Obesity, overweight BMI can raise risk of death by 91% - study

Obesity or excess weight has been found to significantly raise one's risk of death by 22%-91%, far more than what scientists had initially thought, according to a new study.

The researcher from the University of Colorado Boulder behind the study also determined that the risk of death from being underweight is not as substantial as initially thought.

The findings of this study were published in the peer-reviewed academic journal Population Studies.

Busting preconceptions on obesity: The actual risk of a high BMI

Obesity is an increasingly common condition throughout the world. In the US, for example, obesity impacts just under half (41.9%) of all adults, according to the US Centers for Disease Control and Prevention (CDC).

Obesity itself is measured in terms of body mass index (BMI). This measurement is essentially a value derived from a person's body mass and height. In essence, it's one's weight divided by one's height squared. 

To simplify it, a BMI under 18.5 is considered underweight while a BMI over 25 is considered overweight and a BMI over 30 is considered obese. 

It should be noted that BMI is far from a perfect system of measurement for weight. For example, the formula doesn't always take muscle into account, as one can have a high BMI even if a large amount of it is muscle mass. For example, one particularly famous case is that of Tom Cruise, who at one point was given a BMI of 31.5, meaning he was obese, due to his height and extremely muscular physique.

In addition, as noted by a US National Institutes of Health (NIH) article published in the British Journal of General Practice in 2010, BMI was also designed for white people – specifically, Belgians as it was made by Belgian national Adolphe Quetelet to determine the average Belgian male's size – and therefore, does not apply so well to people of color.

Regardless, BMI has still become widespread as a "rule of thumb" of sorts to determine whether someone is underweight, average, overweight or obese.

In particular, numerous studies have also linked one's BMI to mortality risk, with underweight and obesity both being believed to be related to an increased risk of death.

However, it had previously been believed obesity only raises one's risk of death from any cause in particularly extreme cases.

This, in turn, is linked to numerous biases in estimates of the link between BMI and mortality, often overlooking factors like the amount of time spent with a given BMI.

In addition, there is also the matter of what has been dubbed the "obesity paradox." This refers to how the lowest mortality risk among BMIs can be found among overweight people, while obese and healthy weights have the same mortality rate and extremely obese (BMI of 35 and up) and underweight people have the same higher level of mortality.

But this new study claims this previous assumption may not be the case.

How much does obesity raise one's risk of death?

To figure this out, the researcher carried out a statistical analysis of around 18,000 different people, including almost 4,500 deaths, from 1988 to 2015 based on data from the National Health and Nutrition Examination Survey (NHANES).

The study had come with the hypothesis that factors at play behind biases in BMI-mortality link studies included that lower mortality risk among high-BMI individuals were due to recent weight gain and that high mortality risk among low-BMI individuals was due to illnesses causing the weight loss to bring them there in the first place. In other words, this is due to reverse causation.

The findings of the study seem to back it up.

Exactly 20% of subjects with a healthy BMI were overweight or obese in the previous decade. This group had significantly worse health than the other categories specifically because their weight wasn't stable.

Indeed, rapid weight loss-inducing diseases can be caused by being overweight or obese for an extended period of time.

Meanwhile, 60% of those with obese BMI and 37% of those with overweight BMI had been at lower BMI in the preceding decade. Those who recently gained weight were found to be in better health.

By not taking these trends into account, it means that all the data for mortality risk from BMI has been heavily skewed.

So taking all the data into account, the researcher behind the study calculated the numbers again.

The result was that rather than a bell curve where overweight was the healthiest BMI, there is now a straight line, where a healthy BMI has the lowest mortality risk and being underweight has no signficiant increased mortality. 

The result also shows that previous assumptions about death linked to BMI may also be completely wrong. Previous research indicated that out of all adult deaths in the US, 2%-3% have been due to high BMI. 

According to the calculations reached in this new study, though, the actual figure may be eight times higher.

The findings of this study also reaffirmed previous studies on the obesity paradox. 

In particular, one previous study from 2022 regarding the obesity paradox's relationship to developing Alzheimer's disease found that keeping one's BMI stable when in old age may be the best way to prevent Alzheimer's. This, too, builds on the idea that being overweight is considered healthier solely due to it being stable rather than any other inherent quality.

Future studies will need to take into account the study's conclusions regarding BMI biases when examining the subject.

Aaron Reich contributed to this report.