The lower urinary pH of type 2 diabetics is associated with greater levels of undissociated uric acid.


TORONTO—Type 2 diabetics may be at a higher risk for uric acid stones, but not calcium oxalate stones, compared with non-diabetics, according to Texas researchers. 

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“Kidney stones are more common in patients with type 2 diabetes than in non-diabetic individuals,” said lead investigator Naim Maalouf, MD, assistant professor of internal medicine at the University of Texas Southwestern Medical Center in Dallas.


“We found that patients with type 2 diabetes had lower [urinary] pH, which predisposed them to [uric acid] stones.” A low urinary pH can result from a diet high in animal protein or from insulin resistance, he said, but the specific underlying factors have not been well elucidated. 


Dr. Maalouf’s study, which he presented here at The Endocrine Society’s 89th annual meeting, examined lithogenic factors in the urine of 20 patients with type 2 diabetes mellitus and 57 healthy non-diabetic volunteers. No subject was taking any medication that altered stone risk, and all patients provided a single 24-hour urine collection. The investigators calculated the urine content of undissociated uric acid and relative supersaturation ratio (RSR) for calcium oxalate (CaOx) and brushite.


Diabetic patients had significantly greater excretion of calcium than the healthy subjects (219 vs. 154 mg/day), greater excretion of sodium (205 vs. 162 mEq/day), and greater excretion of sulfate (22 vs. 18 mmol/day). The difference in urine calcium excretion, however, was no longer significant after adjusting for urine sulfate and sodium. The 24-hour urine pH was significantly lower in the diabetics (5.55 vs. 6.03).


The investigators observed no significant differences in urine volume, oxalate, citrate, ammonium, phosphate, magnesium, potassium, and uric acid. The RSR for CaOx and brushite were not significantly different between the groups. The urine content of undissociated uric acid was significantly greater in the diabetic patients (204 vs.102 mg/day) due to the lower urine pH levels.


The greater calcium excretion in the diabetic patients was likely related to the higher intake of sodium and animal protein, the researchers concluded. In addition, the higher urine calcium is, in part, offset by smaller differences in urine oxalate and citrate, resulting in a similar RSR for CaOx in the two groups. The greater urinary content of undissociated uric acid in the diabetics suggests that the higher incidence of kidney stones in diabetics is likely due to an increase in uric acid nephrolithiasis.


“Since calcium oxalate stones are the most common types of stones, people tend to forget about the uric acid stones, but if a patient with kidney stones has type 2 diabetes one should look for them,” Dr. Maalouf said. “Most of these stones are not seen on the regular x-ray. You may need to look on a CT scan.”