Fiber is a well known nutrient, but not well understood.
Put simply, fiber refers to carbohydrates that cannot be digested by humans.
They are classified as either soluble or insoluble, depending on whether they dissolve in liquids.
Insoluble fibers function mostly as "bulking" agents and are not very interesting.
However... soluble fiber can have powerful effects on health and metabolism (1).
Several studies show that soluble fiber can help you lose fat... but this fiber must have certain properties.
Let me explain...
It is estimated that around 100 trillion bacteria live in the human gut, primarily in the large intestine (2).
These bacteria are known as the gut flora.
Having bacteria in the gut may sound undesirable, but this is actually a good thing.
Just like other organisms, bacteria need to eat well to stay healthy.
This is where fiber (mostly soluble) steps in... it passes through the digestive system mostly unchanged, eventually reaching the friendly bacteria in the intestine who end up digesting the fiber and turning it into usable energy.
Another type of fiber called resistant starch behaves in a similar manner.
Bottom Line: Fiber does not get digested and tends to reach the large intestine relatively unchanged. There, certain soluble fibers can help feed the friendly bacteria that are absolutely essential for good health.
Gut bacteria have long been known to have an effect on inflammatory pathways (10).
They produce nutrients for the body, including short-chain fatty acids that feed the cells in the colon.
Just to clarify, acute (short-term) inflammation is beneficial as it helps the body fight foreign invaders and repair damaged cells.
However, chronic (long-term) inflammation becomes a major problem when it is inappropriately deployed against the body's own tissues.
Bottom Line: Inflammation is a strong driver of disease, including obesity. Fiber consumption has been linked to reduced inflammation..
While I don't believe that counting calories is necessary, it is a simple fact that we need to be in calorie deficit to lose weight.
That is, more calories (energy) need to be leaving the body than entering it.
Anything that reduces our appetite can make us take in fewer calories without having to think about it.
Fiber is often believed to have this effect... that is, making us feel more satiated so that we eat less.
However, according to the evidence, only a specific type of fiber does this.
A recent review of 44 studies found that while 39% of fiber treatments increased satiety, only 22% actually reduced food intake (21).
If we break it down further, it seems that the more viscous a fiber is, the better it is at reducing appetite and food intake.
Put simply, the viscosity of a substance refers to its resistance to stress - as in, the "thickness" of a liquid. For example, honey is much more viscous than water.
Viscous soluble fibers such as pectins, β-glucans, psyllium, glucomannan and guar gum all thicken in water, forming a gel-like substance that "sits" in the gut (22).
This gel slows down the emptying of the stomach and increases the time it takes to digest and absorb nutrients. The end result is a prolonged feeling of fullness and a significantly reduced appetite (23, 24).
There is some evidence that the weight loss effects of fiber target the belly fat specifically, which is the harmful fat in the abdominal cavity that is strongly associated with metabolic disease (25).
Bottom Line: Fibers with a high viscosity have been shown to increase satiety, reduce appetite and cause automatic weight loss. Fibers with low viscosity appear to have no influence.
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 pretty 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.
But in the real world, it's the synergy of all the nutrients in foods that provides the most benefits, consuming isolated nutrients will never have the same effects.
For this reason, getting your fiber from whole plant foods is best.
Bottom Line: Fiber supplements are mostly ineffective for weight loss, with the exception of a powerful type of fiber called glucomannan. Getting your fiber from whole plant foods is the best and healthiest approach.
Viscous fibers are found exclusively in plant foods.
Rich sources include beans (legumes), flax seeds, asparagus, brussels sprouts and oats, to name a few.
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.
Adding more foods rich in fiber (especially viscous fiber) can be an effective way to lose weight.
But, same as with any other weight loss method, it won't lead to long term results unless it comes with a lasting lifestyle change.
Also, let's not forget that health is about way more than just weight. Eating plenty of fiber from real foods can have numerous other health benefits.