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.

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.

1. 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.

2. Fiber Supports Microbial Diversity

In general, the more species of bacteria you harbor, the healthier you are likely to be. A high-fiber diet contributes to bacterial diversity, possibly in next-to-no time.

3. 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.

The relationship between gut bacteria and the immune system is complex. Moreover, it appears to differ among individuals. A high-fiber diet increases the production of certain enzymes that help with digesting carbohydrates, while triggering different immune system responses among individuals.

4. Fiber Keeps Chronic Illness at Bay

Dietary patterns that are low in fiber, like the Western-style diet, have been linked with systemic inflammation, a driver of chronic disease. 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.

5. Fiber Helps Keep You Thin

You may have heard that fiber helps you to maintain a healthy weight because it fills you up. 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 are powerful anti-inflammatories. Now they are zeroing in on the relationship between gut bacteria and the metabolism. 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.

Very few Americans consume enough fiber. A typical "western" diet provides about 15g 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.