The larger the numbers of the predominant archeon, Methanobrevibacter smithii, in the human gut, the greater the risk of obesity. This is the conclusion of scientists at the Maastricht University Medical Center from a cohort study of almost 500 children aged six to ten years.

These findings confirm an earlier study in rats showing a correlation between M
smithii and weight gain. Moreover, the degree of weight gain in these animals corresponded with the extent of colonization in the small and large intestine with M smithii.

To date, it is unclear what mechanism is behind but M smithii seems to play an important role in the efficient digestion of dietary fibers by consuming the end products of bacterial fermentation like hydrogen. M smithii is a methanogen, and recycles the hydrogen by combining it with carbon dioxide to methane. The removal of hydrogen gas by M smithii is thought to allow an
increase in the extraction of energy from nutrients by shifting bacterial fermentation to more oxidized end products. Thus, M smithii may play a role in caloric harvest. This might explain the findings of the Dutch scientists based in Maastricht.

In 2009, another study showed an increase in M smithii in anorexia nervosa patients compared to lean people. These findings seem contradictory to the abovementioned findings, unless the development of M smithii in anorexia nervosa patients is associated with an adaptive attempt towards optimal exploitation of the very low caloric intake. In any event, this ancient bacterium is an interesting target for future research and potential interventions to help manage weight.