Fiber is one of the main reasons whole plant foods are good for you.
Growing evidence shows that adequate fiber intake may benefit your digestion and reduce your risk of chronic disease.
Many of these benefits are mediated by your gut microbiota — the millions of bacteria that live in your digestive system.
However, not all fiber is created equal. Each type has different health effects.
This article explains the evidence-based health benefits of fiber.
Put simply, dietary fiber is a non-digestible carbohydrate found in foods.
It’s split into two broad categories based on its water solubility:
- Soluble fiber: Dissolves in water and can be metabolized by the "good" bacteria in the gut.
- Insoluble fiber: Does not dissolve in water.
Perhaps a more helpful way to categorize fiber is fermentable versus non-fermentable, which refers to whether friendly gut bacteria can use it or not.
It's important to keep in mind that there are many different types of fiber. Some of them have important health benefits, while others are mostly useless.
There is also a lot of overlap between soluble and insoluble fibers. Some insoluble fibers can be digested by the good bacteria in the intestine, and most foods contain both soluble and insoluble fibers.
Health authorities recommend that men and women eat 38 and 25 grams of fiber per day, respectively.
Summary Non-digestible carbohydrates are collectively known as fiber. They are most often categorized as soluble or insoluble.
The bacteria that live in the human body outnumber the body's cells 10 to 1.
Bacteria live on the skin, in the mouth and in the nose, but the great majority live in the gut, primarily the large intestine ().
About 500 different species of bacteria live in the intestine, totaling about 100 trillion cells. These gut bacteria are also known as the gut flora.
This is not a bad thing. In fact, there is a mutually beneficial relationship between you and some of the bacteria that live in your digestive system.
You provide food, shelter and a safe habitat for the bacteria. In return, they take care of some things that the human body cannot do on its own.
Of the many different kinds of bacteria, some are crucial for various aspects of your health, including weight, blood sugar control, immune function and even brain function (, , , , ).
You may wonder what this has to do with fiber. Just like any other organism, bacteria need to eat to get energy to survive and function.
The problem is that most carbs, proteins and fats are absorbed into the bloodstream before they make it to the large intestine, leaving little for the gut flora.
This is where fiber comes in. Human cells don't have the enzymes to digest fiber, so it reaches the large intestine relatively unchanged.
However, intestinal bacteria do have the enzymes to digest many of these fibers.
This is the most important reason that (some) dietary fibers are important for health. They feed the "good" bacteria in the intestine, functioning as prebiotics ().
In this way, they promote the growth of "good" gut bacteria, which can have various positive effects on health ().
The friendly bacteria produce nutrients for the body, including short-chain fatty acids like acetate, propionate and butyrate, of which butyrate appears to be the most important ().
These short-chain fatty acids can feed the cells in the colon, leading to reduced gut inflammation and improvements in digestive disorders like irritable bowel syndrome, Crohn’s disease and ulcerative colitis (, , ).
When the bacteria ferment the fiber, they also produce gases. This is the reason high-fiber diets can cause flatulence and stomach discomfort in some people. These side effects usually go away with time as your body adjusts.
Summary Consuming adequate amounts of soluble, fermentable fiber is very important for optimal health because it optimizes the function of the friendly bacteria in the gut.
Certain types of fiber can help you lose weight by reducing your appetite.
In fact, some studies show that increasing dietary fiber can cause weight loss by automatically reducing calorie intake (, ).
Fiber can soak up water in the intestine, slowing the absorption of nutrients and increasing feelings of fullness ().
However, this depends on the type of fiber. Some types have no effect on weight, while certain soluble fibers can have a significant effect (, , , ).
A good example of an effective fiber supplement for weight loss is glucomannan.
Summary Some types of fiber can cause weight loss by increasing feelings of fullness, leading to a reduced calorie intake.
High-fiber foods tend to have a lower glycemic index than refined carb sources, which have been stripped of most of their fiber.
However, scientists believe only high-viscosity, soluble fibers have this property ().
Including these viscous, soluble fibers in your carb-containing meals may cause smaller spikes in blood sugar ().
This is important, especially if you're following a high-carb diet. In this case, the fiber can reduce the likelihood of the carbs raising your blood sugar to harmful levels.
That said, if you have blood sugar issues, you should consider reducing your carb intake, especially low-fiber, refined carbs, such as white flour and added sugar.
Summary Foods that contain viscous fiber have a lower glycemic index and cause smaller spikes in blood sugar than foods that are low in fiber.
Viscous, soluble fiber can also reduce your cholesterol levels.
However, the effect isn't nearly as impressive as you might expect.
A review of 67 controlled studies found that consuming 2–10 grams of soluble fiber per day reduced total cholesterol by only 1.7 mg/dl and LDL cholesterol by 2.2 mg/dl, on average ().
But this also depends on the viscosity of the fiber. Some studies have found impressive reductions in cholesterol with increased fiber intake (, ).
Whether this has any meaningful effects in the long term is unknown, although many observational studies show that people who eat more fiber have a lower risk of heart disease ().
Summary Some types of fiber can reduce cholesterol levels. However, most studies show that the effect isn't very large, on average.
One of the main benefits of increasing fiber intake is reduced constipation.
Fiber is claimed to help absorb water, increase the bulk of your stool and speed up the movement of your stool through the intestine. However, the evidence is fairly conflicting (, ).
Some studies show that increasing fiber can improve symptoms of constipation, but other studies show that removing fiber improves constipation. The effects depend on the type of fiber.
In one study in 63 individuals with chronic constipation, going on a low-fiber diet fixed their problem. The individuals who remained on a high-fiber diet saw no improvement ().
In general, fiber that increases the water content of your stool has a laxative effect, while fiber that adds to the dry mass of stool without increasing its water content may have a constipating effect.
Soluble fibers that form a gel in the digestive tract and are not fermented by gut bacteria are often effective. A good example of a gel-forming fiber is psyllium ().
Other types of fiber, such as sorbitol, have a laxative effect by drawing water into the colon. Prunes are a good source of sorbitol (, ).
Choosing the right type of fiber may help your constipation, but taking the wrong supplements can do the opposite.
For this reason, you should consult with a health professional before taking fiber supplements for constipation.
Summary The laxative effects of fiber differ. Some reduce constipation, but others increase constipation. This appears to depend on the individual and type of fiber.
Colorectal cancer is the third leading cause of cancer deaths in the world ().
Many studies have linked a high intake of fiber-rich foods with a reduced risk of colon cancer ().
However, whole, high-fiber foods like fruits, vegetables and whole grains contain various other healthy nutrients and antioxidants that may affect cancer risk.
Therefore, it’s difficult to isolate the effects of fiber from other factors in healthy, whole-food diets. To date, no strong evidence proves that fiber has cancer-preventive effects ().
Yet, since fiber may help keep the colon wall healthy, many scientists believe that fiber plays an important role ().
Summary Studies have associated a high fiber intake with a reduced risk of colon cancer. However, correlation doesn’t equal causation. To date, no studies have proven the direct benefits of fiber in cancer prevention.
Dietary fiber has various health benefits.
Not only does it feed your gut bacteria, fermentable fiber also forms short-chain fatty acids, which nourish the colon wall.
Additionally, viscous, soluble fiber may reduce your appetite, lower cholesterol levels and decrease the rise in blood sugar after high-carb meals.
If you are aiming for a healthy lifestyle, you should make sure to get a variety of fiber from whole fruits, vegetables and grains.