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Fiber Follies

Processed fiber may be implicated in liver cancer — if you are a mouse with a congenital disease, have an imbalance in gut microbes, and gorge on the stuff.

The scary headline scooted around the internet. “Diets high in Processed Fiber May Increase Risk of Liver Cancer in Some People.” Unsurprisingly, this prompted lots of questions, including one from a regular consumer of Kellogg’s Bran Buds. She had been consuming this cereal for its purported health benefits and now was wondering if it contained the “risky” processed fiber. As is so often the case with such stories, the devil is in the details. But to cut to the chase, I don’t think there is any reason to worry about fiber intake.

The benefits of consuming fiber first hit the headlines in 1979 with the publication of Irish surgeon Denis Burkitt’s book, “Don’t Forget Fiber in Your Diet” with the seductive subtitle, “To Help Avoid many of our Commonest Diseases.” Burkitt had spent time in Africa and noted lower rates of cancer, particularly colorectal, when compared with rates in the West. He suggested this could be explained by the African diet being higher in fiber.

Fiber is the component of food, mostly in the carbohydrate family, that cannot be digested by humans. Some of the fiber, mostly the insoluble variety, adds bulk to the stool and goes straight through, carrying potential carcinogens with it. Other fiber components, mostly those that are water soluble, serve as food for the trillions of bacteria that inhabit the gut and are termed “prebiotics.” When some families of bacteria digest fiber, they churn out a number of short chain fatty acids, ethanoic, propanoic and butanoic being examples, that can then be absorbed into the bloodstream and are thought to have beneficial effects on the immune system, as long as they are present in the right proportions. And here comes the catch. In the wrong ratio, these short chain fatty acids may actually promote liver cancer. When would such a situation arise? If the normally healthy ratio of the different microbes in the gut is upset in favour of those that produce an excess of butanoic acid. In such a case, fermentable fiber may feed these bacteria which would be undesirable.

This concern is based on research carried out at the University of Toledo in Ohio that discovered a greater risk of developing liver cancer in mice fed a diet enriched with the fermentable fiber, inulin. But, and an important but, this was only the case for mice that had a rare congenital defect in which blood leaving the intestine goes into the body’s general blood supply instead of going to the liver where potential toxins generated by gut bacteria are filtered out. The liver then senses that there is some digestive abnormality happening and starts producing more bile acids that are needed for digestion. Indeed, the Toledo researchers found high levels of bile acids in the blood of the mice that developed liver cancer.

But as the common expression goes, men are not giant mice. So, what does this study mean for humans? Whether people can suffer from the same kind of rare genetic abnormality, a “portosystemic shunt,” as the mice, isn’t clear. However, bile acid levels in the blood can be measured, and it turns out that levels are higher in men who go on to develop liver cancer. Perhaps, like in mice, the elevated bile acid level is due to the presence of metabolic products of fermentable fiber. This raises the possibility for screening and identifying people who may be at higher risk of liver cancer as a result of consuming fermentable fiber. Once identified, they can be counselled in terms of what sort of foods contain fermentable fiber and should be curbed.

There are a few points to keep in mind. The study that generated the publicity used mice, and actually only a small percentage of the mice developed cancer when fed an inulin-enriched diet. Inulin is normally found in chicory, but extracting it involves processing, hence when added to foods it can be termed to be “processed fiber.” The thesis is that the processing alters inulin’s properties which in turn can alter which microbes have an appetite for fermenting it. Possible, but there is no hard evidence for this. Still, this is what has led to the suggestion that processed fiber added to food may have a different biological effect from fiber that is naturally present in whole grains, fruits and vegetables. Many foods today, such as cereals and crackers, are advertised as “health promoting” because they contain added fiber. If that fiber is of the “processed” variety, the “healthy” claim may not hold for people who may have an imbalance in gut bacteria and may also be afflicted with the “portosystemic shunt” abnormality. Note the repeated use of the term “may.”

Now back to Kellogg’s Bran Buds. Besides wheat bran, it has added psyllium husk, but there is no way to know to what degree this has been processed, or indeed if that really matters. Remember the mouse study was done only with processed inulin, which somehow in the media reports was then extended to mean all processed fiber. In any case, the connection of inulin to liver cancer in humans is very tenuous, whereas the evidence for fiber decreasing the risk of colorectal cancer, diabetes and cardiovascular disease is compelling. The recommendation to consume a total of 40 grams of fiber a day, of whatever kind, is on firm footing. Bran Buds contain 11 grams per serving, which is significant. However, they also contain 7 grams of sugar which we could do without. That’s why I favour Fiber One with its 14 grams of fiber and no added sugar.

What’s our take-away here? It is unrealistic to worry about fiber having any sort of carcinogenic effect based on a study in which a small percent of mice with a congenital abnormality developed liver cancer after being fed a diet containing more of one specific fiber, inulin, than any human would ever consume. We should strive for a diet high in unprocessed fiber as found in whole grains instead of relying on highly processed foods that claim to be healthy due to their added fiber content.


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