Fiber is an important nutrient that’s often overlooked.

Put simply, fiber refers to carbohydrates that cannot be digested by your gut.

It is classified as either soluble or insoluble depending on whether it dissolves in water.

Insoluble fibers function mostly as bulking agents, adding content to your stool. In contrast, certain types of soluble fiber can significantly affect health and metabolism — as well as your weight (1).

This article explains how soluble fiber may promote weight loss.

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An estimated 100 trillion bacteria live in your gut, primarily in the large intestine (2).

Along with other microbes found in your digestive system, these bacteria are often called the gut flora or gut microbiome.

Different species of bacteria play important roles in various aspects of health, including weight management, blood sugar control, immunity and even brain function (3, 4, 5, 6, 7).

Just like other organisms, bacteria need to eat well to stay healthy.

This is where fiber — soluble, for the most part — steps in. Soluble fiber passes through your digestive system mostly unchanged, eventually reaching your friendly gut bacteria that digest it and turn it into usable energy.

The fiber that benefits your gut bacteria is known as prebiotic fiber or fermentable fiber. It is considered very beneficial for health and body weight (8, 9).

Certain insoluble fibers, such as resistant starch, also function as prebiotics.

Summary Fiber does not get digested and tends to reach your large intestine relatively unchanged. There, certain soluble fibers help feed the friendly gut bacteria that are essential for good health.

Gut bacteria are renowned for their effect on chronic inflammation (10).

They produce nutrients for your body, including short-chain fatty acids that feed the cells in your colon.

This leads to reduced gut inflammation and improvements in related inflammatory disorders (11, 12, 13).

Just to clarify, acute (short-term) inflammation is beneficial because it helps your body fight foreign invaders and repair damaged cells.

However, chronic (long-term) inflammation is a serious problem because it may begin to combat your body's own tissues.

Chronic, low-level inflammation plays a major role in almost every chronic Western disease, including heart disease, Alzheimer's and metabolic syndrome (14, 15, 16).

There is also mounting evidence that inflammation is associated with weight gain and obesity (17, 18, 10).

Several observational studies demonstrate that a high fiber intake is linked to lower levels of inflammatory markers in the bloodstream (19, 20).

Summary Inflammation is associated with many lifestyle diseases, including obesity. Fiber consumption has been linked to reduced inflammation.

Viscous fibers occur exclusively in plant foods.

Rich sources include beans and legumes, flax seeds, asparagus, Brussels sprouts and oats.

If you're planning to switch to a high-fiber diet, remember to do it gradually to give your body time to adjust.

Abdominal discomfort, cramps and even diarrhea are common side effects if you ramp up your fiber intake too quickly.

Summary Viscous, soluble fiber is only found in plant foods. Whole plant foods such as beans, asparagus, Brussels sprouts and oats are rich in viscous fiber.

Fiber supplements are typically made by isolating the fiber from plants.

While these isolated fibers may have some health benefits, the evidence for weight control is mixed and unconvincing.

A very large review study found that psyllium and guar gum — both soluble, viscous fibers — are ineffective as weight loss supplements (26).

One notable exception is glucomannan, a fiber extracted from the konjac root.

This incredibly viscous dietary fiber causes modest weight loss when used as a supplement (27, 28, 29).

However, supplementing with isolated nutrients rarely makes much difference on its own.

For the greatest impact, you should combine fiber supplements with other healthy weight loss strategies.

Although glucomannan and other soluble fiber supplements are a good option, it’s best to focus your diet on whole plant foods.

Summary Fiber supplements are usually ineffective for weight loss — with the exception of glucomannan. However, getting your fiber from whole plant foods is better than supplementing.

Eating more foods rich in fiber — especially viscous fiber — can be an effective strategy to lose weight.

However, like many weight loss methods, it won't lead to long-term results unless you pair it with a lasting lifestyle change.

Keep in mind that fiber supplements likely have less of an overall health impact than fiber-rich whole foods.

Additionally, don’t forget that health isn’t all about body weight. Eating plenty of fiber from real foods can have numerous other health benefits.