Not All Fiber Is Equal

 According to a 2017 article by Nicola McKeown in the Journal of the Academy of Nutrition and Dietetics, dietary fiber has a number of misconceptions. This paper reviewed many controlled trials and came up with conclusions regarding fiber’s supposed benefits concerning serum cholesterol levels, improving glycemic control, and normalizing stool form in constipation and diarrhea.

 Misconception #1: All soluble fibers lower elevated serum cholesterol levels.

            While it is true that some soluble fibers can lower elevated cholesterol, it is NOT true that ALL fibers have this effect. Apparently, only fibers that are considered to be highly viscous (that is, sticky or pasty) and form gels have been shown to exhibit this health benefit. Fibers such as beta-glucan, psyllium, and raw guar gum have been shown to have this property. Studies using fiber that was non-sticky (such as inulin) did not show an appreciable difference in lowering cholesterol, though inulin did decrease triglycerides in two studies at a dose of 10 grams per day. 

            The sticky, viscous fibers were thought to lower cholesterol by binding to bile acids, trapping it within the stools, allowing the bile to be eliminated and not re-absorbed. A gel-forming fiber becomes more concentrated as it moves through the intestinal tract which allows it to trap bile. This causes the liver to clear the LDL (bad) cholesterol from the blood and use it for bile production. 

 Misconception #2: All soluble fibers improve glycemic control.

            Similar to lowering blood cholesterol, high-viscosity fiber can improve glycemic control but fiber such as inulin and oligosaccharides do not. This is achieved by consuming fiber (psyllium, for example) along with a meal. The fiber makes the food bolus more viscous and gel-like, which slows the interaction of nutrients with digestive enzymes in the small intestine. Sugars are delayed in being absorbed, and instead of being absorbed in the early portions of the small intestine, they are absorbed in the lower portions, near the colon. Total nutrient absorption is not decreased, only delayed, which moderates blood glucose levels more proportionally. Insoluble fiber such as wheat bran and cellulose, and non-viscous fibers (inulin and dextrin) do not provide these benefits in controlling blood sugar. 

Misconception #3: All fibers provide a regularity benefit.

            There are two mechanisms by which fiber can provide a laxative effect: large, coarse insoluble fiber particles such as wheat bran can mechanically irritate the large bowel. This stimulates mucus and water secretion. Such particles’ actions in the bowel can be imitated using inert plastic particles of the same size and texture as the insoluble fiber. The plastic irritates the bowel in the same manner, demonstrating that the laxative effective is entirely due to mechanical irritation. This is definitely not a preferred method if the gut is damaged or sensitive and may actually lead to diarrhea.

The other method is by using gel-forming fibers (psyllium, again) which absorb water and incorporates into the stool. The increased water content of the stool is then easier to pass. The food bolus, or chyme, is greater than 90% water when it reaches the colon, where the water content decreases to around 75% in the rectum. As stool water content decreases, the stool becomes thicker. Any less water and the stools become much dryer and firmer, making it harder to pass (constipation). 

 I find the issue of fiber digestion to be much more complex than I imagined. I was also impacted by the fact that lack of appropriate fiber may be the reason behind the increase in GI disease. Bottom line, it appears that the best fiber would be psyllium, beta-glucans, and partially hydrolyzed guar gum. Adding foods high in these fibers to our diet would also be beneficial; at a minimum we should be reducing the consumption of processed and refined foods as much as possible. Eat your veggies!