Haryana, Delhi and the states of the Northeast. There are multiple factors that increase the risk of urolithiasis. One of these factors is Obesity and it has been studied to understand the role it plays in disease pathogenesis.
Obesity is described as a condition in which adults have a BMI of 40 kg/m2 or more. BMI is estimation of adult body fat based on height and weight. It is calculated as person’s weight in kilograms divided by the square of height. Obesity prevalence has increased by 27.5% in adults and 47.1% in children between 1980 and 2013. Obesity can be attributed to a number of factors such as genetic predisposition, dietary indiscretions, consumption of certain medicines and pharmacological products, and lack of physical activities. There are other comorbidities linked to obesity such as diabetes, hypertension, and hyperlipidemia. These conditions can individually contribute to risk of developing kidney stones, and if present together, they can cause specific changes in the body organs and systems contributing to kidney stone formation.
It is found that levels of sodium, calcium, sulphate, phosphate, oxalate, uric acid, and urate, which are risk factors for stone formation, are higher in the urine of obese and over-weight patients. Visceral obesity is caused by fat, deep inside the body around the abdominal organs and can be quantified by imaging. Scientists have reported that the risk of developing uric acid stones and or calcium oxalate stones correlates positively with visceral obesity. It has been observed that calcium oxalate stones are more common than uric acid stones in obese patients. Conversely, a higher rate of obesity is observed in patients with uric acid stones than in patients with other types of stones.
Low urinary pH, hyperuricosuria (excretion of excess uric acid in urine) and low urinary volume have been associated with increased kidney stone formation. Low pH occurs due to defect in production of ammonia in the nephron that is used to balance H+ ions and maintain pH by creating ammonium. Insulin resistance can impair ammonia excretion due to ineffective Na+/H+ exchanger (NHE3). Impairment of mitochondrial metabolism may cause recycling of glutamine for purine production causing increased uric acid. Formation of mixed stones of calcium oxalate and urate can occur in patients with hyperuricosuria but normal pH. Body weight/BMI has been reported to be inversely proportional to urine pH.
High amounts of meat in regular diet results in increased purine load and reduction in urine pH ultimately resulting in greater risk of uric acid stone formation. Risk of calcium oxalate stone formation increases due to increase in oxalate excretion in urine, which is shown to have a positive correlation with body weight and body surface area due to increased oxalate synthesis. Losing weight may help obese patients by providing protection against stone formation, however, certain strategies used for weight loss may increase the risk further. Drugs used for weight management have also been associated with kidney stone formation. Bariatric surgery also increases the risk due to increased urinary oxalate excretion. It is therefore recommended to abide with a balanced diet and maintain a healthy weight range to ensure maximum protection against kidney stones and related complications.
