A recent study showed that processed soluble fiber added to low-fiber foods led to liver cancers in mice, probably by preventing the liver from clearing bile from the body (Cell, October 18, 2018;175(3):679-694). Soluble fiber is felt to be one of the most healthful components of foods, and has been shown to lower cholesterol and inflammation, reduce obesity (Am J Physiol Gastrointest Liver Physiol, 2015; 309: G528-G541) and prevent diabetes, heart attacks, and certain cancers. However, it may be harmful to eat soluble fiber such as inulin that has been extracted from a plant source such as chicory root, and added into processed foods or taken as a supplement.
Researchers at the University of Toledo added inulin to the diet of mice that consisted of processed foods which were very low in both insoluble and soluble fiber. The mice received inulin for six months and developed liver cancer at a very high rate. Other data show that a diet of refined processed foods that are low in fiber is associated with increased risk for cancers in humans(BMJ, 2018; 360: k322), particularly colon cancer, and that the extracted soluble fiber, inulin, is associated with increased risk for cancer in mice (Int J Cancer, 2003; 106: 653-660). This new study shows that adding processed soluble fiber to an otherwise low-fiber diet may induce liver cancer in mice.
What Is Soluble Fiber? Carbohydrates are single sugars and chains of sugars that are found in plants. You cannot absorb any carbohydrate until it is broken down into single sugars. Fibers are chains of sugars that are so tightly tied together that humans lack the enzymes necessary to break them down into single sugars. There are two types of fiber:
• Insoluble fiber cannot be absorbed by your body so it passes out from your body in your stool.
• Soluble fiber passes through your upper intestinal tract unabsorbed until it reaches your colon that is loaded with trillions of bacteria, some of which do have the enzymes to break down soluble fiber into healthful Short Chain Fatty Acids (SCFA) that are absorbed and help to prevent diseases and prolong your life.
Inulin is Often Added to Processed Foods Manufacturers of processed foods are aware of the overwhelming research that shows that a low-fiber diet is harmful, so they add soluble fiber to many of their products as a diversionary tactic to make you think that their products are high in fiber and are therefore healthful. Snack bars advertised to help you lose weight, make you an athlete, give you energy or just make you more healthy often contain large amounts of inulin. You will not find inulin in the list of ingredients of brands such as Fiber One, Kashi or Kellogg’s Fiber Plus, but you will see chicory root fiber, the most common processed source of inulin. Kellogg's Fiber Plus has chicory root fiber listed as its first ingredient (ingredients are listed in order of decreasing weight per serving). Inulin, chicory root powder and other soluble fibers are also widely available as supplements marketed to aid digestion or prevent constipation.
How Could Adding Soluble Fiber to a Low-Fiber Diet Cause Cancer? The typical western diet is loaded with processed foods that are high in fat and sugar, and low in healthful fiber (found in vegetables, fruits, whole grains and seeds). This diet is linked to 70 percent of North American adults being overweight and more than 50 percent being diabetic or prediabetic. The authors of this new study have previously shown that soluble fiber is converted to short chain fatty acids that reduce obesity and diabetes.
Your liver helps to prevent cancer by the following mechanisms:
• In the process of converting food to energy, your body accumulates large amounts of toxic waste products that can harm you.
• To remove these toxic compounds, your healthful HDL cholesterol and other protective chemicals carry these toxic products from your bloodstream to your liver.
• Your liver converts these toxic products to components of bile that pass from your liver to your bile ducts to your intestines, and then pass out in your stool.
This new study on mice showed that adding the soluble fiber, inulin, to an otherwise low-fiber diet for six months in mice caused extra bile to accumulate and stay in the liver to damage the liver cells and cause cancer. Other studies show the same results (Mol Aspects Med, 2017; 56: 45-53).
Mice that received the extra inulin for six months and also were given antibiotics to kill the bacteria in their colons did not develop liver cancer, suggesting that the harmful effects of inulin came from bacteria in the colon that grew there because of the low-fiber diet and then fermented the inulin to produce toxins that damaged the liver. Earlier data show that liver cancer may be caused by harmful colon bacteria (Cancer Cell, 2012; 21: 504-516).
• Special acids from the hop plant Humulus lupulus, that inhibit colon bacterial fermentation, also prevented liver cancer in the mice.
• There are reports of jaundice and accumulation of bile in the liver in people who eat large amounts of processed foods that have added extracted soluble fiber.
My Recommendations The evidence is that soluble fiber in plants is healthful. The new concern raised by this study is that extracted inulin may have harmful side effects. I agree with the authors of this study on mice who conclude, "its benefits notwithstanding, enrichment of foods with fermentable [soluble] fiber should be approached with great caution as it may increase risk of HCC" [liver cancer]. I always recommend that you "stay close to nature" and eat a wide variety of plants that are whole or minimally processed.
• Eat lots of fruits, vegetables, whole grains, beans, seeds and nuts; they are full of soluble and insoluble fiber that helps to prevent disease and prolong life.
• Restrict processed foods that have had most of their fiber removed and then have processed soluble fiber added in. Just this year the FDA approved inulin to be added to foods. If this study on mice can be confirmed with further studies, that approval may be withdrawn. I think there is sufficient reason to avoid these products now.
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