My mother wasn’t an expert on nutrition, but she was often spot-on. For instance, her commitment to eating salad. As she saw it, raw vegetables provide "roughage," which helps to keep you regular. Today, this component of plant foods is commonly called insoluble fiber and my mother was right: It bulks up stools, preventing constipation and other digestive problems.

While on the mark for her time, my mother’s wisdom has been left behind by current research which shows that the advantages of consuming fiber spiral far beyond your digestive tract. Plant foods provide various types of fiber, which contribute to benefits like keeping blood sugar or cholesterol under control. Scientists now understand that the body functions as a complex ecosystem. That means the food you eat can activate biological pathways in every part of your body.

Fiber Helps to Build the Foundation for Health

Consider that a thriving universe of bacteria lives on and inside your body. Those residing in your gut constitute the gut microbiome. These invisible settlers impact your health and fiber pulls many of their strings.

Humans don’t digest fiber. It reaches our large intestine intact where it provides a nourishing meal for our microbial friends. Well-fed microbes flourish, helping to keep you in tip-top form. Studies show that the more fiber you consume, the more robust your microbiome and ultimately the better your health.

Fiber Supports Microbial Diversity

In general terms the more species of bacteria you harbour, the healthier you are likely to be. Although studies differ in their conclusions, most research shows that a high-fiber diet contributes to bacterial diversity, possibly in next-to-no time. One study showed that eating a high-fiber diet for as little as 2 weeks significantly jacked up the subjects’ microbial mix.

Diversity Pays Dividends

In your microbial universe, the more the merrier because bacteria have different skill sets depending on their type. They also tend to work as a team. Bacteria produce compounds known as metabolites. Not only are these substances very beneficial for you they also nourish your "good guy" bacteria.

Numerous studies show that bacteria amplify their production of metabolites in response to the fiber you consume. This sets off a complex chain reaction known as cross-feeding, stimulating the growth of various types of bacteria and further boosting diversity.

Fiber Strengthens Your Immune System

Seventy percent of your immune system is in your gut. Your microbiome and your immune system work side by side calibrating how each performs. For instance, one laboratory study showed that eating a high-fiber diet may help you to fight the flu. These researchers found that consuming fiber altered gut bacteria and increased the production of beneficial metabolites, activating a type of white blood cell that helps to protect the body from infections.

The relationship between gut bacteria and the immune system is complex. Moreover, it appears to differ among individuals. One recent study showed that a high-fiber diet increased the production of certain enzymes that help with digesting carbohydrates, while triggering different immune system responses among individuals. Researchers believe these distinct reactions open the door to further research into precision nutrition, using diet to treat specific conditions in individuals.

Fiber Keeps Chronic Illness at Bay

In general terms, dietary patterns that are low in fiber, like the Western-style diet, have been linked with systemic inflammation, a driver of chronic disease. A substantial body of research shows that eating a high-fiber diet lowers the risk of developing numerous conditions, including heart disease, diabetes, and even certain types of cancer. Fiber works its magic in part by stimulating changes in the gut microbiome that reduce inflammation.

Fiber Helps to Keep You Thin

You may have heard that fiber helps you to maintain a healthy weight because it fills you up. But fiber also helps to keep you thin by supporting the growth of beneficial gut bacteria that battle inflammation, which—you guessed it—is associated with obesity. Being overweight has been linked with chronic low-grade inflammation, which is why it’s a precursor for more serious conditions.

Scientists have long known that certain strains of beneficial bacteria (for instance, Lactobacillus reuteri) are powerful anti-inflammatories. Now they are zeroing in on the relationship between gut bacteria and the metabolism. Research shows that most people suffering from metabolic disorders, including obesity, have lower levels of a specific bacterium: Akkermansia muciniphila. Levels of this microbe can be boosted by eating a high-fiber diet, rich in plant foods.

Sadly, very few Americans consume enough fiber. A typical "western" diet provides about 15 g of fiber daily—far less than the recommended amount, which ranges between 25 to 38 grams. The good news is, it’s not hard to boost your intake of this valuable nutrient. A helpful rule of thumb is to ditch the junk food and eat whole foods instead.

Plant foods like fruits, vegetables, nuts, seeds, whole grains, dried beans and lentils are fiber rich. They also contain a kaleidoscope of other nutrients that often work together synergistically, boosting their individual benefits. And they are the favorite meal for your microbial friends, helping to build a robust microbiome that will work to keep you well.

Judith Finlayson is the author of "You Are What Your Grandparents Ate: What You Need to Know About Nutrition, Experience, Epigenetics, and the Origins of Chronic Disease." Visit her at www.judithfinlayson.com.

Selected References
* Oliver, A. et al. High-Fiber, Whole-food Dietary Intervention Alters the Human Gut Microbiome but not Fecal Short-Chain Fatty Acids. ASM Journals 2021.
* Holscher, H. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes 2017
* Reynolds, A. et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet 2019.
* Ma, W. et al Dietary fiber intake, the gut microbiome, and chronic systemic inflammation in a cohort of adult men. Genome Medicine, 2021.
* Verhoog, S. Dietary Factors and Modulation of Bacteria Strains of Akkermansia muciniphila and Faecalibacterium pausnitzli: A Systemic Review. Nutrients 2019.