Why is Fiber Good For You? The Crunchy Truth
"Eat more fiber."
The health authorities constantly tell us to eat fiber.
They want us to load up on fruits, vegetables, whole grains and legumes... all fiber rich foods.
Doing this, they say, will help lower cholesterol, relieve constipation and prevent all sorts of diseases.
But the actual research doesn't back up many of these claims.
Even though fiber has some important benefits, many of the health claims turned out to be false when they were put to the test (1).
Fiber is definitely overrated, but it is still an important part of a healthy, real food based diet. Let me explain why...
Put simply, dietary fiber is indigestible carbohydrate found in foods.
Classically, fiber is split into two categories based on its solubility in water:
- Soluble fiber dissolves in water and can be metabolized by the "good" bacteria in the gut.
- Insoluble fiber does not dissolve in water.
Another more useful way to categorize fiber is "fermentable vs non-fermentable" - that is, whether the friendly bacteria in the gut can use it or not.
It's important to keep in mind that there are many different types of fiber, just like there are many different types of fats.
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.
The health authorities recommend that women eat 25 grams of fiber per day and that men eat 38 grams per day.
Did you know that your body is really just 10% human?
That's because the bacteria that live in the body outnumber the body's cells 10 to 1.
Bacteria live on the skin, in the mouth and nose... but the great majority lives in the gut, primarily the large intestine (2).
There are about 500 different species of bacteria living in the intestine, total number about 100 trillion. The bacteria in the gut are also known as the gut flora.
This is not a bad thing... there is a mutually beneficial relationship between us humans and the bacteria. We provide the bacteria with shelter and a safe habitat, instead they take care of some things that the human body can not do on its own.
There are many different kinds of bacteria... and the type (different species) of bacteria can have a dramatic effect on various aspects of health, including weight, blood sugar control, immune function and even brain function (3, 4, 5, 6, 7).
But what does that have to do with fiber?
Well... like other organisms, bacteria need to eat. They need to get energy from somewhere in order to survive and function.
The problem is that most carbs, proteins and fats get absorbed into the bloodstream before they make it to the large intestine. There is nothing left for the gut flora.
This is where fiber steps in... humans don't have the enzymes to digest fiber and therefore it reaches the large intestine relatively unchanged.
However, the 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 (8).
That way, they increase our levels of the "good" bacteria, which can have various possitive effects on health (9).
These short-chain fatty acids can feed the cells in the colon, leading to reduced inflammation in the gut and improvements in various digestive disorders like Irritable Bowel Syndrome, Chron's disease and Ulcerative colitis (11, 12, 13).
When the bacteria ferment the fiber, they also produce gases. This is the reason high-fiber diets can cause flatulence and stomach discomfort, but this usually goes away with time as your body adjusts.
Bottom Line: 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.
There is conflicting evidence about whether fiber can help people lose weight or not.
Some fibers bind water in the intestine, which can slow absorption of nutrients and increase feelings of fullness (14).
Bottom Line: Some types of fiber can cause weight loss by increasing feelings of fullness and leading to reduced calorie intake.
Foods that have fiber in them tend to have a lower glycemic index (21).
What this means is that they cause smaller spikes in blood sugar after a carbohydrate containing meal (22).
This can be important... but only if you're eating a high-carb diet. In these cases, the fiber can reduce the likelihood of the carbs spiking your blood sugar to harmful levels.
But really... if you have blood sugar issues, then it would make much more sense just to skip the carbs instead of trying to minimize the damage using fiber.
Bottom Line: Foods that contain fiber have a lower glycemic index and cause smaller spikes in blood sugar than foods that are low in fiber.
Some types of fiber can reduce blood cholesterol levels.
However, the effect isn't nearly as impressive as you may think.
A review of 67 controlled trials found that 2-10 grams of soluble fiber per day reduced Total cholesterol by 1.7 mg/dl and LDL cholesterol by 2.2 mg/dl, on average (23).
Whether this has any meaningful effects in the long term is not known, although many observational studies do show that people who eat more fiber have a lower risk of heart disease (27).
Bottom Line: Some types of fiber can reduce cholesterol levels, although the effect isn't very large on average.
One of the main purported benefits of fiber is reduced constipation.
Fiber is claimed to help absorb water, increase the bulk of the stool and speed up movement of stool through the intestine.
Given how almost every health professional believes that fiber can help with constipation, you would think that there was strong evidence behind it.
Some studies show that adding fiber can improve symptoms of constipation, but other studies show that removing fiber improves constipation.
In one study of 63 individuals with chronic constipation, going on a low-fiber diet actually fixed their problem.... the individuals who remained on a high-fiber diet saw no improvement (30).
According to one review of 6 studies, soluble fiber can help with constipation, while insoluble fiber has no effect (31).
For this reason, I think it is questionable to recommend fiber to everyone with constipation. It may help some people, but it can make matters worse for others.
Bottom Line: The evidence about fiber helping with constipation is surprisingly weak and the studies do not agree. This appears to depend on the individual, as well as the type of fiber.
There were some initial studies showing that fiber was associated with a lower risk of colorectal cancer, but higher quality studies haven't found any link (33).
The health benefits of fiber are not as clear cut as you might think, given how enthusiastically many nutritionists recommend that we eat it.
However, it is definitely true that fiber-rich foods tend to be healthier than low-fiber foods. But that's because high-fiber foods tend to be whole, unprocessed foods... which are healthy for many other reasons.
The recommended amount of 25-38 grams of fiber per day may be excessive. There is no evidence that eating less fiber than that has any harmful effects.
For this reason, I don't think there is any reason to load up on whole grains or legumes to get more fiber into your diet... eating plenty of vegetables (and maybe some fruit) should be more than sufficient.
At the end of the day, fiber appears to be overrated.
But fiber is still an essential part of a healthy diet... if only for the sake of feeding the little guys in the intestine that are so important for the optimal function of our bodies.