Sydney Speisel, writing in Slate, has finally provided me with the answer. It's not the chow. It's my bacteria supply:
...the germs inside of us may affect us every day—by helping to determine our weight.Makes sense to me. Now how do I eliminate the little, er, buggers?
New research: Two recent papers from the laboratory of Dr. Jeffrey Gordon at Washington University in St. Louis, one whose lead author is Peter Turnbaugh and the other whose lead author is Ruth Ley, are beginning to unravel this phenomenon. Many of the gut bacteria are representative of two large groups of germs: the Firmicutes (which roughly overlap with the Gram-positives of high school biology) and the Bacteroidetes (typically Gram-negative bacteria, which grow without oxygen and are usually sort of smelly). There are other germs in the bowel, as well, but Gordon's group has been particularly interested in the balance between these two large divisions of intestinal bacteria.
Findings: Studying mice that are genetically predisposed to obesity or leanness (but are otherwise similar), the researchers found that fat mice had an abundance of Firmicutes while their lean littermates had more Bacteroidetes. Here's one reason why: When the bowel in a mouse is inhabited by lots of Firmicutes bacteria, the animal harvests energy from its diet more efficiently. So the same amount of food that fattens the Firmicutes-carrying mice leaves the Bacteroidetes-carrying mice slim. More provocatively, when Gordon's team took germ-free, normal-weight mice and infected them with Firmicutes-predominant bacteria taken from obese mice, they fattened up—whereas similar littermates colonized with intestinal bacteria taken from lean mice did not. Something similar seems to occur in humans: The ecology of the bowel shifts toward the Firmicutes in heavy people and toward the Bacteriodetes in leaner people (including those who become thin as a result of dieting).
Conclusion: These findings, taken together, strongly suggest that intestinal bacteria may well play a significant role in how animals and humans absorb energy from the food we eat and turn it into body fat. How comforting: Germs, not gluttony, may be the cause of my weight gain.