Stool energy density is positively correlated to intestinal transit time and related to microbial enterotypes

Abstract

Background: It has been hypothesised that the gut microbiota causally affects obesity via its capacity to extract energy from the diet. Yet, evidence elucidating the role of particular human microbial community structures and determinants of microbiota-dependent energy harvest is lacking.

Results: Here, we investigated whether energy extraction from the diet in 85 overweight adults, estimated by dry stool energy density, was associated with intestinal transit time and variations in microbial community diversity and overall structure stratified as enterotypes. We hypothesised that a slower intestinal transit would allow for more energy extraction. However, opposite of what we expected, the stool energy density was positively associated with intestinal transit time. Stratifications into enterotypes showed that individuals with a Bacteroides enterotype (B-type) had significantly lower stool energy density, shorter intestinal transit times, and lower alpha-diversity compared to individuals with a Ruminococcaceae enterotype (R-type). The Prevotella (P-type) individuals appeared in between the B- and R-type. The differences in stool energy density between enterotypes were not explained by differences in habitual diet, intake of dietary fibre or faecal bacterial cell counts. However, the R-type individuals showed higher urinary and faecal levels of microbial-derived proteolytic metabolites compared to the B-type, suggesting increased colonic proteolysis in the R-type individuals. This could imply a less effective colonic energy extraction in the R-type individuals compared to the B-type individuals. Notably, the R-type had significantly lower body weight compared to the B-type.

Conclusions: Our findings suggest that gut microbial energy harvest is diversified among individuals by intestinal transit time and associated gut microbiome ecosystem variations. A better understanding of these associations could support the development of personalised nutrition and improved weight-loss strategies. Video Abstract.

Keywords: Energy harvest; Intestinal transit time; Microbial ecology; Personalised nutrition.

Fig. 1

Enterotypes differ in stool energy density, intestinal transit time, microbial alpha-diversity, and body weight. a The study included baseline measurements of 85 overweight subjects. Prior to collection of the stool and urine samples used in the study, habitual dietary intake was estimated based on 4-day dietary registrations and intestinal transit time was estimated using radio-opaque markers from day 1 to 6 where participants maintained their habitual diet and lifestyle. The collected stool sample was used to estimate dry stool energy density as a measure of gut microbial energy extraction, bacterial cell counts, the gut microbiome community structure, and short-chain fatty acids. Microbial-derived metabolites were measured in the urine samples. b Principal coordinate analysis plot using Bray-Curtis distance of bacterial relative abundance on the genus level as distance metric. Symbols are samples, with shape/colour indicating assigned enterotype (red circles: Bacteroides (B-type), n = 35; yellow diamonds: Prevotella (P-type), n = 16; green squares: Ruminococcaceae (R-type), n = 34). Relative abundance of the taxa used for enterotype assignment (black arrows) and values for dry energy, Shannon index and transit time (purple arrows) were plotted supplementary (i.e. projected after ordination). Horizontal and vertical axis explain 20% and 12% of variation, respectively. Subjects stratified into three enterotypes differed in c stool energy density (n = 77), d intestinal transit time (n = 85), microbiome alpha-diversity as reflected by e Shannon Index and f observed richness (n = 85), as well (g) body weight (n = 85). Differences between enterotypes were detected using the Mann-Whitney U test. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001

Fig. 2

Enterotypes differ in proteolytic metabolites in faeces and urine. a Heatmap showing Spearman correlation coefficients of the associations between dry stool energy density, intestinal transit time and faecal short-chain fatty acids (SCFAs). Faecal concentration of SCFAs according to the three enterotypes with respect to b the SCFAs acetate, propionate and butyrate, c the branched SCFAs isobutyrate, 2-methylbutyrate, and isovalerate, as well as d valerate and caproate. e Log-transformed urinary relative levels of the microbial-derived proteolytic metabolites p-cresol sulfate, p-cresol glucuronide, and phenylacetylglutamine according to enterotypes. Differences between enterotypes were detected using the Mann-Whitney U test. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001

Fig. 3

Proposed human gut microbiota-dependent energy extraction. We propose that intestinal transit time diversifies the gut microbiome community structures into preferred community structures captured by enterotypes (B-type, Bacteroides; R-type, Ruminococcaceae; and P-type, Prevotella) with the B-type and R-type being most distinct in terms of transit time and alpha-diversity. The B and R-type enterotypes differ in alpha-diversity and colonic fermentation, which in essence is the trade-off between saccharolytic and proteolytic metabolism, which may affect the enterotypes’ overall efficiency to extract energy from food. This could potentially translate into different body weight. The stars (*) refer to findings from a previously published paper showing that enterotypes differ in metabolic capacity and growth potential [14]