Bariatric Surgery: Its Effects on the Kidneys

Quinn Dombrowskil;; Creative Commons 2.0 License

About 10% of American women and 5% of American men suffer from morbid obesity.  This is defined as a body mass index greater than 40.  The adverse effects of morbid obesity are all well-known.  For severely obese patients, especially with a body mass index of greater than 35, surgical treatment is the only proven and effective weight loss therapy.  Hence, weight loss or bariatric surgery is expectedly an increasingly used intervention to reduce obesity related complications.

Bariatric surgery is however not just about weight loss and there are metabolic changes that happen after a successful surgery.


We know that obesity is associated with a significant increased risk of chronic kidney disease. The mechanism by which this happens is beyond the scope of this article but I have covered this in detail here. After typical weight loss surgery, the very mechanisms that lead to injury to the kidney in the obese person begin to reverse. Therefore, here are some ways by which the kidney function might improve after weight loss surgery:

    Since all these parameters that lead to kidney disease in obese patients are improved post surgery, the kidney function also expectedly improves (this was proven in a study by Navarro Diaz in a prospective trial which showed improved GFR in 61 patients up to 1 year after bariatric surgery).



     Bariatric surgery does cause some deleterious changes in the kidneys as well.  

    1. One of the more significant abnormalities seen after bariatric surgery is increased excretion of oxalate in the urine.  Oxalate is one of the most common chemicals in the urine that can lead to formation of kidney stones.  In rare cases, the amount of oxalate excretion in the urine can be high enough to cause kidney failure.  In fact renal failure secondary to a high oxalate excretion in the urine after a jejunoileal bypass was one of the reasons why this procedure was discontinued in the US in 1980.  The reason why excretion of oxalate in the urine goes up after bariatric surgery has been attributed to a phenomenon called enteric hyperoxaluria.  This can therefore lead to increased risk of forming kidney stones as well as a type of kidney disease called oxalate nephropathy. Modern bariatric procedures have also been linked to hyperoxaluria.  In a recent study in patients who underwent the Roux-en-Y Gastric Bypass Weight-Loss Surgery (RYGB procedure), hyperoxaluria was seen in as much as 74% of the patient up to 6 months or more after bariatric surgery.
    1. There are other adverse changes which could include the kidneys' impaired ability to excrete acid into the blood.  One of the toxins that is excreted in the urine typically is acid which is a waste product of metabolism.  The capacity of the kidneys to excrete acid into the urine could be reduced in people who have had bariatric procedures.  This was demonstrated in a Scandinavian study done on 10 such patients.  The phenomena that developed in this case is something called distal renal tubular acidosis and beyond the scope of this article.
    2. Rarely, a phenomena and which is not really specific to bariatric surgery but to any surgery, called rhabdomyolysis can develop which can in turn lead to acute kidney injury (which used to be called acute renal failure).

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