Fiber - Part Of A Healthy Diet
Fiber is a plant-based nutrient that our body cannot digest or absorb. They mainly belong to the group of complex carbohydrates. Unlike macronutrients, fiber does not provide energy. Fiber passes through the stomach, small and large intestines and is excreted undigested. They are primarily needed for a healthy digestion and to promote regular and soft bowel movements. They are mainly found in grain products, nuts, legumes, as well as in fresh fruits and vegetables (see Table of Fiber-rich Foods).
Types of Dietary Fiber
Below are the most important types of dietary fiber in plant-based foods:
- Plant gums
- Resistant starch
Dietary fiber is classified into different groups. The most well-known are soluble and insoluble fiber, which are usually combined in most foods. It used to be believed that soluble fiber formed more viscous gels and could be processed more easily by intestinal bacteria 1. However, newer research has shown that the solubility of fiber is not a reliable indicator of its physiological effects on intestinal bacteria. Therefore, newer subdivisions of dietary fiber have been made.
Insoluble fiber speeds up the passage of food through the digestive system, which increases the regularity of bowel movements (preventing constipation). This is because it binds to water in the colon 2. The result is that the food swells up, signaling the intestine that it is full and initiating the process of elimination. This, in turn, can help prevent diverticulosis and hemorrhoids. Additionally, it is believed that the increased bowel movements help to expel toxins and cancer-causing carcinogens 3.
Water-insoluble fiber includes 4:
- Some pectins
- Some hemicelluloses
Foods that contain water-insoluble fiber include: nuts, seeds, and grains.
Water-soluble fiber and viscous fiber
This type of fiber is soluble in water and at the same time binds it, creating a viscous mass (gel) in the stomach 5. Soluble fibers generally slow down the digestive process and delay stomach emptying 6. Therefore, they can ensure better absorption of nutrients and contribute to stabilizing blood sugar levels. The following are considered soluble fibers 7:
- Plant gums
- Wheat dextrin
Foods that contain soluble fibers include legumes, oat products, and citrus fruits.
However, not all soluble fibers form gels 8. Therefore, a newer classification into viscous and non-viscous fibers has been introduced. Viscous fibers have gel-forming properties in the digestive tract 9. The following are considered viscous fibers 10:
- Some plant gums
Non-viscous fibers include:
Fermentable and non-fermentable fibers
Some fibers can be easily fermented by intestinal bacteria, producing short-chain fatty acids (acetate, propionate, and butyrate) and gases 11. The fatty acids are then absorbed and metabolized by the bacteria, serving as an energy source for them. Foods containing fermentable fibers include oats, barley, fruits, and vegetables 12. Fermentable fibers include 13:
- Wheat dextrin
Non-fermentable fibers include:
They cannot be utilized by intestinal bacteria.
Functional fiber refers to isolated, non-digestible carbohydrates that have numerous health benefits 14. These include:
- Fructooligosaccharides (such as fructans and inulin)
- Isolated resistant starch
- Isolated plant gum
- Resistant dextrins
Fiber Promotes a Healthy Gut Microbiome
Fiber has numerous positive effects on health. Certain fibers such as inulin, oligofructose, and fructans are prebiotics that influence the growth of beneficial gut bacteria. They serve as a food source for gut bacteria (including Lactobacillus and Escherichia coli), enabling beneficial bacteria to proliferate while suppressing the growth of harmful bacteria that produce toxins. Gut bacteria are essential for maintaining a healthy gut microbiome, aiding in digestion, and producing vitamins. Foods containing prebiotic fibers include asparagus, onions, and bananas. A high-fiber diet is therefore recommended for the treatment of inflammatory bowel diseases such as Crohn's disease 15. Fiber may also help in the treatment of irritable bowel syndrome 16, although results vary and a definitive evaluation cannot be made.
Constant Amino Acid Flow through Fiber
Similar to the slowed absorption of carbohydrates, proteins (consisting of amino acids) also behave in the same way. When they are absorbed more slowly, the amino acids are available to the body for a longer period of time. Thus, fiber can contribute to a constant flow of amino acids. In addition, fiber stimulates the secretion of pancreatic enzymes (digestive enzymes) that improve protein breakdown 17.
Lowering Diabetes Risk with Fiber
A fiber-rich diet is associated with a reduced risk of diabetes 18. The study showed that especially water-insoluble fiber and whole grain products lead to a reduced risk. Therefore, a fiber-rich diet is also recommended for diabetics. Scientists of a Taiwanese study also associate lower fiber intake with a higher risk of diabetes 19. Fiber also slows down the absorption of carbohydrates from food. By slower absorption of carbohydrates, the body can better control insulin levels.
Fiber Helps with Weight Loss
Consuming fiber slows down digestion and keeps the food in the stomach and intestines for longer, leading to prolonged satiety. Additionally, fiber fills up the body without adding calories, making it particularly helpful for weight loss. Feeling full leads to reduced food intake, which promotes weight loss 20. Fiber can also help prevent obesity 21. Studies also show that high fiber intake, mainly through cereal consumption, can prevent weight gain in the waist area 22.
Reduce Cholesterol Levels and Lower Blood Pressure
Other positive properties of dietary fiber include its blood pressure-lowering effect and its ability to reduce bad LDL cholesterol without affecting healthy HDL cholesterol levels 23. According to the latest studies, blood pressure can be particularly well reduced with beta-glucan-rich foods such as oats and barley 24.
Fiber improves kidney function and reduces inflammation susceptibility
A high intake of fiber is associated with better kidney function and generally lower inflammation susceptibility in older men 25. Additionally, the study indicates that high amounts of fiber are associated with a lower mortality risk from cancer, especially in patients with kidney dysfunction (see next section). A greater intake of fiber, fruits, and vegetables is also associated with a reduced risk of kidney stones in postmenopausal women 26. The generally low inflammation levels resulting from fiber consumption are also confirmed by a long-term study 27.
Preventive effects of fiber
Dietary fiber has protective effects against certain gastrointestinal diseases such as constipation, hemorrhoids, colon cancer, gastroesophageal reflux disease, duodenal ulcers, diverticulitis, obesity, diabetes, stroke, hypertension, and cardiovascular diseases. (28, 29, 30).
It has been repeatedly shown that fiber significantly reduces the risk of heart attack 31 32. A high-fiber diet is also associated with a lower risk of cardiovascular diseases 33. A fiber intake of 14 g per 1,000 kcal has been found to lead to a significant reduction (16 to 33 %) in coronary heart disease 34. Additionally, according to study results, high consumption of fiber-rich vegetables is associated with significantly lower overall mortality 35.
Fiber intake is also associated with a reduced risk of inflammatory bowel disease 36.
Dietary Fiber Reduces Cancer Risk
Results from 2013 suggest that higher consumption of dietary fiber and lower intake of sugar are associated with a decreased risk of liver cell carcinoma 37. Additionally, scientists have linked high dietary fiber intake to a lower risk of biliary tract cancer.
Scientists conducting a 2013 meta-analysis found that a higher consumption of dietary fiber was associated with a decreased risk of stomach cancer 38.
In 2014, scientists observed that insoluble dietary fiber from legumes was associated with a decreased risk of prostate cancer 39. Their findings suggest a potential protective role of dietary fiber intake in the development of prostate cancer.
Regarding the preventative effect of fiber on colon cancer, there are differing study results. Scientists in older studies were not able to find a connection, whereas newer studies from 2015 have 40 41 42. Newer research suggests that vegetable fiber may protect against colon cancer.
Recommended Fiber Intake
Women should aim for 25 grams and men 38 grams of fiber per day to meet their needs. After the age of 51, the fiber requirements decrease to 21 grams for women and 30 grams for men. The following are the recommended daily intake amounts for fiber (AI) 45:
|Gender and Age Group||Amount in g|
|over 51 years||30|
|over 51 years||21|
|0 to 12 months||not defined|
|1 to 3 years||19|
|4 to 8 years||25|
|Boys 9 to 13 years||31|
|Boys 14 to 18 years||38|
|Girls 9 to 13 years||26|
|Girls 14 to 18 years||26|
With a balanced vegan diet, there is usually no fiber deficiency. The daily requirement can be easily met with fiber-rich foods.
An upper limit for fiber has not been defined. However, very high consumption levels may reduce the absorption of certain minerals 46.
The only meal in which fiber should be avoided, if possible, is the one after exercising. The nutrients from the food should be absorbed quickly and passed on.
Cooking high-fiber foods has only a minor impact on the fiber content of the food 47.
What should vegans consider?
With a vegan diet, the fiber requirements are very well met, and there is almost an excess of fiber 48 49. One consequence can be a reduced absorption of nutrients. Studies show that with a consumption of more than 50 g of fiber per day, the absorption of minerals, especially iron, calcium, magnesium, and zinc is reduced 50. This is because fiber-rich foods are also rich in phytates, which have a negative effect on the absorption of the aforementioned minerals 51. he absorption of Vitamin B6 from food is reduced by about 5-10 % in the presence of fiber 52. Additionally, increased intake of fiber (especially soluble fiber) can cause bloating and a feeling of fullness. Diarrhea and flatulence are also possible.