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.
23 comments:
Gosh, I skimmed that article and assumed he was going to conclude that we eat too much. I guess I'm biased because my colleagues have shown through their studies of human metabolism and energetic throughput that it's how much we eat, more than how much we burn, that makes us get huge. I was not expecting the mathematician to conclude that we're fat because there's too much food outside our bodies!
Maybe the distinction between inside and outside our bodies is arbitrary on this buffet we call Earth.
P.S. Love the last line.
Did it ever occur to anyone that maybe the food is in charge? It needs us to grow it and digest it and void it after all.
You're being your usual funny self, Holly! But perhaps our concepts of causation really are too rigidly defined by things like the boundaries between 'us' and 'else'. After all, as we've posted in the past, our microbial interconnections blur that boundary today and in evolutionary terms.
If eyes are for perceiving and we're wired to respond, then food outside our bodies is part of us in a truly causal sense.
And similarly, behaviorally we're wired to interact with an environment and interpret it, and manipulate it. So advertising, the true culprit perhaps, is part of our causal world, too.
Or, if human society is viewed as a super-organism, an old anthropological idea with some merit, then the people who work in advertising (generously calling them 'people') and those making corn fructose, and the McFood employees, and supply truck drivers and so on, are all benefiting with healthier lives than they'd have had (if they even existed) a century or two ago (or in our early ancestry!). So the System is, overall, quite salubrious....
Ha! Among other things, I think you've rediscovered Gaia, Holly!
Not to mention the diet book writers, publishers, sellers, the medical care industry that takes care of all the people who've overeaten, and following up on Holly's point that this may well all be driven by plants, all those extra plants that have to be grown and harvested to make all this possible!
Let's be serious: We know from Newton's time, when this became formal, that everything is connected via gravity and so on. No object is independent of any other object.
In science, we draw boundaries to identify units of observation that we think are causally distinct for whatever our purpose happens to be.
An obvious boundary in life is the 'individual' but that is artificial in ways we've been tossing around here, and is certainly causally transitory from an evolution perspective.
So it is not just flighty Gaia-fication to ask how things should be studied to understand causation. We want to know the 'cause' of someone's body weight, and clearly the boundaries are...unclear!
Well, this is serious, too, from an evolutionary point of view. Evolution doesn't 'care' how you proliferate. Plants have 'figured out' how to proliferate by making us eat them and being domesticable so we can help them reproduce phenominally well!
From an evolutionary point of view, corn is a species as successful as almost any on earth.
Absolutely. Along with cows, who share that corn with us and, in a non-negligible sense, have us wrapped around their little hooves in terms of evolutionary success.
Indeed, is there anything in the cow genome that can tell us why they're so successful?
It would be difficult even if we applied selection theory to try to find 'signatures' of selection, and we could certainly not do that without taking humans (an element of the selective part of the cow environment) into account.
So why do we think we can accurately identify the relevant environments in other cases, such as interpreting fossil evolution?
So my waistline is a wasted line.
Insomuch as you're not contributing to corn fitness, yes.
I lost you on this one.
Rather, you lost me! :)
Hang on, whose comments are "Flighty Gaia-fication"? Mine?
Sorry! I just meant that from the point of view of the plant, or the cow, the more we eat the better.
"wasted line" is for unnecessary, unrealistic, not real boundary, etc. (Meh, I tried ;)).
You succeeded, I failed. :)
Last time I saw you, you had no waistline issues, so you are, to corn, an environmental non-responder--so yes, a waste-line. Whether it's an evolutionary wasted line for you, given that you don't have any kids, I guess you're not much use for that either so far!
As far as any other meanings in your question, that I'm too dense to spot, I'll leave that to Anne...
Ha! Thanks you two! I think we've pretty much figured it all out, wouldn't you say? What's next?
You mean you don't want to chew the fat over this issue any longer??
Now's my turn to get serious.
The life of a typical obese person is still probably a whole lot better than the life of someone who doesn't have enough to eat or lacks a steady food source. So what the heck are we doing studying the causes of obesity when (a) we know that overeating is all there is to it (for the vast majority) and (b) that money and effort could go to sharing our excess with those who are sorely lacking?
My short answer: There are more jobs for explainers than there are for sharers. This doesn't make sense to me in an age where we know so much already (so we need fewer explanations) yet we have so much excess, with unequal access to that excess.
Absolutely positively!! It's even worse than you say. Many of the diseases we are plagued with these days are 'luxury' diseases that get us when we've had longer, more completely functional, easier (and, for that matter, reproductively successful) lives than the vast majority of our evolutionary forebears.
'Overweight' means weighing more than magazine models weigh or (if and when you can actually trust the research, which is not so clear after all!) at slightly higher risk of late-onset disease than your more slender peers--even though all living to a grand old age for our species.
And we are pouring funds into researching this, and so much of it in the 'genetics' of this largely non-genetic 'trait', when as you say for most people it's basically overeating....or, perhaps, given our long healthy lives, just indulging in rare luxury--more than we absolutely need, but enough to give us a very good life.
It is also just one more example of a kind of class or social inequity. We may call it 'science' and so on, but to a great extent it is professor welfare. Even worse, once a segment of professor society gets its hands on resources, it won't willingly let go, even when there are serious diseases, indeed many that could benefit from genetic research.
And as you say, there are countless millions in countries around the world who would be happy to live on our table scraps (or the plentiful food we provide our pets).
But I have to say that it does make sense anthropologically, in that humans are pretty uniformly hierarchical when given the chance, and tribal (sharing mainly with their own). This is just one particular manifestation of it.
To me perhaps the saddest side of this, at least from an idealistic point of view is that everyone knows it. We can't say we 'need further research' to know the truth of things. They're hidden in plain sight.
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