An American Institute for Cancer Research and World Cancer Research Fund International report, “Food, Nutrition, Physical Activity and the Prevention of Cancer: a Global Perspective,” concluded that a high-fiber diet can reduce the risk of colorectal cancer.
Found in vegetables, fruits, legumes and other plant foods, dietary fiber may have a direct and indirect effect against co-lon cancer.
According to AICR, foods high in dietary fiber are relatively low in energy density, contributing to a feeling of fullness that makes over-eating less likely and reducing the risk of overweight and obesity, a cause of seven types of cancer. Fiber may also help the body maintain a healthy bacterial cell turnover in the colon. Emerging research continues to explore the cellular functions and mechanisms that may be responsible for the protective effect of dietary fiber.
In searching for fiber’s protective effect, scientists have honed in on one of fiber’s metabolic by-products: butyrate. AICR grantee Leonard Augenlicht, PhD, Professor of Medicine and Cell Biology at Albert Einstein College of Medicine, hypothesizes that one of the keys to dietary fiber may lie in butyrate — a short-chain fatty acid that is the principal energy source for intestinal cells.
When Augenlicht and colleagues treated cancer cells with butyrate, the cells generally stopped growing, which led to apoptosis. Although butyrate inhibits the growth of cancerous cells, it also serves as an energy source for normal cells and stays at relatively high levels in the intestinal lumen, where digested food passes and nutrients get absorbed, AICR said. When butyrate is removed from normal cells, cell growth stops.
“Because butyrate seems to affect normal and cancer cells differently, we refer to it as the ‘butyrate paradox’,” says Augenlicht. “We don’t know why butyrate represses tumor cells, but it seems to inhibit the transcription of certain genes that cause cells to proliferate.” According to Augenlicht, one of these genes may be cyclin D1, which plays a major role in intestinal tumor growth.
“A lot of questions remain about fiber and butyrate,” says Augenlicht. “Dietary fiber appears to be protective, but it hasn’t been shown consistently in studies with people because it’s difficult to do large intervention studies and tease out all of the competing influences.”
Along with fiber, whole grains’ high minerals, vitamins and phytochemical content have made whole grains and cancer risk an active area of research. Thus far, the body of evidence on the cancer-protective effects of whole grains connects the benefits of high-fiber with a healthy weight.
Yet many studies have suggested other roles for whole grains in cancer protection, such as one recent study of nearly 3,000 middle-aged adults. The study found that adults who consumed several servings of whole grains per day, including dark bread, ready-to-eat cereals, oatmeal, popcorn and brown rice, had lower levels of visceral fat, or fat around the waist, when they limited refined grains, said AICR. Visceral fat sits deep within the abdomen and is associated with chronic inflammation and high insulin levels, both linked to increased risk of cancer;
The study, published in the American Journal of Clinical Nutrition, found that visceral fat was 10 percent lower in adults who consumed at least three servings of whole grains per day and consumed fewer than one daily serving refined grains, when compared to those who ate no whole grains.
Conversely, participants who consumed more refined grains, such as pasta, English muffins, white bread, pizza and rice, had higher levels of visceral fat. “This implies that it is important to make substitutions in the diet, rather than simply adding whole-grain foods,” said lead author Nicola M. McKeown, PhD, Assistant Professor at Tufts University.
AICR funds research in diet, physical activity and weight management.