Sunday, April 13, 2008

Kidney Stones, Obesity and Bariatric Surgery

The most common type of kidney stones in the general population, and bariatric surgery is no exception, is "calcium oxalate" stones. Historically, calcium oxalate kidney stones formation was a complication of the obsolete jejuno-ileal bypass (JI Bypass) of the 1970s. The risk for kidney stones, kidney failure, and liver disease led to the abandonment of that surgery more than 20 years ago.

A Mayo Clinic retrospective study showed that by 12 months after gastric bypass, the mean urinary oxalate and calcium oxalate supersaturation were both increased in a group of patients who did not have a history of forming stones.

Notice that, even without surgery, recent data have suggested an increased prevalence of stones with diabetes and obesity. Insulin resistance may lower urinary citrate and increase urinary calcium. Obesity may increase oxalates in urine.

It is safe to say that bariatric surgery can increase the risk of forming kidney stones, and certain recommendations need to be followed. Water and fluid intake, calcium citrate supplements and a low fat diet, go a long way. Remember: Oxalate is not good. Citrate is good. Calcium is good in the intestine but not good in the urine. Sodium is not good. Too much fat in the stool is not good. Here are more details.

Water and fluid intake: Drinking plenty of water is one of the most important measures to help preventing kidney stones. At least 10 glasses (cups) of water (80 ounces = two and a half liters) is recommended. In the presence of a history of kidney stones, even more is required. Lemonade (made from real lemons or a frozen concentrate), is good because it increases the citrates in the urine, which helps preventing kidney stones. Citrate is good; it inhibits the formation and growth of calcium crystals. But grapefruit Juice is not so good in this situation. A number of studies reported an increased risk for kidney stones from drinking grapefruit juice.

Low fat diet: The amount of oxalate in the urine increases with the amount of fat in the stool (fecal fat). After JI bypass the overall fat absorption was reported to be only 15%. So, the amount of fat passing into the stool, because of not being absorbed, is huge, and hence the higher likelihood of developing kidney stones. After purely restrictive surgeries (adjustable gastric band or vertical banded gastroplasty), fat absorption is normal, that is 97%. Biliopancreatic diversion +/- duodenal switch caused only 19% fat absorption. After gastric bypass, it was intermediate (67%) although the study group was very small. Probably the effect is less with proximal than distal bypass.

Restricting foods rich in oxalates: These include chocolate, cocoa, spinach and other dark green leafy items, most nuts, soy products, most berries, beets, beans, and tea. Because oxalates are so common in otherwise healthy food items, it is very difficult to completely eliminate them from a daily healthy diet. You can diminish the effect of oxalate rich foods by accompanying them with dietary sources of calcium to lower oxalate absorption, and by drinking additional fluids along the day.

Dietary calcium and calcium supplements: Calcium in the diet binds with oxalates in the gastrointestinal tract. So, less oxalates will be absorbed in the intestine, and less will be available by the kidney to form stones. Calcium supplements seem to have the same protective effect, but they have to be taken with meals. Calcium citrate is preferred because it helps to increase urinary citrate excretion.

Notice that vitamin C can convert to oxalate. Therefore, vitamin C supplements should be limited to less than 1000 mg/d.

Sodium: Sodium is not your friend. Lowering sodium intake lowers calcium in urine, since calcium excretion is linked to sodium excretion. So, less sodium makes less calcium available in the urine to form stones.

Protein: Animal protein was shown to lower citrate excretion in urine and to increase calcium and uric acid excretion. It is unknown if the malabsorption accompanying gastric bypass (which is why patients are asked to take more proteins), would weaken that bad effect. Also, a study showed that urinary calcium, oxalate, magnesium, citrate, and phosphorus did not differ between a diet of plant protein and beef protein.

Medications: Thiazide diuretics (example, hydrochlorothiazide "HCTZ" ): have been proven to be effective in reducing calcium in urine and stone recurrence. These "water pills" help decreasing the calcium in urine, and lowering the chance of developing kidney stones. Usually patients also receive potassium supplementation, which, in this case, could be potassium citrate to provided more citrate.

Oxalobacter formigenes: This organism relies completely on oxalate as its source of energy . This colonic bacterium could be a promising treatment for oxaluria.


Asplin JR, Coe FL. Hyperoxaluria in kidney stone formers treated with modern bariatric surgery. J Urol. 2007 Feb;177(2):565-9. PMID: 17222634 (Abstract)

Lieske JC, Kumar R, Collazo-Clavell ML. Nephrolithiasis After Bariatric Surgery for Obesity. Semin Nephrol. 2008 Mar;28(2):163-173. PMID: 18359397 (Abstract)

Finkielstein VA, Goldfarb DS.Strategies for preventing calcium oxalate stones.CMAJ. 2006 May 9;174(10):1407-9. PMID: 16682705 (Full Text)

Nelson WK, Houghton SG, Milliner DS, Lieske JC, Sarr MG. Enteric hyperoxaluria, nephrolithiasis, and oxalate nephropathy: potentially serious and unappreciated complications of Roux-en-Y gastric bypass. Surg Obes Relat Dis. 2005 Sep-Oct;1(5):481-5. PMID: 16925274 (Abstract)

Duncan SH, Richardson AJ, Kaul P, Holmes RP, Allison MJ, Stewart CS. Oxalobacter formigenes and its potential role in human health. Appl Environ Microbiol. 2002 Aug;68(8):3841-7. PMID: 12147479 (Full Text)

Lieske JC, Goldfarb DS, De Simone C, Regnier C. Use of a probiotic to decrease enteric hyperoxaluria.Kidney Int. 2005 Sep;68(3):1244-9. PMID: 16105057 (Abstract)