Last week's Science magazine (1 April 2011) has a very interesting article on the role that bacteria play with regard to obesity. Studies are showing that the presence of certain bacteria in the gut of animals (including humans) counteracts obesity. Or to put it the other way around, if certain bacteria are not present in the gut, then there is a significant increase in body fat and obesity.
Evidence of this is seen in domesticated animals (such as cows) which gain significant fat weight when fed antibiotics. The same happens with mice, other mammals, and probably humans. Antibiotics kill off certain types of gut bacteria, which prevents them from inhibiting fat production. In humans, the march toward a vastly reduced biota of gut bacteria began about 60 years ago with the widespread introduction of miracle drugs. It turns out that a single-high dose of antibiotic has similar impacts to the gut microbiota as the long terms low-dosages used in cattle and other domesticated animals - and probably has the same results with regard to fat accumulations (and obesity).
The article says that the idea used t be that the microbiota populations would spring back after a person finished a course of antibiotics. However, that is not the case. The reality seems to be that sometimes once a species is eliminated from the gut, it doesn't return. Studies of "normal" and "obese" humans have found that obese individuals have lost 30% to 40% of the bacteria genes as compared to "normal" individuals, and the variety of bacteria is similarly depressed. It seems likely that the missing genes (and species) could account for the extra weight of the host.
This seems to make sense to me when considering what is going on around us today. It doesn't make sense to me that the "epidemic" of obesity is caused only by bad habits. It makes more sense to me that some environmental factor is changing the weight set points of millions of individuals all at the same time. For many years I have wondered what is causing so many people to get fat? The "funny" thing about obesity is that in many cases it seems to be a change in weight "set point" rather than eating habits. I will use myself as example. I used to be a long, skinny guy - weighing around 150 pounds at 6'5" tall. That persisted for five years or so. Then I did a lot of heavy work, and "reset" my weight to 210 pounds for a decade or so (the extra weight was almost all muscle). No matter how my eating changed, or my work effort changed, I stayed at the same weight. Then I got older and took a desk job and "reset" my weight to 240 pounds (getting pretty heavy by now). For the past decade or so it seems to be level at more like 275 pounds. In each case, there wasn't much of a change in my eating habits (actually, my calorie intake has gone down with increased weight because I would like to not weigh so much) - it was more like a change in my body's set point. I can eat more, or eat less, and within reasonable limits keep the same weight for years at a time. (It seems that every time I tried to diet to reduce my weight, I could get it down, but then it reset my set point about 5 pounds higher than before the diet - with the net result of an increase in weight, and an enforced reduction in food to keep it there. I stopped that several years ago for fear of turning into a spherical shape.)
It seems that some outside influence is causing weights to increase. Maybe not actually "causing" this to happen, but making it much easier to put on additional weight - and harder to take it off again. A change caused by a change in gut microbiota seems sensible, especially when it is so easily shown how this influences the weight of other large mammals. Why should we react any differently than other animals? My guess is that we don't.
So, maybe it will turn out that we need to figure out how to repopulate our guts with the "correct" bacteria following a round of antibiotics. Not just any bacteria, but the specific bacteria that influence weight gain (or other problems that we may be experiencing because of the use of antibiotics).
It will be interesting to see what this line of investigation leads. Maybe it will take some of the shame off of obesity (and the smugness of skinny folks) if a cause, and a cure, is found to be as simple as being more careful with the populations of critters living in our bodies. Maybe we just need to suck on more rocks like many kids do.
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