In one study, researchers at Osako University in Japan found that people who drank more than two sodas a day had more protein in their urine, a hallmark of kidney dysfunction, than those who had fewer or no sodas daily. More than 12,000 people with normal kidney function participated in the three-year study.
Almost 11 percent who said they drank two or more sodas a day had protein in their urine at the end of the three years. Eight percent of those who did not drink any soda and 9 percent of those who drank about one can a day tested positive for proteininuria (protein in the urine). Proteininuria is considered an early, but reversible, marker of kidney damage.
While the researchers aren't exactly sure why, "Oxidative stress and inflammation induced by fructose, which is a more active sugar than glucose, may play an important role," said study author Ryohei Yamamoto, MD, PhD, a professor in the department of geriatric medicine and nephrology at Osaka University. However, fructose alone can't be the culprit, because participants drank both diet and non-diet sodas in the study.
A related study conducted by Case Western Reserve University in Cleveland, Ohio, found that moderate consumption of fructose sugar increases the kidneys' sensitivity to angiotension II, a protein that regulates salt balance. Rats fed moderate amounts of fructose for just 13 days exhibited this effect.
The increased sensitivity leads to more salt reabsorption by cells in the kidneys, a finding that might explain why soda consumption has been linked to diabetes, obesity, high blood pressure and kidney failure, researchers said. (Excess sodium in the body is never good.)
Dr, Anil Agarwal, a kidney specialist at Ohio State University, told HealthDay that the new studies suggest that even people with normal kidney function are at risk if they drink too much soda -- and that soft drinks sweetened with high-fructose corn syrup are the worst.
"Fructose is sweeter than glucose, and doesn't cause feelings of satiety," he said. Even one can of soda a day is too much, he said. "There is no safe amount of soda. If you look at the recommended amounts of sugar we can safely consume every day, one can of soda exceeds the maximum level."
The American Heart Association's recommended daily sugar intake is 9 teaspoons for adult men, 5 teaspoons for adult women and 3 teaspoons for children. A 12-ounce can of non-diet soda contains about 7 teaspoons of sugar, Agarwal told HealthDay.
Both studies were presently recently at the annual meeting of the American Society of Nephrology in Atlanta.
Protein in the urine may be a marker for more than just kidney disease, said Dr. Orlando Gutierrez, a kidney specialist at the University of Alabama at Birmingham. "We now understand that protein in the urine may be a really early marker for heart disease, stroke and heart failure," he said.
Dr. Jaime Uribarri, a kidney specialist at the Icahn School of Medicine at Mount Sinai Medical Center in New York City, told HealthDay that the new findings "reaffirm an association between soda and health problems." Diet soda also can cause health problems, he said.
His recommendation? "Drink water instead of soda."