by Bill Landers
Why do we get fat? Diet and genetics play a large role, but scientists have found an important third factor.
Diet: You are what you eat. Excess calories get stored as fat as a hedge against hard times — a survival trait from our hunter-gatherer ancestors. Without man-made storage, the only way to hoard food was to fatten up when it was plentiful and live off the stored fat when food was scarce.
Human genetics: If your ancestors didn't have a constant food supply, genes that piled on the fat in good times provided a competitive edge. People with food-to-fat genes lived longer in hard times and passed those genes to their children. Those lacking the fat genes tended to die off in hard times and pass fewer of their genes to their progeny. Geneticists call this “genetic conservation.” Conversely, people living in places where food was abundant didn't need as much fat as an insurance policy. Since lean people tend to live longer, lean genes tended to be conserved in populations with ample food all year round.
While fat and lean genes were critical to survival in ancient times, food is now relatively abundant in first world countries, but our fats genes don't know that. They're still industriously converting excess calories to fat every time and every chance they get … and they get a lot of extra calories from yet another genetic source. Trillions of them.
Enteric genetics: Just as bacteria live in our mouths, they also live in our intestines. There are between 500 to 600 species of bacteria that can live in our large and small intestines, about the same number of species that live in our mouths. In fact, there are 10 times more bacteria living in our intestines (enteric bacteria) than there are cells in our entire bodies.
They reproduce so fast that about 60% of our feces are composed entirely of enteric microorganisms. Now, here's where it gets really interesting. The intestinal bacteria are independent organisms with their own genes and genetic abilities. The particular species of bacteria present in your intestines helps determine whether you get fat or not.
The secret of the skinny mice
Jeffrey Gordon (Washington University in St. Louis) became famous in biology circles for breeding skinny mice. They were the same species as fat mice fed exactly the same diet. The only difference was that the skinny mice were totally bacteria free, born and raised in sterile chambers. No gut bacteria at all. Fat foods just passed through their digestive systems without being digested because without the right species of gut bacteria, they couldn't digest the fats.
Humans are not very good at digesting cereals like oats. Our genes can't make the necessary enzymes. Certain species of bacteria, though, are very good at converting oats to sugars. If you happen to have those species in your intestines, you (the host) will get many more calories out of a meal of oats than someone who lacks that bacterial species. Same amount of food, but you'll get fatter and they won't. So, how much weight you gain depends not only on how much you eat and your genetic history, but also which species of bacteria you're carrying around with you and what they eat!
In effect, we have two sets of genes; the ones we inherit from our parents, and the ones we acquire from our environmental bacteria that set up shop in our guts.
Both sets of genes vary from person to person. Traveler's diarrhea results, in part, from not having the right bacteria in your intestine to handle new foods. They just pass right through you. However, there's evidence that our intestinal bacteria can change over time. When people move to a new place and start eating different foods, they also pick up new species of enteric bacteria that are better able to convert the new foods to nutrition.
Inspired by an article in National Public Radio's “All Things Considered” on Nov. 4, 2008.