Whether or not a high-fiber diet lowers the risk of colorectal cancer has been a subject of debate among scientists and confusion among consumers for over two decades. The question is not fully answered, but two new studies published in a recent issue of the Lancet add to the body of evidence that fiber may indeed protect against colorectal cancer.

The PLCO Cancer Screening Trial

In the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial, researchers from the US National Cancer Institute screened volunteers for colorectal cancer. After completing a food questionnaire, the dietary fiber intake of nearly 34,000 people who were cancer free was compared with the fiber intake of about 3,600 volunteers who had a cancerous lesion.

The EPIC study

In a second European study, the European Prospective Investigation into Cancer (EPIC), over 500,000 participants completed an extensive lifestyle and diet profile. For the next 41/2 years researchers tracked the development of colorectal cancer, using health insurance records, cancer registries, and next-of-kin information.

The findings

In both studies, the participants who ate the most fiber - more than 30 to 35 grams a day - reduced their risk of cancer by approximately 25% compared with those who ate less than 15 grams of fiber each day. Fiber appeared be more effective in protecting against colon cancer than rectal cancer.

The mechanism by which fiber may fight disease is thought to have three components. Fiber increases stool weight, decreases transit time, and stimulates the growth of healthy bacteria. The net effect dilutes harmful substances and minimizes their contact with the intestinal lining. However, not all studies have clearly shown that it is actually the dietary fiber that reduces cancer. Keep in mind that people who eat more fiber tend to exercise more, smoke less, drink less alcohol, eat less red meat, and get more folate and calcium, nutrients that may have their own cancer-fighting abilities.

Cancer prevention dietary guidelines

While the fiber-cancer connection continues to be investigated, the American Institute for Cancer Research still suggests aiming for 20 to 35 grams of fiber each day and following their diet and health guidelines for cancer prevention:

• Choose a diet rich in a variety of plant-based foods
• Eat plenty of vegetables and fruits
• Maintain a healthy weight and be physically active
• Drink alcohol only in moderation, if at all.
• Select foods low in fat and salt
• Prepare and store foods safely
• Do not use tobacco in any form