Friday, January 12, 2007

Greedy Gut?

Time for another installment of the ongoing Fat Knowledge look into human gut bacteria.

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.

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. Although individuals can harbour up to a thousand different types of microbes, more than 90% of these belong to one or other of two groups, called Bacteroidetes and Firmicutes. 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.
Interesting, never heard of these Firmcutes and Bacteriodetes.
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 (a form of carbohydrate that mammals have a hard time digesting by themselves) 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.
Cool, lets get this started. I wonder how you change the colonization of you gut?

For people that want to stay thin, lets get them more Bacteriodetes. For the vegans, starving Ethiopians, and the diet for a small planet crowd lets give them more Firmicutes so they can eat less and still maintain their body weight. And lets come up a crapper that can tell you the ratio of Bacteriodetes to Firmicutes in your body.

And while we are at it, lets start messing around with genetically modified bacteria so that we can make this symbiotic relationship of ours even more beneficial.
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.

By comparing the two kinds of mice, they discovered that the gut microbes in the regular mice were tinkering with their hosts' metabolisms, regulating them in at least two different ways.

First, they suppressed production by the mice's bodies of a substance called fasting-induced adipose factor. This encouraged the mice to store fat. Second, they caused lower levels of another substance, called adenosine monophosphate-activated protein kinase, which made it harder for them to burn fat that they had already accumulated. The upshot is that gut microbes not only release energy from food, they also encourage bodies to store that energy as fat and to keep the fat on.
Not quite clear to me what germ-free means here. Weren't we comparing Bacteriodetes to Firmicutes ratios? But it is fascinating that the bacteria go beyond just digestion to somehow affect adipose factor and adenosine monophosphate-activated protein kinase levels. I wonder how they do that?
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.
Hmm, so you can affect the type of bacteria in your get based on your weight? I wonder the mechanism through which this works if it isn't diet. Some more complicated feedback loop it appears.
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.
Or as I like to call it: I am we.

via The Economist

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