What Causes Colon Cancer? More importantly, what prevents it?

Colon cancer incidence --like that of many cancers-- varies several-fold between different countries, indicating that perhaps there are environmental or lifestyle factors that cause or prevent it. Much of this post is based on this excellent review article

Known Influences of Colon Cancer Risk (ranked from most important to most equivocal)

Aspirin: The Women's Health Study of 34,000 women 45 years and older found a 42% reduction in colonrectal cancer in the group that was taking aspirin.

Selenium: Populations with adequate selenium intake (most of the U.S. is OK, but European soils are selenium-depleted so their foods are lower) have lower colorectal cancer.

CLA, or conjugated linoleic acid is protective against cancer and found in grass-fed beef and dairy products, possibly explaining a lower incidence of colorectal cancer among people who eat dairy.

Dietary fat: results are conflicting. While a study based on a large, hospital-based sample of cases and controls, provided no evidence that dietary fat was associated with increased risk15, an equally large and well executed case-control analysis revealed a strong association of dietary fat with colon cancer risk 16.

Fiber: results are conflicting. One of the best studies of diet and colon cancer, the prospective Nurses Health Study, indicated that dietary fiber does not affect the risk of subsequent colon cancer 17. Although some fruits and vegetables, which contribute fiber to the diet, may be protective, dietary fiber alone appears to have no impact on colon cancer risk17.

Vegetarian Diet: while eating a vegetarian diet slightly lowers the risk of all cancers, it has also been found to increase the risk of colon cancer, according to the European Prospective Investigation into Cancer and Nutrition (EPIC) study.

Meat: results are conflicting, and different for different types of meat.  “The available epidemiologic data are not sufficient to support an independent and unequivocal positive association between red meat intake and CRC [colorectal cancer].” - from a review of the evidence regarding red meat consumption and colon cancer in the European Journal of Cancer Prevention [1]. Cited by Dr. Briffa.

However, a number of compounds in meat are known to be harmful.  For example, heterocyclic amines, a known cancer-promoting substance, are produced when meat is grilled or fried at high temperature.  High iron levels may induce oxidative stress and can be harmful.  Meat increases IGF-1, which promotes growth of all cells, esp. cancer. Recently, a sugar in red meat called Neu5Gc has been linked to inflammatory effects.  And even Carnitine, a heart-healthy compound found in red meat, has been found to stimulate bacteria to produce TMAO, a known cancer-causing substance.  However, whether all of these individual compounds, when consumed as part of a healthy diet, cause cancer, has yet to be determined.   Chris Kresser has comprehensive review article that addresses these complexities in greater detail.  

[1] Alexander DD, et al. Meta-analysis of prospective studies of red meat consumption and colorectal cancer. Eur J Cancer Prev. 2011 May 2

"Substance X causes cancer in animals...."

Consider two statements, S1 and S2: S1: ‘‘Would you agree or disagree that the way an animal reacts to a chemical is a reliable predictor of how a human would react to it?’’

The second statement, S2, is a little more specific: ‘‘If a scientific study produces evidence that a chemical causes cancer in animals, then we can be reasonably sure that the chemical will cause cancer in humans.’’

How did you respond? How would a scientist respond?

from P. Slovic, ‘‘Trust, Emotion, Sex, Politics, and Science: Surveying the Risk-Assessment Battlefield,’’ in Environment, Ethics, and Behavior, M. H. Bazerman, D. M. Messick, A. E. Tenbrunsel, and K. A. Wade-Benzoni (eds.) (San Francisco, New Lexington, 1997), pp. 277–313.

Toxic Bodies by Nancy Langston

"Toxic Bodies: Hormone Disruptors and the Legacy of DES" explores why our environment has become saturated with synthetic chemicals that disrupt hormones, and asks what we can do to protect human and environmental health. In this thorough undertaking, Environmental Historian Nancy Langston examines endocrine disruptors as a case study of environmental risk assessment and response. Interesting to compare this book's stance on weight-of-evidence to Our Stolen Future or The Secret History of the War on Cancer by Devra Davis. Compare that to a more formal report by the American Institute for Cancer Research.

Environmental Causes of Cancer

from: Environmental Health News and World Wire News

The President's Cancer Panel reported that "the true burden of environmentally induced cancers has been grossly underestimated" and strongly urged action to reduce people's widespread exposure to carcinogens.

The panel advised President Obama "to use the power of your office to remove the carcinogens and other toxins from our food, water, and air that needlessly increase health care costs, cripple our nation's productivity, and devastate American lives."

The 240-page report by the President's Cancer Panel is the first to focus on environmental causes of cancer. The panel, created by an act of Congress in 1971, is charged with monitoring the multi-billion-dollar National Cancer Program and reports directly to the President every year.

Environmental exposures "do not represent a new front in the ongoing war on cancer. However, the grievous harm from this group of carcinogens has not been addressed adequately by the National Cancer Program," the panel said in its letter to Obama that precedes the report. "The American people --even before they are born-- are bombarded continually with myriad combinations of these dangerous exposures."

The panel, appointed by President Bush, told President Obama that the federal government is missing the chance to protect people from cancer by reducing their exposure to carcinogens. In its letter, the panel singled out bisphenol A, a chemical used in polycarbonate plastic and can linings that is unregulated in the United States, as well as radon, formaldehyde and benzene.

"The increasing number of known or suspected environmental carcinogens compels us to action, even though we may currently lack irrefutable proof of harm." - Dr. LaSalle D. Lefall, Jr., chair of the President's Cancer PanelEnvironmental health scientists were pleased by the findings, saying it embraces everything that they have been saying for years.

Richard Clapp, a professor of environmental health at Boston University's School of Public Health and one of the nation's leading cancer epidemiologists, called the report "a call to action."

Environmental and occupational exposures contribute to "tens of thousands of cancer cases a year," Clapp said. "If we had any calamity that produced tens of thousands of deaths or serious diseases, that’s a national emergency in my view.”

These include:

* Malignant melanoma of the skin in adults has increased by 168 percent due to the use of sunscreens in childhood that fail to block long wave ultraviolet light;

* Thyroid cancer has increased by 124 percent due in large part to ionizing radiation;

* Non-Hodgkin’s lymphoma has increased 76 percent due mostly to phenoxy herbicides; and phenylenediamine hair dyes;

* Testicular cancer has increased by 49 percent due to pesticides; hormonal ingredients in cosmetics and personal care products; and estrogen residues in meat;

* Childhood leukemia has increased by 55 percent due to ionizing radiation; domestic pesticides; nitrite preservatives in meats, particularly hot dogs; and parental exposures to occupational carcinogens;

* Ovary cancer (mortality) for women over the age of 65 has increased by 47 percent in African American women and 13 percent in Caucasian women due to genital use of talc powder;

* Breast cancer has increased 17 percent due to a wide range of factors. These include: birth control pills; estrogen replacement therapy; toxic hormonal ingredients in cosmetics and personal care products; diagnostic radiation; and routine premenopausal mammography, with a cumulative breast dose exposure of up to about five rads over ten years.