Obesity and health—a quick primer

Still coughing and tired, so here’s another one I’m migrating from the old blog. –PalMD

Blogging on Peer-Reviewed ResearchThere has been much talk in the media over the last few years about the “obesity epidemic” in the U.S. This has led to a bit of a backlash among a small but vocal group of critics who don’t believe the evidence linking obesity and poor health. The reasons for their disbelief are not all that clear to me, given the overwhelming amount of evidence linking obesity with both serious health conditions such as diabetes and heart disease, and adverse outcomes, such as premature death. When their arguments are examined in detail, most of these critics appear to be classic denialists, rather than honest skeptics. So let’s examine a small slice of the obesity pie.

When citing scientific research to back up an assertion, it is important to make sure you are not “cherry-picking” studies that support your point of view, and that you are not “quote-mining” the literature, taking statements out of context to change their original meaning. These are tactics commonly used by denialists. The way we interpret scientific literature in medicine is to examine as many studies as possible for quality, methods, and conclusions. We make decisions based on the overall picture, not on any single study. For example, if an overwhelming number of good-quality studies support the idea that smoking causes heart disease, but a couple of small studies do not support this conclusion, this does not mean that smoking does not cause heart disease.

So let’s look at a few examples of the literature on obesity. A large cohort study was published in the prestigious New England Journal of Medicine in 2006 which examined the effect of being overweight and obese on overall mortality. This was a very powerful study, involving tens of thousands of subjects. It revealed that obesity was very strongly associated with increased risk of death in a wide variety of patients. The one exception was in elderly people with very low body weight, a group who also had excess mortality. In the elderly, very low body weight usually indicates serious underlying disease, such as cancer. The study also found increasing risk of death in people who were not obese, but overweight. Some of the strengths of this study include the large sample size, and the good choice of an end point (death, which is a far more interesting end point then, say, blood pressure, as death and disability are the things people generally wish to avoid). Another strength was an analysis that excluded smokers, who have a higher mortality. Removing smokers from the analysis showed an even higher risk of death in the obese patient.

Any time a study finds a statistical link, there must also be a biologically plausible explanation that fills out the overall validity of the study. What are some of the reasons obesity might impact mortality? Obesity is strongly associated with high cholesterol and high triglycerides, both of which increase the risk of heart disease. Type II diabetes is very strongly linked to obesity, and diabetes leads to heart disease and premature death. Hypertension is also strongly linked to body mass, and hypertension leads to heart disease.

Most of these conditions can be mitigated by medication, but also by weight loss. Given how much “natural medicine” proponents hate pharmaceuticals, it’s a wonder that so many of them think obesity is just grand.

The overwhelming data on obesity, disease, and mortality are remarkable for the strength and volume of data. Few associations in medicine are as well documented. Those who continue to deny the link are deluding themselves and others.


Kenneth F. Adams, Ph.D., Arthur Schatzkin, M.D., Tamara B. Harris, M.D., Victor Kipnis, Ph.D.,
Traci Mouw, M.P.H., Rachel Ballard-Barbash, M.D., Albert Hollenbeck, Ph.D., and Michael F. Leitzmann, M.D. Overweight, Obesity, and Mortality in a Large Prospective Cohort of Persons 50 to 71 Years Old. N Engl J Med. 355;8. August 24, 2006. http://content.nejm.org/cgi/reprint/355/8/763.pdf

John D. Brunzell, M.D. Hypertriglyceridemia. N Engl J Med. 357;10.September 6, 2007. http://content.nejm.org/cgi/reprint/357/10/1009.pdf

McGee DL. Body mass index and mortality:a meta-analysis based on personleveldata from twenty-six observationalstudies. Ann Epidemiol 2005;15:87-97.

Manson JE, Willett WC, Stampfer MJ,et al. Body weight and mortality among women. N Engl J Med 1995;333:677-85.

Calle EE, Thun MJ, Petrelli JM, RodriguezC, Heath CW Jr. Body-mass index and mortality in a prospective cohort of
U.S. adults. N Engl J Med 1999;341:1097-105.


  1. Is there any evidence to the assertion made by Barry Glassner and others that the problems of obesity are part due to that fat people (like myself) get flak over it?

  2. FutureMD

    I wouldn’t be surprised if the depression aspect of obesity is mostly caused by the social stigma, but diabetes is from the endocrine derangment caused by having so much fat tissue. Fat is an endocrine organ, too much of it will mess up your hormones.

  3. maddox22

    Forgive me if this is a dumb question. But you spend a lot of time on this blog emphasizing that correlation is not causation. Yet every statement you made in this post is that obesity is “associated” with X, Y, or Z. I don’t deny that; I’m not qualified as a statistician to assess the actual data, so I have to accept that those more qualified than me have done the analysis correctly. But you’ve only stated an association between obesity and mortality (and disease), not a causation. Is there data showing that overweight/obesity is the cause of diabetes/heart disease/etc? If not, isn’t it equally possible that there’s another factor causing a person to both be overweight and diabetic/hypertensive/etc (but that being overweight isn’t the cause of the other illnesses)?

    I think a lot of people who have problems with the discussions of the obesity epidemic are not saying that there are no associations between overweight and health problems. I think the main issues that they have are a) that the assumption seems to be that the excess weight causes the health issues, when in fact only a correlation has been found; b) that it’s assumed that simply losing weight will correct all of the problems; c) that all health problems of overweight or obese individuals is caused by the excess weight; and d) that all a person has to do to “solve” these problems is just eat less–despite studies showing that overweight and obese people generally do not eat massively more calories per day than thinner people (certainly not in alignment with what metabolic studies predict they should have to eat to maintain their weight), and that some thin people eat huge numbers of calories per day and never get fat.

    FutureMD: Isn’t it possible that the endocrine disruption causes the excess fat? Or that some other disruption causes the excess fat? As I said, I think the thing that bothers most people about the discussion of the obesity epidemic is the underlying assumption that weight is completely under a person’s conscious control–that if I’m overweight, I must just eat too much of the wrong foods, or not getting enough exercise. That I could be a size 2 if I just wanted it enough.

  4. D. C. Sessions

    Without reading the study (and without disagreeing with their conclusions, either) I would still suspect that they are using BMI as their metric for “overweight” and “obesity.” Unfortunately for studies like this, BMI contains a height factor: taller people with otherwise equal body proportions (e.g. body-fat percentage) have higher BMIs.

    In one sense this is reasonable, since a whole host of problems are exacerbated by being tall. On the other, it’s not really something we get much choice about and therefore makes for poorer decision-making.

    My favorite example is Shaquille O’Neal. At 7’1″ and 325 pounds, he’s has a BMI of 31.6, the same as a 5’2″ woman weighing 173 pounds: seriously obese. (Perhaps Mr. O’Neal should start an exercise program.)

    Now, I agree that he will probably have fewer orthopaedic problems in the future if he puts less load on his joints and quite possibly will save his heart some extra work by reducing the total amount of muscle mass it has to support, but there are limits. Getting him down to an “ideal” BMI of 21.7 would mean a net weight of 223 pounds, which (given his skeleton) would be flirting with emaciation. Karen Carpenter, anyone?

    Again, getting rid of excess body fat is a Plain Good Idea. I have the same BMI as Shaq, and I’m shorter than he is — if a good bit over 5’2″ No question I need to lose weight, and not just fat; two surgeries this year for leg injuries brings that little fact home quite well.

    I just wish the studies would use a more appropriate metric than BMI that doesn’t have a height bias built in.

  5. D. C. Sessions

    If not, isn’t it equally possible that there’s another factor causing a person to both be overweight and diabetic/hypertensive/etc (but that being overweight isn’t the cause of the other illnesses)?

    Too many intervention studies make the causality in obesity/diabetes, obesity/hypertension, etc. clear.

    NB: I do volunteer work on an Apache reservation, and the lifestyle effect on their obesity and diabetes rates are shocking. Look at photos from a century ago and compare them to today, then look at the (maybe I’m kidding myself) increasing number of young Apaches who are doing a “back to tradition” revival of older diet and exercise patterns.

    Day into night into day again.

  6. Honestly, I’m skeptical of any study using BMI for two reasons.

    1) The formula is wrong. Not “height biased”, wrong. Weight scales to the cube (roughly) of height, not the square. There are 50 years of papers on this, covering hundreds of species (many with size ranges far beyond ours), and I’ve never seen an exponent of less than 2.7 reported for anything.

    2) Where did the cutoffs come from? What data were they based on? I’ve searched and searched and never found any sort of answer. Given their astonishing regularity (15, 25, 30, 35…), the possibility that most easily arises is “they were made up”.

    Of course, this neglects the litany of other flaws of the BMI (doesn’t account for build, doesn’t account for exercise history, etc.), and frankly, I think it’s just a lazy way to do science. “Oh, we can’t be bothered to take blood samples and do triglyceride levels, so we’ll just look through patient files and use Excel…”

  7. DC Sessions:”increasing number of young Apaches who are doing a “back to tradition” revival of older diet and exercise pattern” —

    Remind me again of the *average Apache life span* in the ‘good ole days”?

    As noted above, these are mostly correlational studies. And that is a potential problem. The weight may be an indicator of other problems within the metabolism and simply addressing the weight is treating the symptom.

    Though in a couple of studies (working from memory here) where they followed people who lost weight and found that it did not necessarily result in improved life span. Now it could be that damage was already done, but even if that is the case, why push to make these people suffer, with every meal a struggle?

    I suspect also (and have some sympathy here) that some objection to the weight-loss issue is that, instead of being a private decision (happy vs long life??) it is being done as a kind of social manipulation approach, including propagandization of children in schools to try to shame and pressure people to fit a mold. The image cast (‘for their own good’–sheesh I hate that rationale) suggests that these people are undisciplined, foolish, socially incorrect, and needing guidance from government and others. (I even saw a claim that heavy people help cause global warming by increasing auto fuel consumption)

    No wonder they resist.

    [I say this from an ‘outside’ perspective. I’ve been lucky and never had to cut my weight (at nearly 60 I can still fit clothes I wore at 20). But I see others, who eat much more sparingly than I do struggle, and starve themselves, and I wonder how much quality of life this is providing]

  8. maddox22

    D.C. Sessions, can you give me examples? All of the studies I’ve read about or heard about find correlations, but not causations. Diabetes in particular seems to me (again, based on the studies I’ve learned about) to be at least as strongly correlated with family history (particularly race/ethnicity) as with weight or BMI. (I don’t doubt you. But I’d like to see the studies you refer to, to see if I’ve misinterpreted them.)

    As I said, I’m not arguing that there are correlations between body weight/body mass distribution and health. But it seems to me that most of the studies and treatments out there assume that the excess weight is the cause of all problems–that if you are overweight, you must be unhealthy. It also makes the implicit assumption that there is no genetic or metagenomic control over body weight. As I said, I think that that particular assumption is the thing that many people have a problem with–the implication that they are overweight because they lack willpower.

  9. Paul Murray

    These people have been proof-texting the bible for so long that they honestly think that quote-mining is proper scholarship.

  10. This is an issue that I’ve been wanting to understand for a while now. The facts really do not seem to be Out There, though.

    Correlation isn’t proof of cause, as we all know. I have seen claims recently that the underlying causal agent to both obesity and ill-health is lack of exercise. That fat people who exercise are way better off than thin people who don’t. I’ve also seen suggestions that social stigma can underlie both; that fat people get worse medical care; and that poverty is the underlying causal agent.

    And then every time the “obesity crisis” is mentioned, there’s a picture of a headless fatty. It carries with it the implicit equation of the mildly overweight with the ginormous blimps. Not to mention callous meanness about those ginormous blimp people, who no doubt are suffering a great deal.

    And then we also hear about the muscular athletic types being required to lose weight to fit arbitrary BMI guidelines; and the huge problems of eating disorders in young women; and Hollywood, magazines and the fashion industry labelling underweight women as “too fat”.

    And then 95% of diets fail, and the best predictor for major weight gain is yoyo dieting, and weight loss surgery comes with a very high death toll, perhaps a greater risk than the risk of the obesity it’s supposed to fix. So even if it’s true, what anyone can do about it becomes pretty unclear.

    It’s a massive minefield of conflicting evidence, and I wish that someone that I trust were blogging it. I do look at Junkfood Science and Kate Harding, and I think they both make some good points. I’d like to see the other side well-expressed, rather than represented by magazine nutritionists (no qualifications required) and diet gurus (buy my book/DVD/product). Is anyone aware of scientifically reputable popular sources?

  11. I read this post — or perhaps a similar one — when you first put it up. At that time I had been reading junkfoodscience avidly. Your points really got me thinking, and I realised that I was trawling the internet trying to make myself feel better about my weight and about my newly diagnosed insulin resistance. Anyway, it was one of a number of factors that helped me get serious about the problem and start an exercise programme and get those glucose numbers down. So, thanks!

  12. PS: I will say that Sandy Szwarc’s site at http://junkfoodscience.blogspot.com looks absolutely nothing like most denialists out there. Among others, she links to Quackwatch, Snopes, and this cool statistics guide – http://www.tufts.edu/~gdallal/LHSP.HTM , and has links to major anti-woo books including recent ones Stephen Barrett, and Singh & Ernst. And my old favourite How to Lie with Statistics, and a favourite funny, the DHMO campaign.

    She has a very credible appearance to me. Not a crystal or a drop of magic water in sight. But maybe this just means that she’s very good at it. I do see a sniff of climate change denialism somewhere among the links from the people she links to, but that seems pretty indirect. If you actually do want to start assessing or debunking somewhere, her Obesity Crisis series would be an excellent starting point.

  13. Penn & Teller, in the miserable, fifth season of B****S***! take on the obesity “myth.” As I recall, their primary literature consisted of two books by … lawyers(!), of course.

  14. (Perhaps Mr. O’Neal should start an exercise program.)

    Are you seriously suggesting that a metric specifically designed for people with a sedentary lifestyle should also work for a professional athlete?

  15. @Dougal: That remark was clearly ironic.

    Anyway, from where do you know that BMI is a metric designed for people with a sedentary lifestyle? That was D.C.’s question.

  16. I reckon that a lot of the obesity and health nonsense has its origins in good intentions. There’s far too much pressure on people to look good, particularly adolescents. So we try to tell them that being skinny isn’t healthy and carrying a few extra pounds isn’t a big deal. We also try to prevent bullying of those who are rather more than a few pounds overweight.

    This is in conflict with the idea that being fat is unhealthy. So those who put a lot of effort into discouraging an unrealistic body image are going to be tempted to down play or even deny some of the health issues.

  17. The article in NEJM is actually itself an excellent example of the cherry-picking. Those authors found that overweight was not associated with excess mortality in their sample – usually relative risks below 1. So they stratified by all kinds of things – smoking, illness, age group, length of follow-up, whatever – about 15 different analyses. Overweight wasn’t significantly associated with mortality in any of those analyses – usually the relative risks were still actually below 1. So then they tried using BMI recalled from age 55. (All the BMIs were based on self-report anyway, so this one is based on asking somebody what they weighed at age 55). That still didn’t show anything. So finally they used data for BMI at age 55 for never smokers. Aha! We finally got something! So of these approximately 20 analyses, tested at a p < .05 level, one showed something. Guess which one ended up in the abstract.

  18. SteveMD

    What I find distressing about the whole subject is not the science, disputed or not, but the fact that an “anti-obesity” stance has now been promoted to an acceptable form of bigotry. We don’t treat smokers with the same contempt, they are rarely called names in public or told to just use a little discipline nor are they as widely blamed for using healthcare resources selfishly. Yet the same “excuses” for berating them exist. It cannot be denied that most people are, in some way, concerned about their weight. Either they believe they are overweight, which may or may not be the case, or afraid, if that is not too strong a word, of becoming overweight. Add to this the “tough-love” crap that is piled onto them and this must result in a great deal of emotional stress, possibly creating mental illness.

    I see nothing wrong with people being fat, I am slightly “overweight” myself, but aren’t lifestyle choices upto the individual? Do we berate people who break bones or risk their lives in extreme sports? Inform, educate and treat adults like adults.

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