Please Mind the Prebiotic Fiber Gap, Full Stop.

Dietary fiber is the most deficient nutrient in our diet. Only 5% get enough. Increasing prebiotic fiber can reset metabolism, modulate inflammation, and regulate mood.  Practical advice is provided at the end on how to increase fiber in your diet with low FODMAP options that don’t increase bloating.

Simple yet ‘complex’

So let’s start with the simple question, is all fiber the same in its effect on health? The short answer is no. There you go! Simple right? But if you want, stay for a bit, as the whole story is actually quite fascinating and relevant to making healthy eating decisions. Bare with me and you might be glad you did!

Unlike simple sugars and digestible starches, fiber is a catch-all term for the complex carbohydrates present in plants that our digestive enzymes simply cannot break down.[1] (Yes, it may be surprising, but fiber is a type of complex carbohydrate!) These complex carbs make their way all the way down to the last part of the small intestine and colon, relatively untouched as leftovers and our gut microbes feast on them while producing important molecules for our body and health like butyrate (See blog on butyrate).[2]

Solutions that may not be the whole answer

The fiber indoctrinated might have heard of the terms “soluble” and “insoluble” which are sometimes highlighted on the nutrition label of packaged goods.[3]  Solubility essentially is a measure of how well the fiber dissolves in water. Conventionally, the non-dissolving fibers (insoluble) have been thought to be the roughage, the fiber that helps prevent and treat constipation.  The dissolving fibers (soluble) have been thought to be the fiber that contributes to metabolic health by slowing sugar and cholesterol absorption in the upper intestine. 

But the truth, as is often the case, is more complex.  Fermentability is another property of fiber (present in both soluble and insoluble fibers) that profoundly contributes to health via butyrate a powerful regulator of metabolism and immunity.[4] (See blog on Butyrate here)  Two of the best molecules for being fermented by the microbiome into butyrate turn out to be resistant starch and beta glucan.[5–7] Other fibers that you may have heard of like FOS, GOS, inulin and arabinoxylan, are also good at making butyrate but they can have other properties that may make them less desirable to some people.[8] 

A diet portfolio that is inflation proof 

One less desirable property of other fibers mentioned above (FOS, GOS, inulin) is fermentation that is too rapid and too soon. This can happen in some people to a certain degree in the upper parts of the small intestine and cause considerable discomfort with bloating and loose stools.[9] It is an especially important consideration in those with microbiome imbalances like irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO) that are interrelated conditions that may affect as many as 1 in 5 people in the US.[10]  

These fibers are designated as FODMAPS (fermentable oligosaccharides, disaccharides, monosaccharides and polyols).[11]  If you’ve ever had a high fiber bar or processed food and experienced bloating, one of these fibers is likely to blame. Take a look at the label for FOS, GOS, chicory root, inulin, onion powder, or garlic powder and see if you notice trends with these ingredients. Onions, garlic, wheat, beans, and cruciferous vegetables (broccoli, cauliflower, Brussels sprouts, cabbage) are whole foods that also tend to be high sources of FODMAPs and may cause GI issues in those prone to them.

In fact, there is a special diet developed out of Monash University for those with IBS that limits foods with these fibers (low FODMAP diet) so they can experience happy and normal bowel movements again.[12] The diets work really well in a lot of people, but sadly this means they limit a lot of healthy foods and fibers which otherwise contribute to health.[13] The good news is there are other low FODMAP fibers (e.g. beta-glucan and resistant starch) that are tolerated well by folks with IBS.[14]  Other commercially available synthetic fibers (e.g. methycellulose) are also FODMAP friendly and minimally fermented but they also are missing the metabolic benefit of butyrate.[15]

The plot thickens as do some fibers

Another double edged property of some fibers is the texture or viscosity of fiber.  This is the property that turns fiber into a sludge-like consistency when added to water.[16] If you’ve ever tried psyllium husk (a great source of arabinoxylans) sold under popular fiber brand names, you’ll relate immediately to this visceral experience.  

Well, there’s good news here too. Some fibers are naturally less viscous (e.g. resistant starch) and other fibers can be made relatively less viscous by breaking them into smaller parts (lower molecular weight beta glucans).[17]  So those that have a strong gag reflex to sludge can drink some fibers with pleasure!

Getting practical, piece de resistance & the golden rule

This is all very interesting, but what does this mean practically for someone trying to increase fiber in their diet? The first thing to emphasize is that folks should increase their intake of plant based whole foods.  Michael Pollan puts it best in his book In Defense of Food, “Eat food, mostly plants, not too much.” Indeed, the best sources of fiber are whole grains (think oatmeal, whole grain breads, and brown rice) and legumes of all sorts (think black beans, garbanzo beans, and lentils), followed by nuts, seeds, fruits and vegetables.[1]  

Another way to maximize fiber intake, especially a form of resistant starch called RS3, is to consume your starches reheated (e.g. oatmeal, potatoes, rice, and pasta)![18] Get into the habit of making a large batch for the week, refrigerating and reheating a portion for each meal.

For packaged goods, it’s important to look at the labels and choose foods that have desirable carbohydrate to fiber ratios. 10:1 is great and 5:1 might be even better.[19] Here is a carbohydrate to fiber ratio calculator to help you choose the right foods (Link to Calculator).

For the reasons above, I would focus less on the soluble vs. insoluble distinction and focus just on total fiber, which makes it easier too! In the end, the goal is to get at least 30 grams of fiber in the diet every day (>25g for women and >38 g for men).[4]

Fiber tips for those inspired

The mantra to “go low and slow” is important to help mitigate intolerable GI side effects as the microbiome equilibrates and finds a healthy balance with its new food.  Even with this trick, some people are not able to tolerate some high fiber foods.  If you experience prolonged GI effects, I would recommend you see a nutritionist trained in the low FODMAP fiber diet to help you identify your triggers. Other people find many whole foods less delicious and enjoyable. Still some people find it hard to prepare healthier options with such busy lives.[20] 

For those that are looking to supplement their fiber intake because, try as they might, they just can’t reach that golden number of 30 grams, there are many great options out there!  The ones that are most readily fermented to butyrate might be best for health; and for those with bellies prone to inflation and texture aversions, low FODMAP and lower-viscosity options containing resistant starch and or beta glucan might just fill the gap!

More Articles & Resources

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