Although the two studies were conducted on laboratory animals, the researchers claim to have traced the mechanism that could cause this protective effect.
One study conducted at Rutgers University, New Jersey, focused on skin cancer in mice and found that female mice that had 24-hour access to running wheels and were exposed to ultraviolet B light (UVB) took longer to develop skin tumours, developed fewer and smaller tumours, and had decreased amounts of body fat compared to mice that did not have access to running wheels.
Lead study researcher Dr. Allan Conney, Garbe Professor of Cancer and Leukemia Research, suggests that exercise could actually increase the physiological cell-death process and hence these mice showed delayed development of tumors although they were at significant risk.
The second study at the University of Wisconsin-Madison looked at the development of pre-cancerous polyps in the intestines of male mice and discovered that voluntary exercise and a restricted diet reduced the number and size of polyps and improved survival.
Lead researcher Dr. Lisa Colbert suggests that a negative energy balance, which is produced by increasing energy output through exercise, could be the reason for the inhibition of polyps.
Dr. Colbert says there were on average 16 polyps per mouse in the exercising mice compared to 22 polyps in the control mice, a decrease of 25%.
Conney believes the studies may be the first to suggest a cell suicide mechanism effect linked to voluntary exercise and the development of cancer, which she says deserves further investigation.
The studies are published in the May 13 issue of the journal "Carcinogenesis".
In the complete carcinogenesis model, mice with no running wheel started to develop tumours 20 weeks after the start of UVB exposure, while tumours in the running wheel group started after 23 weeks. The average tumour-free time was 25 weeks for the no running wheel group and 27 weeks for the running wheel group.
Dr Conney said: "The rate of increase in tumour numbers per mouse for the no running wheel group was significantly greater than that for the running wheel group. On average, the tumour size per mouse for the no running wheel group was about 3.5 times more than in the exercise group.
"In both models, voluntary running decreased the number of non-malignant tumours per mouse by 34%. Exercise substantially decreased the size of non-malignant tumours and malignant tumours: in the high risk model, the non-malignant tumour size per mouse was decreased by 54% and the malignant tumour size per mouse by 73%, and in the complete carcinogenesis model, tumour size per mouse was decreased by 75% and 69% respectively."
For the bowel cancer study, Dr Colbert and her co-authors used mice (APC Min mice) that had a genetic mutation that predisposed them to develop intestinal polyps. "Our studies are relevant for humans in that these Min mice have a mutation in one of the same genes, APC, that is also mutated in human colon cancer," she explained. "The protective effect of exercise and lower body weight in our mice is consistent with epidemiological evidence in humans that suggests higher levels of activity and lower body weight reduces the risk of colon cancer."
Mutations in the APC gene in humans are responsible for an inherited condition called familial adenomatous polyposis (FAP). FAP affects about one in 10,000-15,000 people worldwide, 95% of whom will develop numerous polyps in the bowel which eventually develop into colon cancer, usually before the age of 40. The gene is mutated in sporadic forms of colon cancer as well.
The researchers randomly assigned seven-week-old male mice to either voluntary wheel running or to no exercise for 10 weeks. For the first three weeks both groups had the same amount of food and water, but after that the exercising mice were fed the amount that the non-exercising mice had eaten the week before so that their food consumption was unable to rise with their increased activity, thereby producing a negative energy balance.
By the end of the ten weeks, six of the 23 control mice had died due to the number of polyps that had grown and the resulting anaemia, while all the 24 exercising mice were still alive.
"The exercising mice ran an average of 3.8 km a day, and the further they ran the fewer polyps they had. Exercise significantly reduced total polyp number and polyp size, as well as prolonging survival," said Dr Colbert. "On average there were 16 polyps per mouse in the exercising mice compared to 22 polyps in the control mice - a decrease of 25%."
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