Prebiotics and Fiber: Supplemental Treasures or Marketing Ploys?

Pulled from my podcast The Perfect Stool: Understanding and Healing the Gut Microbiome


Because their names are so similar and because they’re often combined in one supplement, people often confuse the terms probiotics and prebiotics. And you’ve probably heard a lot from me so far about probiotics, as I’m a fan, but today I’m going to be writing about prebiotics.

So the term prebiotic can be used officially as a food ingredient if you can scientifically demonstrate that it
• resists gastric acidity, (meaning your stomach acid doesn’t break it down), hydrolysis by mammalian enzymes (hydrolysis is any chemical reaction in which a molecule of water ruptures one or more chemical bonds), and absorption in the upper gastrointestinal tract;
• is fermented by the intestinal microflora; and
• selectively stimulates the growth and/or activity of intestinal bacteria potentially associated with health and well-being.
In other words, we can’t digest it and our good bacteria can.

The most widely accepted prebiotic supplements are the fermentable oligosaccharides fructoligosacrharides (FOS or fructans), inulin (a form of FOS), galactooliogosaccharides (GOS), xylooligosaccharides (XOS) and lactulose. Besides being found in supplements, these beneficial substances are handily located in something we all could stand to eat more of – fruits and vegetables, in particular legumes, as well as in whole grains, nuts and seeds. So the more of these foods you’re eating, in particular fruits and vegetables, and the greater the variety of them, the more and different types of prebiotics you are getting.

Some examples of particularly good food sources of prebiotics are: onions, leeks, radishes, carrots, coconut meat and coconut flour, flax and chia seeds, tomatoes, bananas, garlic, chicory root, dandelion greens, Jerusalem artichoke, jicama, asparagus, and yams.

Both prebiotics and probiotics nurture the good bacteria required by the digestive tract for proper health beginning in your mouth. And while probiotics are live, active, bacterial cultures capable of multiplying in numbers, prebiotics serve as the food source for the bacteria and do not grow or reproduce. As such, prebiotics are considered functional foods because they provide numerous health benefits and aid in the prevention and treatment of various health conditions and diseases.

Prebiotics have been shown to have many positive effects including:
• increasing the health of the intestinal mucous membrane, which improves digestion and gut health in general
• decreasing blood sugar and insulin levels, and consequently the risk of obesity and weight gain
• decreasing inflammation
• lowering cholesterol and the risk for cardiovascular disease
• lowering the stress response
• helping with hormonal balance
• modulating the immune system and helping manage autoimmune symptoms. In particular, prebiotic foods can result in significant changes in the composition of the gut microbiome that help improve immunity, shown by improvements in biomarkers and activities of the immune system, including reduced levels of certain cancer-promoting enzymes and bacterial metabolites in the gut (the byproducts of bacterial digestion).
• Other benefits include preventing traveler’s diarrhea and aiding in the digestion of lactose in those who are lactose intolerant, in particular with the prebiotic GOS.

So now that we’ve talked about some of the benefits, let’s get to some of the potential concerns, especially if we’re talking about supplementing with prebiotics, as opposed to getting them from food.

First, because you’re giving your gut bacteria a sudden, pure form of food, you’ll likely have some bloating from the gasses that the bacteria produce after eating this food, until the balance of bacteria in your intestines adjusts and more bacteria that consume that gas grow to accommodate it. So start slow and build up – either with a supplement or with a new prebiotic food you don’t normally consume (think gas with that occasional meal with beans – it’s because your system isn’t adjusted to their regular consumption).

In his great tome “Healthy Gut, Healthy You,” Dr. Michael Ruscio summarizes the research on prebiotics for various gut conditions.

For IBS (Irritable Bowel Syndrome) and SIBO (Small Intestinal Bacterial Overgrowth), the best evidence suggests prebiotics should be avoided and a low-prebiotic diet like the low-FODMAP diet should be adopted. FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols and it’s a huge list of foods including many fruits and vegetables, whole grains, dairy products and more. But keep in mind that this is a temporary diet and elimination and reintroduction process, not a permanent diet, until you’ve resolved your SIBO, dysbiosis or IBS. I’ve done the low FODMAPs diet on two different occasions and it’s very limiting – excluding two very common food ingredients – onion and garlic, including their powders – so imagine trying to go to a restaurant and not eating FODMAPs. And I actually had a funny thing happen to me during those two low FODMAP diets, and I’m curious if anyone else has experienced this. When I’m excluding FODMAPs, after a couple of days, everyone’s breath starts to smell like garlic to me, to the point that I can’t even kiss my husband. So it’s interesting how much our bacteria and its die-off (for lack of food) can influence our senses. So if you do try low FODMAPs, keep in mind it’s more about managing symptoms, rather than solving the problem. Typically you also need to take antimicrobials or other supplements to bring a dysbiotic gut or bacterial overgrowth into balance.

For IBD (an umbrella term used to describe disorders that involve chronic inflammation of your digestive tract, such as Crohn’s disease and ulcerative colitis), the best evidence suggests that people should avoid supplemental prebiotics as some trials have shown harm from using prebiotic supplementation in IBD.

While in a flared state, you should adopt a low-prebiotic diet like the low-FODMAPs diet or the Specific Carbohydrate Diet (SCD), which is a diet that eliminates all sources of grains and pseudograins like quinoa, even the white ones, all but long-fermented dairy, processed meats, most processed sugar, artificial sweeteners, sugar alcohols and all processed foods.

In terms of weight loss and gain, while a systematic review of twenty-six clinical trials concluded that prebiotics are effective in lowering blood sugar, many studies show prebiotic supplements have little to no effect on weight loss.

Finally, for celiac disease, there are no studies available, however, indirect evidence suggests avoiding prebiotics may be advisable, especially in those who do not fully respond to a gluten-free diet.


Now on to another related topic. While I’m talking about prebiotics, I should also mention synbiotics, which are combination products of probiotics and larger amounts of prebiotics than the nominal amounts typically added to a lot of probiotics. One review published in 2014 concluded that the use of synbiotics may promote an increase in the number of bifidobacteria, glycemic (blood sugar) control, stimulation of the immune system, reduction of blood cholesterol and balancing the intestinal flora, which aids in reducing constipation and/or diarrhea and improves intestinal permeability. The study, linked above, has recommendations on dosages of certain prebiotics and probiotics in order to see benefits.

Fiber Supplements

Another related topic is fiber supplements. So while all prebiotics are dietary fibers, not all dietary fibers are prebiotics.

But first, a little bit about what fiber actually is and why it’s important: fiber is the part of plant-based foods—fruits, vegetables, nuts, seeds, legumes—that give the food its structure. There are many types of fiber, but we’ve separated it into two major types: soluble fiber and insoluble fiber. Soluble fiber dissolves in water and gastrointestinal fluids when it enters the stomach and intestines and then is transformed into a gel-like substance, which helps to positively slow absorption of nutrients during digestion. Insoluble fiber passes through the body and becomes the bulking agent to take up and help eliminate any by-products of metabolic processes, such as bacteria that need to be cleared, excess estrogen, and excess waste. Both soluble and insoluble fiber play an important role in digestion and the health of the microbiome. While soluble fiber feeds the bacteria in the gut, insoluble fiber ensures that nothing harmful lingers too long and creates a state of putrefaction or dysbiosis. You can find soluble fiber in supplements like Metamucil (or its simplest form without additives, psyllium husk), Citrucel (which is methylcellulose) and FiberCon, Fiber Lax, Equalactin, and Mitrolan, which are polycarbophil. You can find insoluble fiber in foods like beans, barley, corn, rice, bran, whole wheat, vegetables and apple and pear skins.

Other supplemental fibers you may want to consider include Sunfiber, which is made from the guar plant and is actually a low FODMAP fiber, glucomannan (made from Konjac root), acacia, pectin and inulin. In foods, you’ll find some of these same and other fibers as well, including cellulose, found in legumes, nuts, and bran; inulin, found in bananas, garlic, onions, asparagus, wheat, barley and rye; pectins, found in apples, strawberries, citrus fruits, carrots, and potatoes, and in smaller amounts in legumes and nuts; beta glucans, found in oats, barley, shiitake mushrooms, and reishi mushrooms; and lignin, found in whole grain foods, legumes, green beans, cauliflower, zucchini, avocado, unripe bananas, and nuts and seeds.

So you’ve probably heard that we all need more fiber and you’ve likely heard of the Recommended Daily Allowance of fiber, which is is 30 to 38 grams a day for men, 25 grams a day for women between 18 and 50 years old, and 21 grams a day if a woman is 51 and older. Or generally, you should get 14 grams of fiber for every 1,000 calories in your diet. But it’s important to remember that, while fiber can help some and probably most people feel better, especially if you’re currently eating a standard American diet, it can make others feel worse, especially those with inflammatory bowel disease during a flare. But for anyone considering supplementing with fiber, it’s a great first step in trying to solve lower level bowel issues like constipation, but be sure to do it with a full glass of water or other liquids.

Resistant Starch

Another related topic is resistant starch, which is like fiber, in that it feeds your healthy gut bacteria. It’s called resistant because it resists digestion by us but not by our bacteria. Resistant starch is naturally found in foods like green bananas and their flour; banana skins; cooked and then cooled potatoes, rice and pasta; beans and legumes; raw potato starch; hi-maize flour and oats. And I like to think of resistant starch as free food because the amazing thing is that by cooking and cooling and then optionally reheating to no more than 130 degrees foods like potatoes and rice, you can save yourself many calories but eat the same foods. This is because the starches convert into resistant starches, so we don’t absorb the calories! They also promote feelings of fullness so you end up eating less. One study showed that men who had a meal with resistant starch versus a placebo ate 90 fewer calories. One great trick I learned is that if you buy organic bananas and use them in smoothies, leave the skin on and you’ll get a nice serving of resistant starch and it will fill you up a lot more without adding calories. I also tend to make my rice ahead of time and reheat it throughout the week as I eat it, but of course that’s primarily because no one else in my house will eat any whole grain rice so I have to make it for myself while they make white rice.

Benefits of Fiber

In terms of the research on the benefits of fiber, Michael Ruscio also sums it up in his book. In terms of digestive-tract cancers, the overall impact of supplemental fiber, including resistant starch, on colorectal-cancer risk appears minimal at best. Most of the data shows no positive impact.

For IBD, randomized controlled trials have shown fiber to be helpful for IBD, but again, fiber is best used when IBD is in remission. Low-fiber diets have been shown to be helpful for active or flaring IBD.

For IBS, fiber has been shown to help IBS symptoms, including stool frequency and consistency and quality of life. However, high-fiber intake can be problematic for some IBS patients. You may have heard that something like 80% of IBS is believed to be caused by small intestine bacterial overgrowth (SIBO), so as you can imagine, if your small intestine bacteria is overgrown, adding more fuel to the fire wouldn’t be ideal. As a result, low-fiber diets like low FODMAPs have also been shown to be helpful in IBS, at least until the SIBO is cleared, if that’s your underlying cause. And in general, fiber has the most benefit for those with IBS-C or constipation, although I have to say that I have tended more toward the IBS-D and found that supplementing with psyllium husk gave me much better quality stool as it absorbed some of the excess water in my colon.

For celiac disease, there is no quality data available for supplemental fiber’s impact.

For type 2 diabetes, high-level science shows supplemental fiber can help lower fasting blood glucose by about thirty-five points and hemoglobin A1C by about 1% in patients with type 2 diabetes. But the healthier your blood sugar already is, the less effect supplemental fiber will have.

For heart health, supplemental fiber may cause a small decrease in blood pressure and cholesterol levels. However, there does not appear to be a clear benefit for heart disease from fiber supplementation.

For obesity and weight loss, a review paper showed that the average weight loss from fiber supplementation was around 4.2 pounds. Viscous fibers (gel forming) might be best for weight loss but also may carry the highest risk of digestive side effects. Overall, supplemental fiber does not appear to be hugely effective for weight loss.

Now I’m going to mention a study from 2018 that seems to have gone virtually unnoticed but definitely didn’t escape my notice. Researchers at the University of Toledo found a link between refined dietary fiber, gut bacteria, and liver cancer, at least in mice. The study challenges conventional wisdom that dietary fiber is good no matter how you get it. Chicory root is used as a source of inulin to fortify a lot of processed foods with fiber. In the study, the mice were given chicory root inulin and they developed liver cancer. And the ones that developed liver cancer had altered and elevated gut bacteria, or dysbiosis. Interestingly, the researchers observed no evidence of liver cancer in inulin-fed mice that were treated with broad-spectrum antibiotics to deplete gut bacteria. So given this was on mice and was just one study, probably the biggest takeaway is that fortifying processed foods with refined, soluble fiber may not be safe or advisable for people with SIBO or dysbiosis, whose abnormal fermentation of this fiber could potentially increase their susceptibility to liver cancer.

On the other hand, there are studies supporting the positive effects of fiber, in particular in kidney disease. A 12-week, single-blind study published in 2014 in the Journal of Renal Nutrition found that supplemental inulin was beneficial, in particular with increasing stool frequency. Many people with chronic kidney disease don’t get enough fiber, because many fiber sources are too high in potassium and phosphorus, so if you do have kidney disease and try to get more fiber, be sure to do it carefully and with your doctor’s advice and support.

Speaking of stool frequency, in case you weren’t sure, ideal stool frequency is around 1-2 bowel movements a day, so it’s beneficial to keep the gastrointestinal tract moving. This also helps prevent diverticulosis or pockets inside the colon, which can become diverticulitis when they become inflamed or infected. This is a very painful condition and can lead to bowel surgery, so keeping those bowels moving regularly is very important.

Now you may be asking what to do with all this information if you’re just an average person? Well, my general approach to fiber and prebiotics has been to get it from my diet rather than trying to supplement, although I will say that prior to solving all my bowel issues, I did have success with a tablespoon of psyllium husk in my smoothies, and I now get it in my keto bread. I hear that Jason Hawrelak, a Naturopath, probiotic researcher and university lecturer, recommends aiming for 40 different whole plant foods week. So next time you’re shopping, pick one new fruit or vegetable you’ve never tried, and incorporate it into your diet. Then add another one the following week. Just because you like broccoli and have finally gotten your kids to eat it, don’t just buy broccoli every week – switch it up, get something different, try new recipes. And you can find some of my favorite vegetable recipes at the bottom of my podcast show notes.

If you’re someone with IBD and you’re flaring, switch to a low fiber diet, but once your flare is over, slowly increase your fiber again to protect yourself from future flares.

If you’re considering investing in a prebiotic or synbiotic supplement, look for research to support its effectiveness. But don’t be like these folks I’ve seen in some Facebook groups who are selectively supplementing with certain prebiotics in hopes of increasing specific gut bacteria. We really don’t know enough about individual gut bacteria to be targeting them. Rather, think more about how well your gut is functioning overall.

If you want more help with your gut, autoimmune or other health issues, you can set up a free, 30-minute Breakthrough Session with me (Lindsey) to share what you’ve been going through and decide whether my 5-appointment gut health coaching program or a longer program for autoimmunity or weight loss is a good fit for you. Individual 1-hour consultations may be scheduled directly here.

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