Almost all Americans need to increase their fiber intake by eating foods containing fiber as well as using supplemental fiber if needed. I do and will continue to do so with one caveat, which I’ll talk about later. Let’s get into the research that caused the headline about fiber and cancer.

Genetic Defects Were the Underlying Problem

Based on observational studies, researchers examined the role of soluble fiber on metabolic syndrome (MetS) in mice—not just any mice but mice that had a specific defective gene called Toll-like receptor 5 (TLR5). These mice develop MetS when fed a modern diet. Researchers used several types of soluble fiber, including inulin and pectin. While the symptoms associated with MetS were decreased, they discovered that close to half the TLR5 mice developed liver cancer while the normal controls did not. Why? One thing they noted was that the TLR5 mice became jaundiced due to bile acids in their blood.

Based on prior research, researchers suspected that something was causing bile acids to enter the blood stream. There’s a strain of mice, C57BL/6 mice, that has a defect which causes portosystemic shunt (an abnormal vein connecting the blood supply returning from the intestines to the vein returning blood to the heart, bypassing the liver). That allows bile acids to build up in the blood stream, damaging the liver, and causing liver cancer. The use of high soluble-fiber diets speeds up that process because of the fermentation of the fiber by probiotics. Hence that’s why the results associated fiber with cancer. But is it an issue in humans?

Soluble Fiber and Liver Cancer in Humans

I think the risk to humans is negligible for two reasons. Based on the latest data, the risk for portosystemic shunt in humans is about one in 30,000. The researchers suggest it may be higher, but we don’t test for it. The caveat I mentioned? Bile acids can be detected by a simple blood test; if you want to be sure you don’t have an issue, just request that test to eliminate the shunt or any other cause of bile acids in your blood.

The second reason is that the researchers fed the mice 10% of their diet as fiber. In one trial the inulin content was 2.5% while in another it was 7.5% of the mice’s dietary intake. The Daily Value for humans eating 2,000 calories per day is 1.4% or 28 grams, and that’s mixed fibers. We currently average 0.7%. It doesn’t seem likely that even people who supplement their food with fiber supplements will reach a comparably high level of intake.

The Bottom Line

Don’t for a minute think that this research isn’t important because it is. We live in a society that believes if some is good, more must be better. What they’ve shown is that some soluble fiber is good for weight loss and the symptoms of MetS, as well as keeping your bowels moving smoothly. We don’t need to overdo anything; we just need to do the average. Try to get about 25 to 35 grams of fiber daily from foods and supplements, get the blood bile test to be extra cautious, and you’ll be fine.

What are you prepared to do today?

        Dr. Chet

References:
1. https://doi.org/10.1016/j.cell.2018.09.004
2. https://doi.org/10.1053/j.gastro.2022.08.033