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Fruits, Vegetables, and Colorectal Cancer
Studies have shown that a diet high in fruits and vegetables can help you avoid a number of types of cancers, including oral cancers, skin cancer, and prostate cancer. But the effect of a diet high in fruits or vegetables has not yet conclusively linked to the incidence of colon or rectal cancers.
Sex, Fiber, and Colon Cancer
Research into the effect of fiber on colon cancer has shown first that more fiber in your diet protects you from colon cancer, then other studies seem to show that it doesn't. Researchers in Arizona (Am J Clin Nutr 2006;83:343-349) recently combined and analyzed the results of two studies to find that the effects of fiber intake appears to be gender-specific (bet you thought the headline was about something else!).
Avoid Colorectal Cancer: Drink Your Milk!
A study of 45,306 men between the ages of 45 and 79 and without a history of cancer were followed for seven years by researchers in Sweden (Am J Clin Nutr 2006;83:667-73). The study assessed their level of dairy product intake and correlated the subjects' intake to the incidence of colorectal cancers of various types: colorectum, colon, proximal colon, distal colon, and rectum.
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Colon and rectal cancers are the second leading cause of cancer deaths in the United States and among the 6 leading causes of cancer deaths worldwide. Naturally there's been a great deal of interest in links between diet and colorectal cancers: here at Dr. Gourmet we've seen evidence that fruits and vegetables in general, fiber in particular, beans, and even dairy products have helped reduce the risk of colorectal cancers, while eating red and processed meats have appeared to increase that risk.
With such ingredient- and nutrient-specific evidence in hand, researchers at Loma Linda University in California decided to explore the larger picture: were those who followed vegetarian (or vegan) diets - diets that avoid animal proteins - less likely to develop colorectal cancers than those who ate animal proteins (JAMA Intern Med doi:10.1001/jamainternmed.2015.59)?
Fortunately a large-scale, long-term study includes over 77,000 Seventh Day Adventists, who are directed by their faith to follow a diet that avoids meat and includes "legumes, whole grains, nuts, fruits, and vegetables" (Adventist.org). Those Adventists participating in the study were recruited from all 50 states in the U.S. as well as all of the Canadian provinces and include (for this analysis) adults 25 and older. The baseline questionnaire for all participants included an extensive food frequency questionnaire that allowed the researchers to classify the participants into 5 different dietary patterns:
Vegan (defined as consuming eggs, dairy, fish, or other meats less than once per month);
Lacto-Ovo Vegetarian (defined as consuming eggs or dairy at least once per month and consuming fish or meats less than once per month)
Pescovegetarians (consuming fish at least once per month but eating all other meats less than once per month)
Semivegetarians (ate animal proteins less than once per week);
Nonvegetarians (ate animal proteins 1 time per week or more).
After an average of 7 years of follow-up, the researchers could compare the dietary patterns of those who developed colorectal cancers with those who did not.
You might expect that vegans would be the least likely to develop colorectal cancers, given its avoidance of animal protein and high levels of fruits and vegetables along with legumes and whole grains. Not so!
After taking into account age, Body Mass Index, family history of colorectal cancer or peptic ulcer, Vitamin D or or calcium supplement intake, and total fiber consumption, among other variables, those who saw the lowest risk of colorectal cancers were the pescovegetarians with a 42% reduction in risk. Vegans, however, reduced their risk by only 14%, while lacto-ovo vegetarians lowered their risk by 17%.
Once again, prospective studies like these do not necessarily prove a causal link, but it's hard to escape the conclusion that the only difference between two of the dietary patterns - one eating fish, the other not - must have significant impact. What exactly that might be (omega-3 or omega-6 acids? iodine content? something else?) is still up for some debate. In the end, this tells me that a more restrictive diet is not necessarily better for you.
First posted: March 11, 2015