Modulating the human intestinal microbiome in healthy adults and elderly through dietary supplements

The human intestinal tract plays an important role in human health, especially in food digestion and fermentation, as well as in nutrient absorption and immune modulation. Along its entire length the intestinal tract is inhabited by a large number of microbes, collectively called intestinal microbiota. The intestinal microbiota, composed of bacteria, archaea, viruses and eukaryotes (e.g. fungi and protozoa), interacts with the host through many ways, including (but not limit to) immune modulation and metabolite production (e.g. short chain fatty acids and vitamins), and hence, plays an important role in human health.

The work described in the thesis aimed to decipher the composition and function of the GI tract microbiota in adults, especially elderly, as well as the response of the microbiota to different dietary fibre supplementations and how this is related to health. This thesis comprises seven chapters, including one review (Chapter 2) and four research chapters. Three of the research chapters focus on different dietary interventions (Chapter 3-5), whereas one research chapter focusses on the metabolic capacity of intestinal microbiota (Chapter 6).

Chapter 2 dived into the literature and provided a detailed description of changes in the intestinal microbiota in the ageing population, highlighting the importance of health status. Based on the available literature, we hypothesized that observed changes in intestinal microbiota are not based on chronological ageing, but alterations in health status during ageing. Chapter 3 showed high similarity in the microbiota of healthy adults and that of healthy elderly, aside from a few genera that differed in relative abundance. Chapter 4 compared the microbiota of healthy adults and pre-frail elderly and revealed significant differences between age groups with the relative abundance of Bifidobacterium being lower in elderly. Moreover, in Chapter 6, comparing to the microbiota of healthy adults, the microbiota of pre-frail elderly demonstrated decreased metabolic capacity in carbohydrate degradation based on a subgroup of subjects in Chapter 4. Dietary interventions are often used in studies aiming to change microbiota composition. In Chapter 3, the supplementation of sugar beet pectin, however, did not alter faecal microbiota composition or microbial diversity in healthy adults and healthy elderly. In Chapter 4, the supplementation of GOS changed the microbiota of healthy adults and pre-frail elderly with significant increases in the relative abundance of Bifidobacterium and decreases in microbial diversity in both age groups. These changes did not result in alterations in faecal and breath metabolites, nor in immune and oxidative stress markers. This could attribute to the fact that recruited pre-frail elderly are still healthy and hence leave little room for improvements. Using in vitro incubation experiments described in Chapter 6, we found that a large number of microbes changed in their relative abundance in response to different carbohydrates, with altered genera depending on the type of carbohydrate. Moreover, the increase in the relative abundance of Bifidobacterium was most pronounced in the microbiota of adults, compared to pre-frail elderly. In Chapter 5, we identified significant differences between the microbiota of the small intestine and that of faeces. Moreover, the microbiota composition changed gradually from upper, middle to lower sections of the small intestine. Aside from decreased microbial diversity in duodenum, two weeks of synbiotic supplementation did not alter overall microbiota composition in the small intestine and faeces, whereas within a day, the synbiotic ingestion induced remarkable transient changes in small intestinal microbiota composition.