A previous study by Reijnders et al. has added to the body of research calling into question some direct and simple associations found between the gut microbiota and metabolic disorders.
Now a recent study, led by Dr. Nicola Santoro from the Department of Paediatrics at Yale University in New Haven, Connecticut (USA), has found that the gut microbiota of obese youth may drive a higher accumulation of energy than that of lean adolescents through an elevated production of short-chain fatty acids (SCFAs) and a higher capability to oxidize carbohydrates.
The researchers analysed the gut microbiota of 84 children and teenagers (7-20 years old; the participants included 27 youth who were obese, 35 who were severely obese, 7 who were overweight and 15 who were normal weight); in these individuals, body fat distribution was measured by fast-magnetic resonance imaging, de novo lipogenesis (DNL) was quantitated using deuterated water, and the capability of gut microbiota to ferment carbohydrates (CHO) was assessed by 13C-fructose treatment in vitro in faecal material. The participants also provided blood samples and kept a three-day food diary. Researchers measured plasma SCFAs but not faecal SCFAs.
The composition of the gut microbiota was associated with the degree of obesity and the distribution across body fat depots. Specifically, researchers found a significant association between the Firmicutes to Bacteroidetes ratio, and the abundance of Bacteroidetes and Actinobacteria, with body mass index (BMI), visceral and subcutaneous fat. These data show that children and teenagers who are obese have different gut microorganisms than their lean counterparts.
In addition, plasma levels of acetate, propionate, and butyrate were associated with weight gain (expressed as changes of BMI/year) over a 2-year period independent of age, gender, and ethnicity (84 subjects were analysed at baseline and 72 subjects were analysed at follow-up). Strikingly, plasma SCFA concentrations were also linked to hepatic DNL in obese adolescents. Besides this, the rate of CHO fermentation from the gut microbiota was higher in obese than in lean subjects.
In conclusion, this study has shown for the first time that for similar amounts of dietary energy, in children and adolescents the composition of the gut microbiota is associated with increased fat deposits. The gut microbiota of obese youth may drive a higher accumulation of energy than that of lean counterparts through an elevated production of SCFAs as measured in the plasma, which in turn could enhance hepatic DNL causing lipid accumulation in all the fat depots.
This trial has highlighted the need for more follow-up studies to more deeply investigate the relationship between changes in gut microbiota composition and activity and both microbial metabolites and host metabolism. Although this study showed levels of plasma SCFAs were linked to obesity, other work tends to show SCFAs are beneficial for health.