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Lipids

Fat‐rich diets including saturated fats from animal foods negatively affect the gut microbiota, leading to poor metabolization of the nutrient and ultimately leading to obesity. Studies involving murine models showed depletion of Bacteroidetes and Bacillus bifidus abundance and enrichment of Firmicutes and Mollicutes in mice fed with a high‐fat diet [93]. However, such effects on the microbiome are less severe in mice fed with moderate amounts of polyunsaturated fats such as omega‐3, omega‐6, and omega‐9 [94]. Similarly in human studies, a high‐fat diet increases the abundance of anaerobic microbes and Bacteroides [95, 96]. Patients adapting a low‐fat diet showed the increased fecal abundance of Bifidobacterium and decreased proportion of Faecalibacterium prausnitzii [95]. Table 1.4 summarizes the effect of dietary fat on the gut microbiota.

Table 1.4 Effect of dietary fat on gut microbiota.

	Lacticacid bacteria	Bifido bacteria	Clostr idiales	Bacter oides	Bilo phila	Faecalibacterium prausnitzii	Akkermansia muciniphila	References
High fat	↓		↑	↑				[70, 95, 97, 98]
Low fat		↑						[95]
High saturated fat				↑	↑	↑		[95, 96]
High unsaturated fat	↑	↑					↑	[95, 99]

      Lactic acid bacteria include Lactobacillus and Streptococcus.

      Source: Based on Walker et al. [92].

      1.2.2 The Socioeconomic Impact on Diet‐Related Microbiome Changes

Access to proper living conditions, sufficient nutrition, and clean water severely affect the host microbiota [100]. Maslow's hierarchy of needs can be generally divided into three main needs categories: basic, psychological needs, and self‐fulfillment (Figure 1.2a) [102]. Poorer nations with lower gross domestic product (GDP) per capita have limited access to different types of foods, comprising mostly simple carbohydrates. They are also granted limited access to clean water, resulting in poor sanitary conditions. This can be seen in a number of nations in the African continent and some Southern American countries (Figure 1.2b–e). People within these wealth groups require basic needs and are usually underfed, where some might be under‐nourished. Citizens from countries within the middle‐income range have access to foods that can meet the dietary requirements. These people have psychological needs, where despite having the same dietary access as countries with high GDP, these people can satisfy their needs adequately due to their limited spending power [103]. Citizens of the developed countries with high GDP are generally regarded as having self‐fulfillment needs. Due to a surplus of foods and greater spending power, citizens in these countries adopt dietary habits based on their personal preferences. This results in higher consumption of meat, dairy products, sugary products, and processed foods resulting in increased incidences of diabetes and obesity [104]. Countries falling under these categories are nations from Northern America, Western Europe, and East Asian (Figure 1.2b–e).

Figure 1.2 The socioeconomic impact on dietary habits. (a) The schematic of Maslow's hierarchy of needs and the different categories of needs. (b) The global distribution of average income generated per annum wealth [101]. (c) The global percentage of obese patients in different nations. (d) The global average consumption of sugar per individual. (e) The global average consumption of meat per individual.

      Source: Based on LaMagna [101].

On top of access to different diets, individuals of wealthier groups also have access to certain types of dietary luxuries due to their purchasing power. Such dietary luxuries include access to alcohol, tobacco, and fatty foods (e.g. foie gras, caviar). The consumption of alcohol and tobacco has been known to perturb the gut microbiota populations [105]. The exposure of compounds found in drinking and chewing tobacco can influence the mucosal layer in the GI tract that serves as the initial protective barrier against pathogenic microbial colonization [106]. Chewing or smoking tobacco was found to increase the abundance of anaerobic bacterial species in the oral cavity and upper GI tract, where there was observable perturbation in the oral microbiota from the genera of Actinobacillus, Porphyromonas, Lautropia, and Bifidobacterium [107, 108]. Similarly, the chronic exposure of alcohol to the oral and GI microbiome reduces the abundance of Akkermansia muciniphila (Verrucomicrobia phyla) that exhibits anti‐inflammatory properties [109]. Thus, based on the wealth of the individual, the dietary habits influence the gut microbiome and the host health as a whole.

      1.2.3 Age Groups and Dietary‐Related Microbiome Changes

The composition of the average human's microbiome changes over the span of their lifetime, where these changes are attributed to the individual's basal metabolic rate (BMR), host biochemistry, lifestyle, and dietary habits. The BMR of an average individual peaks around the late teenage years and declines as the individual ages. Coupled to the eating habits, the gut microbiota changes depending on these factors, where the composition of the gut microbiota shows vast differences at different growth stages (Figure 1.3).

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