A new large-scale study has provided more strong evidence linking the consumption of red and processed meats to an increased risk of cancer.

Researchers from the U.S. National Cancer Institute examined data on 494,000 participants in the National Institutes of Health (NIH)-AARP Diet and Health Study. Participants filled out a detailed questionnaire on their dietary habits at the beginning of the study, and were then followed for eight years. At the end of this time, researchers compared the cancer data on the 20 percent of participants who ate the most red and processed meat with the data on the 20 percent who ate the least.

All participants were between the ages of 50 and 71. The highest red meat consumption group had an intake of 62.5 grams (2.2 ounces) of red meat for every 1,000 calories consumed. The processed meat consumption of the high group was 22.6 grams (0.8 ounces) per 1,000 calories.

Red meat was defined in the study as any meat originating from a mammal, including beef, pork and lamb.

Researchers found that people who consumed the most red meat had a 25 percent higher risk of developing colorectal cancer in the study period compared with those who ate the least, and a 20 percent higher risk of developing lung cancer. The risk of esophageal and liver cancer was increased by between 20 and 60 percent.

Those who ate the most processed meat had a 20 percent higher risk of being diagnosed with colorectal cancer and a 16 percent higher risk of lung cancer. Increased intake of red and processed meat was also correlated with an increased risk of pancreatic cancer in men.

According to the researchers, one in 10 cases of lung or colorectal cancer could be averted by limiting red meat intake.

Prior studies have also linked meat consumption to increased cancer risk, particularly the risk of colorectal and stomach cancer. Other studies have found associations between meat intake and the risk of bladder, breast, cervical, endometrial, esophageal, glioma, kidney, liver, lung, mouth, ovarian, pancreatic and prostate cancers.

Scientists have said that roughly 35 percent of cancers can be attributed to diet, a connection as strong as that with smoking. A November report from the World Cancer Research fund called alcohol and obesity the two biggest causes of cancer, and warned that the intake of processed and red meat should be limited.

Because high meat intake correlates with cancer risk factors like high calorie intake and obesity, as well as the consumption of other unhealthy foods, the researchers could not determine for sure whether the meat was causing cancer or merely contributing to other risk factors. But research has suggested that meat contains a number of carcinogenic compounds, including some that are formed during cooking or processing.

The researchers noted that meat also contains other potential carcinogens, including heme iron (the type of iron found in meat), nitrates and nitrites, saturated fat, hormones and salts. All of these substances have been observed to affect hormone metabolism, increase cell proliferation, damage DNA, encourage insulin-like growth hormones and promote damage of cells by free radicals, all of which can lead to cancer.

"Future research should also examine particular nutrients within meats (e.g., iron) or carcinogenic components (e.g., heterocyclic amines, nitrosamines) that are created as a result of certain cooking techniques, particularly among the rarer and less studied cancers," the researchers wrote. They also noted that certain animal agriculture practices, such as the use of antibiotics, may contribute to meat's carcinogenicity.

Lung and colorectal cancers are the first and second leading causes of cancer death, respectively. The risk of developing either disease over the course of a lifetime in the Western world is approximately one in 20.

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