There was a time when fiber seemed like the magical nutrient.
Health professionals touted the benefits of whole grains. Eating fiber-rich foods could help you lower your cholesterol and improve your heart health, not to mention your digestive health.
However, studies show that fiber isn’t necessarily all it’s cracked up to be.
Don’t get me wrong; fiber does have some health benefits. However, you might be surprised when you find out what they are.
For example, fiber might not be as beneficial as some people think when it comes to diseases of the colon or even constipation (1).
Find out the ways in which fiber can benefit your health as well as some of the myths surround it.
Fiber 101
What is fiber? It’s a carbohydrate that is found in foods. When you eat it, your body can’t digest it.
There are two types of fiber, soluble and insoluble fiber.
Soluble Fiber
Soluble fiber dissolves in water. However, it creates a thick sludge that can prevent nutrients from being absorbed in the small intestine and impede mobility of waste through the digestive tract (2).
While it may prevent dietary cholesterol from being absorbed through the intestines, soluble fiber can also hinder the enzymes necessary to help you digest food properly (3).
Foods that are high in soluble fiber include (4):
- Legumes
- Grains
- Fruit
- Vegetables
- Flaxseeds
Insoluble Fiber
Insoluble fiber cannot be digested at all. It passes through the gut largely unchanged. The more you eat, the more you must pass as stool.
Insoluble fiber can also ferment as it passes through the colon. This can lead to gas and bloating.
Foods that are high in insoluble fiber include (5):
- The outer husk or peel of many fruits and vegetables
- Nuts
- Barley
- Flaxseed shell
- Oat and Oat bran
Resistant starch is a type of fiber that cannot be broken down by enzymes in the gut.
However, when you eat more foods with resistant starch, you don’t have more stools. This indicates that resistant starch may be completely fermented by microbes in the gut (6).
Resistant starch can serve as a prebiotic. This means that it can supply food to the beneficial bacteria in the digestive tract.
However, the fermentation process can lead to gas and bloating.
Some foods that contain resistant starch are (7):
- Seeds
- Nuts
- Legumes
- Grains
- Raw potatoes
- Green bananas
- Potatoes and rice that have been cooked and cooled.
Pure insoluble fiber has no nutritional value, because it cannot be absorbed at all. However, many foods contain all three of these types of fiber.
According to the National Academy of Sciences, men ages 50 and under should consume about 38 total grams of fiber a day. Women of the same age should eat 25 grams. The recommended intake for people over 50 is 30 grams for men and 21 grams for women.
Fiber Helps Feed Beneficial Bacteria
Your body is filled with trillions of cells that don’t belong to you.
These cells make up the bacteria that live all over your body, including on your skin, in your mouth and in your nose. The majority of these microbes live in your intestines (8, 9).
The bacteria that make up your gut flora are there to help the body perform numerous functions.
They synthesize certain vitamins, help the walls of the intestines remain healthy and block out unwanted substances and send out antimicrobial agents to combat harmful bacteria (10, 11, 12, 13, 14, 15).
Gut flora can also affect your weight, ability to metabolize blood sugar, immunity and cognitive function (16, 17, 18, 19, 20).
Those bacteria must be fed in order to function properly. They can get energy from the foods that you eat.
However, most of the nutrients in your diet are absorbed so that your body can use them as fuel.
The fiber that can’t be digested by the body continues through the digestive tract to provide food for the beneficial bacteria in the gut.
Your body may not be able to digest certain fibers, but the bacteria in your gut can. By feeding those good bacteria, certain fibers serve as prebiotics (21).
Probiotics are the live microbes that can colonize your intestines. Prebiotics are the food that the probiotics need to live. Prebiotics can actually help bifidobacteria and Lactobacillus, the good bacteria, proliferate (22).
As fiber is fermented by the friendly bacteria, it produces short-chain fatty acids, or SCFAs. These include (23):
- Acetate: the main SCFA in the colon; increases the creation of cholesterol.
- Propionate: used mainly by the liver; reduces the creation of cholesterol.
- Butyrate: nourishes the colonic mucosa; may help protect the colon against cancer
These SCFAs improve the health of the cells in the large intestine and have been linked to reduced inflammation.
SCFAs can also help improve symptoms of gastrointestinal conditions, such as irritable bowel syndrome, Crohn’s disease and ulcerative colitis (24, 25, 26).
As the bacteria ferment fiber, gas is released. If you feel bloated or experience flatulence after eating high-fiber foods, now you know the reason.
KEY POINT: Fiber can feed the good bacteria in the gut that have been shown to have many health benefits.
Fiber May Help You Lose Weight
Research about fiber and weight loss is controversial.
As soluble fiber binds water in the intestines, it slows digestion. This can help you feel full for a longer amount of time (27).
The idea behind this as a mechanism for weight loss is that if you feel fuller, you may eat less. Lowering calorie intake can help you lose weight.
In fact, studies show that eating just 14 more grams of fiber per day for more than two days can result in a 10% reduction in the number of calories consumed (28).
There is significant research that points to lower weight and less body fat in people who eat more fiber. No matter how much fat people eat, they tend to have a lower body mass index when they eat more fiber compared to people who don’t consume enough roughage (29).
One study showed that participants who ate a meal that contained fiber and protein ate the same amount as the participants who ate mainly protein. However, people who ate the meal with fiber and protein ate less during their next meal than the other group (30).
Different types of fiber have different effects on weight loss, however:
- One study found that guar gum doesn’t seem to reduce body weight (31).
- Another study found that guar gum did lower hunger and reduce calorie intake (32).
- Psyllium husk and glucomannan (konjac) can help you lose weight (33).
KEY POINT: Some kinds of fiber can help you stay full after eating, leading you to eat less later.
Fiber Can Help You Manage Your Blood Sugar
Foods that contain carbohydrates are rated by how they affect your blood sugar levels. This is called the glycemic index (34).
Although the importance of the glycemic index in managing blood glucose is controversial, it can still be a useful tool.
Some types of fiber can lower the glycemic index of food (35).
Because insoluble fiber doesn’t seem to affect the glycemic index much, a slice of whole-wheat bread is rated similarly to white bread. However, foods with a lot of soluble fiber, like oats and beans, are lower on the glycemic index.
Even so, many studies show that it’s insoluble fiber that’s associated with a lowered risk of diabetes (36).
Carbohydrates that have lower glycemic index levels are metabolized better by the body and don’t cause blood sugar levels to increase rapidly. This can be important if you’re eating an imbalanced diet that relies mainly on carbohydrates (not healthy by the way).
Instead of adding fiber to a high carbohydrate diet, people who have problems with blood sugar should preferably be limiting their carbohydrates in the first place.
KEY POINT: Foods that are high in soluble fiber can help prevent your blood sugar from spiking if you’re eating large amounts of carbohydrates without balancing them with proteins and fats.
Fiber May Have Minor Cholesterol-Lowering Properties
Many studies have linked some kinds of fiber with lowered cholesterol levels. However, that’s not as big a deal as you may think.
Bear in mind, there is no concrete evidence that lowering cholesterol will help you live longer (37).
Researchers looked at 67 controlled trials to determine just how significant fiber can lower total cholesterol. On average, adding 2 to 10 grams of soluble fiber to the diet each day only reduced total cholesterol by 1.7 mg/dl (38).
These researchers concluded that it’s impractical to add enough fiber to the diet to make a significant difference in cholesterol levels.
On the other hand, some studies have found significant connections between certain types of fiber and cholesterol. In one study, people taking glucomannan (konjac) without changing their lifestyle or diet reduced their LDL levels by more than 7% (39).
Another study found significant cholesterol-lowering effects of fiber. However, the researchers admitted that the amount of fiber that the participants consumed was higher than that which is normally recommended (40).
One study looked at a very specific type of fiber that is unrealistic to add to the general population’s diet (41).
Researchers aren’t sure exactly how fiber acts upon cholesterol or whether this is a meaningful health benefit. However, several observational studies have connected people who eat high-fiber diets with a reduced risk of cardiovascular disease (42).
KEY POINT: Although some types of fiber can lower total cholesterol levels, researchers aren’t sure if this is a meaningful health benefit. The reduction is on average minor.
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Truth About Fiber and Constipation
Many advertisements claim that fiber’s ability to improve stool regularity is a great benefit.
The idea is that if you’re constipated, consuming soluble fiber can help your stool absorb water.
If the stool is heavier and bulkier, your intestines will move it out of your body more quickly. Insoluble fiber adds more waste product to your intestines, making you have to eliminate more frequently as well.
Although this is the way the intestines work in people with normal bowel movements, bulkier stool may not improve frequency in constipated people (43).
The evidence behind this theory is somewhat weak.
One study found that there were no effects of fiber on constipation in elderly participants (44), though this study had several flaws.
A meta-analysis of numerous studies found that although fiber may increase stool frequency in constipated individuals, it may not work to soften the stool or make passing bowel movements less painful (45).
Other researchers have found that while high-fiber diets don’t improve symptoms of constipation, low-fiber diets do (46). In this study, people who stopped eating fiber altogether began having bowel movements every day instead of every 3.75 days.
A systematic review of six different studies found that soluble fiber may help people with chronic constipation. However, results from insoluble fiber are conflicting (47).
Most researchers agree that more randomized controlled trials need to be conducted in order to recommend a high-fiber diet to people with constipation.
KEY POINT: Although most people associate fiber with laxative effect, studies show that it may not be as beneficial as expected in improving constipation.
Fiber Has Not Been Proven to Reduce The Risk of Colorectal Cancer
Colorectal cancer is one of the 5 most common types of cancer in both males and females (48).
Some experts believe that dietary fiber can reduce the risk of colorectal cancer by absorbing carcinogens, improving bowel movement frequency, changing the pH of the colon and increasing the level of short-chain fatty acids in the gut (49).
Although some early observational studies linked fiber with a reduced risk of this type of cancer, better quality studies were inconsistent (50). Most people with an increased risk of colorectal cancer also had risk factors other than a low-fiber diet.
Other studies have also found little evidence of a protective effect of fiber on colorectal cancer (51, 52, 53).
KEY POINT: Although initial research showed promising effects of fiber on the prevention of colorectal cancer, newer studies have not strengthened a correlation.
Bottom Line
Fiber may not be the digestive cure-all that many doctors, nutritionists and advertisements report it to be. However, it’s still important to consume a balanced diet that includes enough fiber.
Many foods that contain fiber also contain other beneficial nutrients. In addition, as foods become more processed, more of the fiber is removed. Therefore, when you’re eating high-fiber foods, you’re often consuming more unrefined nutrients.
Do you have to consume the suggested 25 to 38 grams of fiber each day? Not necessarily. In fact, eating too much fiber can be dangerous (54).
Instead of forcing yourself to consume a great deal of fiber, focus on eating vegetables with every meal. Have fruit as a snack. If you do that, you should consume enough fiber for your body and your health.
How are you incorporating fiber into your diet? Let me know in the comments below.