Background: Alterations of the gut microbiome may accelerate the course of Alzheimer's dementia through the enhancement of inflammatory and neurodegenerative processes. Prebiotic galactooligosaccharides increase the abundance of Bifidobacterium species and may have beneficial effects on disease progression.
Methods: In a group of Alzheimer patients taking a galactooligosaccharide preparation for 12 weeks, the effects on bacterial abundances and Shannon index of the gut microbiome as well as neurofilament light-chain serum concentration were assessed.
Results: There was an increase of the abundance of the genus Bifidobacterium and an increase of the Shannon index when taking galactooligosaccharides. The increase of the Shannon index was correlated with a decrease of the neurofilament light-chain serum concentration.
Discussion: These findings suggest a beneficial effect of galactooligosaccharides on the gut microbiome and an inhibition of neurodegeneration in patients with Alzheimer's dementia. Further clinical studies may be warranted.
Alterations of the gut microbiome in Alzheimer patients have been observed by different researchers [1-3]. Reproducible alterations identified so far are a decrease in microbial diversity and changes in abundance of the genera Bifidobacterium and Escherichia [4]. Various mechanisms have been suggested by which gut microbiota may influence the course of Alzheimer’s dementia. There may be anti-inflammatory effects of butyrate-producing species mitigating the course of the disease [5]. Other bacteria or bacterial components may lead to inflammation and increased permeability of the colon epithelium. They may consequently promote the transgression of bacteria or bacterial components into the systemic circulation and enhance inflammatory and neurodegenerative processes [6] as well as amyloid-β pathology [7].
Neurofilament light-chain (Nfl) is a part of the neuronal cytoskeleton and is released in the case of axonal damage [8,9]. CSF and serum levels of Nfl are highly correlated [8]. Nfl is increased in Alzheimer’s dementia and mild cognitive impairment [10]. In patients with Alzheimer’s disease, the level of Nfl is correlated with the extent and the further progression of cognitive impairment [11,12]. Therefore it may be used as a marker of neuronal destruction quantitatively reflecting neurodegeneration in Alzheimer’s disease.
Prebiotics are substrates selectively utilized by host microorganisms conferring a health benefit [13]. Galactooligosaccharides (GOS), a component of human mother’s milk, are complex carbohydrates and act as physiological prebiotics [14]. They are resistant to digestion in the upper gastrointestinal tract and reach the colon intactly. There they have a bifidogenic effect [15,16] and may mitigate the symptoms in inflammatory diseases of the colon [17] as well as of other organ systems [18]. They act as soluble decoy receptors, thus preventing the adhesion of pathogens to epithelial cells and they stimulate tight junctions. Thus, GOS may also help to mitigate inflammation und slow down neurodegeneration in Alzheimer’s disease. Against this background, we conducted a pilot study on the effects of a GOS preparation (Dynabion ® ) in Alzheimer patients.
The assessments were carried out in a group of out-patients with probable mild Alzheimer’s dementia according to the NINCDS-ADRDA criteria [19]. None of them was suffering from inflammatory bowel disease or diabetes mellitus, had had an infectious disease during the last three months or had been under antibiotic treatment during the last six months.
The patients took app. 5 grams of a galacto-oligosaccharides powder daily with added small amounts of curcumin and berberine (Dynabion ® ) for 12 weeks. The powder was dissolved in a glass of water and taken before breakfast.
Stool samples were obtained before and at the end of the GOS intake, immediately frozen at -20 °C and later processed by means of PCR amplification and subsequent 16S-rRNA sequencing using the commercially available MiSeq ® system (Illumina Inc.). Bacterial diversity was measured by means of the widely used Shannon index, which quantifies entropy and increases with the number of detected species and the evenness of their abundances [20]. Further we assessed the abundances of the genera Bifidobacterium and Escherichia , which have been reported to be significantly altered in Alzheimer’s dementia [4]. At both time points, serum samples were obtained by peripheral venipuncture. Nfl was quantified by an ultrasensitive single molecule array (Simoa) on a Quanterix SR-X Analyzer using the NF-light ® assay.
Differences of Shannon index, bacterial abundances and Nfl serum concentrations before and during GOS intake were assessed by paired Student’s t-tests. Bacterial abundance values had been ln-transformed in order to normalize distributions. The relationships between gut microbiota and serum concentration of Nfl were analyzed by means of Spearman rank-correlation coefficients. Given the exploratory nature of this study, no adjustment for multiple testing was performed.
The assessments were carried out in 10 Alzheimer patients, 5 women and 5 men at ages between 71 and 85 years (mean: 78 years) with a body mass index (BMI) between 19.7 and 29.3 (mean: 25.2). Their MMSE scores were between 18 and 26 (mean: 21.2). The duration of dementia ranged between 15 and 36 months (mean: 24.4 months).
After GOS intake, the abundance of the genus Bifidobacterium increased from 1.33% (mean; range: 0.004-8%) to 1.70% (mean; range: 0.001 to 5%) (p = 0.042), mainly in the patients with initially low abundances (Figure 1a). There was also an increase of the Shannon index from 2.64 ± 0.69 (mean ± SD) to 2.98 ± 0.47 (P = 0.047). The increase was found particularly in the patients with initially low Shannon indices (Figure 1b). The abundance of the genus Escherichia and the serum concentration of Nfl were not significantly altered.
Figure 1: Effects of galactooligosaccharides on the gut microbiome. (A) Effect of galactooligosaccharides on the abundance of Bifidobacterium. The abundance of the genus Bifidobacterium (in%, logarithmic axis division) is shown for each individual patient before and after taking galactooligosaccharides for 12 weeks; (B) Effect of galactooligosaccharides on the Shannon index. The Shannon index is shown for each individual patient before and after taking galactooligosaccharides for 12 weeks.
View Figure 1
After taking GOS, there was a significant negative correlation between the changes of the Shannon Index and those of the Nfl serum concentration (r = 0.750; p = 0.021) (Figure 2). No significant correlations between the abundances of Bifidobacterium or Escherichia and the Nfl serum concentration could be ascertained.
Figure 2: Correlation between the changes of Shannon index and Neurofilament light-chain when taking galactooligosaccharides. The changes of the serum concentration of neurofilament light-chain (in ng/l) are plotted over the changes of the Shannon index for each individual patient. The regression line is given.
View Figure 2
A bifidogenic effect of GOS has been reported in healthy elderly subjects [15,21] and in patients with ulcerative colitis [17]. We could confirm this finding in patients with Alzheimer’s disease. Bifidobacterium , which is reduced in the gut microbiome of elderly subjects [22,23], protects the colonic mucus layer enhancing mucus production [24] through the modulation of goblet cells [25].
The increase of Bifidobacterium when taking GOS has health promoting effects. The production of the pro-inflammatory cytokines IL-6, IL-1β, and TNF-alpha is reduced [21]. We also observed an increase of microbial diversity reflected by an increase of the Shannon index. The increase of the Shannon index was correlated with a decrease of the neuronal destruction marker Nfl. Thus, taking GOS may inhibit neurodegeneration in patients with Alzheimer’s disease. In particular, patients with a dysbiosis of the gut microbiome, which is manifested by a low microbial diversity, may benefit from this treatment.
This assumption is supported by some clinical findings. Oligomannate, another prebiotic, has been extensively studied in Chinese Alzheimer patients [26] and has shown disease-modifying effects in this patient group [27]. A disease-modifying effect of Bifidobacterium has also been found in a double-blind controlled Japanese trial [28]. Adults with MCI at ages between 50 and 79 years, who were treated with a Bifidobacterium strain for 16 weeks, showed significant improvements in cognitive performance as reflected by the RBANS. Thus, controlled studies with GOS in patients with Alzheimer’s disease may be warranted.
Both authors have contributed to this study, accepted responsibility for the entire content of this manuscript and consented to its submission to the journal, reviewed all the results and approved the final version of the manuscript.