Prebiotics

The health benefits of consuming dietary fiber have long been known. Both soluble and insoluble fiber play a crucial role in digestive health, aiding both digestion and elimination. Scientific research has shown that a high intake of dietary fiber correlates with a lower risk of developing digestive disorders and several other conditions, including type 2 diabetes, obesity, coronary artery disease, hypertension and stroke (1). A common misconception is that many people believe that the terms "fiber" and "prebiotic" are interchangeable and mean the same thing. However, while all prebiotics are dietary fibers, not all dietary fibers are prebiotics (2). Compared to the broader classification of dietary fiber, science is just beginning to confirm the unique health benefits of prebiotics (3). Prebiotics provide nutrients to the bacteria in the gut flora, which they can use as a source of energy.

What are prebiotics?

The current definition describes a prebiotic as "a non-digestible compound that, when metabolized by microorganisms in the intestine, influences the composition and/or activity of the intestinal flora, thereby conferring a beneficial physiological effect on the host (3)." Basically, prebiotics are indigestible carbohydrates that have unique characteristics and health benefits. To be classified as a prebiotic, a dietary fiber must have the following characteristics:

It must resist normal digestion and absorption in the digestive tract.
It must be fermentable by the intestinal bacteria.
It must provide health benefits to the host by selectively stimulating the growth and/or activity of microorganisms in the gut (4, 5).

Prebiotics vs. probiotics

Although prebiotics and probiotics are often referred to as "synbiotics" because they work together synergistically to support gut bacterial health, the two have very different functions. Prebiotics are indigestible fibers that are fermented by microorganisms in the digestive tract and basically serve as food and promote the growth of beneficial bacteria. Probiotics, on the other hand, are live microorganisms that have beneficial effects for the host (6). Probiotics can be taken in the form of supplements or fermented foods such as yogurt, fermented vegetables (e.g. kimchi and sauerkraut), miso and kombucha (7). It is important to note that unlike probiotics, prebiotics can only support the growth of bacteria that are already present in the gut (8).

Different types of prebiotics

There are many different types of prebiotics. The majority of these are a subgroup of the carbohydrate category and most prebiotics belong to the oligosaccharide group. However, there is also evidence that there are prebiotics that do not belong to the carbohydrate group. While inulin, fructooligosaccharides (FOS) and galactooligosaccharides (GOS) are among the best known prebiotics (3), a number of other compounds have also been shown to have prebiotic effects and may be associated with health benefits. These include:

arabino-oligosaccharides (3)
Beta-glucans (3)
Fructans (9)
Galactomannan (3)
Oligosaccharides in breast milk (10)
Isomaltooligosaccharides (3
Lactulose (3)
Mannan (11) and glucomannan (12)
Pectin (13)
Xylooligosaccharides (3)

Here is a brief overview of the most important categories of prebiotics:

Fructans

This category consists of inulin and fructooligosaccharides or oligofructose. Until recently, studies only indicated that fructans could only selectively stimulate lactic acid bacteria. However, there is now some evidence that the chain length of fructans is an important criterion for determining which bacteria ferment them (14). This means that other bacteria can also be stimulated directly or indirectly by fructans.

Galacto-oligosaccharides

Galacto-oligosaccharides can stimulate the growth of bifidobacteria and lactobacilli. Enterobacteria, Bacteroidete and Firmicutes are also stimulated by galacto-oligosaccharides, albeit to a lesser extent than bifidobacteria. There are some galacto-oligosaccharides derived from lactulose, the isomer of lactose, which are considered prebiotics (14). However, the effects of galacto-oligosaccharides of the raffinose family on the intestinal flora are still unknown.

Resistant starch and glucose-derived oligosaccharides

Resistant starch is resistant to digestion in the upper digestive tract. It can promote the production of high levels of butyrate, which is why it has been classified as a prebiotic. Polydextrose is an oligosaccharide derived from glucose. It consists of glucan with many branches and glycoside compounds. There is evidence that polydextrose can stimulate bifidobacteria, but this has not yet been definitively confirmed (15).

Other oligosaccharides

Some oligosaccharides are derived from a polysaccharide known as pectin.

Oligosaccharides that do not belong to the carbohydrate group

Although carbohydrates tend to meet the criteria of the definition of prebiotics, there are some compounds such as flavanoids found in cocoa that are not classified as carbohydrates but meet these criteria. In vitro and in vivo studies have shown that flavanols can stimulate lactic acid bacteria (16).

Benefits of prebiotics

Prebiotics are fermented in the digestive tract and act as nutrients for beneficial gut bacteria (17). They promote a healthy bacterial profile in the gut flora and offer a range of associated health benefits (18). The products of degradation of prebiotics by gut bacteria are mainly short-chain fatty acids, which are small enough to diffuse through the gut wall and enter the bloodstream. They also serve as a source of energy for the cells of the inner intestinal wall.

Prebiotics for inflammatory bowel diseases

There are several studies that have investigated the effects of prebiotics in irritable bowel syndrome and Crohn's disease. Irritable bowel syndrome is characterized by chronic abdominal pain and altered digestive behavior, while Crohn's disease is a chronic, recurrent inflammatory disease of the digestive system that can affect any part of the digestive system from mouth to anus. Both conditions have been observed to have reduced populations of bifidobacteria and other beneficial gut bacteria (19, 20). Study results on the effects of prebiotics in these diseases are mixed. Two studies came to the conclusion that neither 6 nor 20 grams of fructooligosaccharides per day could alleviate the symptoms of irritable bowel syndrome (21, 22).

In contrast, two more recent randomized double-blind studies have shown that symptoms of irritable bowel syndrome could be alleviated by consuming 5 grams of fructooligosaccharides for 6 weeks (23) or 3.5 grams of galacto-oligosaccharides per day for 12 weeks (24). A 2006 study reported that supplementation with 15 grams of fructooligosaccharides per day for 3 weeks increased the number of bifidobacteria in the stool and alleviated the symptoms of Crohn's disease (25). However, other randomized, placebo-controlled, double-blind studies showed no benefits in Crohn's disease patients with either supplementation with 15 grams of fructooligosaccharides or supplementation with 20 grams of oligofructose-enriched inulin per day (26, 27).

Prebiotics and colorectal cancer

Prebiotic fermentation products such as butyrate have been shown to have protective effects on the risk of developing colorectal cancer and to inhibit the progression of colorectal cancer by inducing apoptosis of cancer cells (28, 29, 30). In addition, symbiotic therapy (Lactobacillus rhamnosus and Bifidobacterium lactis plus inulin) has shown the potential to reduce the risk of colorectal cancer by improving the integrity and function of the epithelial barrier (28, 31, 32).

Prebiotics and the immune system

The consumption of prebiotics can improve immune function by increasing the population of protective microorganisms. Studies conducted with animals and humans have shown that prebiotics can reduce the populations of harmful bacteria (33, 34, 35, 36). Mannose, for example, can reduce intestinal colonization by pathogens by promoting mannose adherence to Salmonella (37). In addition, pathogens are unable to attach to the epithelium in the presence of oligosaccharides. Prebiotics can also induce the expression of immune molecules - especially cytokines. Furthermore, it has been shown that a mixture of oligofructans and inulin can improve antibody responses to viral vaccines such as influenza and measles vaccines. Fructooligosaccharides have also been shown to improve the antibody response to influenza vaccination while reducing the side effects of vaccination (38, 39). This category of prebiotics can also reduce diarrhea-related fevers in infants, reduce the use of antibiotics and reduce the duration of illness (40, 41).

Prebiotics and the nervous system

The digestive tract is connected to the central nervous system via the so-called "gut-brain axis" (42). Thus, administration of prebiotics to piglets reduced gray matter to improve neuronal pruning (43). However, the regulatory effects of prebiotics in the human brain have not yet been fully investigated. Gut flora influences the brain via three routes, including neuronal pathways, endocrine pathways, and immune pathways (42, 44, 45).

a. Neuronal pathways

The products of fermentation of prebiotics can affect the brain via the vagus nerve (45). Some prebiotics such as fructooligosaccharides and galactooligosaccharides have regulatory effects on neurotrophic factors, neurotransmitters and synaptic proteins (47, 48).

b. Endocrine pathways

The hypothalamic-pituitary-adrenal axis is a neuroendocrine pathway. The growth of gut flora in mice can appropriately induce corticosterone and adrenocorticotrophic hormones (49). In addition, prebiotics act as regulators of other hormones such as plasma peptide YY (50).

c. Immune pathways

As mentioned above, prebiotics can influence different aspects of the immune system.

In addition to neurological factors, prebiotics are also able to influence mood, memory, learning and some psychiatric disorders by altering the activity and/or composition of gut bacteria (45).

Prebiotics and mood

Stress hormones are able to influence behavioral patterns associated with anxiety (51, 52). Stress hormone levels have been shown to increase in germ-free mice in response to controlled stress. After administration of Bifidobacterium infantis, the levels of these stress hormones fell back to normal (53).

Prebiotics and memory, concentration and learning

A number of studies have shown the relationship between memory performance and the administration of fermentable compounds in humans and animals (54). Studies with prebiotics indicate an improvement in memory performance in middle-aged adults (55, 56). Some prebiotics such as arabinoxylan and arabinose may improve general alertness and slow the accumulation process of proteins associated with dementia (57). A randomized, placebo-controlled, double-blind study from 2015 concluded that galactooligosaccharides at a dosage of 5.5 grams increased salivary cortisol levels and improved concentration in adults, while fructooligosaccharides showed no such effect (58).

A randomized, placebo-controlled, double-blind study showed that administration of non-starch polysaccharides at a dose of 3.6 grams per day for 12 weeks improved the memory process in middle-aged adults (55, 56). In two further studies, it was observed that administration of inulin-enriched oligofructose improved mood, memory and short-term memory (after 4 hours), while this prebiotic was not able to improve long-term memory (after 43 days) (59, 60). In other studies, administration of a polydextrose-galactooligosaccharide mixture reduced anxious behavior in male piglets and promoted social interactions in rats (43, 61). In addition, consumption of this mixture improved memory performance (61, 62).

Prebiotics and the skin

The consumption of prebiotics can reduce both the risk of developing allergic skin diseases such as atopic dermatitis and their severity (63, 64). In hairless mice exposed to UV light, administration of galactooligosaccharides for 12 weeks increased water retention in the skin and prevented the development of skin redness (65). In addition, galactooligosaccharides can strengthen the skin barrier.

Probiotics and the cardiovascular system

According to statistics, about 30% of all deaths in the western world are due to cardiovascular diseases. The main reason for this trend is changes in lifestyle and dietary habits. Many scientists have therefore investigated the influence of dietary fibers and prebiotics on cardiovascular diseases. However, no direct functions of prebiotics have yet been found in this respect, but there are a number of indirect effects. Prebiotics are able to reduce the risk of cardiovascular disease by reducing inflammation. Several studies have also demonstrated an improvement in the blood lipid profile through the consumption of prebiotics. In a randomized, placebo-controlled, double-blind, cross-over study, healthy volunteers were treated with 10 grams of inulin for 3 weeks.

It was observed that this treatment reduced blood triglyceride levels and lipogenesis in the liver, but had no statistically significant effect on cholesterol levels (66). Other studies came to similar conclusions (67, 68). One study, however, found that lactulose increased blood cholesterol levels by up to 10% (69). In a randomized, placebo-controlled, double-blind, cross-over study in overweight subjects with risk factors for metabolic syndrome increased by at least a factor of 3, administration of Bimuno® galacto-oligosaccharides for 12 weeks reduced circulating levels of cholesterol and triglycerides and improved the ratio of total cholesterol to HDL cholesterol (70). In older people, however, this prebiotic had no effect on the ratio of total cholesterol to HDL cholesterol (71). A meta-analysis investigated the effects of beta-glucan on the blood lipid profile and concluded that beta-glucan consumption could lower total cholesterol and bad LDL cholesterol levels (72). Another meta-analysis of randomized controlled trials concluded that fructooligosaccharides can lower blood triglyceride levels by an average of 7.5% (73).

Prebiotics and calcium absorption

Statistics show that in the USA alone 28 million people suffer from osteoporosis or low bone mass and that in Europe one in eight adults over 50 suffers a spinal fracture every year (74). There are clinical studies on the influence of prebiotics on the absorption of minerals such as calcium, but the results are contradictory. Some studies have shown that the consumption of lactulose or inulin + oligofructose in doses of 5 to 20 grams per day significantly increases calcium absorption, while no such effects have been observed with galactooligosaccharides and fructooligosaccharides (75).

Possible side effects

The consumption of prebiotics in the form of food or supplements is not associated with any side effects when consumed in the correct doses. Flatulence and diarrhea only occur when prebiotics are overdosed (76). People who suffer from irritable bowel syndrome or other digestive tract disorders should talk to their doctor to get personalized recommendations for including prebiotics in their diet.

Dosages

Most people can consume an adequate amount of prebiotics by aiming to consume the recommended amount of fiber. This amount is 25 to 38 grams per day for adults. Consuming whole grains and plenty of fruits and vegetables is often the best way to achieve this. Many prebiotic supplements provide a dose of around 4 to 5 grams per day. If you are using such a supplement, you should start slowly with a low dosage to see how your body reacts to the supplement. If bloating occurs, then you should halve the dose. Many people combine prebiotics with probiotics for a better effect.

Conclusion

Prebiotics are dietary fibers that have unique characteristics and health benefits. To date, only a small number of dietary fibers have been classified as prebiotics, although many more have been shown to have prebiotic effects. Supplementation with certain prebiotics has also been shown to have benefits for people with digestive problems, cardiovascular disease and other health problems, as well as improving immune system function.