It′s long been known that exercising can reduce the risk of cancer.
And now scientists believe they have found another reason why - and even say it could pave way for new treatments.
Those who stay fit are better able to process a by-product of running, jogging and pumping iron, research shows.
Lactate ? which makes muscles stiffen after exercise ? is a key driver of cancer growth and spread, experts claim.
Dr Inigo San Millan, of the University of California, Berkeley, said: ′With this paper, we open a whole new door for understanding cancer, showing for the first time that lactate is not only present, but mandatory for every step in its development.
′We hope to sound the alarm for the research community that to stop cancer you have to stop lactate.′
Exercising is known to reduce the risk of breast, bowel, colon and womb cancer.
The researchers wanted to understand why, and built upon previous research about the ′Warburg effect′ - when cancer cells quickly rake in more glucose than normal cells.
Despite the cause not previously being known, the new study, published in Carcinogenesis, sought to explain why.
They found lactate interferes with the body′s immune response to cancer, and helps the tumour to spread.
When exercising, muscles use glucose for energy.
When there isn′t enough oxygen to keep up with the amount of energy needed, the body produces lactate as a by-product.
It therefore recycles the lactate for a beneficial use, including turning it into a key fuel source for the brain, muscles and organs.
But, the study found the recycling system broke down for cancer cells.
A fault in the recycling system was less likely to happen in athletes, as their body was more used to converting lactate to beneficial fuel sources.
The results could lead to new exercise and dietary prescriptions for cancer patients, the researchers said.
New diagnostic tools could be made to find a break down in the lactate recycling system, to help doctors identify brewing cancer, they said.
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