Three studies reported alpha diversity across an urbanization gradient whereby one study compared urban and rural (Oduaran et al., 2020), the second compared urban, semi-urban, and rural (Lokmer et al., 2020), and the third one compared hunter-gatherers versus rural communities-pastoralists and subsistence farmers (Rubel et al., 2020). Lokmer et al. showed an overall decrease in alpha diversity along the urbanization gradient (Lokmer et al., 2020). These findings were echoed by Oduaran et al. where alpha diversity was higher in rural compared to urban communities (Oduaran et al., 2020). The hunter-gatherers in Rubel et al. (2020) showed higher alpha diversity compared to other communities while the pastoralists had the lowest alpha diversity measures (Rubel et al., 2020). Overall, the alpha diversity was higher in hunter-gatherers/rural compared to urban and lower in pastoralists compared to other rural communities.

Beta diversity was presented in five studies (Gomez et al., 2016; Ayeni et al., 2018; Afolayan et al., 2019; Angelakis et al., 2019; Oduaran et al., 2020). Two studies compared hunter-gatherers versus urban communities (Gomez et al., 2016; Angelakis et al., 2019), and both showed a significant difference in beta diversity revealing greater diversity in hunter-gatherer populations. Differences in the diversity measures between urban and rural were also shown in Oduaran et al. (p=0.001 for Bray-Curtis dissimilarity measure) and Ayeni et al. (p<0.05 for Unweighted and Weighted UNIFRAC distances) with higher diversity in rural communities (Ayeni et al., 2018; Oduaran et al., 2020). This is similarly reported in Afolayan et al. where the pastoralists had a more diverse gut microbiome compared to the urban residents’ (Afolayan et al., 2019).

Reporting of the composition of gut microbiota varied between studies. Therefore, complete mapping of the gut microbiota across the hunter-gatherers, rural and urban gradients is limited. Out of the included studies, 5 (4 studies included rural and 3 in hunter-gatherers’ settings) reported 6 phyla in common (higher taxonomic level); Firmicutes, Bacteroidetes, Cyanobacteria, Spirochaetes, Actinobacteria and Proteobacteria (Schnorr et al., 2014; Gomez et al., 2016; Afolayan et al., 2019; Tang et al., 2019; Chen et al., 2021). Firmicutes and Bacteroidetes were the most abundant phyla in both settings. The range for Firmicutes in rural vs hunter-gatherers areas was (46.9-77.63%) vs (49.5-72%) respectively; the range for Bacteroidetes also among rural and hunter-gatherers areas was (13.8-33%) vs (17-44.4%) respectively (Schnorr et al., 2014; Gomez et al., 2016; Afolayan et al., 2019; Tang et al., 2019; Chen et al., 2021). This was followed by Proteobacteria which was higher in rural settings compared to hunter-gatherers, with a range of (5.5 -17.73%) in rural vs (6-8.49%) in hunter-gatherers. The presence of Actinobacteria was relatively low in both rural (1.27-1.79%) and hunter-gatherer populations (less than 0.54%). Spirochaetes were reported in all 3 studies focusing on hunter-gatherer populations, but their abundance was relatively low. In contrast, only 50% of the studies done in rural areas reported the presence of Spirochaetes, with their abundance ranging from low to high. Cyanobacteria were the least commonly reported in 1 study in each setting (0.29% rural) vs (0.7% Hunter-gatherers). Most phyla did not differ between hunter-gatherers and rural in Morton et al. except for Proteobacteria where the relative abundance was higher among hunter-gatherers. The details of the direction of the relative abundance at the phyla level reported across the gradient are shown in Figure 2 .

Eight studies reported gut microbiota at the family level (Dugas et al., 2018; Afolayan et al., 2019; Hansen et al., 2019; Tang et al., 2019; Afolayan et al., 2020; Doumatey et al., 2020; Even et al., 2021; Parbie et al., 2021). The Prevotellacea family was the most frequently mentioned in 5 studies (Dugas et al., 2018; Afolayan et al., 2019; Hansen et al., 2019; Tang et al., 2019; Parbie et al., 2021) and is also mentioned to be the most prevalent in the African populations by Hansen et al. (2019).

Eight studies compared the relative abundance across hunter-gatherers, rural, semi-urban, or urban areas at the genus level as shown in Figure 3 . Complex carbohydrate digesters Bacteroides were the most frequently reported and appeared in five studies (Morton et al., 2015; Afolayan et al., 2019; Lokmer et al., 2020; Oduaran et al., 2020; Rubel et al., 2020). The relative abundance of Bacteroides according to the urbanization level of the assessed population varied across studies. They had a higher relative abundance among hunter-gatherers and rural farmers (Morton et al., 2015), pastoralists (Rubel et al., 2020), and urban (Afolayan et al., 2019). In contrast, their relative abundances were low in hunter-gatherers (Rubel et al., 2020), rural fishers (Morton et al., 2015), pastoralists (Afolayan et al., 2019), and agro-pastoralists (Oduaran et al., 2020; Rubel et al., 2020) and decreased over the urbanization gradient from urban to rural (Lokmer et al., 2020). Hence, the direction of the relative abundance of Bacteroides does not follow a clear gradient. Similar to Bacteroides, there is no clear difference between urban and rural residents for Bifidobacterium, microorganisms known to digest carbohydrates and regulate gut health (Naumova et al., 2020). This component of the microbiota was compared across the urbanization gradient where it had a high relative abundance among rural fishers (Morton et al., 2015) and urban residents (Oduaran et al., 2020) but a low relative abundance among hunter-gatherers (Morton et al., 2015), and farmers and agro-pastoralists (Oduaran et al., 2020).

Blautia was more enriched among rural pastoralists and agro-pastoralists but low in urban (Ayeni et al., 2018; Afolayan et al., 2019) while Blautia (species - obeum) was high in urban and low among agro-pastoralists (Katsidzira et al., 2019). The relative abundance of Bilophila (species - wadsworthia) (linked with disrupted glucose metabolism in mice) (Natividad et al., 2018), Cetobacterium, Ruminobacter, and Turicibacter did not differ between rural agropastoralists and urban residents (Katsidzira et al., 2019; Oduaran et al., 2020).

The relative abundance of the Treponema genus was high in hunter-gatherers compared to rural and was also detected in urban (Oduaran et al., 2020) contrary to the findings in (Angelakis et al., 2019) where no Treponema was detected in urban areas. Out of the six named core genera, Faecalibacterium was the most frequently reported in the comparison (4 out of 8 studies), followed by Blautia (3 studies), Clostridium (2 studies), Ruminococcus (2 studies) and Eubacterium (1 study) (Dehingia et al., 2015). Roseburia was also reported in one study only where its relative abundance increased from rural to urban (Lokmer et al., 2020).

Similar to previously mentioned genera, Faecalibacterium did not follow a consistent pattern. It showed a positive correlation with the urbanization gradient from rural to urban (Lokmer et al., 2020), and displayed a high abundance among pastoralists (Afolayan et al., 2019) and urban (Ayeni et al., 2018; Oduaran et al., 2020), but a low abundance in agro-pastoralists (Ayeni et al., 2018; Oduaran et al., 2020) and urban residents (Afolayan et al., 2019). The relative abundance of Fusobacterium was similar for rural agro-pastoralists and urban residents (Katsidzira et al., 2019), low in agro-pastoralists but high in urban (Oduaran et al., 2020) and vice versa (Ayeni et al., 2018).

Carbohydrate fermenters and producers of the short-chain fatty acid butyrate (Ezaki, 2015), Coprococcus were more abundant in urban residents and low in agro-pastoralists (Ayeni et al., 2018) and increased with the urbanization gradient from rural to urban (Lokmer et al., 2020).

The genus Prevotella was reported in several studies including Prevotella, Prevotella_1, Prevotella_2, Prevotella_9, and Prevotella stercorea. In this review, Prevotella was more abundant in Hunter-gatherers (Gomez et al., 2016; Rubel et al., 2020), agro-pastoralists (Rubel et al., 2020), and urban (Oduaran et al., 2020). The genus decreased in relative abundance across the urbanization gradient from rural to urban (Lokmer et al., 2020) and was less abundant in rural farmers, pastoralists, and agro-pastoralists (Gomez et al., 2016; Oduaran et al., 2020; Rubel et al., 2020). Prevotella_1 was more abundant among hunter-gatherers (Gomez et al., 2016) and urban (Oduaran et al., 2020) and less abundant among farmers (Gomez et al., 2016) and agro-pastoralists (Oduaran et al., 2020). Prevotella_2 and Prevotella_9 were both abundant among pastoralists compared to urbanites (Afolayan et al., 2019) while Prevotella stercorea were more abundant among hunter-gatherers and low among pastoralists (Rubel et al., 2020). Refer to Figure 3 for further details on the distribution of the relative abundance of the genera across the gradient.

Diet was assessed directly or indirectly in 11 studies using 24-hour recalls (Tang et al., 2019; Lokmer et al., 2020), food frequency questionnaires (FFQ) (Schnorr et al., 2014; Morton et al., 2015; Lokmer et al., 2020), narrative reports (Afolayan et al., 2019; Ocvirk et al., 2020), and nutritional questionnaires (Dugas et al., 2018; Rubel et al., 2020). Diets of urban residents consisted of dairy products, meat, sweets, and soft drinks (Lokmer et al., 2020) (Cameroon), low fiber-diet, processed foods, high-fiber diet, fermented drinks (Afolayan et al., 2019) (Nigeria), cereals, refined grains, fruits, and tubers, moderate use of antibiotics, use of treated water with a high level of hygiene (Ayeni et al., 2018) (Nigeria). In contrast, the rural residents reported consumption of tubers, leafy vegetables, peanuts, and palm wine (Lokmer et al., 2020) (Cameroon), grains, tubers, fruits and soups (okra, melon) and traditional soups, regular fish intake with less meat intake, a variety of fermented foods, use river water and rely on local herbs (Ayeni et al., 2018) and cassava, fish and meat (Morton et al., 2015) (Cameroon). Pastoralists reported eating high-fibre-based foods and fermented drinks while agro-pastoralists reported grain and vegetable consumption. Their livestock was mostly for selling purposes (Afolayan et al., 2019). Hunter-gatherers showed high-fiber diets and other food items from hunting and foraging (Rubel et al., 2020) (Cameroon), and high meat and nut consumption (Gomez et al., 2016) (Central Africa) but also low meat consumption (Morton et al., 2015) (Cameroon). Carbohydrate and fiber intake were observed to be similar in urban and rural settings with a high intake of protein in urban (Zimbabwe) (Katsidzira et al., 2019).

The relationship between diet and gut microbiota was assessed in 4 studies only (Morton et al., 2015; Smits et al., 2017; Tang et al., 2019; Rubel et al., 2020). For the Hadza community (Hunter-gatherers) there was a strong link between seasonal changes in diet and gut microbiota composition (Smits et al., 2017). During the wet season, Hadza consumes a plant-based diet (berries, honey, baobab, and tubers) and has access to more meat during dry seasons. Consequently, there was a decline in the population of Bacteroidetes (Prevotellaceae family) were depleted during the wet season and reappeared in the dry season. In contrast, Firmicutes remained stable throughout the seasons.

Second, after adjusting for the presence of Entamoeba spp, Morton et al. (2015) found a persistent low gut microbiota diversity among the fishing community compared to hunter-gatherers and subsistence farmers. Lastly, the presence of genus Ruminoccocus was associated with meat and iron intake while a high relative abundance of Faecalibacterium and Succininvibrio was positively correlated with fish and/or insect intake and vitamin A-rich green vegetables and fruits (Tang et al., 2019). Further details are given in Supplementary Material Appendix 3 .

We observed a difference in both alpha and beta diversity across the urbanization gradient with a higher diversity in hunter-gatherers/rural compared to urban communities. However, the taxa level and diversity metric analysis used differed between studies. The results we found are similar to a meta-analysis on the impact of urbanization on the gut microbiota globally (including a subset of the same studies from SSA included in this review) that showed a lower alpha diversity in urban populations compared to rural/hunter-gatherers settings (Rosas-Plaza et al., 2022). In this meta-analysis beta diversity at family and genera, levels showed similar trends, with the urban and Hunter-gatherers communities at the extremes (lowest vs highest diversity) (Rosas-Plaza et al., 2022). Several studies have evaluated this in other parts of the world with varying results (Clemente et al., 2015; Das et al., 2018; Jha et al., 2018; McCall et al., 2020; Rosas-Plaza et al., 2022). For example, a study in Tibet China showed differences in beta diversity and not alpha diversity across the urbanization gradient, contrary to the studies included here with differences in both measures (Li et al., 2018). Likewise, a study conducted among populations transitioning from foragers to farmers in the Himalayan region did not find any variations in alpha diversity (Jha et al., 2018). Conversely, a study done in South America, which explored the overall and chemical variation across the spectrum from traditional villages to the city center, did not find any differences in diversity along the urbanization gradient (McCall et al., 2020). In general, evidence indicates that gut microbiota in higher-income/industrialized exhibit lower diversity compared to lower-income/non-industrialized/rural/traditional communities (Obregon-Tito et al., 2015; Mancabelli et al., 2017; Rosas-Plaza et al., 2022). Nevertheless, there is a need for additional studies to fully understand the distinctions in gut microbiota among individuals residing in geographically close areas.

Bacteroidetes and Firmicutes are known to dominate the gut microbiota. Six other phyla are proposed to form the 8 core gut microbiota by the healthy gut microbiota (GutFeelingKB data and metadata); Proteobacteria, Actinobacteria, Spirochaetes, Plancytomycetes, Euryarchaeota and Cyanobacteria (King et al., 2019). Similar, to the above-established core gut microbiota, Firmicutes and Bacteroidetes were the most abundant phyla in the included studies. Further, there are differences in the relative abundance of gut microbiota across the urbanization gradient although not uniform for all the phyla/genera as some were relatively more abundant while others were less abundant, as shown in Figure 2 .

The microbiome of the rural (Hunter-gatherers/traditional communities and non-urban) is associated with a low prevalence of diseases including inflammatory bowel diseases, colorectal cancer, and metabolic diseases (Zuo et al., 2018; Cui et al., 2021). Perturbations in a few phyla have been linked to health outcomes such as the Firmicutes: Bacteroidetes ratio where higher Bacteroidetes levels are seen in lean compared to obese individuals (Johnson et al., 2017). Also, a higher relative abundance of Bacteroidetes and Proteobacteria compared to Firmicutes was reported among patients with chronic kidney disease (Lun et al., 2019). In this review, the ratio (Firmicutes: Bacteroidetes) was reported in one study only where it was 5:1 in rural and 1:1 in the Hunter-gatherers’ community (Gomez et al., 2016).

A study in South America from the jungle to the city center by McCall et al. showed a high proportion of Proteobacteria with increasing urbanization (McCall et al., 2020). Similarly, in this review, the relative abundance of Proteobacteria was higher in rural compared to hunter-gatherers’ settings. In contrast to the findings in South America, where the relative abundance of Actinobacteria decreased with urbanization (higher in hunter-gatherers), our review did not identify any clear trend for this phylum (McCall et al., 2020).

Most studies reported the gut microbiota differences at the genus level. Existing evidence has linked individual genera to disease conditions and health outcomes. In this review, there are no clear differences in the relative abundance of identified genera across the urbanization gradient except for Coprococcus where the relative abundance was consistently low in rural and higher in urban (Ayeni et al., 2018; Lokmer et al., 2020). Bacteroides were the most frequently reported in the studies that compared the relative abundance across the gradient. They are known to maintain gut health, digest complex carbohydrates, metabolize bile, maintain the immune system and gut barrier, and produce Vitamins K and B (Xu and Gordon, 2003). This genera together with Faecalibacterium and Blautia were more abundant in urban herdsmen in Tibet contrary to the findings of this review where their relative abundance did not follow a clear gradient (Li et al., 2018).

The relative abundance of the Lactobacillus genus was reported to be high among the Mongolian pastoralists compared to the urban Mongolians possibly due to the high consumption of dairy products (Iebba et al., 2016). However, in our review, Lactobacillus was reported in two rural communities (Iebba et al., 2016; Tang et al., 2019) and there was no difference between pastoral and non-pastoral communities (Afolayan et al., 2019). The biological importance of Lactobacillus is not fully understood as their relative abundance is both negatively and positively correlated with disease outcomes (Heeney et al., 2018).

A study assessing the gut microbiota based on diet, ethnicity, and urbanization in China identified four types of enterotypes including one dominated by the Escherichia genus (Lu et al., 2021). This genus was also reported in a study in Korea contrasting gut microbiota between rural villages and urban (Wu et al., 2011). However, in this review, the Escherichia genus was not reported in any of the studies that compared gut microbiota across the gradient.

The relative abundance of the genus Prevotella varied across the urbanization gradient with a high and low relative abundance of the genera across the gradient without clear direction. Prevotella and its sub-genera were reported across the gradient with alternating relative abundances. The abundance of the Prevotella genus has been shown to correlate with carbohydrate consumption, which is documented in the included studies across the urbanization gradient (Sonnenburg et al. ). A higher relative abundance of Prevotella is often associated with plant-based diets and improved glucose metabolism (Kovatcheva-Datchary et al., 2015; Ley, 2016). The presence of Prevotella throughout the urbanization gradient indicates a potential trend between urbanization and industrialization, as the abundance of Prevotella is nearly absent in industrialized societies (Ley, 2016). This observation suggests that industrialization may have a greater effect on the abundance of this genus compared to the process of urbanization seen in SSA.

The composition and relative abundance of Bifidobacterium in mice fed with fish oil were higher compared to those eating lard (David et al., 2014). In line with this finding, this review found Bifidobacterium to have a high relative abundance among the fishing and urban communities compared to hunter-gatherers and agro-pastoralists (Oduaran et al., 2020). Bifidobacterium has been used as an important probiotic in several food products including yogurt and supplements and is believed to enhance a healthy gut (Naumova et al., 2020).

Bilophila wadsworthia is known to digest taurine a naturally occurring compound in meat and milk fat and produce hydrogen sulfide an inflammatory agent (Yazici et al., 2017; Hanson et al., 2021). Also, their ability to produce secondary bile is linked with the occurrence of colorectal cancer (Turnbaugh, 2012). The relative abundance of Bilophila wadsworthia did not differ between rural agro-pastoralists and urban residents (Katsidzira et al., 2019; Oduaran et al., 2020). This may signify a similarity in animal-based diet consumption among rural agro-pastoralists and urban residents (Mehta et al., 2017).

Blautia was reported to be low among rural pastoralists and agro-pastoralists but high in urban (Ayeni et al., 2018; Afolayan et al., 2019) while Blautia Obeum was high in urban and low among agro-pastoralists (Katsidzira et al., 2019). In Kim et al, the relative abundance of Blautia was low in urban Korea compared to rural villages (Wu et al., 2011). This genus is associated with better host health and reduced risk of metabolic syndrome (Liu et al., 2021).

Coprococcus, a butyrate producer, is positively correlated with a high-quality diet and negatively correlated with hypertension (Mancabelli et al., 2017; Laitinen and Mokkala, 2019). Two studies found Coprococcus were more abundant in urban residents and low in agro-pastoralists (Ayeni et al., 2018) and increased with the urbanization gradient from rural to urban (Lokmer et al., 2020). This is contrary to a study in Japan where Coprococcus were least abundant in urban (Naito et al., 2019). Further, without following the urbanization gradient, the Coprococcus genus was found to dominate the Brazilian population compared to Cameroonians (Schaan et al., 2021).

Fusobacterium is an important gut commensal organism that plays a key role in protecting the body from pathogens (Brennan and Garrett, 2019; Stokowa-Sołtys et al., 2021). However, its isolation from disease-related samples (clinical) has indicated a potential role in disease causation and progression questioning the direction of its effect on health (Brennan and Garrett, 2019). Fusobacterium is linked to an increased risk of colorectal cancer and its high relative abundance is associated with the consumption of Westernized diets (Mehta et al., 2017). In this review, the distribution of Fusobacterium was mixed with some studies reporting high relative abundance in urban areas while others reported a low abundance or no difference between urban and rural residents (Katsidzira et al., 2019; Afolayan et al., 2020; Oduaran et al., 2020).

Similar to Fusobacterium, Faecalibacterium strengthens the gut mucosal integrity and maintains the health of the gut and organism physiology through the production of different anti-inflammatory metabolites such as butyrate and salicylic acid (He et al., 2021). It is recognized as the most ubiquitous species due to its presence in the gut of most mammals including humans and was similarly the most frequently reported genus in this review (He et al., 2021). Faecalibacterium was more abundant in the mountainous rural compared to urbanizing and urbanized areas in Ningxia China (Du et al., 2021). We did not find a clear gradient for Faecalibacterium in the included studies. Like Fusobacterium, Fecalibacterium also exhibits a bidirectional impact on health, with a low relative abundance being associated with irritable bowel syndrome and a high relative abundance being linked to colorectal cancer (Rajilić-Stojanović et al., 2011; Keku et al., 2013).

Diets and individual foods influence the diversity and composition of the gut microbiota as shown in several reviews (Frame et al., 2019; Willis and Slavin, 2020; Kimble et al., 2022; Wang et al., 2022). A scoping review of randomized controlled trials on the effects of meat on the gut microbiota showed an increase in Ruminococcus, Roseburia, Bacteroides, and Anaerostipes genera with a high intake of meat while Faecalibacterium decreased (Wang et al., 2022). A review of the adherence to the Mediterranean diet and gut microbiota did not show any relationship at the higher taxonomic level (phyla) but at the genera level with Ruminococcus, Bacteroides, and Faecalibacterium (Kimble et al., 2022). Another systematic review of the effect of fiber supplementation on gut microbiota showed a significantly higher relative abundance of Bifidobacterium and Lactobacillus compared to other genera (So et al., 2018).

In this review, we could not conclude on the differences in diets across the urbanization gradient and its effect on the gut microbiota as only a few studies used standard methods of dietary assessment. One study reported the link between diet and gut microbiota where similar to (Wang et al., 2022) Ruminococcus was linked to meat and iron intake and Faecalibacterium to fish, Vitamin A-rich green vegetables and fruits similar to (Kimble et al., 2022). Also, there was evidence of changes in the gut microbiota in response to changing diets and seasons. This phenomenon was described in one study only and not across the gradient. Hence, it is evident that the effects of diet on the diversity and composition of the gut microbiota are still under-researched in the SSA. Included studies have mostly named the macronutrients without further quantification of the consumed diets, the micronutrient contents, or the analysis of dietary patterns. For example, the high-fiber diets reported in both rural and urban communities may differ significantly in quantity and quality hence their effect on the gut microbiota.

Intestinal parasites are associated with the composition and diversity of the gut microbiota. Their low prevalence in industrialized communities is thought to contribute to the lower diversity of the gut microbiota, although the mechanism underlying this association is poorly understood (Chabé et al., 2017). A study in Malaysia showed a higher microbial diversity among helminths positive compared to negative colonized individuals (Lee et al., 2014). The evidence from this review partly affirms this finding as it shows a relationship between gut microbiota and intestinal parasites. The presence of intestinal parasites was associated with a high diversity of the gut microbiota in two studies (Rubel et al., 2020; Even et al., 2021) but the effect size was not shown in some studies. Further, the use of anti-helminths was linked to a long-term loss of gut microbiota diversity (Rosa et al., 2018). As the relationship between gut microbiota and intestinal parasites is increasingly seen further studies to inform the public health efforts of mass deworming are needed. The absence of intestinal parasites has also been linked to an increased risk of inflammatory bowel diseases and Crohn’s disease (Panelli et al., 2020).

Evidence links metabolites and enriched metabolic pathways of the gut microbiota with several negative and positive health outcomes. Metabolites and pathways, such as the production of short-chain fatty acids in the colon, amino acid metabolism, carbohydrate metabolism, and secondary metabolite biosynthesis pathways, have been shown to positively influence health outcomes (Lin et al., 2017). Gut microbiome metabolites such as trimethylamine N-oxide (TMAO) and hydrogen sulfide are thought to affect gut and cardiovascular health in both positive and negative ways (Wu et al., 2021). However, evidence is still building on other gut microbiome by-products such as “p-cresol” being a potential neurotoxic (Tevzadze et al., 2017; Tevzadze et al., 2018). In this review, differences in pathway enrichment between rural and urban were reported in two studies where amino acid and carbohydrate metabolism pathways were enriched in the urban population (Afolayan et al., 2019) while carbohydrate metabolism gene pathways were depleted in the hunter-gatherers population (Gomez et al., 2016). The differences in the enrichment were not stated in most studies, however, we noted a uniform reporting of the amino acid metabolism pathways.

Low dietary protein intake is linked with the body using essential amino acids produced by the gut microbiome (Wu et al., 2021). Some bacteria species (e.g. Clostridium acetobutylicum) are known to have a complete gene set for essential amino acids biosynthesis (Nolling et al., 2001). Hence, the wide reporting of the amino acid metabolism pathways may signify a diet lacking all the essential amino acids by the included population. However, given the complex relationship and interpretation of metabolic pathways other factors may explain the pattern of the reported amino acid metabolism pathways. A study done in China in two adjacent rural and urban provinces reported 266 out of 1108 differences in metabolites between the two sites (Wang et al., 2020). Six metabolites were twice the amount in urban compared to rural for Xanthine metabolism (derived from caffeine) (Wang et al., 2020). Other metabolites and pathways enriched in urban included those causing inflammation and metabolic derangements (Wang et al., 2020). Characterization of the gut microbiome metabolic pathways and endogenous and exogenous metabolites is still a growing field. Thus, we did not find a large body of evidence comparing the differences along the urbanization gradient to draw robust inferences.