Cancer patients are being urged to consider the impact of diet and exercise as experts launch the world's biggest trial into how weight loss may stop the disease coming back.
It is already known that being overweight contributes to the development of 11 different types of cancer but researchers believe it also affects how some people respond to treatment and the risk of recurrence.
Evidence presented at the American Society of Clinical Oncology (ASCO) conference in Chicago shows being overweight results in poorer outcomes, while exercise appears to show promise in helping people beat the disease.
Dr Jennifer Ligibel, assistant professor of medicine at Harvard Medical School, is launching a major trial looking at whether losing weight can cut the chance of breast cancer coming back.
Although previous observational studies have found a link, hers will be the first randomised controlled trial with a specific aim of seeing how weight loss affects cancer.
The new trial will enrol 3,200 women from the US and Canada who are overweight and obese with a BMI of at least 27.
They will be randomly split into two groups, with the intervention arm receiving weight-loss advice to lose 10% of their body weight alongside an exercise programme, for two years.
Dr Ligibel said that of 100 women, around 23 would normally be expected to have a recurrence of cancer, but this should drop to 19 in the weight-loss group.
Women on the diet will have around 1,200 to 1,500 calories per day and exercise for about 150 minutes a week for the first six months, increasing to 250 minutes where possible.
She said brisk walks would count as exercise, adding: "It's a very accessible form of exercise. It doesn't cost anything. You just need a pair of shoes.
"This will be the largest study that's ever tested the impact of weight loss on cancer - any form of cancer."
Another study is also being launched that will look at the role of exercise on the risk of disease coming back in men with prostate cancer.
Professor Fred Saad, a cancer specialist at the University of Montreal, who is leading the trial, said cancer patients should be given a personal trainer alongside their medication.
He said exercise and weight loss "increase survival rates", and high intensity exercise - spurred on by a trainer - is more effective than exercising alone.
Professor Melinda Irwin, associate director of cancer prevention and control for Yale Cancer Centre, who is involved in eight studies on exercise and cancer, said she has found a "strong connection between exercise after diagnosis and mortality afterwards."
Women with breast cancer who have never been active before can reap benefits, she believes, adding that although the studies were observational she is "in the camp that says this association is causal and reversible".
She added: "After treatment, weight loss is the most powerful thing you can do. It's the next best pill to treatment and it's free and has no side-effects."
Carolyn Rogers, clinical nurse specialist at Breast Cancer Care, said: "We know that trying to maintain a healthy weight could make a difference when facing breast cancer and reduce the risk of it returning. We look forward to seeing the results of further larger studies."
Fran Woodard, executive director of policy and impact at Macmillan Cancer Support, said being a healthy weight and taking exercise gives women the best chance of survival.
"But cancer treatment is gruelling and fatigue can seriously hamper someone's best intentions," she said.
"Asking a person with cancer about their lifestyle habits when they have just been diagnosed can be challenging and must be done sensitively. However this is an issue too important to tiptoe around."