FIGURE 4-1 Difference in body fat gain between initially germ-free mice that receive a microbiota transplanted from either an obese donor (either a genetically obese donor [ob/ob] or a diet-induced obese donor [“Western”]) or a lean donor (again, either a genetically lean donor [+/+] or a diet-induced lean donor [“CHO”]).
NOTE: DIO = diet-induced obesity.
SOURCE: Turnbaugh et al., 2006, 2008.

Figure 4-1). Again, the mice that gained more fat tissue did so even though they were consuming the same amount of calories as the mice with microbiota from lean donors. These results suggest that microbial communities derived from obese versus lean mice impact the energy balance of their new hosts in different ways.

Human twin data provide additional evidence to support the hypothesis that the microbiome impacts host energetics. Using 16S rRNA data from both monozygotic and dyzygotic twin pairs, Turnbaugh and colleagues (2009a, 2010) found greater than 300 microbial genes associated with obesity. Many of these genes “make sense,” Turnbaugh said, given the shift in the relative abundance of Firmicutes and Bacteroidetes associated with obesity in mice. Some of them also indicate a microbial contribution to host carbohydrate and other metabolic pathways.

In another human study, Greenblum et al. (2012) analyzed metagenomic data from twin pairs and other individuals and identified network-level differences in microbial metabolism genes between obese and lean individuals. That is, they identified specific networks of genes that were associated with obese individuals and other networks associated with lean individuals. Most of the differences between the obese- and lean-associated networks were on

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