Background: Resting metabolic rate (RMR) has significantly declined over the last 100 years. Several factors, including a lowered body temperature and decreased immune activity, have, in part, explained this decline. However, recently it was shown that replacing refined grains with whole grains is associated with a normalization of RMR. As consuming more whole grain fiber can promote increased obligate anaerobic bacterial fermentation activity in the gut microbiome, resulting in increased short chain fatty acid (SCFA) production, it is possible that the microbiome may mediate the association between whole grain consumption and RMR normalization. Currently, many countries are experiencing an epidemiological transition by adopting diets high in ultra-processed foods and low in fiber. Indeed 10 Year NHANES trend for fiber and whole grain intake show that less than half of the US meet the current fiber recommendations. Methods: We explored the associations between measured RMR, dietary fiber intake, gut microbiota (ASVs) and fecal SCFAs in 104 METSMicrobiome adults from Ghana (N=33), South Africa (SA, N=42) and US (N=29). Results: Ghanaians had the lowest average BMIs and obesity prevalence, followed by SA and then US participants, and both Ghanaians and SAs had higher measured RMR compared to the US. Ghanaians consumed almost twice as much fiber (25.1 vs. 16.9 g/d) compared to US, which associated with greater fecal SCFA concentrations. Across all sites, meeting dietary fiber recommendations (14g/1000kcal) resulted in a greater microbial alpha diversity and the enrichment of 18 ASVs mostly associated with the SCFA-producing class Clostridia. Similarly, participants who's measured RMR met their predicted RMRs using the Cunningham equation, had significantly different microbial beta diversity and were enriched for 43 ASVs mostly from the class Clostridia when compared to those who did not. Conclusions: In an international cohort of African-origin adults, country of origin is significantly associated with both fiber intake, fecal SCFAs and measured RMR, which is reflected by differential abundance and diversity in the gut microbiota. As countries experience the epidemiologic transition, the importance of a dietary fiber should be emphasized as a low-cost method to maintain metabolic health.