Chapter 115 Prebiotics, Synbiotics, and Colonic Foods
A prebiotic is defined as “a nondigestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon.”1 For food ingredients to be classified as prebiotics, they must:
1. Neither be digested nor absorbed in the stomach or small intestine.2
2. Act as a selective food source for one or a limited number of potentially beneficial commensal bacteria in the large intestine.2
3. Change the colonic microflora ecosystem towards a healthier composition.2
4. Induce luminal or systemic changes that improve the health of the host.3
Most emphasis at this stage has been on finding and studying food sources that are used by lactic acid-producing bacteria. This is due to the health-promoting properties of these organisms.2 The best known lactic acid-producing bacteria belong to the genera Lactobacillus and Bifidobacterium.
Not all food ingredients that make it to the colon undigested will be prebiotics. Only compounds that are selectively consumed by beneficial members of the microflora, while being ignored by potentially pathogenic ones, can truly be termed prebiotics. When prebiotics reach the colon, they are preferentially utilized by those organisms that have the capacity to hydrolyse their bonds. The metabolism of the prebiotic results in the increased growth and activity of the beneficial organism(s), often at the expense of other components of the microflora.4 Food ingredients that make it to the colon undigested, but lack selectivity of fermentation are termed “colonic foods” and are discussed in more depth at the end of this chapter.
There are numerous compounds with the potential to be termed prebiotics. Table 115-1 lists these compounds and the type of microorganisms whose growth they promote. However, the best-researched prebiotics are fructooligosaccharides (FOSs), galactooligosaccharides (GOSs), and lactulose.
|PREBIOTIC COMPOUND||FOOD SOURCES||TARGETED MICROORGANISMS|
|Fructooligosaccharides||See Table 115-2||Bifidobacterium spp.|
|Galactooligosaccharides||Cow’s milk; yogurt; human milk||Bifidobacterium spp.|
|Galactosyl lactose||Human milk||Bifidobacterium spp.|
|Lactitol||None known||Lactobacillus spp.; Bifidobacterium spp.|
|Lactulose||UHT milk||Lactobacillus spp.; Bifidobacterium spp.|
|Polydextrose||None known||Lactobacillus spp.; Bifidobacterium spp.|
|Raffinose||Legumes; beets||Lactobacillus spp.; Bifidobacterium spp.|
UHT = Ultra-high-temperature.
Prebiotic substances and the organisms whose growth they promote.5–8
FOSs are linear or branched chains of fructose and glucose molecules.9 The number of fructose units contained in the compound determines the name of the FOS. Oligofructose is generally composed of between 2 and 7 units, whereas inulin is composed of up to 60.10 FOSs are found in varying percentages in foods and have been discovered in over 36,000 plant species, where they function primarily as storage carbohydrates.11 FOSs are found in many common vegetables, including asparagus, onion, leek, garlic, artichoke, Jerusalem artichoke, and chicory root. However, it is from the chicory root (Cichorium intybus) that most of the commercially produced inulin and oligofructose is manufactured. Short-chain fructans, such as oligofructose, are produced from inulin through a process of partial enzymatic hydrolysis.9
There are two ways in which FOS can be consumed, in supplements and in foods.
The most commonly employed method to purify and concentrate FOS for supplement use is via a hot water extraction of fresh chicory roots. This process results in inulin (also known as Raftiline, a large-chain FOS) as the end product. Some manufacturers utilize enzymatic hydrolysis to produce oligofructose (also known as Raftilose, a medium-chain FOS) from inulin,10 although other manufacturers synthesize FOS from sucrose using the fungal enzyme fructosyltransferase (from Aspergillus niger). This latter process involves chemical synthesis of a new compound (called Neosugar or Actilight, a short-chain FOS) from two other natural compounds (fructose and glucose). The finished compound is similar to naturally obtained FOS, only smaller in size.9
FOSs are resistant to digestion in the upper gastrointestinal tract (GIT) because of the β-configuration of the bonds between the fructose units. Human digestive enzymes are specific in requiring α-linkages; thus, FOSs are classified as nondigestible oligosaccharides.9
Food Sources of Fructooligosaccharides
As previously mentioned, FOSs are common food ingredients. Individuals consuming the standard Western diet consume an average of 5.1 g/day of FOS.12 However, this can easily be increased if foods rich in FOS are consumed on a daily basis. Foods containing FOS are outlined in Table 115-2.
|FORM||AVERAGE PERCENTAGE OF FRESH WEIGHT|
|Jerusalem artichoke||Tubers (raw)||18.0%|
|Roasted (as coffee)||41.7%|
The FOS content of some commonly consumed foods.12,13
Other FOS-containing foods include honey,14 beer, Chinese chives, and maple syrup.15
There is no particular advantage to the consumption of FOS in supplements compared with FOS-rich foods. As long as the amount of FOS consumed in a food is similar to that used in clinical studies, it will promote identical effects, as all FOSs consumed reach the colon intact, whether ingested in whole foods or in supplements.
The health benefits claimed for FOS mainly stem from their ability to increase numbers of beneficial organisms in the colon, to reduce numbers of potentially pathogenic microorganisms (PPMs), and to stimulate short-chain fatty acid (SCFA) production. These health benefits include:
• Enhanced resistance to enteric pathogens due to colonization resistance provided by the increased growth of lactic acid bacteria, and increased resistance to infections due to the nonspecific stimulation of the immune system16
• Modification of carcinogen metabolism16
• Enhanced absorption of minerals16
• Improved serum lipid parameters16
• Treatment of atopic eczema and prevention of atopy development18
• Increased intestinal mucin production and trophic effects on the colonic epithelium, secondary to the increased production of SCFAs19
This review focuses primarily on the ability of FOS to enhance bifidobacterial growth, improve immune response, enhance mineral absorption, possibly treat and prevent atopic diseases, and improve satiety.
Enhanced Growth of Bifidobacteria
In a human trial utilizing oligofructose (8 g/day over a 2-week period), Mitsuoka et al15 found a 0.9 log unit increase in bifidobacteria numbers with oligofructose consumption (P <0.005). The trial also showed a decrease in enterobacteria numbers.15
In an attempt to determine the optimal dose of oligofructose, in terms of maximizing bifidobacteria numbers and minimizing side-effects, Bouhnik et al20 designed a trial that utilized five different dosage levels. The dosages used were 20, 10, 5, and 2.5 g/day of oligofructose and 0 g/day as the placebo. The trial lasted 7 days. The data indicated that bifidobacteria counts did not change in subjects who received 0 or 2.5 g/day of oligofructose, but that counts in those subjects who ingested 5, 10, and 20 g/day were significantly greater (P <0.05) at day 8 than at baseline. A significant correlation between the ingested dose of oligofructose and fecal bifidobacterial counts was observed at day 8 (P <0.01). In terms of side effects, all groups, including the placebo group, experienced mild abdominal symptoms, such as bloating, excess flatus, borborygmi, or mild abdominal pain. In general, the higher the dose of oligofructose, the more side effects experienced. Bouhnik et al20 concluded that 10 g/day was well-tolerated and that this dose was probably the optimal dose of oligofructose, since it led to a significant increase in colonic bifidobacteria with minimal side effects.
In a study reported by Gibson et al,21 both oligofructose and inulin were studied to assess their effects on bifidobacteria levels and the populations of other members of intestinal microflora. Eight subjects participated in the 45-day trial in which they ate controlled diets. For the initial 15 days, all the subjects ate 15 g/day of sucrose. For the middle 15 days, the sucrose was substituted with 15 g of oligofructose. For the final 15 days, four of the participants were given 15 g/day of inulin. A marked effect was noted in bifidobacteria numbers, which increased 0.7 log units on oligofructose (P <0.01) and 0.9 log units on inulin (P <0.001). There was also a significant decrease in bacteroides (P <0.01), clostridia (P <0.05), and fusobacteria (P <0.01) in the oligofructose-fed subjects, whereas gram-positive cocci levels decreased in the inulin-fed group (P <0.001). Inulin administration also increased lactobacilli numbers, although not significantly (P <0.075). In both groups, bifidobacteria became the numerically predominant species in the feces (see Figure 115-1).21
FIGURE 115-1 The prebiotic concept: the effects of sucrose, oligofructose, and inulin on the intestinal ecosystem. Pie diagrams illustrate how the microflora can develop during the feeding of sucrose and the prebiotics, oligofructose and inulin.
(From Gibson GR. Dietary modulation of the human gut microflora using prebiotics. Br J Nutr. 1998;80 [suppl 2]:S209-S21, used with permission.)
Improved Immune Response
Prebiotics, like FOS, have long been suggested to have immune enhancing effects.22,23 It is, however, only recently that human clinical trials with hard outcomes have been completed. One of the first of these trials investigated the effects of a prebiotic mixture (containing a combination of FOS and GOS) in protecting against infections during the first 6 months of life in formula-fed infants. In this randomized, double-blind, placebo-controlled trial, infants allocated to the prebiotic group experienced significantly fewer episodes of all types of infections combined (P = 0.01) compared with those in the placebo group. There was also a trend for fewer episodes of upper respiratory tract infection (P = 0.07) and fewer infections requiring antibiotic treatment (P = 0.10) in the prebiotic group. Additionally, the cumulative incidence of recurring infection and recurring respiratory tract infection was 3.9% and 2.9% in the prebiotic group versus 13.5% and 9.6% in the placebo group, respectively (P <0.05).24
This same infant cohort was followed up over the next 1.5 years. Those infants that received the prebiotic-enhanced formula for the first 6 months of life experienced significantly fewer episodes of physician-diagnosed respiratory tract infections (P <0.01), fever episodes (P <0.00001), and fewer antibiotic prescriptions (P <0.05) compared with those in the placebo group.25
In a randomized, placebo-controlled, open-label study, Bruzzese et al26 also investigated the efficacy of a prebiotic-enhanced infant formula (a combination of GOS and FOS) on infection incidence in infants. The prebiotic-enhanced formula or a standard infant formula was consumed over the initial 12 months of life. During this period, the incidence of gastroenteritis was 59% lower in the prebiotic group (P = 0.015). Additionally, the number of children with recurrent upper respiratory tract infections tended to be lower in the prebiotic group (28% vs 45%; P = 0.06) and the number of children prescribed multiple antibiotic courses per year was also lower (40% vs 66%; P = 0.004).26
In a randomized, double-blind, placebo-controlled trial looking at older children (aged 7 to 19 months) attending daycare centers, short-term administration (21 days) of FOS was found to significantly reduce the number of infectious diseases requiring antibiotic treatment (P <0.001), episodes of diarrhea and vomiting (P <0.001), and episodes of fever (P <0.05) compared with controls.27
In another randomized, controlled trial investigating the effects of FOS in toddlers (4 to 24 month olds) attending daycare, FOS administration was found to significantly reduce antibiotic use (32% reduction; P = 0.001) and daycare absenteeism (61% decrease; P = 0.025) compared with those in the control group. There was also a 34% reduction in episodes of fever in combination with any cold symptoms (P = 0.001) and a 61% decrease in episodes of fever in association with diarrhea (P <0.05) in the FOS-supplemented group.28
Enhanced Mineral Absorption
Studies utilizing animal models demonstrated that microfloral degradation of FOS significantly increased calcium and magnesium absorption.29–31 Human studies also showed that FOS consumption improved calcium absorption.32–34 The proposed mechanism by which mineral absorption is enhanced by FOS is via the action of the protonated SCFA. Protonated SCFAs are absorbed across the apical membrane of colonocytes by direct diffusion. Due to the low pKa values of the SCFA in relation to the intracellular pH, the SCFA dissociates once it enters the cell. This results in the release of a hydrogen ion. The hydrogen ion is subsequently secreted from the cell in return for a cation, which may be a magnesium or calcium ion. The hydrogen ion is then available to protonate another SCFA and enable it to diffuse into the cell.32 However, other mechanisms may also be involved, such as the decrease in colonic pH, which results in increased solubility of calcium, increased colonic venous blood flow, enlarged colonic villi, and enhanced expression of calbindin-D9k (the active calcium transport route).35 These effects are more pronounced in the colon, some of which are calcium-specific, explaining why calcium absorption is increased, while there is very little impact on the absorption of other minerals (e.g., iron and zinc).36
Improved Bioavailability of Phytoestrogens
An interesting animal (rat) study recently found that concurrent consumption of FOS and the soy isoflavones genistein and daidzein significantly improved the bioavailability of these compounds. The relative absorption of genistein was approximately 20% higher in FOS-fed rats than in controls. In addition, the presence of both phytoestrogens in serum was maintained longer in FOS-fed rats than in controls, suggesting that FOS enhanced colonic absorption of these compounds.37
This result may be especially relevant to women after antibiotic therapy, when the metabolism and subsequent absorption of phytoestrogens appears to be impaired.38 Bifidobacteria were demonstrated to possess β-glucosidase activity, and FOS administration resulted in enhanced β-glucosidase activity in animal models,39 as well as improved phytoestrogen bioavailability.40 Therefore, FOS consumption not only aids in the reestablishment of a healthy gut microflora, but its consumption also increases colonic β-glucosidase activity, resulting in enhanced deglycosylation and, thus, increased colonic concentrations of the medicinally active aglycones.
Treatment of Atopic Eczema and Prevention of Atopy Development
A number of studies found an aberrant composition of the GIT microflora in infants who later developed food allergies and atopic eczema.41–43 More specifically, the development of atopic eczema was correlated with increased colonic concentrations of Bacteroides spp., Clostridia spp., and Escherichia coli, with a decreased concentration of bifidobacteria. This change in microbial composition was theorized to deprive the developing immune system from counter-regulatory signals against T-helper 2 (Th2) mediated responses, and therefore, promote Th2-type immunity.43 Infantile atopic eczema is characterized by a Th2-dominated immune response, as well as excessive intestinal inflammation and aberrant macromolecular absorption across the intestinal mucosa.44–46 These latter characteristics may also be caused by the dysbiotic condition. Bacteroides, clostridia, and E. coli all have the potential to trigger inflammatory responses in the gut and can release toxins that can impair intestinal permeability, leading to increased exposure to potential antigens.43 Supplementation with FOS was demonstrated to decrease colonic concentrations of both bacteroides and clostridia, as well as promoting a bifidobacteria-dominated colonic flora.21 FOS use may thus bring the aberrant intestinal flora back into balance and improve gut barrier function. The promotion of an intestinal flora dominated by gram-positive bacteria may also promote a shift towards Th1 immunity via enhanced production of interleukin-12 and interferon-γ.47
No trials appear to have assessed FOS in isolation in the treatment or prevention of atopic eczema. One trial was conducted, however, which assessed the efficacy of a FOS and GOS combination in the prevention of atopic dermatitis in infants.
In a randomized, double-blind, placebo-controlled trial, Moro et al48 evaluated the effects of a prebiotic-enhanced infant formula on the incidence of atopic dermatitis during the first 6 months of life in formula-fed infants at high risk of atopy development. Atopic dermatitis developed in 23% of infants in the control group compared with 10% in the prebiotic group (P = 0.014) over the 6-month intervention period.48 Subjects in the prebiotic group were also found to have significantly reduced plasma levels of total immunoglobulin (IgE) (P = 0.007), IgG2 (P = 0.029), and IgG3 (P = 0.0343), as well as cow’s milk protein-specific IgG1 (P = 0.015), suggesting that FOS and GOS supplementation induced an antiallergic antibody profile.49
These same infants were followed up over the next 18 months. Those infants that received the prebiotic-enhanced formula for the first 6 months of life were at reduced risk of developing atopic disease over the follow-up period. Infants in the control group experienced a significantly higher rate of atopic disease, such as atopic dermatitis (27.9% vs 13.6%), recurrent wheezing (20.6% vs 7.6%), and allergic urticaria (10.3% vs 1.5%) compared with infants in the prebiotic group (all P <0.05).25
Promotion of Satiety
In the wake of some interesting preliminary animal research,50 a single-blind, crossover, placebo-controlled trial was performed to assess the effects of FOS on satiety and energy intake in humans. Subjects ingested either 8 g of FOS twice daily (with breakfast and dinner) or a placebo. FOS supplementation was found to significantly increase satiety at breakfast and dinner (both P = 0.04), but not lunch. At dinner, FOS supplementation was also found to reduce hunger (P = 0.04) and prospective food consumption (P = 0.05). Energy intake at breakfast (P = 0.01) and lunch (P = 0.03) were also found to be significantly reduced after FOS supplementation, resulting in a 5% decrease in total energy intake per day.17
A randomized, double-blind, placebo-controlled trial investigated the effects of FOS supplementation in overweight and obese subjects. Over a 12-week period, subjects consumed either a placebo or 21 g/day of FOS. Subjects in the FOS group experienced a mean reduction of 1.03 kg body weight compared with a 0.45 kg increase in weight in the control group (P = 0.01). FOS consumption was also associated with a lower area under the curve for ghrelin (P = 0.004) and a higher area under the curve for peptide YY (P = 0.03), suggesting an upregulation of satiety hormone secretion. These changes coincided with a reduction in self-reported caloric intake (P ≤0.05). Serum glucose concentrations and insulin levels also significantly improved in the FOS group compared with baseline measures (both P ≤0.05).51
Studies showed a bifidogenic effect in dosages of 4 to 40 g/day of FOS. The optimum dosage in adults, in terms of side-effect profile and increases in bifidobacteria, is considered to be 10 g/day.20 However, it may be a good idea to start off with a lower dose (e.g., 3 g/day) and slowly increase it to reduce chances of adverse GIT reactions. Dosages of less than 3 g/day in adults are unlikely to cause significant alterations in the GIT microecology. Studies in infants and toddlers generally administered between 1 and 3 g/day of FOS.
FOSs are components of many common foods. There are no genotoxic, carcinogenic, mutagenic, teratogenic, or toxicologic effects associated with the ingestion of any FOS.16,52 Oligofructose and inulin are officially recognized as natural food ingredients in most European countries and have a self-affirmed “Generally Regarded as Safe” status in the United States.9
Recently, a published case study described an instance of anaphylaxis attributed to inulin found in vegetables and processed foods. This was later confirmed with skin-prick testing and blinded food-provocation testing.53 This allergy does appear to be extremely rare, however, considering the widespread consumption of FOS-containing foods.
The only side effects noted with administration are mild digestive symptoms, such as flatulence, borborygmi, abdominal bloating, and abdominal discomfort. However, these effects are dose-dependent and occur less regularly in smaller doses. Over time, these symptoms will diminish as the intestinal flora adjusts to the greater amount of substrate available.54 However, some individuals may continue to experience mild abdominal bloating and discomfort even with continued use.
There have been some concerns in the literature regarding the ability of Klebsiella pneumoniae to utilize FOS as a growth substrate.55,56 FOS was shown to stimulate the growth of K. pneumoniae in Petri dishes. However, this occurred only when Klebsiella was grown in isolation, with no other competing organisms present. In mixed culture experiments, where Klebsiella was grown in the presence of many other human GIT microorganisms, this did not occur.10 In these situations, which more closely resemble the environment of the human GIT (where over 500 species of bacteria compete for available growth substrates),57 FOS did not stimulate the growth of Klebsiella.9 In addition, no human or animal experiment has ever reported an increase in the Klebsiella concentration in the GIT after FOS consumption.
Galactooligosaccharides (GOSs) are chains of galactose molecules, typically containing between three and eight units, with a glucose molecule at the reducing terminus. Galactose-containing oligosaccharides are found in all mammalian milks.58 Commercially, they are produced synthetically from lactose using the bacterially derived enzyme β-galactosidase.59 GOSs are neither broken down nor absorbed in the upper GIT, reaching the colon intact after ingestion.