Two new studies suggest fiber does have a role in protecting against colon cancer. The findings are intriguing because several previous large, well-conducted studies have found no such link.

The works are published in the Lancet (Vol. 361, No. 9368: 1491-1495 and 1496-1501).

In the first study, researchers Ulrike Peters, PhD, and colleagues at the National Cancer Institute examined the relationship between the amount of fiber in the diet and the incidence of colorectal polyps, the precursors to colon cancer. The researchers asked nearly 34,000 people about the foods they ate, and examined them for polyps.

They found that people who had the highest amounts of fiber in their diets (36 grams a day or more) had the lowest incidence of colon polyps. Their risk of getting polyps was 27% less than that of the people who ate the least amount of fiber (12 grams a day or less). The effect was especially strong in people whose primary source of fiber was from grains and fruits, rather than vegetables and beans.

In the second study, researchers in Europe, led by Sheila Bingham, PhD, of the UK Medical Research Council, looked at the dietary habits of more than half a million people in 10 countries. They found that people who ate more fiber (33 grams a day) had less chance of getting colon cancer than those who ate the least amount (12 grams a day). In fact, they determined that people who ate little fiber could reduce their risk of colon cancer by 40% by doubling their fiber intake.

The researchers in both groups speculate that fiber may help lower risk by binding potential carcinogens and helping move waste through the colon more rapidly, giving harmful substances less time to damage the cells in the colon. Fiber also stimulates bacteria in the colon that produce compounds thought to slow cell proliferation that could lead to cancer.

But the authors of both studies acknowledge that fiber may not be the only nutrient causing the protective effect. An American Cancer Society expert notes that it’s difficult to pinpoint exactly which element gives the most benefit.

“Fiber is correlated with other healthy components of a plant-based diet and it’s hard to disentangle what the responsible factor or factors might be because they’re so closely tied,” said Marji McCullough, a nutritional epidemiologist with the ACS.

For instance, many high-fiber foods also contain folate and other vitamins, which may be protective. And many people who consume a high-fiber diet are also more likely to eat less red meat, drink less alcohol, smoke less, and get regular exercise – all healthy behaviors that can reduce cancer risk.

However, the European study did not control for many of these risk factors, which may explain the inconsistencies between this research and previous studies. Large US studies (the Nurses’ Health Study, and the Health Professionals Follow-Up Study), found no beneficial association between fiber consumption and colon cancer risk, or only a very weak one, after controlling for many of these factors. Studies from Finland and Sweden also have found no protective effect from fiber.

Another possibility is that fiber is protective only because it works in combination with the other nutrients contained within high-fiber foods. Past studies that looked at fiber from supplements found it did not lower the risk of colon polyps.

“We just have to be hesitant to attribute any association with a particular nutrient unless we’re really sure and it’s been shown consistently and across different study designs,” McCullough said. “We don’t want people to start taking fiber supplements (based on this research) because previous studies have shown that certain types of fiber may even increase the risk of polyps.”

The European researchers also note that their results cannot be extended to fiber supplements, but only pertain to fiber from foods.

Despite the lingering questions about the benefits of fiber, an accompanying editorial by Lynnette Ferguson and Philip Harris, of the University of Auckland in New Zealand, finds one clear message from the studies.

”Whatever the reasons for the results reported by the two studies, eating a diet rich in plant foods, in the form of fruit, vegetables and whole-grain cereals, probably remains the best option for reducing the risk of colon cancer, and for more general health protection,” they write.