A new study by researchers at the National Cancer Institute (NCI) suggests being overweight could double a woman's risk of developing one type of cervical cancer, known as cervical adenocarcinoma. Although infection with certain strains of the human papillomavirus (HPV) is the primary risk factor for cervical cancer, the latest research could help shed light on why some women with HPV develop cervical cancer while others don't.

The findings also give women another reason to try to avoid being overweight or obese. "This might add a small bit of evidence that trying to maintain a healthy body weight would be desirable," said lead researcher James Lacey, PhD.

An American Cancer Society expert agreed. "We're starting to refine our knowledge about obesity and various cancers," said Eugenia Calle, PhD, director of analytic epidemiology at ACS. Calle published research earlier this year that attributed 90,000 cancer deaths in the US each year to excess weight.

Lacey's study was published in the journal Cancer (Vol. 98, No. 4: 814-821).

Obesity is already a well-known risk factor for the most common gynecological cancer, endometrial cancer (cancer of the uterus). It was this relationship, in part, that led the NCI researchers to investigate how obesity might affect cervical adenocarcinoma, which accounts for 10% to 20% of cervical cancers (squamous cell cancers account for most of the rest).

Studies in the 1980s suggested that these two types of cancer had similar risk factors, "and that made sense because of where these tumors occur in the cervix," said researcher Lacey, of the NCI's Division of Cancer Epidemiology and Genetics. Cervical adenocarcinomas typically develop high in the cervical canal, close to the endometrial tissue of the uterus.

The research was also prompted by a better understanding of the role HPV plays in cervical cancer development. Although most HPV infections clear up on their own, certain strains of the virus can cause changes in the cervix that can lead to cancer if the body is not able to eliminate the virus.

Now that HPV's role in cervical cancer is clearer, Lacey said, researchers can look for other factors -- like obesity, sexual history, smoking, or hormone use, for example -- that might influence whether the virus leads to cancer.

Lacey and colleagues compared 124 women with cervical adenocarcinoma to 139 similar women with squamous cell cervical cancer, and 307 similar women without cervical cancer. The researchers calculated their body mass index, or BMI, a ratio of height to weight. A BMI between 18.5 and 24.9 is considered normal weight; a BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or above is considered obese.

Women with a BMI of 25 or higher had about twice the risk of getting cervical adenocarcinoma as women of normal weight. Weight did not seem to influence whether women developed squamous cell carcinoma.

The researchers weren't certain why only adenocarcinoma seemed to be influenced by obesity. It could be that hormonal factors play a greater role in this type of cervical cancer, as they are thought to do in endometrial cancer. However, more research would be needed to make that determination.

Another theory is that screening differences play a role, Lacey said.

Regular Pap tests can detect changes in cervical tissue that could lead to cancer, allowing them to be treated before that happens. But because adenocarcinomas develop higher in the cervix, it may be more difficult to get a good tissue sample for the test, so precancerous changes can be overlooked. This problem may be compounded in overweight and obese women, because excess abdominal fat may make it even more difficult to get a good tissue sample.

Until more is known about just how excess weight relates to cervical cancer, experts say it is wise for women to watch their weight by getting lots of regular exercise and eating a healthful diet. The American Cancer Society's guidelines for healthy eating and physical activity can help.

ACS recommends eating at least five servings of fruits and vegetables every day, as well as plenty of whole grains. Red meat should be eaten in moderation; poultry, fish and beans are better choices. Adults should also get at least 30 minutes of moderate exercise five days a week or more. Longer, more vigorous exercise is even better.

In addition, ACS recommends regular Pap tests to detect any changes in the cervix before they become cancerous.