About half of all women will experience a urinary-tract infection by the time they are in their late 20s, usually because of sexual activity.
But a new study shows that other factors, such as a woman's race, the type of contraception she uses or even whether she drinks carbonated beverages also can affect her risk of the painful infection.
Michigan researchers looked at 374 college students, 86 of whom had a urinary-tract infection. They found that women whose partners used condoms were almost twice as likely to get a urinary-tract infection, compared to women who used oral contraceptives. Diaphragm use tripled a woman's risk of the infection.
Using deodorant sanitary napkins or tampons also increased the risk of infection, as did drinking carbonated beverages.
And black women were more than five times as likely as white women to get a urinary-tract infection, according to the study, which appeared in the March issue of the journal Epidemiology.
The researchers speculated that condom use, especially without lubrication, could increase the trauma associated with vaginal intercourse, which could cause the symptoms of a urinary-tract infection. And organisms that cause such infections may be more virulent in black women, they suggested.
The link between the infections and soda intake, however, remains a mystery.
The researchers thought the caffeine in the soda might be the culprit, but women who drank other caffeine-containing beverages, such as coffee or tea, did not have an increased risk of infection, says study author Dr. Betsy Foxman.
"Some say soda consumption is protective. Others say it is a risk factor," said Dr. Foxman, an assistant professor of epidemiology at the University of Michigan School of Public Health in Ann Arbor.
"It's a suggestive finding," but it is hard to know how drinking soda may be affecting women, she said.
"However, if a woman drinks 10 cans of soda in a day, as some women in the study did, and she has lots of urinary-tract infections, it's not unreasonable to stop" drinking soda, the Michigan researcher said.
Women in the study who drank cranberry juice or took vitamin TC regularly were less likely to get urinary-tract infections.
Cranberry juice is a well-known folk remedy for these infections, and several studies have shown that it seems to both treat and prevent them.
The juice may have a protective effect because it increases the acidity of the urine, or it may contain substances that inhibit bacterial growth, Foxman said. Vitamin C was not as protective as the juice, but it also may increase the acidity of urine, she said.
"About 3 percent of women get chronic urinary-tract infections, and they get quite desperate," she added. "If they know that some of these factors make a difference, then they may try to change them. These are all relatively benign changes."