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 (soluble) or not (insoluble).
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.
This article explains how soluble fiber may promote weight loss.
Fiber feeds your friendly gut bacteria
An estimated 100 trillion bacteria live in your gut, primarily in the large intestine.
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.
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, which 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.
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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.
Good bacteria help fight inflammation
Gut bacteria are well known for their effect on chronic inflammation.
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.
While acute (short-term) inflammation is beneficial because it helps your body fight foreign invaders and repair damaged cells, chronic (long-term) inflammation is a serious concern because it may begin to attack your body’s tissues.
Chronic low-level inflammation plays a major role in almost every chronic Western disease, including heart disease, Alzheimer’s disease, and metabolic syndrome.
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Several observational studies demonstrate that a high fiber intake is linked to lower levels of inflammatory markers in the bloodstream.
Summary: Inflammation is associated with many lifestyle conditions, including obesity. Fiber consumption has been linked to reduced inflammation.
Viscous fiber may reduce your appetite, helping you eat less
You need to be in a calorie deficit to lose weight. This means that more calories (energy) need to be leaving your body than entering it.
Counting calories helps many people — but it may not be necessary if you choose the right foods.
Anything that reduces your appetite can decrease your calorie intake. With less appetite, you may lose weight without even having to think about it.
Fiber is often believed to suppress your appetite. However, evidence suggests that only a specific type of fiber has this effect.
A recent review of 44 studies showed that while 39 percent of fiber treatments increased fullness, only 22 percent reduced food intake.
The more viscous the fiber, the better it is at reducing appetite and food intake. The viscosity of a substance refers to its thickness and stickiness. For example, honey is much more viscous than water.
Viscous soluble fibers such as pectins, beta-glucans, psyllium, glucomannan, and guar gum all thicken in water, forming a gel-like substance that sits in your gut.
This gel slows the emptying of your stomach, increasing digestion and absorption times. The result is a prolonged feeling of fullness and a significantly reduced appetite.
Some evidence indicates that the weight loss effects of fiber specifically target belly fat, which is the harmful fat in your abdominal cavity strongly associated with metabolic disease.
Summary: Fibers with high viscosity may provide increased fullness, reduced appetite, and automatic weight loss. Fibers with low viscosity appear to not influence these factors.
Are fiber supplements effective for weight loss?
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.
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A very large review study found that psyllium and guar gum — both soluble, viscous fibers — are ineffective as weight loss supplements.
One notable exception is glucomannan, a fiber extracted from the konjac root.
This incredibly viscous dietary fiber shows promise for weight reduction in people with overweight and obesity.
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 — except for glucomannan. However, getting your fiber from whole plant foods is better than supplements.
Rich sources of viscous fiber
Viscous fibers occur exclusively in plant foods. Rich sources include:
- beans and legumes
- Brussels sprouts
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.
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Summary: Viscous, soluble fiber is found only in plant foods. Whole plant foods such as beans, asparagus, Brussels sprouts, and oats are rich in viscous fiber.
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.Last updated on January 5, 2022, and last reviewed by an expert on December 23, 2021.