My weight’s been bugging me cont…..

As predicted in a previous post, gut flora does appear to contribute to obesity.

In two papers published in today’s issue of Nature, the relationship between gut bacteria and weight gain was further explored.

In one study, mice brought up in a sterile, microbe free environments had gut bacterial populations seeded from either obese or skinny mice. Those seeded with microbes from skinny mice put on 25% in body fat, those seeded with obese microbes put on 45%.

In another study, the bacterial flora found in the gut of obese people was compared to that found in slender people. The proportion of different types of bacteria varied between groups. It was proposed that the bacteria found in higher proportions in overweight people (Firmicutes) was more efficient in converting food to energy. After a year of (low fat or low carb) dieting, the gut flora in obese people became more like that found in slim people.

So it seems that there is a relationship between weight and gut flora.

This relationship is a little unclear however. The mouse study shows that bacteria from an obese individual will cause weight gain. However dieting in obese humans will alter the bacterial content of the human gut to that of a leaner person. So is obesity and weight controlling gut flora or gut flora controlling weight and obesity?

It would be interesting to see if reverting back to a regular diet after dieting would then alter the gut bacteria back to obese proportions. Could diet be influencing the bacteria directly, or could people who can stick to a year of dieting keep the weight off because the flora has changed semi permanently?

From an evolutionary perspective, it seems that maintaining efficient gut bacteria would be beneficial if you are underweight and less useful if you are overweight. However the reverse seems to be the reality. People who need less energy (they have large amount of fat reserves) have more efficient bacteria than skinny people. When they become thinner by dieting, their gut bacteria becomes less effective.
What ever the relationship, I am sure weight loss centres will soon be pushing gut cleansing as a weight loss remedy.


Ley, R. E., Turnbaugh, P. J., Klein, S. & Gordon, J. I. (2006) Microbial ecology: Human gut microbes associated with obesity Nature 444, 1022–1023

Turnbaugh, P. J., Ley, R. E., Mahowald, M.E., Magrini, V. Mardis, E.R. and Gordon, J.I. (2006) An obesity-associated gut microbiome with increased capacity for energy harvest, Nature 444, 1027–1031


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