Staying slim and fit is especially important for cancer survivors, because obesity raises the risk of cancer coming back, the American Cancer Society said in new guidelines issued on Wednesday.

“The evidence really is quite strong for the need for cancer survivors to achieve and maintain a healthful weight,” Wendy Demark-Wahnefried of Duke University Medical Center, one of the report’s authors, said in an interview.

The recommendations, updating advice issued in 2001 and 2003, were published in the society’s “CA: A Cancer Journal for Clinicians.”

The report said obesity is a well-established risk factor for some of the most common forms of cancer, including breast cancer in post-menopausal women and cancers of the colon, esophagus, liver, gallbladder, pancreas, kidney, uterus and prostate.

It also cited increasing evidence that being overweight raises the risk for recurrence and reduces likelihood of survival for many cancers.

Demark-Wahnefried said cancer survivors also face a greater risk of heart disease and diabetes, adding, “Obesity is a big risk factor for those diseases as well as second cancers.”

The American Cancer Society said nearly two-thirds of U.S. cancer patients live more than five years after diagnosis, and more than 10 million Americans now alive have been diagnosed with cancer at some time in their lives.

The report said vegetarian diets can have many benefits because they tend to be low in saturated fat and high in fiber and vitamins.

“However, no direct evidence has determined whether consuming a vegetarian diet has any additional benefit for the prevention of cancer recurrence over an omnivorous diet high in vegetables, fruits, and whole grains, and low in red meats,” the report stated.

The report noted that preliminary evidence indicates that for some types of cancer, one to three hours per week of exercise can cut the risk of cancer recurrence and death.