Dietary fiber is a type of carbohydrate found in plant foods like whole grains, vegetables, fruits, and beans. People have been eating high-fiber diets for millions of years. Ancient humans used to eat over 100 grams of fiber a day.
The gut microbiota (the community of bacteria in our intestines) helps us digest fiber, create vitamins, break down toxins, and make other important substances.
Modern Diets and Fiber
In recent centuries, people have started eating much less fiber, especially in industrialized countries. Our gut bacteria can adapt quickly to changes in our diet, but our human genes do not change as fast. Changes in the gut bacteria due to low-fiber diets are linked to more health problems, such as obesity, diabetes, cancer, and heart disease.
Types of Dietary Fiber
There has been debate about what counts as dietary fiber. Oligosaccharides (fiber made of 3–9 sugar units) were especially debated. Now, most countries and experts agree that oligosaccharides are a type of fiber.
Dietary fiber can be: Non-starch polysaccharides (NSPs): like cellulose, hemicellulose, pectins, and inulin. Resistant starches (RS): found in foods like whole grains, potatoes, and under-ripe bananas. Resistant/nondigestible oligosaccharides (ROS): like galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS).
How Much Fiber Do People Eat?
The average person in the world eats between 15 and 26 grams of fiber a day. Most health recommendations suggest 20–35 grams per day. People in Northern and Central Europe and Australia eat the most fiber, likely because of their traditional whole grain foods. In places like Japan, people eat less fiber, which may be due to different foods and smaller average body size. Some countries suggest fiber intake based on the amount of calories eaten (e.g., 14 grams per 1,000 calories in the US).
What Happens to Fiber in the Body?
Fiber is not digested in the upper parts of our digestive system. Instead, it is broken down by bacteria in the colon (large intestine). The way fiber is structured (size, solubility, etc.) affects how easily and by which bacteria it is fermented. Some fibers are broken down quickly, others more slowly. When bacteria break down fiber, they produce gases and short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate.
Why are Short-Chain Fatty Acids (SCFAs) Important?
SCFAs can be used as energy by cells in our gut. They also get into the blood and act as signals that help regulate metabolism, immune function, protect the gut lining, brain function, bone health, and reduce inflammation and cancer risk. SCFAs are believed to help improve blood sugar control, for example by increasing hormones such as GLP-1 and PYY.
How Does Fiber Affect Gut Bacteria?
Eating more fiber changes the types and amounts of bacteria in the gut. People who eat fiber-rich diets have different gut bacteria compared to those who eat low-fiber, Western diets. High-fiber diets are linked to more diverse gut bacteria, which is thought to be healthier. Some studies show that people in rural or traditional societies have more gut bacterial diversity than people in modern, urban areas.
Different Fibers Feed Different Bacteria
Certain types of fiber help certain bacteria grow:
Inulin, GOS, FOS, and others often increase Bifidobacterium. Some fibers boost Faecalibacterium, Ruminococcus, or Lactobacillus.
The changes can happen after just 1–2 weeks of eating more fiber and usually stay as long as the fiber is eaten. Not all studies agree on how fiber affects gut bacterial diversity, and effects can depend on the type of fiber, the amount, and the person.
How Much Fiber is Needed for Gut Health?
Some studies have tried to find the "cutoff" amount of fiber that changes gut bacteria. For example, inulin may increase Bifidobacterium at 5–8 grams per day. Other types of fiber have different effective doses. The starting makeup of a person’s gut bacteria can also affect how much they respond to fiber.
Fiber and Diabetes
Changes in gut bacteria are linked to diseases like diabetes. Eating more fiber can help people with type 2 diabetes by increasing helpful bacteria and improving blood sugar control, though results vary between individuals and types of fiber. Some studies show that after eating more fiber, diabetic patients’ gut bacteria make more SCFAs, which helps lower blood sugar.
Conclusion
Eating more dietary fiber is good for health, mainly because it feeds helpful gut bacteria and leads to the production of healthy substances like SCFAs. Modern diets low in fiber are linked to more chronic diseases. The type, amount, and duration of fiber intake all matter for gut health. Learning more about how fiber and the gut microbiome work together can help develop better ways to prevent and treat diseases.