Excess weight or obesity boosts risk of death by anywhere from 22% to 91%鈥攕ignificantly more than previously believed鈥攚hile the mortality risk of being slightly underweight has likely been overestimated, according to new CU Boulder research.
The findings, published Feb. 9 in the journal , counter prevailing wisdom that excess weight boosts mortality risk only in extreme cases.听
The statistical analysis of nearly 18,000 people also shines a light on the pitfalls of using body mass index (BMI) to study health outcomes, providing evidence that the go-to metric can potentially bias findings. After accounting for those biases, it estimates that about 1 in 6 U.S. deaths are related to excess weight or obesity.
鈥淓xisting studies have likely underestimated the mortality consequences of living in a country where cheap, unhealthy food has grown increasingly accessible, and sedentary lifestyles have become the norm,鈥� said author Ryan Masters, associate professor of sociology at CU Boulder.
鈥淭his study and others are beginning to expose the true toll of this public health crisis.鈥澨�
Ryan Masters
Challenging the 鈥榦besity paradox鈥�
While numerous studies show that heart disease, high blood pressure and diabetes (which are often associated with being overweight) elevate mortality risk, very few have shown that groups with higher BMIs have higher mortality rates.
Instead, in what some call the 鈥渙besity paradox,鈥澨齧ost studies show a U-shaped curve: Those in the 鈥渙verweight鈥� category (BMI 25鈥�30) surprisingly have the lowest mortality risk. Those in the 鈥渙bese鈥� category (30鈥�35) have little or no increased risk over the so-called 鈥渉ealthy鈥� category (18.5鈥�25). And both the 鈥渦nderweight鈥� (less than 18.5) and extremely obese (35 and higher) are at increased risk of death.
鈥淭he conventional wisdom is that elevated BMI generally does not raise mortality risk until you get to very high levels, and that there are actually some survival benefits to being overweight,鈥� said Masters, a social demographer who has spent his career studying mortality trends. 鈥淚 have been suspicious of these claims.鈥�
He noted that BMI, which doctors and scientists often use as a health measure, is based on weight and height only and doesn鈥檛 account for differences in body composition or how long a person has been overweight.听
鈥淚t鈥檚 a reflection of stature at a point in time. That鈥檚 it,鈥� said Masters, noting that Tom Cruise (at 5 feet 7 inches and an extremely muscular 201 pounds at one point), had a BMI of 31.5, famously putting him in the category of 鈥渙bese.鈥� 鈥淚t isn鈥檛 fully capturing all of the nuances and different sizes and shapes the body comes in.鈥�
Duration matters
To see what happened when those nuances were considered, Masters mined the National Health and Nutrition Examination Survey (NHANES) from 1988 to 2015, looking at data from 17,784 people, including 4,468 deaths.
He discovered that a full 20% of the sample characterized as 鈥渉ealthy鈥� weight had been in the overweight or obese category in the decade prior. When set apart, this group had a substantially worse health profile than those in the category whose weight had been stable.听
Masters pointed out that a lifetime carrying excess weight can lead to illnesses that, paradoxically, lead to rapid weight loss. If BMI data is captured during this time, it can skew study results.听
鈥淚 would argue that we have been artificially inflating the mortality risk in the low-BMI category by including those who had been high BMI and had just lost weight recently,鈥� he said.
Meanwhile, 37% of those characterized as overweight and 60% of those with obese BMI had been at lower BMIs in the decade prior. Notably, those who had only recently gained weight had better health profiles.
鈥淭he health and mortality consequences of high BMI are not like a light switch,鈥� said Masters. 鈥淭here鈥檚 an expanding body of work suggesting that the consequences are duration-dependent.鈥澨�
By including people who had spent most of their life at low-BMI weight in the high-BMI categories, previous studies have inadvertently made high BMI look less risky than it is, he said.
When he looked at differences in fat distribution within BMI categories, he also found that variations made a huge difference in reported health outcomes.
Exposing a public health problem
Collectively, the findings confirm that studies have been 鈥渟ignificantly affected鈥� by BMI-related bias.
When re-crunching the numbers without these biases, he found not a U-shape but a straight upward line, with those with low BMI (18.5鈥�22.5) having the lowest mortality risk.
Contrary to previous research, the study found no significant mortality risk increases for the 鈥渦nderweight鈥澨齝ategory.
While previous research estimated 2 to 3% of U.S. adult deaths were due to high BMI, his study pegs the toll at eight times that.
Masters said he hopes the research will alert scientists to be 鈥渆xtremely cautious鈥� when making conclusions based on BMI. But he also hopes the work will draw attention to what he sees not as a problem for individuals alone听to solve听but rather a public health crisis fueled by an unhealthy or 鈥渙besogenic鈥� environment in the U.S.听
鈥淔or groups born in the 1970s or 1980s who have lived their whole lives in this obesogenic environment, the prospects of healthy aging into older adulthood does not look good right now,鈥� he said. 鈥淚 hope this work can influence higher-level discussions about what we as a society can do about it.鈥�
