An epidemiologist is likely to say it's gene by environment interaction, though these days an epidemiologist may well finger only genes, given that identifying environmental risk factors for complex traits, even those for which environmental factors are clearly primary, has so often proven to be daunting. Many epidemiologists excuse their jumping onto the genomic bandwagon with the rationale that yes, genetic effects are weak, but we need to know them anyway because once they are identified they can be regressed out of the search for the real (to them, environmental) effects.
A nutritionist might say it's diet, and/or not enough exercise, and the girth of the diet section of any bookstore tells you how many ways that explanation can be parsed. Indeed, that girth itself is an indicator of the size of the problem -- a simple and cheap epidemiological stand-in!
Some people pick out a single component of the diet -- sugar is a big one these days; we blogged about that here -- as the culprit. A lipid scientist might chalk it up to leptin, a hormone involved in regulating appetite and metabolism. A person struggling with his or her weight might say it's personal weakness.
And now mathematics weighs in. An interview with an applied mathematician at the NIH was reported in yesterday's Science Section of the New York Times Times. Carson Chow is at the National Institute of Diabetes and Digestive and Kidney Diseases, where they have a growing interest in mathematical study of obesity. Why not, math is technical, opaque, and so has sex appeal! Chow was hired in 2004, at which time he had little knowledge of obesity, but quickly learned.
I could see the facts on the epidemic were quite astounding. Between 1975 and 2005, the average weight of Americans had increased by about 20 pounds. Since the 1970s, the national obesity rate had jumped from around 20 percent to over 30 percent.
The interesting question posed to me when I was hired was, “Why is this happening?”When he first arrived, Chow worked with a mathematical physiologist who had developed a model of obesity that involved "hundreds of equations", including all the usual variables -- height, weight, exercise, caloric intake, and so on. Chow says he pared it down into a simple equation, the essential message of which is that the obesity epidemic has been caused by "the overproduction of food in the United States."
This is interesting, but not the first time this explanation has been proffered. 'Food chain journalist' Michael Pollen has also blamed obesity on overproduction of food. In particular, the excess of nitrogen after World War II, and its subsequent use as a fertilizer, which meant more corn being grown, and thus more corn-based processed food, all dependent on farm subsidies. And, it has long been felt that the generally post-war epidemic of obesity and related disorders in Native Americans was due to sedentary, depressed lifestyles and the open-ended availability of cheap calories.
But, overproduction of food can't really be the answer by itself, because excess corn can sit in the field until the cows come home if no one is going to buy it or what's made from it. Someone had to convince the consumer to buy, and then eat the stuff. So then, maybe it's the advertising industry that's responsible for the obesity epidemic. Those paid-deceivers lie (so to speak) at the heart of many of the more serious problems in the US these days, after all. Indeed, Chow believes that if the industry stopped marketing food to children, that would be a start. (Oh, no, can't limit free speech, bleats Madison Avenue, claiming in effect that making you obese is their first-amendment right!) Further, Chow says, "You simply have to cut calories and be vigilant for the rest of your life." Vigilant in resisting the appeal of all that food that's being flashed at you wherever you look.
But maybe there is no single cause of obesity. Maybe obesity is yet another complex trait and, collectively, everyone is right. Perspective is important -- if you're studying leptin, what matters to you is not why there's so much excess food in the marketplace, but why people want to eat it. If you're a geneticist, nothing matters except your next GWAS. It's like the question of what causes AIDS -- is it HIV, needle sharing, poverty? It's all of the above.
This is, in a way, a lesson for much of life -- including evolution and genetic causation. Life is not about single factors. If it were, or had been, it would be far too vulnerable to extinction. The buffer of complexity spares life, while the complexity of buffets generates spare tires.