Figure 1.3 The composition of the gut microbiome in people at different age stages.

      Source: Based on Koenig [110] and Biagi et al. [111]

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The initial development of the human gut microbiota is shaped during birth through microbial colonization introduced by the environment. During gestation, fetuses are generally considered germ‐free in utero, where the gut microbes of the individual microbiota are introduced post‐delivery. The microbiome is shaped by initial microbes introduced during childbirth, where infants delivered through natural birth and Caesarean‐section (C‐section) have different microbiota composition [18, 112, 113]. The GI tract of infants delivered by natural birth is primarily colonized by maternal vaginal and fecal bacteria with the enriched abundance of Lactobacillus and Bifidobacterium spp. [114], whereas the GI tract of C‐section infants is colonized by other environmental bacteria [112]. The microbiome is further shaped by the infants' diet, where the breast‐fed infants have more heterogeneous microbiota with higher taxonomic diversity than formula‐fed babies [115]. These variations in the delivery method and diet contribute to the maturation of the infant's immune system through the gut microbiome development [116]. Breast‐fed infants have further exposed microbes present in the milk and breast surface, accounting for over 700 species of bacteria [117] made up primarily of Streptococci and Staphylococci [118]. Breast milk is also rich in complex oligosaccharides that stimulates the growth of beneficial microbial groups such as Staphylococci [118] and Bifidobacteria [119]. In comparison, the microbiota of formula‐fed babies adapts a microbiota similar to that of an adult, with an increased abundance of E. coli, Clostridium difficile, Bacteroides fragilis, and Lactobacilli [120, 121]. The microbiota during the age of 0–3 years old is highly dynamic, which stabilizes after the age of 3 years [122].

      Children (3–10 years old) undergo massive changes in the microbiota composition, particularly due to the introduction of solid dietary foods. Food solids comprise various nutrients and fibers that facilitate the colonization of various microbial groups including butyrate producers such as Bacteroides and certain Clostridium species [110, 123]. The diet introduced during the pre‐adolescence phase influences how the microbiome takes shape, where children provided with a balanced diet (meat/fish, fruit, vegetables, eggs/beans, and bread/pasta) showed different microbiota shift compared to those given an unhealthy diet (processed, sugar‐rich, and fatty foods) [124]. A study conducted in Japan discovered that Ruminococcus and Bacteroides were found to be enriched in children provided with unprocessed foods (e.g. meat/fish, fruit), whereas Blautia and Clostridium were abundant in the GI tract of children provided with processed food. Additionally, micronutrients provided through nutritional beverages were found to influence the microbiota population. Children provided with the Growing Up Milk‐Lite (GUMLi) was found to have increased bifidobacterial abundance compared to natural bovine milk and other milk formulations [124], indicating that micronutrients can be used to alter the microbiota.

The microbiota diversity in adults is similar to the children gut microbiome, but varies in the abundance of the various groups where adults showed a lesser abundance of Actinobacteria, Bacilli, Bifidobacterium, Faecalibacterium spp., Clostridium cluster IV (Ruminococcaceae), and Bacteroidetes [125, 126]. Clostridium cluster XIVa (i.e. Butyrivibrio crossotus and related bacteria), Firmicutes, and Bacteroides were more abundant in adults than children [127–129]. Other phyla showing a lower abundance in average adults include Proteobacteria, Verrucomicrobiota, Actinobacteria, and Euryarchaeota; where the various microbiota members play a role in microbes maintaining the host immune homeostasis [130]. The adult microbiota is relatively stable but can be perturbed by changes in diet, physical activity, illness, and changes in hormonal cycles and medical therapies. Alternations of the microbiome may positively or negatively impact the host health, where the microbiome is linked to various medical issues [131]. This will be discussed in the following subchapter 1.3.

      The composition of the intestinal microbiota of people in their golden age (>65 years) differs largely between individuals [132]. These microbiota differ even further compared to the diversity of core microbiota in younger adults [111, 132]. The gut microbiome of elderlies has increased abundance of facultative anaerobes (such as Proteobacteria and Bacilli) and decreased abundance of F. prauznitzii and Clostridium cluster XIVa bacteria. It was also reported that centenarian's microbiota shows decreased abundance of Bacteroides, Bifidobacterium and Enterobacteriaceae; and enriched Clostridium spp. abundance [133].

      The composition of the microbiota is certainly influenced by age; however, the dietary habits during infancy and pre‐adolescence play an important role in shaping the diversity of the microbiota. The dysbiosis of the microbiota during adulthood alters the host biochemistry, resulting in the changes of the host immune system, behavior, and susceptibility to disease.

      1.2.4 Continental Dietary Difference and Its Effect of the Local Microbiome

      1.2.4.1 Asia

      Dietary habits in Asia are often influenced by rice consumption, which is widely cultivated in Southeast Asia. Other than rice, there is a large diversity of food depending on the agricultural activity within the region [134–136]. While interstate trade supplements domestic production, the main dietary denominator remains in the regional agricultural activities [137]. On top of this, many developing countries in Asia have governmental recommended dietary allowances (RDAs) that also influence eating habits. A study conducted in Zhejiang, China, showed that the mean daily nutrient intake by urban women met the national RDA, meeting the required levels of macronutrients (energy, carbohydrate, protein, and fat). The Chinese government regulates the national food supply to ensure that each state receives foods that meet the nutritional requirements [138].

      Additionally, fermented foods that are rich in prebiotics and probiotics are heavily consumed in Asia‐Pacific countries. Such local foods include tempeh, tempoyak (Southeast Asia), natto (Japan), and fermented tea (China and Taiwan); and influence the gut microbiota. Asia‐Pacific children are noted to have higher Bifidobacteria abundance [139], due to supplementary fermented foods in the diet such as Japanese fermented milk products and Korean kimchi [140, 141].

      1.2.4.2 Europe

Due to extensive animal‐based husbandry in Europe, tight regulations are enforced to control the release of anthropogenic greenhouse gas emissions (GHGEs) accounting for 25% of total GHGE in Europe [142, 143]. Even so, the main agricultural produce in Europe is red meat and dairy products [144, 145], thus making red meat (processed and unprocessed) and dairy products as part and parcel of the integral diet in Europe. This led to a subtle change in the Western/European microbiome often showed a higher abundance of Prevotella and Bacteroides than Asia‐Pacific microbiome that favors Actinobacteria [146].

      1.2.4.3 Australia

      The Australian continent agricultural activity focuses on producing wheat, barley, canola, chickpeas, and oats in the winter while producing sorghum grain in the summer. On top of this, other agricultural activities are focused on farming sugarcane, leaving limited farming grounds for orchards and vegetables. This results in lower consumption of fruits and leafy vegetables that are a rich source of prebiotics [147]. Australian diet is also heavily influenced by meat and dairy products [148]. This leads to close to 20% of the adult population being classified as obese as reported by the WHO in 2012 [149, 150]. It is possible that the dietary pattern influenced the increased incidences of Clostridium difficile infection and increased rates of ulcerative colitis (UC) observed in Australia [151]. Additionally, it was found that the dairy‐rich diet in children also influenced enriched Firmicutes‐affiliated and Bifidobacterium lineages [124].

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