Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

Scientists Continuing to Study How Fiber Aids Gut Microbiome

Dear Doctor: I know that all sorts of studies show that fiber is good for you, but has anyone figured out the reason why?

Dear Reader: You're right -- the upside of fiber has been with us for quite a while now. Not only has the list of proven health benefits continued to grow, but also the language itself has changed: It used to be referred to as "roughage." But whatever it's being called, fiber is basically the carbohydrates in our food that resist digestion. And though there has been plenty of educated guessing about why we do measurably better over the short and long haul with a regular supply of fiber in our diets, a definitive answer has proven elusive. Now, however, ongoing research into the gut microbiome is offering up some intriguing answers.

First, some of what researchers had already believed about fiber's benefits continue to hold true. Foods that are high in fiber help to slow the absorption of sugars during digestion, which is a boon in regulating blood sugar levels. As a result, fiber in the diet can be a hedge against developing diabetes. Fiber makes you feel full after a meal, helps lower inflammation, and its presence in the intestines aids in elimination. Research shows that a diet high in fiber results in lower rates of heart disease and certain cancers, which may be why eating lots of fiber has been linked to a longer life span in general.

What's really interesting is scientists are now discovering that the fiber we consume feeds the billions of microorganisms each of us hosts in our bodies. Better known as the gut microbiome, it's becoming clear that they play an important role in keeping us healthy. If that number sounds like a lot, you're right. It's now estimated that these tiny hitchhikers make up between 1 and 3 percent of a person's total body weight. That means if your scale says 135 pounds, up to 4 pounds of that weight is beneficial bacteria. And since both quantity and diversity of beneficial bacteria are equally important, making sure they stay fed and happy is key.

Recent studies suggest that the way to cultivate a strong gut microbiome is keeping it supplied with plenty of fiber. When mice were put on a diet loaded with fat and low on fiber, the number and diversity of bacteria in their guts plummeted. At the same time, the mice themselves underwent changes that adversely affected their health. Not only did their intestines shrink, the protective coating of mucus along the intestinal walls became dangerously thin. This woke up the immune system and triggered an inflammation reaction. In time, this chronic inflammation was associated with weight gain and unhealthy blood sugar levels.

Bottom line -- the fiber you consume feeds the beneficial gut bacteria that help to keep us healthy in ways we don't yet fully understand and have yet to discover. So, when you're having that salad of leafy greens, a handful of walnuts, an apple or banana, or a serving of beans, you're not just eating for one -- you're eating for a few billion.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)