Greedy guts?
（The Economists Jan 4th, 2007）
ALTHOUGH most people prefer not to think about it, human guts are full of bacteria. And a good thing, too. These intestinal bugs help digestion, and also stop their disease-causing counterparts from invading. In return, their human hosts provide them with a warm place to live and a share of their meals.
Now it is working rather too well. A group of researchers led by Jeffrey Gordon, of the Washington University School of Medicine, in St Louis, has found that some types of microbes are a lot better than others at providing usable food to their hosts. In the past, when food was scarce, those who harboured such microbes would have been blessed. These days, paradoxically, they are cursed, for the extra food seems to contribute to obesity.
Dr Gordon’s research is outlined in a paper published in this week’s Proceedings of the National Academy of Sciences (PNAS) and two others published last month in Nature. In the Nature papers, he and his team reported that obese people have a different mix of gut microbes from that found in lean people—a mix that is more efficient at unlocking energy from the food they consume. The researchers sequenced bacterial DNA from faecal samples taken from volunteers and discovered that those who were obese had a higher proportion of Firmicutes than lean people did.
Bugs in the system
This also turned out to be true in mice, and working with these rodents, the researchers discovered that the types of Firmicute found in obese animals are more efficient at converting complex polysaccharides into simple, usable sugars such as glucose. In effect, the Firmicutes made more energy available from the same amount of food. The researchers were even able to make mice that had been raised in a germ-free environment fatter or thinner by colonising their guts with microbes from either obese or lean mice.
It sounds simple enough. Unfortunately, further probing showed that the story is a little more complicated, for Dr Gordon did not merely count the gut bacteria of fat and thin people—he then put some of the fat ones on a diet. As these once-obese humans lost weight over the course of a year, their mix of gut microbes changed to reflect their new, svelte status. Why this happened is not clear. It does not seem to have been a result of the composition of the diet, since the effect was the same whether people lost weight with a low-fat diet or a low-carbohydrate diet. Nevertheless, this part of the experiment suggests it is weight that determines gut biodiversity, not the other way round.
The paper published in PNAS, though, supports the idea that the bacterial mixture is cause not effect, by adding yet another element to the story. In this study, Dr Gordon took normal mice and germ-free mice, and fed both groups a “Western” diet that was high in fat and sugar. The normal mice gained weight; the germ-free mice stayed lean.
The findings do emphasis how profound the relationship is between people and their gut bacteria. These bacteria can be thought of as an additional digestive organ. Alternatively, humans might view themselves as a sort of collective organism—a human casing surrounding a vast colony of microbes. It is just a pity that this colony is working so hard on behalf of its casing that, in an era when food comes from the supermarket rather than the savannah, the result is rather too good.
1. By “Now it is working too well” (line 1, paragraph 2), the author implies .
A. guts bacteria help digestion and prevent diseasecausing counterparts from invading
B. some guts microbes work well to provide a lot of usable food to human
C. the extra food provided by some guts microbes can cause heavy weight
D. when food is scarce, some guts microbes can make human blessed
2. Dr Gordon’s research has found that .
A. all guts microbes are good at providing usable food to their hosts
B. Firmicutes account for a higher proportion in slim people than in heavy people
C. it is weight that determines one’s mix of guts microbes
D. the more food, the more energy Firmicutes can make available
