Nov. 2, 2009 -- Diet soda may help keep your calories in check, but drinking two or more diet sodas a day may double your risk of declining kidney function, a new study shows.
Women who drank two or more diet sodas a day had a 30% drop in a measure of kidney function during the lengthy study follow-up, according to research presented Saturday at the annual meeting of the American Society of Nephrology in San Diego.
"Thirty percent is considered significant,'' says researcher Julie Lin, MD, MPH, assistant professor of medicine at Harvard Medical School and a staff physician at Brigham and Women's Hospital in Boston. That's especially true, she says, because most study participants had well-preserved kidney function at the start of the study.
Diet Soda and Kidneys: Study Details
The researchers evaluated 3,256 women already participating in the Nurses' Health Study who had submitted dietary information, including their intake of sugary beverages -- sugar-sweetened drinks, sugar-sweetened soda, and artificially sweetened soda. Sugar-sweetened drinks included soda, fruit juices, punch, and iced tea.
Information was also available on measures of kidney function. Their median age was 67.
Lin's team looked at the cumulative average beverage intake, derived from food questionnaires completed in 1984, 1986, and 1990. The women replied whether they drank the beverages less than once a month, one to four times a month, two to six times weekly, once daily but less than twice, or twice a day or more often.
Diet Soda and Kidneys: Study Results
When the researchers compared kidney function of the women in 1989 and 2000, they found that 11.4% or 372 women had a kidney function decline of 30% or more. When they looked at the diet information, they found that the 30% decline in kidney function was associated with drinking two or more artificially sweetened sodas a day. This was true even after taking into account factors such as age, high blood pressure, diabetes, and physical activity.
Put another way: the women who drank two or more diet sodas a day had a decline in their glomerular filtration rate, a measure of kidney function, of 3 milliliters per minute per year. ''With natural aging, kidney function declines about 1 mL per minute per year after age 40," Lin says. No link was found with the other beverages. And less than two sodas a day didn't seem to hurt. "We didn't see any association up to two artificially sweetened beverages a day," Lin says.
''A serving was reported as either a glass, a can, or a bottle of a beverage," Lin tells WebMD. ''It was not more specific than that."
''The mechanisms aren't clear," Lin says of the association she found. In another study she presented at the meeting, she found higher salt intake is also associated with faster kidney function decline.
All of the participants were women, so Lin can't say for sure that the association holds for men, although she says there is ''no biological reason to think it wouldn't."
Diet Soda and Kidney Function: Industry Input
Asked to review the study findings, Maureen Storey, senior vice president of science policy for the American Beverage Association, says in a prepared statement: "It's important to remember that this is an abstract presented at an annual meeting." She notes that the research needs further scrutiny by researchers.
Diet Soda and Kidney Function: Dietitian's View
In reviewing the study, Connie Diekman, RD, director of university nutrition for Washington University, St. Louis, wonders if the link might have come about because of long-term consumption, as many of the participants were older adults.
The link found, she says, "calls for more studies where actual intake can be assessed, rather than taking the information from food frequency questionnaires, which could be subject to mistakes."
Diet drinks, she says, are ''generally low in important health-promoting nutrients, so keeping them as a small part of your eating plan would be a smart step."