Do a few extra pounds really add a few extra years of life? Or is something rotten?
Here’s a quick quiz for you. Don’t worry, there’s only one question that you need to answer, and it’s totally hypothetical:
You’re a doctor at a hospital. Two patients are in two private rooms in front of you. You can’t see into the rooms, and you don’t know anything about the patients except that the left patient is overweight, and the right patient is not.
Which patient is more likely to die?
Did you guess that the overweight patient is more likely to die? If so, I’ve got some bad news for you; you’re wrong, according to a 2013 meta-analysis published in the Journal of the American Medical Association.
How does this make sense? We’ve always heard about how carrying excess weight is unhealthy. Why is an overweight person less likely to die than someone who is not overweight? Is this shoddy science, or is this study proving that everything we know about health and nutrition is wrong, and we should all be snarfing cheeseburgers for every meal?
What the study says
In 2013, researchers at the National Center for Health Statistics looked at 97 different studies, performing an analysis of these studies (called a meta-analysis, since it’s an analysis of existing analyses) that encompassed a total of 2.88 million total individuals.
They separated their findings by BMI range, into normal-weight, overweight, and 3 increasing grades of obesity. They found:
Relative to normal weight, both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality.
In other words, if we consider someone of normal weight (BMI 18–25) to be our baseline:
- Overweight people (BMI 25–30) were less likely to die than a normal-weight person.
- Obese people in grade 1 (BMI 30–35) were as likely to die as a normal-weight person.
- Obese people in grades 2 and 3 (BMI > 35) were more likely to die than a normal-weight person.
Seems like a clear conclusion, doesn’t it? Someone who’s overweight is, based on this analysis, less likely to die than someone who is at a “healthy weight” BMI.
And this statistic has been cited in areas like the “Healthy At Every Size” (HAES) movement, stating things like “on average, overweight people live longer than healthy-weight people”.
But are we missing something?
Correlation vs. causation: this study is missing context
A number of reviews have looked at this paper and, while the statistics are fine, there are challenges to the conclusions. Most critics don’t have an issue with the actual study, but instead argue that it’s interpreted by popular news articles in the wrong way.
Consider this: what medical conditions kill people?
We can probably name a few, off the top of our heads: heart attacks, stroke, cancer, dementia, Parkinson’s, chronic respiratory diseases, and infections/pathogens/bacteria/viruses.
Some of these diseases can kill quickly (heart attack, stroke). But many of them kill slowly, and have a long period of progressive degeneration.
The result of these diseases is that, once people become sick, they are often sick for an extended length of time. They’ll have pain, they’ll often be in and out of the hospital — and they experience significant weight loss.
To put it another way, being overweight doesn’t protect you against getting disease. But when you get a disease, you’ll usually lose significant weight before you die.
Remember, the meta-analysis isn’t arguing that there’s any causation. It is NOT saying that overweight people live longer. It is ONLY saying that, when a person dies, they are most likely obese (died quickly) or at a BMI that is not overweight (died more slowly).
For a metaphor, consider sky diving. Did you know that practically all skydiving deaths occur at ground level? No one dies in midair. Therefore, the only place in skydiving where you should be concerned is the moment you land.
It’s true, but it’s missing important context. Even if you won’t die until you hit the ground, you probably will place a lot of importance on the integrity of your parachute — even though that deploys way up in the air, a long way from where you’re at greatest risk of death.
Excess weight is more likely to cause conditions that result in weight loss (and then death)
With context, we start to see that, despite the weight at the end of the journey (death), there are reasons to also want to control weight at earlier stages in the journey (life).
For example, let’s think back to that list of causes of medical death that we listed earlier. Many of these, such as cancer, blood pressure, type II diabetes, and heart failure, are more likely to occur when someone carries excess weight.
Not only are obese individuals more likely to die sooner than their non-obese counterparts, but they also incur, on average, significantly higher medical expenses. They’re both living shorter lives and living less healthy lives, more likely to require more medical treatment.
Here’s a great figure from a similar 2010 paper that followed 1.46 million adults over 10 years:
Note that we can see the same observations we noted above. People who are at a “healthy-weight” BMI (between 22 and 25) are least likely to die. We see higher death rates in people who are either underweight or are overweight.
The underweight individuals are more likely to have an illness that increases their risk of death, while the overweight individuals are at a higher risk of developing a deadly illness.
Additionally, it’s important to note that, even in our original study, obese individuals were at a higher risk of death than normal-weight individuals, and that risk went up as the BMI climbed. Someone who is overweight is at greater risk than someone at a healthy weight, but they are still at a lower risk than another individual who is fully obese.
In summary: many die underweight due to disease, not because they’re less healthy
As always, we need to be careful when we see a fact that is suspicious. Is causation being improperly applied?
That’s what is happening in this case. Yes, it’s true that people are less likely to die when overweight. But being overweight is not protecting them from death; rather, people tend to lose weight when they get a debilitating, deadly disease.
Normal-weight people aren’t dying faster; dying makes people lose weight.
And when it comes to years of healthy, chronic disease free life, aiming for a BMI between 20 and 25 is still the best option.
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