Effects of Bariatric Surgery
Improvement in Renal Function
The improvements in the glomerular filtration rate and proteinuria were outlined 1 year after bariatric surgery in a controlled prospective trial. Subsequently, the 3-year follow-up data from the second phase of the Longitudinal Assessment of Bariatric Surgery study, for 1691 patients who had Roux-en-Y surgery, reported remission rates for diabetes mellitus, dyslipidemia, and hypertension at 67.5%, 61%, and 38.2%, respectively. Nearly half of these rates also were reported in 588 patients who underwent the laparoscopic adjustable gastric band procedure. The findings of these studies support the presumption that insulin resistance, compensatory hyperinsulinemia, inappropriate activation of the renin-angiotensin-aldosterone system, and hypertension may lead to microalbuminuria, matrix proliferation, and renal injury in obesity and that the correction of these factors may actually reverse obesity-related CKD.
Hyperoxaluria, Nephrolithiasis, and Oxalate Nephropathy
Hyperoxaluria is the most significant abnormality seen in urine chemistry after bariatric surgery because of the enhanced absorption of free oxalate from the gut. This is the result of less oxalate binding in the gut to calcium, which is already bound to free fatty acids produced by the resultant malabsorption. Severe hyperoxaluria that follows jejunoileal bypass may well precipitate acute kidney injury. This procedure is no longer performed.
Modern bariatric procedures also have been linked to hyperoxaluria. In one study, hyperoxaluria was reported in 74% of the patients, one-third of them had severe levels (urine oxalate excretion >100 mg/day) ≥6 months after bariatric surgery. It is worth noting, however, that purely restrictive bariatric procedures such as gastric banding and sleeve gastrectomy, as opposed to malabsorptive procedures, were associated with extremely low risk for stone formation. Patients who have undergone modern bariatric surgery developed an adjusted mean urine oxalate excretion of 83 mg/day compared with 39 mg/day for patients who routinely form kidney stones and 34 mg/day for normal subjects. These values compare favorably with 102 mg/day reported in jejunoileal bypass.
When oxalate nephropathy develops, it usually presents with acute renal failure (ARF) or nephrolithiasis, often superimposed on CKD. This can rapidly progress to ESRD requiring dialysis, as was reported in some patients after Roux-en-Y gastric bypass; their renal biopsies revealed acute and chronic tubulointerstitial injury with abundant oxalate crystals. As such, the available evidence suggests that although bariatric surgical intervention may produce salutary effects similar to those of diet-induced weight loss, it is at the expense of surgical morbidity and the potential of hyperoxaluric complications.
Other Renal Effects
Rhabdomyolysis was a common complication following bariatric surgery that correlated with the length of time on the operating table, although progression to ARF was rare, presumably resulting from aggressive intravenous fluid support. Gluteal muscles were especially involved, likely because of the pressure applied by the weight of the body. This problem is prevented by gluteal padding and perioperative intravenous fluid support.
Although ARF subsequent to bariatric surgery is not infrequent, dialysis support is seldom needed. In this respect, angiotensin-converting enzyme inhibitors and angiotensin receptor blockers were identified as significant risk factors associated with the development of ARF in this patient population; therefore, in addition to gluteal padding and fluid support, withholding these agents in advance of bariatric surgery makes the risk for developing postoperative ARF minimal.