Cardiorespiratory fitness is a measurement of how well the body uses oxygen during intense exercise. Good cardiorespiratory fitness is well known to be associated with better cardiovascular health, but less is known about its effect on cancer risk, the study team writes in the British Journal of Sports Medicine.

“We understand that many cancers are due to unhealthy lifestyle factors and many cancers are therefore preventable,” lead author Dr. Magnus Thorsten Jensen told Reuters Health by email.

“There have previously been reports that higher levels of physical activity may reduce the risk of cancer, which is one of the reasons why physical activity is encouraged by the World Health Organization,” said Jensen, a researcher at Holbaeck Hospital in Denmark.

Jensen and his colleagues analyzed data from the Copenhagen Male Study, which recruited middle-aged men who worked in Copenhagen starting in 1970.

The researchers followed 5,131 men who were about 49 years old, on average, and cancer-free when the study began. All of the participants underwent VO2 Max tests along with regular physical examinations at that time.

VO2 Max gauges the maximum amount of oxygen the body can process to make energy during exercise and is measured in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min).

Over the next 44 years, 4,482 participants died, 1,527 of them from cancer.

Researchers found that for every 10 mL/kg/min increase in VO2Max at the baseline test, there was a 17 percent lower risk of cancer death and an 11 percent lower risk of death from any cause during the follow up period.

“Our results show that the greater shape you are in, the longer you live and the lower is the risk of dying from cancer,” Jensen said.

That correlation was not seen, however, between self-reported physical activity and risk of cancer death.

“We find that it is not enough just to be physically active, you have to actually be fit,” Jensen said. "Very often, of course, physical activity and physical fitness go hand-in-hand, but our study shows the most important factor is your actual fitness level.”

When the researchers looked at specific cancer types, the risk reduction with greater cardiorespiratory fitness held, except for prostate cancer, where no difference was seen.

Despite its large size and long follow-up, the study is limited by the fact that undetected cancers or other illnesses could be the reasons for some men’s poor fitness levels, the researchers acknowledge.

Having such a long follow-up time allowed the study to exclude men who died within the first 20 years of the study from the analysis, Jensen noted. "This did not change the results, and underlying cancer is, therefore, a very unlikely explanation of our findings,” he said.

“I find our results very optimistic and encouraging,” he said. “Good general fitness can be achieved by everyone and doesn’t necessarily require expensive equipment or major investments."

There are several biological reasons why someone who is more physically fit would be less likely develop cancer, said Dr. Kim Dittus, an oncologist at the University of Vermont College of Medicine in Burlington.

"Someone who`s more fit is likely to be normal weight because exercise can help control weight," said Dittus, who was not involved in the study.

Overweight and obese men and women are likely to have higher estrogen levels, she said. Excess weight also influences insulin and insulin resistance. These hormonal differences have been tied to increased risk of different types of cancers.

Exercise is also important for people who have been diagnosed with cancer, Dittus added.

"We have a number of epidemiologic studies suggesting that those individuals who are more physically active are less likely to have their cancer come back, so they have better cancer outcomes than those who are inactive," she said.

Dittus added that individuals with better cardiorespiratory fitness might be better able to tolerate chemotherapy as well.

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