Sept. 30, 2010 -- Excess caffeine increases the likelihood of urinary incontinence in women, according to a new study that echoes the findings of previous research.

''Women who consume high levels of caffeine are 70% more likely to have urinary incontinence than women who don't," says Jon Gleason, MD, an instructor and fellow at the University of Alabama at Birmingham Medical School's Division of Women's Pelvic Medicine and Reconstructive Surgery.

He is slated to present the findings Friday at the American Urogynecologic Society's annual meeting in Long Beach, Calif.

Caffeine sources include coffee, tea, beverages, foods, and some pharmaceuticals.

In Gleason’s study, women who reported taking in 329 milligrams of caffeine a day -- about three cups of coffee -- or more had a 70% higher likelihood of having the bladder problem.

Urinary incontinence, or the unintentional loss of urine, affects more than 13 million Americans, mostly women. Women are most likely to develop it either during pregnancy or childbirth or with menopause and its hormonal changes, due to pelvic muscle weakness. But it's not a natural consequence of aging, experts say.

Caffeine and Bladder Problems: The Study

Previous research has produced conflicting findings about caffeine and bladder problems, Gleason says. So he looked at a larger group of women than those studied in previous research.

Gleason evaluated data on 1,356 women, aged 20 to 85, who had participated in the 2005-2006 National Health and Nutrition Examination Survey, a large nationally representative survey.

The women kept food diaries on two separate days and also answered questions about bladder function.

Moderate caffeine intake, defined as about 182 milligrams a day, was not linked to urinary incontinence. But higher intakes -- 329.5 milligrams or higher -- were.

People who had a high caffeine intake were more likely to be aged 40-59, to drink alcohol, and to be non-Hispanic white.

Gleason found no links between urinary incontinence and body mass index, vaginal childbirth, or high intake of water.

What's Behind the Link?

''Only high levels of caffeine were associated with urinary incontinence," Gleason says.

What is it about the caffeine? "There is evidence that caffeine has a diuretic effect," he says. The diuretic effect increases the amount of urine you make.

Caffeine may also make the muscles that contract when you void overactive, he says.

Caffeine and Bladder Problems: Second View

The study findings are similar to those found by Lilly Arya, MD, an associate professor of obstetrics and gynecology at the University of Pennsylvania School of Medicine in Philadelphia, who published a study on caffeine and incontinence nearly a decade ago.

''What these studies are really finding [is that] high levels of caffeine are associated with urinary incontinence," she says.

Does it cause or aggravate the condition? "That's not known," says Arya, who reports doing consultant work for Pfizer, Astellas, and Duramed, makers of incontinence drugs or products.

Caffeine and Bladder Problems: Advice

The new study findings lend weight to advice many doctors already give, Arya says.

''For the woman with [bladder] problems, it should ideally be zero caffeine," she says. But she's realistic, knowing ''you have to be able to get through the day." So she tells these women, "Have a cup, one small cup, but make it as small as you can, definitely less than 8 ounces."

For women without bladder problems? "Up to two cups a day is generally fine. No bigger than 12 ounces, and the second cup should preferably be decaf."

"If you review all the caffeine literature, it seems like, in the U.S., up to two cups of coffee a day is considered moderate." What’s more difficult, she says, is defining a cup.

Years ago, she tells WebMD, researchers considered 5 ounces to be a ''standard" cup. "That's your grandma's china cup," she says.

These days, many researchers view an 8-ounce cup standard, she says. But coffee sold at outlets is often served in a much bigger cup.

At Starbucks, for instance, a short is 8 ounces but a tall is 12, a grande is 16, and a venti, 20 ounces.