Lightening the load of heavy weight research

There’s a new study out on weight and mortality risk this week.  What is it saying?

It depends on who you ask.

If you ask the press, they’ll say this:

Carrying some extra pounds may not be good after all

Or this:

If you're overweight at any point, you're raising your risk for an early death

Yuck!  That sounds just dreadful.  Why are they saying this, what does this mean, and is it true?

First, let me fill in some back story.  In 2013, prominent epidemiologist Katherine Flegal and co-authors published a paper examining relationships between body weight and all-cause mortality (risks of death from all causes).  What they found was a lower mortality risk in the so-called overweight BMI category of 25-30, and not-increased risk in the so-called obesity I BMI category of 30-35.  Their results ran contrary to conventional wisdom (so much for conventional wisdom…).  They also unleashed a furious and very rude backlash among prominent and heretofore relatively well-behaved public health  and obesity researchers.  Here are a few reactions:

“It’s a horrific message to put out at this particular time. We shouldn’t take it for granted that we can cancel the gym, that we can eat ourselves to death with black forest gateaux.”
UK National Obesity Forum

“You’d hate to have the message get out there that it’s good to be overweight. The reality is that people who are overweight very often become obese and that’s clearly not good.”
Mercedes Carnethon, Northwestern Univ. School of Medicine

Since the Flegal et. al. 2013 article, some researchers who disagree with those findings have been trying to explain how being “overweight” (I use the quotes because I’m referring to the BMI category of 25–30 here, not any description of a person’s body) can lower your mortality risk.  Andrew Stokes, a population health researcher at Boston University, has been working on trying to tease out what’s going on with weight changes over time and mortality (death by any cause).  In a bunch of recent papers he and his coauthors have looked not just at BMIs and death rates, but at maximum BMI of individuals and possible relationships between that max, trends in their BMIs over time, and death rates.  (side note: my friend Dan and I are working on an article addressing Stokes’ work, which is in progress.  I’ll certainly blog about our work when we have results).

This newest paper looks at population data from three very big longitudinal studies and concludes that we can explain the so-called “obesity paradox” (that BMI 25–30 confers lower mortality risk rather than increased mortality risk) by looking at maximum BMI.  Those with maximum BMI of 25 or greater had increased mortality risk compared to those with maximum BMI of <25.

Ah– so being fatter really is bad for you.  Whew; public health and medicine don’t need to change all that signage after all.

a mind map of phrases connected with risks of more rather than lessbody weight

Well, maybe they do.  Looking at this article, I found some complicated and interesting results (which I’ve seen in other such articles, but aren’t splashed across the headlines.)

Interesting result one:  being “underweight” (BMI <18.5) carries a much greater mortality risk than being “overweight” (BMI 25–30).  For a lot of age/sex categories, it carries a much great mortality risk than being “type I obese” (BMI 30–35).  For instance, for non-smoking men< 70 years old, the mortality risk was almost the same for <18.5 BMI as for >35 BMI (2.89 and 3.19 respectively).  That is, people at the far ends of the weight spectrum measured both had much increased mortality risk.  Again, we are talking about maximum BMI here (just to be precise).

Interesting result two:  the mortality risks from a particular max BMI shift as the population ages.  The details are pretty complicated, but here’s an example:  if my max BMI is say, 31, then these results show how my mortality risks may go up and down as I age.  This is interesting and important for patients and health care providers.  Given some max BMI, the medical advice might be different depending on the age of the patient (and other features of her medical history).  Of course, many medical practitioners act on this already by paying special attention to many features other than BMI in caring for their patients.

Interesting result three:  the results are based on three very large samples (about 225,000 people) of white people– they made up more than 91% of the sample.  We already know that BMI distributions vary across racial categories, so these results (if they turn out to be correct), would not apply in a simple way to other groups.

Interesting result four: In the article, the authors point out that their targeted group (BMI 25–30) is pretty diverse with respect to body fat percentage and waist circumference.  They’re also going to be pretty diverse with respect to their eating and physical activity practices (like every other BMI group).  The authors think that they can use max BMI to identify who in the BMI 25–30 group is at increased risk.  But to what end?  It’s not like medical practice has any currently effective procedures for bringing about and sustaining weight loss over time (except maybe some forms of gastric bypass, which aren’t indicated for the population targeted in the article).  So, what is an appropriate response to this information from patients and providers, other than more moral panic?

For me, my response to this article is to dig into the details, talk to my colleague Dan about our article, and attend to my health-as-I-define-it in the best ways I know how.  I’m not convinced these folks are right.  And I’m not convinced that we even agree on how their being right might reasonably translate into anything medically useful or practical.  However, we all know that science, medicine and health care are super-complicated, so while we’re waiting for the fog to clear, let’s just do nice things for ourselves.  So I’m headed out for a bike ride now!



About catherine w

I'm an analytic philosopher, retooled as a public health ethicist. I'm interested in heath behavior change, particularly around eating and activity, and how things other than knowledge affect our health decisions.I'm also a cyclist (road, off-road, commuter), squash player, x skier, occasional yoga-doer, hiker, swimmer and leisurely walker.

9 thoughts on “Lightening the load of heavy weight research

  1. Helen says:

    I know this was for the previous study but it emphasises what drives me mad
    “We shouldn’t take it for granted that we can cancel the gym, that we can eat ourselves to death with black forest gateaux.”

    What drives me mad is this presumption that the only reason people go to the gym is for weight loss or to maintain a certain weight instead of the DOZENS of other reasons it is good to keep physically active. If we could just STOP making physical activity about weight that would be a giant step in the right direction for the health of the population (whatever damn weight they are)

    Liked by 1 person

  2. Great article anf you have brought all opinions into one place for reference. I mean there is no doubt that society has slowly been gaining the dreaded extra pounds. Much can be attributed to what we eat and the amounts. That few extra pounds that we gain here and there are much harder to lose as we age. Exercise and proper eating take hard work.
    I look at many of these studies as one persons opinion. Questions lie are they credible come to mind. Are they simply published to gain a coveted spot in journals. Recall that study a few years back that said eggs were bad for you? Now they are ok.
    Myself personally I listen to my own body, pay attention to what I eat and how much I eat and well the exercise comes from being on the move. Study or no study I know if I have gained those few extra pounds… basically pay attention to what your body is telling you. Drink a half gallon slurpy and it will speak to you immediately and the results will show very soon after.


  3. This is why BMI can be a rubbish way of measuring health.


  4. Sam B says:

    I think one of the things we both find puzzling is what this means for medical advice. Suppose being 5’7 meant I was more likely to die earlier than someone who was 5’5. We’d carefully watch for the conditions and health problems that would cause my earlier death, sure, but there would be no expectation that I’d do anything about it. Weight isn’t exactly like height in it’s unchangeable nature but it’s not far off.


  5. Kim Solga says:

    I’m curious about the BMI 25-30 category: it includes people who might be a bit overweight and not exercising, people who might be a bit overweight and exercising, as well as many muscular athletes. I think about the women I row with: I think the majority of them are probably BMI 26-27. I know this sounds naive and simplistic, but could part of the story with the “exceptionalism” of this category in some research findings have to do with this spread of body types across athletes and non-athletes? And yes: further evidence that BMI is more harmful than helpful, to us but also to science!

    Liked by 1 person

  6. laufvergnügen says:

    Absolutely agree about it being much more dangerous to have a BMI less than 18.5 — anorexia has a high mortality rate.


  7. Ange B says:

    Perhaps the mortality we should be concerned with is that of the concept of BMI, or at least the current categories depicting a level of ‘fatness’.
    I am curious whether any studies have looked at BMI in relation to risk of death due heart disease or all cause mortality or diabetes and determined that a BMI of ’30’ is actually significant point at which risk increases – why not 32.5 or 36.15?
    Where does this rather exact ‘5.0kg/m2 equals the difference between healthy/normal bodyand an obese one’ equate with and mark also a significant point in the scale of the risk of disease and dying? (Does that make sense?) 5kg/m2 seems to be a convenient, not necessarily a scientific, measure – the mere point that the results reported in this study confound the “experts” and cause them to get defensive would seem to add weight to this?

    I’m looking forward to reading your future assessment of this research too.

    I love telling people how much I weigh (incidentally my BMI is currently 32.5) Just for the astounded “but you’re not a big donut scoffing couchbound unfit fattie” look on their trying-hard-to not-be-judgemental faces. Very entertaining.


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