HEIDI SPLETE (Senior Writer)
Mounting evidence linking high consumption of fructose to an increased risk of kidney disease could have clinical implications for the treatment of high-risk patients.
Data from recent studies suggest high fructose intake causes elevated uric acid levels, which might contribute to the development of kidney stones and kidney disease.
“It is not yet clear whether fructose—either natural or in the form of high-fructose corn syrup—itself is problematic, or if consuming large amounts of any type of sugar is bad for the kidneys,” David Shoham, Ph.D., an epidemiologist at Loyola University Chicago, Maywood, said in an interview.
He presented results from a preliminary population-based study at a meeting last year, showing an increased rate of albuminuria in people who reported drinking several sugary (nondiet) soft drinks daily, compared with those who reported drinking one or fewer sugary soft drinks per day. Sugar-sweetened sodas contain high-fructose corn syrup, which contains mostly fructose but also contains glucose, he noted.
“It is safe to say Americans consume too much sugar and too many calories in general, and reducing overall sugar intake would be a good thing, because this is playing a role in the obesity and diabetes epidemics,” Dr. Shoham said.
Achieving a reduction in overall sugar intake might be more feasible than avoiding fructose altogether because it is ubiquitous in the American diet. Fructose occurs naturally in fruit, and high-fructose corn syrup can be found in a range of packaged foods, including jam, bread, hot dogs, and even reduced-calorie salad dressing, he said.
But even a small reduction in sugar intake might help reduce the risk of kidney problems, especially in individuals with other risk factors.
More research is needed to learn more about how fructose behaves in the body, but findings from studies suggest that fructose has a potentially unique ability to cause increased uric acid levels, Dr. Richard J. Johnson of the University of Florida, Gainesville, and his colleagues wrote in a review article (Am. J. Clin. Nutr. 2007;86:899–906).
Dr. Johnson and his colleagues cited studies in which increased fructose intake led to increased uric acid production. Fructose is the only sugar that raises the concentration of uric acid, and this might be the mechanism that links fructose to the development of kidney disease and other conditions, the authors noted.
“Hyperuricemia in humans is also strongly associated with endothelial dysfunction, and lowering uric acid concentrations has consistently been shown to improve endothelial function after several weeks,” they wrote.
Separating the effects of fat from those of sugar remains a challenge in studies of kidney disease. Dr. Johnson and his colleagues noted that many Americans have been consuming less fat for the past 20 years but also have been consuming more sugar, and obesity rates have soared during the same time period.
Data from thousands of participants in the Nurses' Health Study and the Health Professionals Follow-Up Study support a link between increased fructose consumption and an increased risk of kidney disease.
In the follow-up study, Dr. Eric N. Taylor and Dr. Gary Curhan of Harvard Medical School, Boston, reviewed questionnaires from 93,730 older women, 101,824 younger women, and 45,984 adult men about fructose intake.
Overall, the researchers identified 4,902 incident kidney stones during a combined 48 years. Based on the responses to the food questionnaires, the relative risk of kidney stones was significantly higher in individuals whose responses put them in the highest quintile of total fructose consumption, compared with those in the lowest quintile. Nonfructose carbohydrate intake was not significantly associated with an increased risk of kidney stones in any subset of the study population (Kidney Int. 2008;73:207–12).
The researchers calculated the total fructose intake based on a combination of free fructose (found in high-fructose corn syrup) and conjugated fructose (found in table sugar).
“Fructose may be absorbed more quickly when it is free. We are studying this now,” Dr. Johnson said in an interview. “Low-sugar and low-fructose diets may be another way to help prevent kidney stones, and might also have a potential cardiovascular benefit, possibly reducing the development of hypertension, obesity, and diabetes.”
But more research is needed to verify an association between a high fructose intake and an increased risk of renal and cardiovascular disease. Dr. Johnson said that he is part of a team preparing to test whether a low-fructose diet can slow the progression of kidney disease. Studies also are needed to examine whether a low-fructose diet can promote weight reduction, reduce blood pressure, and improve insulin resistance, he added.
