Effect of prebiotics on the human gut microbiota of elderly persons

Abstract

The colonic microbiota undergoes certain age related changes that may affect health. For example, above the age of 55–65 y, populations of bifidobacteria are known to decrease markedly. Bifidobacteria are known inhibitors of pathogenic microbes and a decrease in their activities may increase susceptibility to infections. There is therefore interest in trying to reverse their decline in aged persons. As the gut microbiota responds to dietary intervention, both probiotics and prebiotics have been tested in this regard. Probiotics are live microbes in the diet, whereas prebiotics are fermentable ingredients that specifically target components of the indigenous microbiota seen to be beneficial. We have published a recent paper demonstrating that prebiotic galactooligosaccharides can exert power effects upon bifidobacteria in the gut flora of elderly persons (both in vivo and in vitro). This addendum summarizes research that led up to this study and discusses the possible impact of prebiotics in impacting upon the gut health of aged persons.

Keywords: :

• AGE
• bifidobacteria
• colonic fermentation
• gut flora
• prebiotics

This article refers to:

Introduction

It is estimated that well over 1,000 different species of bacteria exist in the gut of a healthy adult, the majority of which are strict anaerobes.¹ Bacteroides, Lactobacillus, Clostridium, Bifidobacterium, Fusobacterium, Eubacterium, Enterococcus, enterobacteria and Escherichia are examples of genera that are present in the colon, which is by far the most heavily colonised area of the gut. Generally, gut bacteria may be categorized into beneficial, detrimental or neutral groupings. There is much evidence that both lactobacilli and bifidobacteria are positive for gastrointestinal health.^2-6 As such, they are the usual targets for dietary intervention studies that aim to modulate the gut microbiota. Dietary substrates reaching the large intestine are able to influence the composition and activities of bacteria present therein through their fermentation capacities. Modulation of the intestinal microflora by dietary means is the basis for the probiotic⁷ and prebiotic⁸ concepts. Various strains of bifidobacteria and lactobacilli as probiotics have also been shown to exert immunostimulatory properties.⁹ In elderly persons, a distinct alteration in the composition of intestinal microbiota is evident and it has been suggested that bifidobacterial numbers decrease markedly.^10,11 It is unknown why bifidobacteria decrease with age but this could be related to dietary changes, antibiotic use or alteration of receptor sites in the gut. Nevertheless, it is possible that decreased populations of bifidobacteria can contribute to increased disease risk as we age. The use of probiotics and prebiotics to reverse this change has been used and studies are summarized below.

Probiotics

Research has indicated that the administration of probiotics (live microorganisms which are beneficial to the health of the host) are able to optimise gut microbiota leading to prevention of diseases as well as enhanced immunity. The most well known probiotics are species of Lactobacillus and Bifidobacterium. These are found naturally in the gut and used by the food industry in products such as yogurt, juices, cheeses, capsules, pills and dairy drinks.¹² Some studies have been performed looking at the effect of probiotics on the elderly gut microbiota. Of these, a notable rise in Bifidobacterium and Lactobacillus levels and a reduction in more deleterious microorganisms following probiotic consumption was apparent (Table 1).

Table 1. Effect of probiotics on gut microbiota composition of elderly humans

Study design	Subjects	Age	Probiotic	Result	Reference
Randomized, placebo-controlled	n = 33 (placebo) n = 33 (probiotic)	83 ± 7 y 84 ± 8 y	Fermented oat drink containing Bifidobacterium longum and B. longum	⇑B.catenulatum ⇑ B. bifidum ⇑ B. breve	(Lahtinen et al.^13)
Randomized, double-blind, placebo-controlled	n = 20 (placebo) n = 20 (low dose probiotic) n = 20 (medium dose) n = 20 (high dose)	> 60 y	Reconstituted skimmed milk containing Bifidobacterium lactis	⇑ bifidobacteria ⇑ lactobacilli ⇑ enterococci ⇓ enterobacteria	(Ahmed et al.^14)
Randomized, double-blind, placebo-controlled	n = 9 (placebo) n = 9 (synbiotic)*	71 y (mean) 73 y (mean)	Bifidobacterium bifidum and B. lactis (probiotics) together with an inulin-based prebiotic	⇑ bifidobacteria ⇑ lactobacilli ⇓ B. bifidum	(Bartosch et al.^15)

* a synbiotic is a probiotic and prebiotic mixture

Prebiotics

A somewhat less well documented and more recent concept is prebiotics. A prebiotic is a non-viable food component that confers a health benefit on the host associated with modulation of the microbiota.¹⁶ Prebiotics are naturally available in breast milk and in certain vegetables (in low amounts) but can also be synthetic oligosaccharides of which the best known and the most researched examples include fructooligosaccharides (FOS) and galactooligosaccharides (GOS).¹⁷ As with probiotics, few studies have focused on the influence of prebiotics on the elderly gut microbiota however current evidence is promising, showing distinct increases in bifidobacteria and some decreases in pathogenic bacteria (Table 2). One additional concept, where research is more sparse is that of synbiotics.⁸ Here, mixtures of probiotics and prebiotics are used together. The aim is that the prebiotic will help to fortify probiotic survival in the gut.

Table 2. Effect of prebiotics on gut microbiota composition of elderly humans

Study design	Subjects	Age	Prebiotic	Dose	Result	Reference
Randomized double-blind, parallel study	n = 10 (prebiotic inulin) n = 15 (lactose)	76.4 y (mean)	Chicory inulin (prebiotic)	20 g/day (Days 1–8) 40 g/day (Days 12–19)	⇑ bifidobacteria, ⇓ enterobacteria ⇓ enterococci	(Kleessen et al.^10)
Randomized double- blind, placebo controlled, crossover study	n = 37	> 50	GOS in juice	4g/day	⇑ bifidobacteria ⇑butyrate no change in fecal water genotoxicity	(Walton et al.^18)
Single treatment study	n = 19	85 ± 6 y	FOS powder	8g/day	⇑ bifidobacteria	(Guigoz et al.^19)
Randomized double-blind, placebo-controlled, crossover study	n = 44	69.3 ± 4.0 y	GOS mixture	5.5g/day	⇑ bifidobacteria ⇑ Lactobacillus-Enterococcus spp ⇑ C. coccoides-E. rectale group ⇓ C. histolyticum group ⇓ E. coli ⇓ Desulfovibrio spp	(Vulevic et al.^20)
Single treatment study	n = 12	69 ± 2 y	FOS powder	8g/day	⇑ bifidobacteria	(Bouhnik et al.^21)

(FOS, fructooligosaccharides; GOS, galactooligosaccharides)

Our recently published study,¹⁸ to which this article is an addendum, tested a prebiotic GOS in vitro and in vivo for its prebiotic effects in elderly persons. A total of 37 volunteers completed a randomized, double-blind, placebo-controlled crossover trial. The treatments—juice containing 4 g GOS and placebo—were consumed twice daily for 3 weeks, preceded by 3-week washout periods. To study the effect of GOS on different large bowel regions in vitro, three-stage continuous culture models were performed in parallel using faecal inocula from three volunteers. The model was set up to reflect microbial events in the proximal, transverse and distal colon. Microbial samples were enumerated by molecular based procedures. In vivo, following GOS intervention, bifidobacteria were significantly more elevated compared with post-placebo (p = 0·02). Similarly, GOS supplementation had a bifidogenic effect in all vessels of the in vitro systems. Prebiotic GOS supplementation significantly increased bifidobacteria numbers in vivo and in vitro. We also noted increased production of the bacterial fermentation product butyrate. This is seen as a desirable metabolite but does not arise from the activities of bifidobacteria. Further work is necessary to determine the microbial source of butyrate in these studies.

While the noted bifidobacterial decline with age can be reversed by prebiotics, clarity on the specific health outcomes of this are not wholly apparent. Bifidobacteria are recognized inhibitors of gut pathogens,²² so one beneficial property of increasing their numbers and activities is almost certain to be improved resistance to infections. This has particular relevance for elderly persons. A further facet may be impact upon immunosenescence.

Immunosenescence

Immunosenescence refers to the deterioration of the immune system bought on by the advancement of age. As we grow older the epithelial barriers of the skin, lung and gastrointestinal tract degrade enabling the invasion of delicate mucosal tissues by pathogens.²³ A significant reduction in humoral response is therefore observed after vaccination or infections.²⁴

It is well known that prebiotics support the growth of potentially beneficial bacteria (bifidobacteria). However, research with prebiotics in order to bring about an immunostimulatory effect is sparse, with most data originating from animal models.²⁵ Hosono et al.²⁶ showed that consumption of FOS in mice increased phagocytic capacity of macrophages promoting the destruction of foreign organisms. One study, where individuals aged 77–79 y received 8g FOS daily for three weeks, showed increased bifidobacterial levels accompanied by significant rises in total lymphocyte count (CD⁴⁺ and CD⁸⁺ cells).¹⁹ Contradictory to the observations found by Hosono et al.,²⁶ in animal models, reduced phagocytic activity of polymorphs and monocytes were noted in this study. The expression of pro-inflammatory cytokine IL-6 mRNA in peripheral blood mononuclear cells (PBMCs) was observed and attributed to a general decrease in inflammation. Immunostimulatory effects of GOS have been documented. One trial investigated the effects of GOS on the elderly. 44 healthy elderly subjects (65–80 y old) were randomly assigned a prebiotic (GOS) or placebo. Results showed that B-GOS increased levels of bifidobacteria, phagocytosis, NK cell activity and IL-10 cytokines while reducing the pro-inflammatory cytokines (IL-6, IL-1β and TNFα).²⁰

In summary, these findings indicate that prebiotics may influence several effects upon immune function:

• Promoting the growth of beneficial bacteria leading to inhibition of pathogenic growth and therefore reducing the stimulation of pro-inflammatory cytokines (IL-6, IL-1β, TNFα). This lowers the risk of developing chronic inflammation.

• Initiation of inflammatory response due to foreign body invasion is improved through elevated levels of IgA, lymphocyte counts and cytokines.

• Promoting the destruction of pathogens through increasing phagocytic cell and NK cell activity.

• Counteracting chronic inflammation by increasing levels of IL-10 (anti-inflammatory cytokine).

Future Aspects

It must be stated that definitive proof of the health impacts of prebiotics generally needs more research. Specifically for elderly persons improvements in immunosenescence and general resistance to infections may be appropriate.

Similarly, the science of metabonomics offers the possibility of determining the more functional impacts of microbiota changes. Metabonomic studies using a range of analytical techniques (including ¹H NMR spectroscopy, HPLC-MS, UPLC-MS and GC-MS) have been applied to the analysis of urine, faeces and plasma to assess all the metabolites present therein. The microbial genome interacts strongly with the mammalian metabolic system and results in the formation of metabolites that may impact upon host health. In order to investigate the potential for such interactions a combination of metabonomic and microbial analyses may be applied to future human studies that use prebiotics to modulate the microbiota. One question that can be addressed is the impact of the microbiota on the overall human metabonome (and can this be altered by using prebiotics to affect microbial composition and activity)? Our current human studies are addressing this in concert with health markers like immunosenescence, including in the elderly.

Abbreviations:
FOS	=	fructooligosaccharides
GOS	=	galactooligosaccharides