Do the bacteria in your digestive tract – known as your gut microbiota – hold the key to reducing inflammation and chronic disease? Discussions of prebiotics, probiotics, and other steps targeting healthy gut microbes fill magazines, books and other media. Yet questions at my presentations and in informal conversations show lots of confusion.
Previous posts in this series on carbohydrate quality looked at evidence and take-home strategies on what glycemic index and eating more pulses mean for health. In this third post in the series, I’ll address some top questions on prebiotics, the microbiota and health. If you hear the buzz words frequently, but have trouble pinning down exactly what it all means, read on to see what we know and where the many unanswered questions remain.
What is a Prebiotic?
People often think of bacteria and other microorganisms as harmful “germs,” yet many microorganisms help our bodies function properly. Bacteria, fungi and viruses can be found throughout the body. Microorganisms in the body outnumber human cells by 10 to 1. The largest community lives in the gastrointestinal (GI) tract, whether referring to the gut “microbiota” (the microbes themselves) or “microbiome” (the genes inside those microbial cells). Mutually supportive, we feed them, and they support our health, helping to destroy disease-causing microorganisms and producing vitamins, for example.
What’s the difference between a probiotic and a prebiotic?
Probiotics provide live microorganisms (often bacteria) that bring a health benefit when present in adequate amounts.
In the know: You may hear fermented foods ranging from yogurt, kefir, tempeh, and kimchi to sauerkraut and pickles referred to as probiotics. Some foods claimed as a source of “live, active cultures” may not actually benefit health because they don’t provide enough of the right bacteria (such as certain species of Bifidobacterium and Lactobacillus). An expert consensus statement recommends that only foods and supplements with evidence of a health benefit belong in this category.
Prebiotics do not provide live microbes. A prebiotic is a substance (usually some type of carbohydrate) that selectively supports specific gut microorganisms that provide a health benefit. Under a 2017 consensus panel definition, a prebiotic could support the microbiota found in several spots in the body (including the skin), but this Smart Bytes® post focuses on prebiotics feeding the gut microbiota.
In the know: Heat destroys probiotics’ live cultures, but it does not seem to affect prebiotics.
Synbiotics provide both probiotics (live microbes) and prebiotics (material to nurture existing and newly established microbes).
Is a prebiotic different from dietary fiber?Is all dietary fiber a #prebiotic? Here's why not... Click To Tweet
Dietary fiber includes several kinds of carbohydrate that are edible but not digestible by human gut enzymes. Some types of fiber help keep bowels moving regularly; other types help lower blood cholesterol or reduce blood sugar spikes after meals. Some can be fermented by gut bacteria; some cannot.
Prebiotics include specific types of dietary fiber fermented in the digestive tract that support changes in the number, variety and/or activity of gut microbes that promote health. Not all dietary fiber is prebiotic.
In the know: It’s not as simple as it might seem to identify what works as a prebiotic. A dietary fiber (such as cellulose) can be prebiotic in ruminants but not in humans, or prebiotic in the mouth but not in the gut. And just as not all dietary fiber is prebiotic, not all prebiotics are dietary fiber. Some polyphenols (a group of natural plant compounds) pass through the digestive tract to the colon undigested, where they are metabolized by bacteria. They can enhance growth of certain healthful bacteria (like Bifidobacterium and other lactic acid-forming bacteria), reduce growth of some harmful types, and produce compounds that provide additional healthful effects. We’ll circle back to this important point later….
How does a prebiotic work?
Different kinds of gut bacteria flourish under different circumstances. When people change what they eat — for example, by boosting the proportion of whole plant foods and decreasing meat – their gut microbiota begins to change within just 24 hours, and reverts back to baseline within 48 hours if they discontinue the new eating habits.
In the know: Do more prebiotics mean more intestinal gas? Not necessarily, although some people in intervention trials report more “rumbling” at first. Many of the bacteria linked to health benefits (such as Bifidobacteria and Lactobacillus) reportedly do not produce gas. So especially if prebiotic dietary fiber increases gradually, that gives these bacteria time to multiply and be ready for it.
What’s the Benefit of Prebiotics?
Prebiotics seem to protect health through direct effects on the gut microbes they support and through substances those gut microbes produce from the prebiotic.Prebiotics may promote health nurturing good gut bacteria & then forming more protectors. Click To Tweet
Health-promoting microbes in the colon produce vitamins and essential amino acids. Emerging studies in mice and in humans suggest that the gut microbiota also acts through several mechanisms to strengthen the immune system.
Moreover, as they break down prebiotic carbohydrates, gut bacteria produce butyrate and other short chain fatty acids (SCFA) that nurture the cells lining the colon and help strengthen the barrier that protects them. Fermentation and SCFA production also lower the pH in the gut, which reduces breakdown of protein components that form ammonia and other toxic compounds, and inhibits growth of harmful microbes.
Metabolic Health — Diabetes and Heart Protection
A clear benefit of prebiotics from the perspective of Patrice Cani, PhD, is that they may help reduce inflammation resulting from metabolic endotoxemia, which is characterized by elevated blood levels of LPS (lipopolysaccharide, a component of certain bacteria cell walls). Dr. Cani is a professor and researcher at the Université catholique de Louvain in Belgium. His studies and those of others have identified a range of prebiotic amounts that decrease blood levels of LPS (lipopolysaccharide), which also triggers insulin resistance.
In rodents, prebiotics can shape the population of gut microbes to reduce development of low-grade chronic inflammation by helping to strengthen the barrier protecting gut walls with increased mucus thickness and tighter junctions between cells. In humans, prebiotics can promote growth of gut microbes linked with lower inflammation and better health, but ability to affect development of type 2 diabetes and heart disease has not yet been clearly demonstrated.
Multiple studies show that the particular microbes that make up the gut microbiota can differ between people with obesity and those who are lean. Prebiotics that change microbiota composition have led to decreased calorie consumption and weight loss in rodent and some human studies. Scientists tentatively pin these effects to changes in appetite-regulating hormones released from the gut. For example, a 12-week randomized controlled trial found that oligofructose prebiotic given to overweight and obese adults led to self-reported decrease in calorie consumption that was tied to changes in two appetite-related hormones. The prebiotic also led to weight loss that was statistically significant compared to the control group of subjects. Averaging about 2 pounds in 3 months for the group as a whole, this is promising, but not what would be considered “clinically significant” weight loss.
Although prebiotics show potential to support weight loss, it’s a benefit that can’t be taken for granted — at least not in all people and all circumstances. Dr. Cani and colleagues conducted a three-month trial in 30 obese women. Half were given a daily supplement mixture of two prebiotic carbohydrates to mix into drinks, and the others were given a similar-tasting non-prebiotic carbohydrate to serve as placebo. Women receiving prebiotics showed slight drops in BMI (body mass index, a way of expressing weight in proportion to height) as well as in percent body fat and waist/hip ratio compared to women given placebo. But none of these changes were statistically significant (meaning they could have occurred by chance). Likewise, prebiotic treatment did not significantly change elevated insulin or blood sugar. However, prebiotics did produce changes in bacterial species in the gut microbiota that reflected the kind of differences other studies have shown between obese and lean people. Moreover, these changes in gut microbes were associated with changes in body fat and blood levels of LPS that are used as markers of endotoxemia.
A small pilot study provided either a prebiotic (inulin) or water (as placebo) before test meals to seven sedentary, overweight or obese adults. The amount of inulin provided was in the range shown in other studies to affect gut microbiota composition. However, in this study looking at short-term effects, it did not reduce calorie consumption, amount of food eaten, or appetite. The researchers involved suggest that perhaps the changes in microbiota expected from prebiotics take more time to produce changes in appetite.
In short, researchers consider potential benefit on weight very real, yet more research is needed on whether individual differences in eating habits or baseline microbiota could make prebiotics more likely to help particular groups of people reach and maintain a healthy weight.Can #prebiotics help weight loss? Maybe. More research is needed. Click To Tweet
Research is mounting showing that the gut microbiota may affect anti-inflammatory and other defenses against cancer. According to Johanna Lampe, PhD, RD, of Fred Hutchinson Cancer Center, some microbes may increase risk, and others seem to have tumor-suppressive effects, acting locally in the colon and perhaps with broader effects on cancer risk. Prebiotics could help reduce risk by promoting growth of more protective microbes.
Moreover, Dr. Lampe, also a Research Professor at the University of Washington in Seattle, explains that prebiotic sources of dietary fiber that result in higher amounts of butyrate (a short-chain fatty acid) may be important. Butyrate is a fuel source for colon cells and a signaling molecule related to cancer pathways. She notes that dietary fiber that is more soluble and more easily fermented has been shown to reduce colon cancer development in animal models, but that it is less clear whether this holds up in relation to colorectal cancer in humans.
Are Prebiotics a Concern for Anybody?
For people with some intestinal diseases or sensitivities, prebiotics can be counterproductive. Irritable bowel syndrome (IBS) and Crohn’s disease are associated with changes in gut bacteria and greater inflammation in the intestinal lining. Studies in Crohn’s disease have not shown benefit on the microbiota population or symptoms of inflammation with prebiotic supplements. For IBS, some studies have found that low-dose prebiotic supplements can produce beneficial changes in gut bacteria and reduced symptoms. Larger doses, however, can be without benefit or even worsen symptoms.
Quite the opposite approach to IBS involves a low FODMAP diet that avoids foods with fermentable carbohydrates. (FODMAP is an acronym for types of these carbohydrates: Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols.) For people with IBS, restricting these sources of prebiotics can be effective in improving symptoms. But discussions at a 2017 world summit on the gut microbiota noted that this reduces levels of some healthful gut microbes and can raise pH (which could encourage growth of harmful bacteria). Research is needed on how a low FODMAP diet might be adjusted to provide prebiotics that support bacteria that produce protective substances for long-term health while avoiding gas-forming carbohydrates that prompt IBS symptoms.
Bottom Line on Prebiotics & Gut Microbiota:
The microbes in our gut warrant care and feeding, since they seem to play a powerful role in health. Emerging research suggests that prebiotics in food or supplements could play a role in this. Come back for part two on this topic, when we’ll circle back to questions raised here and look at how healthy eating and smart carbohydrate choices might lay important groundwork.
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My sincere thanks to Patrice D. Cani, PhD, and Johanna Lampe, PhD, RD, for graciously lending their expertise to this post through personal interviews.
Top photo credit: purchased from 123rf.com, https://www.123rf.com/profile_nerthuz’>nerthuz / 123RF Stock Photo