Dietary Fiber: Forms, Foods, Side Effects and Benefits Backed by Science

groups of various grains

Dietary fiber is a component of plant foods whose consumption exerts various beneficial effects on our organism.

The peculiarity of these substances is that they are not digested by the gastrointestinal system of humans and reach intact in the large intestine, where they undergo a smaller or greater degree of fermentation than the bacteria that normally colonized the intestine.

Fiber includes non-starchy polysaccharides (cellulose, hemicellulose, pectins, gums), inulin, lignin, and other components.

Soluble And Insoluble Fiber

We get dietary fiber from vegetables, fruits, whole grains, legumes, and nuts. It is customary for dietary fiber to be divided into two categories, the “soluble” and the “insoluble“. In food, there are both these types of dietary fiber, but in various proportions.

Soluble Fiber

Soluble fibers dissolve in water and include pectins, gums, beta-glucans, inulin, some hemicelluloses, some oligosaccharides.

Most soluble fibers are fermented (to a much greater extent than insoluble ones) in the large intestine by the bacteria – natural microflora of the intestine – nourishing them and thus maintaining the balance of this flora (prebiotic action).

This type of dietary fiber can absorb a large amount of water and form gel, increase the residence time of the intestinal contents in the digestive tract and slow down the absorption of nutrients, such as sugars. We find them in more important quantities in legumes, vegetables, fruits, nuts, barley, oats, and psyllium seeds.

Insoluble Fiber

Insoluble fiber doesn’t dissolve in water. Celluloses, some hemicelluloses, lignin, and resistant starch (a type of starch resistant to digestion) belong to the insoluble fibers.

Insoluble fibers have the ability to absorb water and increase the volume of stool, as well as accelerate the passage of contents through the intestine, thus facilitating stools.

This action is enhanced by increased water consumption. Rich sources of insoluble fiber are whole grains, fruits, and vegetables.

Foods With Dietary Fiber

Peas (Boiled) 1 Cup8,8
Broccoli (Boiled) 1 Cup 5,2
Spinach (Boiled) 1 Cup 4,3
Cauliflower (Boiled)1 Cup2,8
Baked potato1 Average2,3
Cabbage (Raw)1 Cup2,2
Onion (Raw) 1 Average 1,9
Carrot (Raw)1 Average 1,7
Tomato (Raw)1 Average 1,5
Dried Plums1 Cup 12,4
Apple (with peel)1 Average 4,4
Banana1 Average 3,1
Orange1 Average 3,1
Peach1 Average 2,2
Plum1 Average 0,9
Apricot1 Average 0,7
Starchy foods, cereals
Whole wheat bread1 slice1,9
Rye bread1 slice 1,9
Bread white1 slice 0,8
Whole grains cereals1 Cup8,9
Oatmeal cereals1 Cup4,6
Corn Flakes type cereals1 Cup1,3
Boiled brown rice1 Cup3,5
Boiled white rice1 Cup0,6
Whole grain pasta boiled1 Cup3,9
White pasta boiled1 Cup2,5
Lentils (boiled)1 Cup15,6
Chickpeas (boiled)1 Cup12,5
Beans (boiled)1 Cup11,3
Almonds1 Cup17,9
Hazelnuts1 Cup13,1
Walnuts1 Cup6,7
Source: USDA National Nutrient Database

Benefits Of High Fiber Diet

Research and clinical studies have managed to show significant physiological effects of these particular ingredients, which although not nutritious, in the sense that they are not digested, are nevertheless necessary for the prevention of diseases and the maintenance of overall health.

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Improve Bowel Function

Increasing bowel function is the most obvious action of dietary fiber. Fiber increases weight and softens the texture of stool and facilitates their passage through the intestine.

Therefore, they facilitate defecation and prevent constipation, while conditions such as hemorrhoids, diverticulosis, and perhaps colon cancer can be prevented (1).

When the fibers are fermented in the large intestine, by the ”good” bacteria, some ”products” are produced, including gases and short-circuit fatty acids.

The production of the latter leads to a decrease in the pH in the large intestine, thus preventing the growth of pathogenic microorganisms (2).

Thus, the fermentation process is especially important for maintaining a healthy intestinal microflora, which is also of value in the prevention of diarrhea. (3)

Glucose Control – Management of Diabetes Mellitus

It has been found that fiber, mainly soluble, slows down the absorption of glucose (sugar), resulting in the subsequent increase in its concentration in the blood to be smoother than its sudden increase that usually occurs after meals poor in dietary fiber. (4)

Delaying the emptying of the stomach and the increased time of stay of intestinal contents in the colon (large intestine) are some of the ways in which this is achieved.

Studies have shown that the presence of fiber in the meal reduces blood glucose and/or insulin levels (5). This is of particular value in terms of dietary treatment of diabetic patients (type I and II) and better glycemic control since a diet rich in fiber improves blood glucose levels and insulin sensitivity, while possibly reducing insulin needs.

In addition, high intake of dietary fiber, in general, has been associated with a decrease in the onset of diabetes mellitus, which indicates that the protective effect of fiber against the disease is also possible.

Lowering Cholesterol Levels

Soluble fiber has been linked to a decrease in total and LDL (“bad”) cholesterol levels in the blood (6). More importantly, oatmeal beta-glucans and psyllium have been used as sources of soluble fiber.

The mechanisms of hypocholesterolemic action of soluble fibers are not completely specified. Their ability to bind bile acids, which are then excreted, and thereby reduce the formation of micelles that are useful for the absorption of lipids is a mechanism that has been proposed.

Also, from the fermentation that the fibers undergo from the bacteria in the colon, propionic acid is produced (among other things, short-circuit fatty acids), which has an inhibitory effect on the liver composition of cholesterol, and this is another suggested mechanism of action.

According to some researchers, the hypocholesterolemic effect of fiber is more clear in people suffering from hyperlipidemia, than in people with normal cholesterol levels (7).

Weight Control

Fiber yields (depending on the degree of kneading) very small amounts of energy, that is, calories.

Foods that are rich in fiber, although usually bulky, provide few calories, which of course depends on the way they are consumed and prepared.

The role of fibers in weight control is mainly auxiliary and is related to the control of food intake.

Foods with a large fiber content require more chewing time and the produced saliva increases, resulting in a feeling of filling of the stomach and the person feeling satiated sooner.

This helps to control the amount of food one consumes and controlling the amount of food is an important parameter in an effort to maintain or reduce a person’s body weight.

Reduction of Cardiovascular Risk

Hyperlipidemia, diabetes mellitus, but also excess body weight, and obesity have long-term negative effects on the health of the heart and blood vessels.

Since a diet adequate in fiber can improve these conditions to some extent, it is easy to understand how these nutrients have some involvement in reducing cardiovascular risk.

In combination with the fact that in some studies there has been a moderate effect (decrease) on blood pressure, especially in hypertensive people, it becomes obvious that fiber should have a place in a diet that aims at the health of the cardiovascular system.

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Too Much Fiber Side Effects

Fiber may also have some side effects.

Some types of fiber seem to reduce the absorption of minerals such as iron, calcium, and zinc, while very often coexist in the same food with phytic acids, which bind the minerals and prevent their absorption by the digestive system.

Also, a diet high in fiber can worsen the symptoms of some patients suffering from irritable bowel syndrome.

In addition, a small percentage of the population seems to be particularly sensitive to the intake of fiber and to show symptoms in the digestive tract in amounts of fiber less than 10 grams/day.

The sharp increase in the amount of fiber in the diet is sometimes accompanied by digestive discomfort, flatulence, pain, or diarrhea.

When such situations occur, the undesirable effects of fiber should be taken into account, while the assistance of a dietitian is necessary.

However, the general rule is that the intake of fiber should be increased gradually and not abruptly.

How Much Fiber Per Day

According to the European Food Safety Authority EFSA, the intake of 25 grams of fiber per day, for adults, is considered sufficient for the smooth functioning of the intestine, while higher intakes have been linked to other benefits, such as reduced cardiovascular risk, better management of type II diabetes and better weight control.

☝ We could say that the diet of a healthy adult should provide 25-30 grams of dietary fiber/day.

Adequate intake of fiber by children is less well thought out and various positions on this issue have been formulated.

To calculate the sufficient amount of fiber in the diet of children has been proposed, among other things, the rule of “age in years + 5” grams of fiber. That is, if a child is 5 years old, then it must have 5(years) + 5 = 10 g. fiber.

How To Get More Dietary Fiber

From what has been mentioned, it seems that both soluble and insoluble fibers exert beneficial effects on the human body. Therefore, the care for a diet with a variety of foods that contain fiber is what helps us to reap the maximum benefits from the combined action of these substances. But how is this achieved? Some everyday ways are:

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