Exercise may significantly reduce your risk for many types of cancer, including some of the most lethal forms of the disease, a large review suggests.

Working out for even a couple of hours a week appears to shrink the risk of breast, colon and lung cancer, said researchers who looked at 1.4 million adults.

"Those are three of the four major cancers that affect Americans today," said Marilie Gammon. She is a professor of epidemiology with the University of North Carolina at Chapel Hill Gillings School of Public Health.

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And fitness buffs, take heart -- your cancer risk appears to continue to decline as you rack up hours of physical activity, with no apparent upper plateau, said study lead author Steven Moore, an investigator with the U.S. National Cancer Institute.

"The more activity, the more the benefit," Moore said. "As people did more, their risk continued to lower."

It should be noted, however, that the study only found an association between exercise and reduced cancer risk; it did not prove a cause-and-effect relationship.

In the study, regular exercise wound up being linked to a reduced risk of 13 cancers in all, the researchers said. The others were leukemia, myeloma and cancers of the esophagus, liver, kidney, stomach, endometrium, rectum, bladder, and head and neck.

Current federal guidelines for exercise -- 150 minutes of moderate-intensity activity a week, or 75 minutes of vigorous activity -- are aimed at heart health but also serve well for cancer prevention, Moore said.

Moderate-intensity exercise involves pursuits such as brisk walking or tennis, while vigorous intensity exercise involves heart-pumping activities such as jogging or swimming laps, according to the U.S. Office of Disease Prevention and Health Promotion.

For this study, Moore and his colleagues focused on leisure-time physical activity performed outside work or household chores. "This is voluntary physical activity typically done to improve health," he said.

About half of all American adults don't meet the federal minimum recommendation for exercise, the study authors said in background information.

Prior research has linked exercise to reduced risk of breast and colon cancer, but no study has attempted to look at the effect of physical activity on many different types of cancer, Moore said.

The researchers pooled data from 12 U.S. and European studies to create a database of 1.4 million adults, aged 19 to 98. They then examined whether self-reported physical activity made a difference in risk of 26 cancers.

Exercise was associated with a reduced risk for half of the cancers considered by the investigators, and that reduction remained significant for nearly all, even after accounting for factors such as obesity and smoking history.

Overall, a higher level of physical activity was associated with a 7 percent lower risk of total cancer, the researchers reported.

The range of reduced risk ran from 42 percent for esophageal cancer to 10 percent for breast cancer, the study authors said. For colon and lung cancer, risk was lowered 16 percent and 26 percent, respectively, the findings suggested.

"This suggests that physical activity may have a role to play in population-wide cancer prevention efforts," Moore said.

The findings were published online May 16 in the journal JAMA Internal Medicine.

No one is certain why exercise seems to help fend off cancer, Moore and Gammon said, but there are some leading theories.

Physical activity reduces levels of hormones, such as estrogen, that have been linked to different cancers, and helps control levels of insulin and insulin-like growth factor, Moore said.

People who work out also tend to have lower levels of inflammation, Moore said. Their cells appear to be subject to less oxidative stress, and are more capable of repairing damaged DNA that might cause cancer, said Gammon, co-author of an editorial accompanying the study.

Gammon said she was most pleased with the 42 percent risk reduction found in esophageal cancer.

"That's pretty amazing, because it's a very deadly tumor," she said. "I think the average length of survival is 11 to 12 months after you're diagnosed."

Other very deadly cancers that appear to become less common with exercise include those of the liver, stomach, kidney, and head and neck, Gammon said.

"Having a strategy to help reduce risk of those cancers is very good, because your outlook is not optimal once you're diagnosed," she said.