Study proves it’s not unhealthy to be overweight? Nah.

Researchersat the University of Manitoba have released a study on therelationship between body mass and health problems. Here’s a typicalmass-media reporton the findings, headlined, “Overweight people don’t havebigger health problems, study finds.”

It’san epidemiological study, which means researchers looked back at abunch of historical data about people and then tried to drawassociations between those people with certain characteristics orbehaviors, and how they fared over time. Epidemiological studies aremuch cheaper and easier and usually quicker to do than almost anyother type of study, and they seem impressive because they ofteninvolve large numbers of people. But they also are some of the leastreliable studies, being typically plagued with just about everyproblem that a study can have, including confounders (what happens topeople in the study may have little or nothing to do with the causesthat are being looked at), confusion of cause and effect (whathappens to people in the study may actually be causing the factorsthat are being studied, rather than being caused by them),unrepresentative samples (the people being studied may not be typicalof the population we care about), trying to draw conclusions fromsmall differences, and more. Most studies about the relationshipbetween health and weight we hear about are epidemiological studies,and we’ve had ample evidence over time that these are some of theleast reliable among generally unreliable epidemiological studies.
Butthere may be something else to worry about with the conclusions ofthis Manitoba study, something that makes me wonder if the findingsare actually right–but not for the reasons implied in reports of thestudy. 
Lastyear I met with Oberlin College biologist Keith Tarvin, who studiesforaging behavior in animals. Tarvin explained to me that throughoutmost of the animal kingdom, animals will generally put on excessweight when provided with plenty of calorie-rich food and not givenany reason to be physically active, such as having to forage or huntfor food, or having to avoid predators. (Animals that need to keepweight down in order to survive are sometimes an exception–birds,for example, can’t fly if they get too heavy.) Situations whereanimals lose weight, on the other hand, are usually associated withsome pathological state or threat–as for example when fish keepweight off in order stay small to be less visible to and appealing topredators, or to slow their maturation in order to preserve fertilityand lower energy needs during times of drought, extreme temperatures,food shortages or other environmental pressures.
Inother words–and at this point I’m merely expressing a conjecturethat came up in my chat with Tarvin—it may be that, weight-relatedhealth factors aside, having a stronger appetite is associated withbeing well-suited to the environment. Or to put it differently,keeping weight off when there is plenty of rich food around and nopressing reason to be physically active is, in a sense, apathological state. Apply this theory to human society today–andat this point I’m degenerating into my own conjecture–and we mightwell predict that being overweight is associated with being, in someways at least, healthier than those who keep weight off.
Doesthis mean we should stop worrying about being overweight? Absolutelynot. To conclude as much would be confusing cause and effect. If thisconjecture is right, it means that being overweight is in someways aneffect ofbeing healthy; it doesn’t mean that being overweight confers anyhealth benefits. And in fact, the possible rightness of thisconjecture should have no bearing whatsoever on the well-establishedfact that carrying excess fat is generally unhealthy, and that peopleimprove their health and lower their health risks when they loseexcess fat.
Evenif it’s true that people who are overweight are in many caseshealthier in some ways than many people who are not overweight, theselucky overweight would still on average even further improvetheir health by losing the excess weight, and those who are notoverweight would become less healthy by putting on excess weight.That doesn’t mean that weight loss is a pathological state, eventhough it’s usually associated with one in the animal kingdom. Weshould understand that the loss of excess weight is a good thing,even if, in a way, it is an unnatural thing in our society ofplentiful, overly stimulating, calorie-dense food and sedentarylifestyles. That’s why our goal should be, in effect, to changewhat’s natural in our society–so that people are pushed byeverything they see around them to avoiding excess weight, ratherthan being pushed to consume rich food and avoid physical activity asthey are today.
Tohave even a decent chance of proving that losing excess weight willmake you healthier–or to solidly prove just about any theory aboutthe relationship between excess weight and health–we’d need arandomized controlled trial in which people are randomly assigned toeither become fatter than they are, or lose weight, or maintain theirweight. That trial is never going to happen, for what I hope areobvious reasons. And this brings us back to the problems withepidemiological studies like the one done in Manitoba. They rarelyanswer the questions we really want to ask. Instead, they give usanswers that only raise more questions. There’s nothing wrong withthat–it’s how science operates. We should be grateful scientists areconducting these studies, and appreciate the fact that they add toour knowledge base. But we should also be aware of the extremelimitations and qualifications that attach to the findings.
Inthe case of the Manitoba study, I think it’s fair to say the study atbest tells us little about the healthfulness of avoiding beingoverweight, and at worst is, at least in how it’s being reported,extremely and even dangerously misleading. One of the biggestproblems with the obesity crisis is the lack of awareness andmotivation on the part of a sizable percentage of overweight people.When a study like this gets press claiming that being overweightseems to be as healthy as not being overweight, we take a big stepbackwards, and cause real damage that could in principle be measuredin the loss of many years of life in the population, not to mentionthe drop in quality of life experienced on average by the overweight.I wish scientists and journalists would start taking these issuesinto account in their reports to journals and the mass media.

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