Overweight or obese women who lost weight and exercise have lower levels of proteins in the blood that help cancer tumours grow, a study found.
And the more weight a woman sheds the better protection against tumours forming she has.
Carrying too many pounds and not exercising has been linked with an increased risk for developing certain cancers, but the reasons for this relationship was not clear.
Drugs not the answer
Scientist founds diets, or a combination of diet and exercise, reduced proteins that play a role in angiogenesis.
This is is a vital function where new blood vessels are formed, for example, during wound healing.
But it can promote the growth and survival of cancer cells.
It had been proposed preventing angiogenesis may be a way to stop cancers by starving tumours, but drugs that block this process have potential adverse effects.
The study by Fred Hutchinson Cancer Research Centre found losing weight and exercise was nature's way to prevent angiogenesis in a safe and healthy way.
Exercise alone - without losing weight - did not have the same effect.
The study examined whether these cancer-promoting proteins changed when overweight, sedentary, postmenopausal women lost weight through diet or diet and exercise over the course of a year.
It involved 439 healthy women aged 50 to 75 who did not have cancer and were overweight, sedentary and postmenopausal.
They were either placed on a calorie and fat restricted diet, did 45 minutes of moderate to vigorous aerobic exercise five days a week or a combination of diet and exercise.
There was also a control group.
Blood samples to measure three proteins known to enhance tumour-related angiogenesis were taken at the start and end of the study a year later
It was found on average, women in the diet arm, exercise arm, and diet and exercise arm lost 8.5, 2.4, and 10.8, per cent of body weight respectively.
This was significantly higher than the average weight loss for women in the control arm at just 0.8 per cent.
Those in the diet arm and the diet and exercise arm had significantly lower levels of the angiogenesis-related proteins, but such effects were not apparent in those in the exercise-only arm.
And the more weight a woman lost, the greater the reductions in their blood angiogenesis-related protein levels.
Principal staff scientist Dr Catherine Duggan said: "We know that being overweight and having a sedentary lifestyle is associated with an increase in risk for developing certain types of cancer.
"However, we don't know exactly why.
"We wanted to investigate how levels of some biomarkers associated with angiogenesis were altered when overweight, sedentary, postmenopausal women enrolled in a research study lost weight and/or became physically active over the course of a year.
"Our study shows that weight loss is a safe and effective method of improving the angiogenic profile in healthy individuals.
"We were surprised by the magnitude of change in these biomarkers with weight loss.
"While we can't say for certain that reducing the circulating levels of angiogenic factors through weight loss would impact the growth of tumours, it is possible that they might be associated with a less favourable milieu for tumour growth and proliferation
"Exercise is important for helping to prevent weight gain, and to maintain weight loss, but does not cause a large amount of weight loss on its own.
"Our study shows that making lifestyle changes--in this case simple changes to the diet to reduce weight--can lower the risk factors for cancer."
The study was published in Cancer Research.