So. You get the worst news of your life: Cancer.

You dutifully sign on for chemo, surgery, radiation. You also vow to eat better. More fruits and veggies, less saturated fat –- all that good stuff should tip the odds in your favor, right?

There’s actually surprisingly little evidence that such dietary changes prolong survival -- except perhaps for colon cancer.

What is crystal clear, though, is the importance of exercise and weight control. Gone is the folk wisdom that people with cancer should avoid getting too thin. The real threat, say cancer nutritionists, is becoming or remaining overweight. At a basic metabolic level, excess weight and lack of exercise may not only add diabetes and heart disease to your cancer troubles, but can impair immune function and even boost levels of hormones, including insulin and estrogen, that may drive some tumors.

For cancer patients who had been hoping that a good diet might improve their survival odds, some seriously disappointing news came out earlier this summer when scientists from the University of California in San Diego reported long-awaited results from the Women’s Healthy Eating and Living study. This randomized, controlled trial followed more than 3,000 women who had been treated for early stage breast cancer. After an average of 7.3 years of follow-up, the researchers found that women randomly assigned to a diet very high in vegetables, fruit, and fiber and very low in fat (15 to 20 percent of calories) did no better in terms of recurrence or death than women who simply stuck to a “5-a-day” diet with five servings of fruits and veggies.

Somewhat better news came last December with publication of a different study, called the Women’s Intervention Nutrition Study, led by Dr. Rowan T. Chlebowski, a medical oncologist at the Los Angeles Biomedical Research Institute. This team studied 2,400 women who had been treated for early-stage breast cancer and randomly assigned them to a dietary fat reduction group or regular diet group. After five years of follow up, there were significantly fewer recurrences among members of the lower fat group, most of whom lost weight.

The trouble is, said Chlebowski, it’s not clear whether it was the low fat diet per se or losing weight that conferred the benefit. And cues from other research suggest that losing weight, in part because it brings insulin levels into better control, may be the real key.

“Obesity is linked to worse outcomes in a variety of cancers, especially cancers of the breast, colon and prostate,” said Dr. Matthew Smith, director of genito-urinary medical oncology at Massachusetts General Hospital. For instance, in men with prostate cancer, “obesity is associated with a greater risk of prostate cancer recurrence after surgery or radiation,” said Smith. And  nfortunately, the hormone treatment that is often used to fight prostate cancer can itself contribute to obesity.

“Many cancer survivors and their families worry about weight loss as a manifestation of advanced cancer, when in fact, weight loss -– intentional weight loss –- and maintenance of ideal body weight may be one of the most effective strategies to improve overall health and the reduce the risk of recurrence,” Smith said.

“Weight gain, especially fat gain, can also impair immune responsiveness and in women with breast cancer, weight  gain may stimulate production of estrogen, which drives some breast tumors,” said Dr. Richard Rivlin, a nutrition specialist at Weill Medical College of Cornell University.

Even more discouraging, some drugs, such as tamoxifen, that women take to reduce breast cancer recurrence, can actually cause weight gain, said Dr. Lee Kaplan, director of the weight center at Massachusetts General Hospital.

Regardless of what you weigh, exercising is key. A study published in 2005 by Harvard Medical School researchers on nearly 3,000 women with breast cancer showed that women who walked the equivalent of three to five hours a week at an average pace had a lower risk of dying from their cancer. Two studies on people with colon cancer showed that walking six hours a week significantly reduces the risk of recurrence.

So, what to do? Two-thirds of the food on your plate should come from fruits, vegetables, whole grains and beans and no more than one-third from meat, fish or chicken, Karen Collins, nutrition advisor to the American Institute for Cancer Research, as nonprofit research group based in Washington, D.C. It’s not that the evidence is there to prove all this will prolong your life if you’ve already got cancer, but this stuff is so good for you it just makes sense to eat this way 

Eating less meat and more fruits and veggies may make a difference if you’ve got colon cancer, said Dr. Jeffrey Meyerhardt, a gastrointestinal oncologist at the Dana-Farber Cancer Institute. In one of his studies, Meyerhardt showed that people with colon cancer who eat more meals of a typical Western diet –- with lots of red meat, refined grains and sugary foods -- have three times the risk of recurrence or death than those who eat less of these foods.

Dr. Lidia Schapira, a breast cancer specialist at Mass. General, put it this way: “Even though the data are imprecise and conflicting, we can’t wait to eat until better data are in."