The microbiome consists of trillions of bacteria living in and on the human body. Bacteria cover the skin and all of the body’s external surfaces, with the majority in the gastrointestinal (GI) tract. The bacterial populations are influenced by the environment, including diet and the bacteria in the soil that sometimes make their way into foods. The body has developed immune defenses against bad bacteria and the means to encourage colonization with good bacteria. It is no surprise that the microbiome and immune function are closely related. However, substances produced by the microbiome not only affect GI health, but can travel to the brain, where they can affect control of food intake and serve as antioxidants in the brain.
The majority of bacteria are located in the GI tract, where they form a tight-knit community often called the “forgotten organ.” The gut bacteria eat what the body does not digest, including soluble fibers, polyphenols and prebiotics, which cause them to produce a variety of substances which enter the bloodstream and affect the function of the body’s organs and cells. Nutrition science has been forever changed by the discovery of the microbiome and research over the last decade.
Obese individuals and those eating an animal protein-based or Western diet have a different microbiome than those eating a plant-based diet. When obese individuals lose weight, their microbiome changes in the direction of what is found in lean individuals.
The gut bacteria of an obese twin and lean twin with identical human genes were placed into germ-free mice, and when these mice were fed an identical diet, the mice that received the bacteria from the obese twin became fat, while the mice that received the bacteria from the lean twin remained lean. (Science. 2013;341:1241214.) Transplanting the bacteria from fat to lean caused the lean mice to become fat, but the reverse did not occur. Thus, the bacteria in the gut reflect the diet and also the individual genome.
Prebiotics are those substances in the diet or dietary supplements that bacteria digest and that influence their growth and behavior. Breast milk naturally contains a complex family of milk oligosaccharides that help to establish the microbiome and immune system of the infant. Bacteriodes infantilis is an “inside eater” which, unlike other bacteria, does not share its food. This bacterium dominates the diaper at a time when immune tolerance to new foods is being developed. The benefits of breastfeeding include the establishment of a stable microbiome, setting the stage for the immune system to function properly.
Seventy percent of the immune system throughout life is located near the intestines in what is called the gut-associated lymphoid tissue and is influenced by the microbiome. Intestinal disorders, poor diet, inactivity and stress all influence the microbiome. Over long periods of time, the microbiome is stable; within an individual, the microbiome composition is based on an interaction of human and bacterial genes. Various conditions, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), can affect the microbiome, and in turn, the microbiome changes may contribute to disease pathology. However, for healthy individuals, maintaining a diverse microbiome is essential to good health. With aging, the diversity of bacteria in the microbiome is reduced; there are also functional changes which accompany the changes in bacterial populations.
There is an opportunity for prebiotics to influence health by changing the microbiome. A number of prebiotics are currently on the market as dietary supplements, and their effects on gut bacteria will continue to be reviewed. One interesting observation from our laboratory is that xylooligosaccharides (XOS) increase Bifidobacteria, but not Lactobacilli, and affect glucose metabolism. Recent research indicated not only fibers and oligosaccharides affect the microbiome, but that polyphenols from pomegranate, cranberry and spices can act as prebiotics, as well. They increase Akkermansia muciniphila, a bacterium in the small intestine which is lower in obese than lean individuals and rises with weight loss. Some of the beneficial effects of a diet rich in fruits and vegetables may be due to their prebiotic and postbiotic effects (see below for an explanation of postbiotics).
Probiotics are live bacteria provided in foods or dietary supplements to enter the gut bacterial community. They face the challenge of passing through stomach acid to populate the intestines. Spore-forming bacteria and increasing supplement preparations to include billions of colonies are contrasting approaches to this challenge.
Postbiotics are those substances made by the bacteria. Some short-chain fatty acids (SCFAs) such as beta-hydroxybutyrate nourish the colonic cells, while acetate and propionate enter the circulation. Polyphenols like pomegranate and cranberry are metabolized to urolithins, but people vary in the ability of their microbiome to produce urolithins leading to the identification of metabotypes for urolithin A and urolithin B, which have aging and disease-risk implications.
Much research remains to be done, but the opportunities for improving human health through diet and lifestyle will depend on including studies of the microbiome in research going forward. Nutrition science will never be the same as it was before the discovery of the microbiome.
David Heber, M.D., Ph.D., FACP, FASN, is professor emeritus and the founding director of the UCLA Center for Human Nutrition at the David Geffen School of Medicine, where he directed multiple National Institutes of Health (NIH)-funded research programs, including the UCLA Clinical Nutrition Research Unit, and the UCLA Dietary Supplements Research Center: Botanicals. He is currently chairman of the Herbalife Nutrition Institute. Heber has published more than 250 research papers and best-selling books for the public, including “What Color Is Your Diet?” and “The L.A. Shape Diet,” both of which have been translated into multiple languages internationally.
Prebiotics at SupplySide West
Dr. Heber will be sharing more about prebiotic research during the “Market Opportunities for Prebiotics” workshop on Friday, Nov. 9 at 2 p.m. at SupplySide West in Las Vegas.