What are prebiotics?
Probiotics have become increasingly popular over the last several decades, marketed as pills, supplements, or even yogurt products (remember the Activia commercials?).
Probiotics are blends of bacteria believed to boost our levels of gut bacteria (bacteria found in our stomach, intestines, and colon). They have been used to improve conditions, including poor digestion and autoimmune disease, which can be caused or made worse by a shortage of gut bacteria (1).
To learn more about probiotics, take a read through our “The Best Time to Take Probiotics” article. But, what are prebiotics, and what foods contain them?
While probiotics are the healthy bacteria needed to keep our body running, prebiotics are the food the bacteria that we already have need to function (2). These include various micronutrients, sugars, and compounds.
The first official definition of a prebiotic was “a nondigestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or various bacteria in the colon, and thus improves host health” (3).
Our gut bacteria are important, and certain varieties (particularly Bifidobacteria and Lactobacilli) have been implicated in a number of disorders such as cancer, low blood ammonia, and bacterial infections (4).
The idea that the bacteria in our gut play an important role in our health far beyond our digestive system is still relatively new. Lactobacilli have since been shown to improve digestion, reduce constipation, help us resist bacterial infections like E. coli and Salmonella (what most of us generically call “food poisoning”), and prevent traveler’s diarrhea (nothing ruins a tropical getaway quite like that one!).
Bifidobacteria studies have shown they can stimulate the immune system, inhibit pathogenic infections, reduce cholesterol, and restore our normal gut flora (bacterial variety) after antibiotic treatments (5).One of the most important prebiotic constituents is fiber, appreciated for its effect on digestion and overall health for quite some time.
It turns out, our gut bacteria love fiber as much as the human body in general does (6). Currently, the vast majority of prebiotics are carbohydrates (this includes sugars), that feed our gut bacteria.
Fiber is a complex carbohydrate, which means it’s composed of many sugar molecules that take several steps to break down, resulting in long-lasting energy. This dietary addition keeps your gut bacteria fed and healthy and keeps digestion flowing smoothly.
This area of prebiotic and probiotic research and development has taken on so much popularity and interest that the International Scientific Association for Probiotics and Prebiotics (ISAPP) was established in 2016, to regulate the field.
The ISAPP defines what actually counts as a probiotic or prebiotic, and what doesn’t (7). Many synthetic and natural supplements are marketed as one of these healthful dietary additions, but do not truly function as such (they are either ineffective, or the incorrect dose to have the effect claimed).
Even with this regulatory body in place, you should do your research before purchasing and trying different probiotic or prebiotic supplements, and always consult your doctor or pharmacist if you have any concerns or questions.
Health benefits of prebiotics
We’ve covered some of the various health benefits or prebiotics, but there are a few more that deserve some attention. Prebiotics are essentially food and nutrients for your gut bacteria. Common prebiotics to watch out for are inulin, pectin, beta-glucan, and fructooligosaccharides (a naturally occurring sugar).
Together, these four are responsible for the most significant prebiotic-associated effects (8).The effect of prebiotics on the immune system has been studied extensively to improve health naturally (i.e., without the need for synthetic pharmaceutical drugs).
One study in mice examined the immune response to the influenza vaccine (higher response to the vaccine means more protection against the virus during the flu season) and found the mice who consumed prebiotics had a higher increase in antibodies (translating to lasting immunity) after receiving the vaccine.
Mice who were antibiotic-compromised were dosed with prebiotics before being challenged with C. difficile bacteria, and they had higher levels of macrophages (immune defense cells) compared to those that received no prebiotics.
When measured in animals, levels of immunoglobulins, antibodies, and immune cells were generally higher if they’d been pretreated with prebiotics, compared to those not treated.
The likely explanation for these results is the prebiotics made the gut bacteria healthy and robust, thereby inducing the immune system to respond even stronger, resulting in higher immunity from vaccines and medical interventions (9).
Another perhaps underrated effect of gut bacteria is their ability to ferment food byproducts that reach the digestive tract (10). This process releases various metabolites that can be absorbed into the body and used for energy generation, growth, or health maintenance.
It would be taxing on our bodies to get by with enough energy without these processes, and studies into the mechanisms and specific outputs of these processes are still ongoing.
These fermentation processes also result in a decrease in pH in the colon, which prevents the growth of harmful bacteria and pathogens in the gut (11).Clearly, the microbiome in the gut is a precarious and dynamic thing.
There are many effects that have yet to be discovered, and complex systems and processes that need working out. However, it has been clear as studies progress that the bacteria in the gut contribute to overall health.
That said, it stands to reason that feeding those bacteria, and ensuring through nutrition that the numbers of bacteria in your system are appropriate, is one way to maintain a healthy body and prevent many diseases.
Now that we know what prebiotics are and some significant health benefits and effects they have, it’s time to discover common and popular foods high in prebiotics and find out how we can boost our prebiotic levels through diet (12).
Natural sources of prebiotics (13, to be exact!):
1. Chicory root
Chicory root is high in inulin, a soluble fiber that has in the past shown positive effects on health, including prebiotic effects. Inulin has shown positive effects on the colon microbial load in our gut, an essential part of our digestive and microbial health systems.
A recent study showed that the microbes throughout the colon grew to healthy proportions when chicory root inulin was present (13).Chicory root was also found to suppress and regulate appetite in a pilot study with mice (14), alluding to its potential as a weight control supplement.
Many weight control supplements are based on fiber, which suppresses appetite and regulates digestion, and fiber is believed to be the reason behind chicory root’s success. The study examined some human satiety hormones (hormones that keep us feeling full), and found these were increased in mice that received chicory root.
2. Jerusalem artichoke
A human study asking participants to consume a glass of vegetable puree containing Jerusalem artichoke showed a modest increase in the Lactobacillus and Enterococcus gut bacteria (15). These bacteria are correlated to improved health in many ways, including preventing urinary tract infections, improving digestion, and contributing to disease prevention.
Studies in mice showed various effects when Jerusalem artichoke was consumed, including improved iron bioavailability (increased ability of the intestine to absorb iron), reduction of plasma glucose levels (stabilizing glucose levels, preventing diabetes), improved calcium and phosphorous levels, and lowered cholesterol (16).
Together, these results suggest a modest addition of Jerusalem artichoke to the diet might have overarching nutritional and health effects in the intestine and colon.
Garlic is a health superfood, containing prebiotics, antibacterials, fructans, and other healthy compounds. Garlic has been shown to improve insulin resistance, liver injury, and obesity-related conditions (17).
Consuming garlic in moderate amounts has shown protective effects in the mucosal defenses of the digestive tract, a common prebiotic effect. While it remains unclear what components of garlic cause these effects, garlic is known to boost the levels of Lactobacillus acidophilus, one of the beneficial lactic acid bacteria found in our gut (18).
Onions are a known prebiotic food (19), but how the prebiotic’s work remains a mystery. That is, we know many of the healthy and beneficial effects elicited by onions, but don’t exactly know which constituents cause these effects.
Onions have been shown to be an effective remedy for cardiovascular disease, type 2 diabetes, thrombotic disorders, and many other conditions. Onions are also known antimicrobial, antioxidant, anticarcinogenic, antiasthmatic, and immunomodulatory vegetables (20).
Prebiotics are likely responsible for some of these effects, and the research to determine which compounds in onions elicit these effects is underway.
Leeks are another common source of prebiotics (especially when eaten raw) (21). They are high in inulin (that soluble fiber we talked about at the beginning of this article), which is responsible for many of the prebiotic effects (22).
Inulin-dosed diabetic rats in a recent study had lower blood glucose levels and reduced liver damage (23), which hints at some other health benefits hiding in this long, green vegetable.
This newly-realized prebiotic food has a drastic effect on our gut microbiome. Mice given asparagus products in addition to a high-fat diet had lower cholesterol and a higher abundance of Firmicutes,
Bacteroidetes, and Actinobacteria than those mice that didn’t receive asparagus (24). Asparagus products have also been tested in cell models for their prebiotic abilities, and it was found that growth of Bifidobacteria, Lactobacilli, Chlostridia, Coliform, and bacterial loads were higher when asparagus inulins were present (25).
Inulin is a continuing theme in prebiotics, having a significant effect on the growth and abundance of many of the beneficial species of bacteria in the gut, and their downstream health benefits.
Bananas contain various healthful compounds, but should be consumed in moderation due to their high sugar content. However, growth of several strains of bacteria have been increased by banana compounds (26).
The specifics of this process remain relatively unknown. That said, some prebiotic compounds that promote Lactobacillus or other gut bacteria are decreased by cooking bananas.
Germinated barley (also called malted barley), is the process of immersing barley in water and encouraging grains to sprout, then drying the product (27). Germinated barley appears to contain higher nutritional value than non-germinated barley.
Studies performed on barley prebiotics have focused primarily on germinating barley and gut microbiota (28).
In particular, Bifidobacteria, a species of bacteria that help your body digest food (29), absorb vital nutrients, and keep harmful bacteria from growing, was found to grow to higher and healthier quantities in older volunteers when they included barley in their diet (30).
Another application of prebiotics in germinated barley is cancer prevention—specifically, colon cancer. Germinated barley has been suggested to contain prebiotics that prevent colon cancer development (31), but these clinical studies are still underway.
Clinical studies have so far shown that germinated barley modulation of the gut bacteria may have some effect on remission or ulcerative colitis, an inflammatory disorder of the large intestine (32).
Oats are high in fiber, whole-grains, and many other healthy ingredients, including prebiotics! The effects of oats on cardio metabolic health and the gut microbiota have prompted several studies, one of which found that whole-grain oats behave as a prebiotic food in their effect on human health (33).
While the study did not isolate prebiotic compounds, this finding suggests its potential as a prebiotic food. Later studies determined that non-starch polysaccharides (long chains of sugars—what we know as carbohydrates) elicit some prebiotic effect of oats (34).
In clinical studies, oats were found to reduce blood glucose and cholesterol levels. The researchers identified β-glucan as the primary prebiotic behind this effect (35). So far, β-glucan and polysaccharides are considered the main prebiotics compounds in oats.
Apple skin and pulp are known to have various health benefits (an apple a day keeps the doctor away!), and it turns out they do also have some effects on our gut microbes.
Mice who were fed dehydrated apple slices with agave fructans or oligofructose had changes in their short-chain fatty acid production (36).
These fatty acids are used as food for gut microbes including Lactobacilli, and Bifidobacteria, which in turn increases with apple consumption (37). Cashew apples in particular have been noted to have prebiotic properties, promoting Lactobacillus growth (38).
Cocoa is high in flavanols, which are metabolized by microbes in the large intestine.Supplementing the diet with flavanol in cocoa increased Bifidobacteria and Lactobacillus populations in the gut of human clinical trials participants (39).
A similar effect was noted in rats, where the cocoa also increased some immune cell products like immunoglobulins (40), which fight disease. The final effect of these cell improvements is unknown, but presumed to be involved in the immunity provided by the microbial community in our guts, preventing infections.
The polyphenols in cocoa are also a rich source of prebiotics that feed our gut microbes (41). In addition to having antioxidant properties and other benefits, cocoa polyphenols do seem to have some microbial interplay with our gut microbes, but the specifics of these interactions are still under investigation. For more information about the effects of cocoa on your health, check out our article “10 Incredible Health Benefits of Cacao.”
12. Wheat bran
The two main prebiotic components of wheat bran are cellulase bran and arabinose. Together, these components were found to increase intestinal villi height (the tiny hairs in our intestine whose purpose is nutrient absorption and protection from pathogens) and increase numbers of goblet cells (a cell that secrets’ mucus, thereby protecting our body from pathogens and ensuring correct digestion and body functioning) in the digestive tract (42).
Another extract of wheat bran called Arabinoxylan-Oligosaccharide (or AXOS for short) has been found to have direct effects on the gut microbiota by increasing Bifidobacterium levels. Some other effects were found, including softened stool and blood glucose regulation, but these responses are still being studied (43).
Wheat bran fibers were found to also directly increase levels of Proteobacteria and Lactobacteria in our gut (44), which we now know the importance of for our health and immune protection.
Clearly, wheat bran has many direct and indirect effects on our digestive system, our intestinal walls, and the microbes that reside in our gastrointestinal system.
Seaweed is an underrated nutrient source that grows abundantly around the world. For this reason (as well as its low cost of production), varieties of seaweed have become popular bases for snack foods and food additives.
Seaweed is high in fiber, as well as oligo- and polysaccharides (45). This article has detailed the importance of all three of these compounds to our gut microbes, particularly the fact that the bacteria themselves can use them as food.
Seaweed may be the next step forward in the search for effective, sustainable, low-cost prebiotic supplements. Human clinical trials have been successful (46), also support this possibility. Cell model studies using seaweed extract showed increases in Bifidobacterium and Lactobacillus bacterial species due to short chain fatty acids and other prebiotics found in seaweed (47).
While probiotics have gotten the spotlight for many years, it’s time for the dawn of prebiotics.
Rather than supplementing our bodies with the bacteria we think it needs, try consuming foods (including the 13 listed in this article!), known to encourage growth of the balanced bacterial species in your gut—because happy gut bacteria means a healthier, happier you! And that, after all, is the purpose of nutrition.