玉米油提高结肠癌风险，反式脂肪升高肠癌的风险

Corn oil raise colon cancer risk, trans fat ups intestinal cancer risk

A study published in the Journal of National Cancer Institute in 1985 suggested that eating too much corn oil can increase risk of colon cancer while eating high fat diets with trans fat can increase risk for intestinal cancer.

The study led by Bandaru S. Reddy and colleagues was conducted to examine how dietary corn oil and trans fat may influence the carcinogenesis induced by a chemical called azoxymethane in female rats.

For the study, all female rats were fed a low fat diet containing 5% corn oil (considered as low fat control diet) from age 5 weeks to 6 weeks.  Beginning at 7 weeks, all study groups of rats received the cancer causing chemical through under-skin injection for three weeks.

After the injection, groups of rats were fed semi-purified diets containing 13.6% corn oil, or 23.5% corn oil or high-fat diets containing either low trans fat, or intermediate trans fat, or high trans fat.

After 34 weeks, all rats were killed and examined.  The study found that rats fed 23.5% corn oil developed more colon cancer than rats eating diets with either 5 or 13.6% corn oil.  There was no difference in the colon cancer incidence between rats fed 5% corn oil diet and 13.6% corn oil diet.




Compared to rats fed high corn oil diet, those that were fed high fat diets with low trans fat, intermediate trans fat and high trans fat developed fewer liver cancer and colon cancer, but more small intestinal tumors.

In another report published in Lipids in 1992, Reddy also mentions that in addition to corn oil, diets with 23% safflower oil, or lard or beef tallow (high-fat) can also increase risk of colon cancer in animals exposed to cancer causing agents.

On the other hand, Reddy says that diets containing 23% coconut oil, olive oil, or fish oil, or high-fat diets with varying levels of trans fat, did not enhance the risk of colon cancer, compared to the control low fat diets.  The control low fat diets contain either 5% corn oil, or safflower oil, or lard or beef tallow.

In fact, fish oil which is high in long chain omega 3 fatty acids such as DHA and EPA is considered to be an anticancer supplement because of its anti-inflammatory activity.  In case reports, omega 3 fatty acids found in fish oil were found effective against certain cancers like lung cancer.

It is not fully understood why high fat diets with corn oil, safflower oil or lard increase risk of colon cancer. It is possible that these oils may influence the risk through different mechanisms.  For instance, animal fat may prolong the staying of toxic waste in the colon. It is also possible certain fats promote the production of secondary bile acids which can in turn promote cancer development. (David Liu)

References:

1. J Natl Cancer Inst (1985) 75 (4): 791-798.
2. Lipids  October 1992, Volume 27, Issue 10, pp 807–813