Fiber is an amazing compound that not only keeps you full, but keeps you regular, controls insulin and keeps your good bacteria happy and healthy.
What most people don’t know is that fiber is a actually a form of carbohydrate. Unlike other carbohydrates, such as sugar or starch, it is not digested in the small intestine. Instead, it travels to the right side of the colon (known as the cecum) and is fermented by bacteria.
There are two forms of fiber: soluble and insoluble. Soluble fiber is fiber that can dissolve in water, this fiber is fermented in the colon and is responsible for satiety by slowing gastric emptying. Insoluble fiber is fiber that does not dissolve in water, this is the fiber that is classed as “bulky”.
It is used to absorb water, and is effective at improving stool consistency.
Most of the fiber that we consume during the day is from our diet. Foods such as wholegrain bread, cereals, oats, and pasta are common sources of fiber. Other examples of high fiber foods include: fruits, vegetables, nuts and seeds, and pulses (beans, lentils and peas).
The recommended amount of fiber per day is around 15-20g per 1,000 calories, so if you are supposed to be consuming 2,000 calories you should have 30-40g of fiber (1).
According to a study in the Journal of the Academy of Nutrition & Dietetics the average fiber intake for Americans was 15.6g per day between 1999 and 2008(2).
You’d think that the increased popularity of diet books over recent years would have helped. But actually the reverse has happened. Thanks to some popular diet scams, dietary fiber levels are even lower than normal as carbohydrates have been vilified (3).
Many people could benefit from increasing their intake of dark green leafy vegetables, pulses, nuts and wholegrain sources (the highest sources for dietary fiber). But for people with poor nutrition, fiber supplementation may be an option. Natural supplements such as chia seeds and psyllium husk/powder are the most commonly used.
1Dietary fiber can reduce cardiovascular disease (CVD) risk. There seems to be a large amount of evidence that backs up the claim that dietary fiber can reduce your risk of cardiovascular disease, but the mechanism for how is still unknown.
There are quite a few possibilities. Fiber can help lower cholesterol, reduce your risk of strokes, lower blood pressure, help with weight loss, and possibly prevent inflammation.
A 2002 study found that a higher intake of dietary fiber led to a reduced risk of CVD and of myocardial infarction (4).
Another study in 2003 found that consuming more fiber (particularly water soluble fiber) reduces the likelihood of coronary heart disease (5).
A 1999 study found that a high fiber diet could protect against obesity and CVD by lowering insulin levels (6).
A 2006 study compared the effectiveness of dietary fiber and fiber supplements.It found that while fiber supplements were more effective at reducing CVD risk factors, there was a lower incidence of CVD linked to diets that featured a lot of food-based fiber primarily from whole grains, fruits and vegetables (7).
One of the possible reasons why fiber is so effective at protecting the heart, is the fact that fiber is protective against high C-reactive protein (8).
C-reactive protein (CRP) is produced in the liver as a response to inflammation, and many doctors see high CRP levels as an indicator of a possible heart attack or stroke. Studies have found that fiber can reduce C-reactive protein levels and this could be responsible for the protective effect it has on the heart (9).
2Fiber may help with weight management. There is quite a lot of evidence that high fiber diets can help with weight management, but not too much consensus on how exactly. It could be due to increased satiety, studies have shown that foods containing resistant starch and corn bran can help treat obesity (10, 11).
Fiber could also help reduce absorption of macronutrients, which is sort of what some carb blocking supplements claim.
Whatever the reasons, increasing fiber does seem to help people lose weight. The most likely reason for this is increased satiety, but also the transformation of a diet that has very low fiber to one that is higher in fiber is also more nutrient-rich.
3Fiber may help improve gut health through increasing “friendly” bacteria. Because fiber isn’t digested in the small intestine it is left in the colon and ferments.
This creates a perfect environment for gut microbiota, and could explain why fiber has been repeatedly linked with an increase in friendly bacteria, and improved gut health (12).
A study in Science (2011) found that the microbiome of 10 subjects changed within 24 hours of ingesting a low fat/ high fiber diet (13).
A study that looked at the mechanism behind dietary fibers effect on the microbiome, they hypothesized that “dietary fiber resists digestion in the small intestine, and enters the colon where it is fermented to produce [short chain fatty acids] that may enhance the healthy composition of gut microbiota.” (14).
4Fiber can regulate blood sugar and prevent diabetes. Fiber helps to control blood sugar levels by slowing absorption of sugars. Because of this, high fiber diets are promoted as a preventative measure for type II diabetes (15).
This diet is also used to help people who already have type II diabetes as fiber helps “improve glycemic control, decreases hyperinsulinemia, and lowers plasma concentrations” (16).
5Increasing fiber can help improve chronic constipation. Chronic constipation affects 20% of the population. A study in The American Journal of Gastroenterology (2004) estimated that it affected 63 million people in North America alone (17).
While it is not as simple as throwing fiber at the problem, studies have indicated that increasing fiber can help improve chronic constipation, though this depends on what the cause of the constipation was in the first place.
A study on children with chronic constipation found that not consuming the recommended daily amount of fiber was a risk factor for chronic constipation (18).
A meta analysis of the effects of dietary fiber on chronic constipation found that while fiber was effective at increasing stool frequency, it did not improve “stool consistency, treatment success, laxative use, and painful defecation” in the five included studies (19).
In other words, fiber can help, but it won’t solve the problem completely.
A study on elderly people with chronic constipation came to the same conclusion. While fiber helped improve their situation and sped up colonic transit time, it did not cure the underlying cause (20).
6Fiber can reduce your risk of strokes. Following on from the last point, there is also a lot of evidence that dietary fiber is effective at stroke risk reduction. A 2013 study by Threapleton found that high fiber intake was “significantly associated with lower risk of first stroke” (21).
Another study, in 2005 found that while diets that were high in refined carbohydrates increased stroke risk, a “high consumption of cereal fiber was associated with lower risk of total and hemorrhagic stroke” in women (22).
The studies all found that fiber was a contributing factor, but so was magnesium, calcium and potassium (25).
After reviewing the evidence, it seems apparent that the evidence for fruits, vegetables, and grains being a good protection against strokes is good (26). Supplementing with fiber should also be an effective strategy.
7LDL-cholesterol, and total cholesterol are both lowered by high fiber diets. There seems to be a lot of evidence that fiber can help reduce LDL or “bad” cholesterol, and lower total cholesterol levels. Soluble fibers such as β-glucan have been shown to reduce serum cholesterol in people with high cholesterol levels (27).
But it wasn’t just β-glucan that had a positive effect on the reduction of total cholesterol, a meta-analysis in the American Journal of Clinical Nutrition found that “various soluble fibers reduce total and LDL cholesterol by similar amounts” (28).
Fiber reduces cholesterol by reducing the amount of bile absorbed in your intestines. The bile is then eliminated through stool which causes an increase in bile salts production, made from cholesterol. LDL receptors are increased to capture more cholesterol from the blood stream to use for the bile salts (29). The result is overall reduced cholesterol levels.
8Fiber may prevent colorectal cancer (also known as bowel cancer). The debate about whether fiber helps reduce colorectal cancer risk or not has still to be settled. While older studies failed to find substantial evidence that dietary fiber protects against colorectal cancer, newer research is more convincing (30, 31).
A study on Finnish men in 1999 found that dietary fiber was not associated with a reduced risk of colorectal cancer (32).
On the other hand, a study in 2007 found that intake of fiber-rich foods is inversely associated with colon cancer in men (33).
Probably the best explanation of what is going on is in this 2001 study in the Journal of the National Cancer Institute (34).
It found that “individuals who consume very low amounts of fruit and vegetables have the greatest risk of colorectal cancer” but that consuming relatively high amounts of fiber from non-vegetable or fruit sources did not lower the risk. In other words, fruits and vegetables are irreplaceable for long-term health.
Another study found that there was a correlation between dietary fiber intake, and reduced risk of colorectal cancer (35).
However, when other lifestyle factors were taken into account the correlation became non-significant. People who have bad diets are more at risk of colorectal cancer, and they also have low fiber, but this does not necessarily equal that high fiber intakes are effective at protecting against cancer. Other lifestyle factors are likely also to influence risk.
A 2007 study found that increased whole grain consumption was associated with a small reduction in cancer risk, but that total fiber intake was not (36).
While a further meta-analysis found evidence that “the majority of studies gave support for a protective effect associated with fiber-rich diets” (37).
While the esophagus is certainly higher in the gastrointestinal tract that the colon, research supports a high fiber diet for reduced risk of esophegeal cancer (38).
On the whole, the evidence for dietary fiber preventing colorectal cancer is slim. But increasing your fiber intake if your current level is low would be a really good idea. High fiber may not prevent colon cancer by itself, but low fiber especially from whole grains, fruits and vegetables as well as other unhealthy lifestyle choices may increase your risk for many diseases.
Finally, we’ll take a quick look at four common fiber supplement ingredients: Chia seeds, Psyllium, and Yacon.
- Chia Seeds create appetite suppression and lower blood glucose (39)
- Psyllium reduces LDL and total cholesterol (40, 41, 42)
- Yacon reduces LDL cholesterol and body weight (43). It also increases the speed of food transport through the body (intestinal motility). It’s good for people with chronic constipation (44).
- Chicory root inulin is a prebiotic fiber that feeds healthy gut bacteria and may improve constipation (45).
1Fiber can sometimes worsen chronic constipation. In an article published in the Journal of Gastroenterology the authors write: “A diet poor in fiber should not be assumed to be the cause of chronic constipation. Some patients may be helped by a fiber-rich diet but many patients with more severe constipation get worse symptoms” (46).
2High fiber diets may lead to nutritional deficiencies. Studies have found that a high fiber diet, without heme iron sources (like red meat) in the diet as well, can actually inhibit the absorption of iron and zinc (47). This is due to the presence of anti-nutrients like phytates, lignans and other substances found in high fiber foods that reduce the absorption of some nutrients.
A 1992 study found that these anti-nutrients, more than the fiber present in the bread, prevented the absorption of iron. The study also determined that fermentation significantly reduced the anti-nutrients’ effect on iron absorption (48).
A supplement (not diet) study found that certain fibers can reduce the absorption of carotenoids (an organic pigment that is found in foods such as carrots and tomatoes which can help protect against certain cancers) (49).
Currently the American National Academy of Sciences and the European Commission recommends 38g of fiber per day for men who are between 14 and 50 years old, and 30g for men 50+ years old. Women 19-50 years old should consume 25g per day while women 50+ years old should consume 21g (50, 51) .
Does fiber help you poop? Insoluble fiber adds bulk to your stool, helping the stool pass more quickly through the intestines.
Does fiber block protein absorption? No studies to date have confirmed fiber’s effect on protein absorption, but it’s likely a small effect.
Does fiber make you fart? An increase in total fiber, especially a jump too quickly can cause gas and bloating. The best rule of thumb is to take it slow and gradually increase your fiber intake.
Does fiber make you fat? Dietary fiber doesn’t make you gain weight, but it may help you lose weight by increasing satiety and nourishing gut bacteria.
Does fiber cancel out sugar? While fiber does not cancel out carbs or sugar, high-fiber foods are typically digested slower, which makes them less likely to spike insulin or be stored as body fat.
Does fiber count as calories? There’s no clear consensus on fiber’s caloric contribution since it often cannot be digested and absorbed in the small intestine like other nutrients. Fiber does feed gut bacteria so there is potential energy or calories that may be discovered in future studies. The calorie intake is likely very minimal though compared to other macronutrients.
Which foods are high in fiber? Foods that are good sources of fibre include most beans, lentils, peas, whole fruits, potatoes with skin, chia seeds, cruciferous vegetables (broccoli, cauliflower, cabbage, kale), nuts, seeds, quinoa, barley, avocado and dark-leafy greens.
The debate on how effective high fiber diets are on a number of conditions will rage on and on, but the simple fact is that a diet that is low in fiber has been shown to increase risk of cancer, CVD, chronic constipation, and diabetes.
Whether that is due to the lack of fiber, or due to poor overall nutrition that happens to include low fiber is irrelevant. Every adult should be aiming to hit their daily target of at least 18g of fiber with high fiber food sources.
Though fiber supplements definitely seem to have their place, you should really be concentrating on improving your fiber intake through diet. Excellent fiber-rich sources include legumes, vegetables, berries, nuts, seeds, and some whole grains. A low carb diet has its place in weight and disease management, but it may also result in a low fiber diet if not carefully planned. This is true for ketogenic diets as well as others.
A high fiber diet, one where you are consuming significantly more than the recommended daily intake should be reconsidered. There appear to be no significant benefits to increasing fiber past your recommended levels, and there could even be some side effects.
The effect of fiber on the absorption of iron should be of particular concern to women who are menstruating or pregnant as iron is an important micronutrient that prevents anemia.
As is almost always the case, people hear that there are benefits to a type of food or supplement and decide to overdo it. Instead of just hitting their targets for the day, they believe that doubling their intake will double the benefits. Nutrition is never that simple.