The colon is home to a large number of species of microflora that play an important role in health. Colonic microflora possess enzymes not present in humans, and are therefore able to metabolise a wide variety of nutrients that would otherwise be excreted. For example, humans do not possess the enzymes necessary to digest fructooligosaccharides and certain forms of resistant starch, but these can be metabolised to short chain fatty acids (SCFA) in the colon by microflora. The SCFA such as lactate, propionate and acetate can then be absorbed, and contribute to the energy needs of the individual. The energy contribution of this colonic fermentation to human metabolism is perhaps the least studied component of the energy balance equation. Research suggests that the type and amount of microflora present in the colon determines the amount of energy that is fermented and absorbed or eliminated in the stool.
Researchers1 have investigated the effects of energy intake on the colonic microflora population in 12 lean and 9 obese individuals. In addition, the authors of the study measured how the changes in microbial populations changed the energy absorbed from the diet. Subjects were fed a weight maintenance diet for 3 days, after which they were randomly selected to receive either 2400 or 3400kcal/d for 3 days. After a 3 day washout period, the subjects crossed-over to the other protocol. Bacterial genes present in the stools of each subject were measured via pyrosequencing of 16S rRNA genes, and ingested and excreted energy was measured via bomb calorimetry. At baseline, there was no significant difference between the 3 main dominate phyla of bacteria seen in lean and obese individuals. These were made up of bacteroidetes, firmicutes and actinobacteria, with bacteroidetes and firmicutes accounting for 97% of pyrosequencing.
Changing the energy content of the food eaten by the subjects caused rapid changes in the microflora of the colon with the 2400 and 3400kcal diets being associated with an increase in firmicutes and a decrease in bacteroidetes. These changes in microbial populations showed associations with the amount energy lost in the stool. For example, a 20% increase in the firmicutes population and a corresponding decrease in the bacteroidetes population was associated with a ≈150kcal increase in the amount of energy absorbed from the diet. However, overfeeding resulted greater energy absorption from the food in lean subjects, when compared to obese individuals. This suggests that the degree of over nutrition relative to maintenance energy needs may play a role in determining the efficiency of energy absorption. Colonic microflora may therefore contribute significantly to the energy balance of individuals in the long-term.
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