Colorectal cancer is the third leading cause of cancer death for American men and women alike … and the disease kills nearly one in three people diagnosed with it.
The National Cancer Institute estimates 143,460 new diagnoses in 2012 … and that colorectal cancers will kill 51,690 Americans this year, with most deaths occurring after age 65.
These statistics help explain why – despite increasing rates of colonoscopies – colorectal cancer remains a leading cancer threat.
A large body of evidence indicates that lifestyle factors influence the risk and progress of colorectal cancers … and lab studies provide possible reasons for those associations.
Epidemiological studies link high intakes of red and processed meat (more than 18 ounces a week) or alcohol (more than two drinks a day) to a higher risk for colorectal cancer.
How much fish does it take?
If fish really helps curb colorectal cancer – and we'd need more clinical evidence to prove that – how much fish does it take to lower the risk?
Frankly, it's impossible to say, since the ranges of intake for “low” and “high” consumers of fish varied substantially across the included studies … and because fish vary widely in their omega-3 content.
However, the studies provide enough data to suggest that it's wise to enjoy at least two servings of fatty fish a week.
Fish that offer the most omega-3 fatty acids include salmon, tuna, sablefish, mackerel, blue fish, sardines, and anchovies. (See our Seafood Nutrition Chart.)
And, based on the pilot clinical studies mentioned here, it appears that a daily fish oil supplement may offer substantial colorectal benefits … although that remains uncertain.
Whether the beneficial effects of fish stem solely from omega-3 fatty acids or partly from other nutrients found in fish – such as selenium and vitamin D – is unproven.
Conversely, the epidemiological evidence generally links exercise, lower body mass, and diets higher in fiber, calcium, or magnesium to a lower risk for colorectal cancer.
And growing evidence suggests that the omega-3 fatty acids in fish may help protect against colorectal cancers.
Omega-3s and colon health: The story so far
People who eat lots of fish – such as Swedish fisher-folk and people living in Finland – enjoy very low rates of colorectal cancer … compared with places where fish is an infrequent item on the home menu.
However, although some studies have shown an inverse relationship between fish consumption and colorectal cancer, others have not.
Consequently, the association between fish consumption and the risk of colorectal cancer has been unclear.
Omega-3 fats from fish (EPA and DHA) have reduced the size and number of colorectal cancer cells in animal studies (Calviello G et al. 1999).
Earlier this year, researchers at Tennessee's Vanderbilt-Ingram Cancer Center (VICC) reported that women who averaged three 3.5 ounce servings of fish a week – versus less than half a serving – were one-third less likely to develop colorectal polyps with cancerous potential (adenomas) … see “Female Fish-Lovers had Fewer Colon Polyps.”
Possible reasons include the anti-inflammatory actions of omega-3s – especially omega-3 EPA, which was used in the UK trial and has produced anti-inflammatory effects in the colon tissues of children with ulcerative colitis (Shimizu T et al. 2003).
(Of course, people who eat lots of fish tend to eat less red and processed meats, which are linked to increased colorectal cancer risk.)
Now, a Chinese team's review of the available epidemiological evidence suggests that people who eat fish often are significantly less likely to develop colorectal cancer.
This new review did not include the Tennessee study, presumably because it was published pretty recently … which means that the risk reduction may be a bit greater than the Chinese team calculated.
Chinese study finds most studies link fish to lower colon risk
The new evidence review comes from researchers at the Fourth Military Medical University (FMMU) in Xi'an, China … one of China's leading biomedical research centers (Wu S et al. 2012).
The Chinese team pooled results from 41 studies that measured fish intake, tracked cancer diagnoses, and accounted for factors known to influence the risk of colorectal cancer risk … family history, body weight, physical activity, smoking, alcohol use, and various dietary factors.
(The studies came from the U.S., Canada, Britain, Australia, Italy, Japan, China, Norway, Finland, Sweden, and the Netherlands.)
Encouragingly, people who ate fish regularly were 12 percent less likely to be diagnosed with any kind of colorectal cancer.
The protective effect was strongest for rectal cancer. Compared to participants who reported eating the least fish, those who reported eating the most fish were 21 percent less likely to have developed rectal cancer.
Obviously, we'll need substantial clinical evidence to prove that fish and/or supplemental omega-3s help prevent colorectal cancers.
But since it's clear that both support heart health, it only makes sense to shift the protein part of your diet in favor of fatty fish, in the wholly realistic hope that this move will deliver myriad benefits.
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