ATLANTA — A cancer diagnosis often inspires people to exercise and eat healthier. Now the experts say there’s strong evidence that both habits may help prevent the disease from coming back.

New guidelines issued Thursday by the American Cancer Society urge doctors to talk to their cancer patients about eating right, exercising and slimming down if they’re too heavy.

That’s not something most doctors do, said Dr. Omer Kucuk, an Emory University oncologist who has researched the effect of nutrition on prostate cancer. They’re focused on surgery, chemotherapy or other treatments for their patients, he added.

“Usually the last thing on their mind is to talk about diet and exercise,” Kucuk said.

Cancer society officials have long encouraged healthy eating and exercise as a way to prevent certain cancers. They and others have tried to spread that gospel to cancer survivors as well. Indeed, the cancer society has a certification program for fitness professionals who work with cancer survivors.

But until now, the group didn’t think there was enough research to support a strong statement for cancer survivors.

Hastine Reese, a breast cancer survivor, says she began to exercise because her husband — not her doctor — pushed her. Besides being good for her health, he thought it might help pull her out of the depression that followed her diagnosis and double-mastectomy.

“When you’re first diagnosed with cancer, you go into a dark place,” said Reese, as she finished a one-hour exercise class this week at DeKalb Medical Center in Decatur, Ga.

Exercise has changed that. “I’m coming into the light, and it’s getting brighter and brighter,” she said.

Being overweight or obese has long been tied to an increased risk of several types of cancer, including cancers of the colon, esophagus, kidney, pancreas and — in postmenopausal women — breast. But there hadn’t been much evidence on the effects of diet and exercise for people who had had cancer.

The last five years saw more than 100 studies involving cancer survivors, many of them showing that exercise and/or a healthy diet was associated with lower cancer recurrence rates and longer survival.

Most of the research was on breast, prostate and colorectal cancer. The evidence is more meager when it comes to other cancers, including the deadliest kind, lung cancer. Also, most of the work involved observational studies, which can’t prove a cause and effect. Still, the volume of research was compelling.

“We’ve got enough data now to make these recommendations,” said Colleen Doyle, the organization’s director of nutrition and physical activity.

At least two other medical groups have strongly recommended exercise and healthier eating for cancer survivors, but the cancer society’s new guidelines are expected to have much greater impact. It’s the nation’s largest cancer charity in both donations and the number of volunteers, and it funds more cancer research than any other non-governmental agency.

There was a time when cancer patients were thought of as gaunt and dying souls. Many cancers were diagnosed at a late stage, after the disease had ravaged the body and caused weight loss.

But better screening and treatment has made early diagnosis of cancer more common and survival more likely. Today, more than two-thirds of cancer patients live at least five years. The ranks of cancer survivors have grown, with more than 12 million Americans identified as cancer survivors.

Meanwhile, obesity has boomed. More than two-thirds of U.S. adults are now considered overweight or obese.

The guidelines recognize that for some people just eating enough food is a priority, and that diet advice can vary during treatment. The cancer society also notes that some people may be too weak at times for vigorous exercise. But experts say that even modest activities, like lifting soup cans while watching TV, can help.

Women seem to take to exercise and diet recommendations more readily than men, or to push their spouses to follow the advice, some doctors said. Most of Reese’s classmates were women.

“I find women to be very, very proactive,” said Dr. Allen Lawhead, a gynecologic oncologist at DeKalb Medical Center. “Men, we traditionally go back into our man cave and hide.”

Lawrence Genter, a survivor of non-Hodgkin’s lymphoma, was one of three men in Reese’s class of about a dozen. “I’m here because of my wife,” he said.

For another cancer survivor, exercise came easy but eating healthy was a challenge. During chemotherapy, nausea is common and food can seem unappetizing.

“The key thing is to eat period — whatever you can get down and keep down,” said Bob Falkenberg of Alpharetta, Ga., who was a marathoner and long-distance cyclist before he was diagnosed with leukemia.

What did he eat during chemo? Mexican food. Hamburgers.

“I had people bring in pizza at one point,” he laughed.