Nutritional Pearls: Does a Vegan Diet Protect Against Colon Cancer?

Jonathan is a 32-year-old man concerned about his risk of colon cancer.

He tells you that in response to online reports of the dangers of red and processed meat, he is thinking of becoming vegan to further cut his cancer risk. He asks if you think this is a good decision. 

How do you advise your patient?

What is the correct answer?
(Answer and discussion on next page)

Dr. Gourmet is the definitive health and nutrition web resource for both physicians and patients. Resources include special diets for coumadin users, patients with GERD/acid reflux, celiac disease, type 2 diabetes, low sodium diets (1500 mg/d), and lactose intolerance.

Timothy S. Harlan, MD, is a board-certified internist and professional chef who translates the Mediterranean diet for the American kitchen. He is an associate clinical professor of medicine and executive director of The Goldring Center for Culinary Medicine at Tulane University in New Orleans. Dr Harlan is the 2014 co-chair and keynote speaker at the Cardiometabolic Risk Summit in Las Vegas, October 10-12. 

Answer: A more restrictive diet is not necessarily better for our patients.

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: at Dr. Gourmet I have reported on evidence that fruits and vegetables in general, fiber, 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.
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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?

The Research

The investigators were able to utilize the large-scale, long-term study including 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."^1,2

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 questioning 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: defined as consuming fish at least once per month but eating all other meats less than once per month.

• Semivegetarians: defined as those who ate animal proteins less than once per week.

• Nonvegetarians: defined as those who ate animal proteins 1 time per week or more.

The Results

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 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 reduced their risk by only 14%, while lacto-ovo vegetarians lowered their risk by 17%.

What’s the “Take Home”?

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 debate. In the end, this tells me that a more restrictive diet is not necessarily better for our patients.

References:

1. Orlich MJ, Singh PN, Sabate J, et al. Vegetarian dietary patterns and the risk of colorectal cancers. [epub ahead of print] JAMA Intern Med. doi:10.1001/jamainternmed.2015.59
2. Adventist.org: https://www.adventist.org/vitality/health/. Accessed March 17, 2015.

For further reading:

• https://www.who.int/mediacentre/factsheets/fs297/en/
• https://www.drgourmet.com/bites/2007/121207.shtml
• https://www.drgourmet.com/column/dr/071006.shtml
• https://www.drgourmet.com/bites/2006/0523.shtml
• https://www.drgourmet.com/bites/2006/0428.shtml
• https://www.drgourmet.com/bites/2006/0428.shtml