There’s no question that America is faced with multiple health crises, none of which are easy to solve. Two of those – opioids and obesity – have been varyingly suggested as reasons why the US death rates year on year aren’t improving. Although we still can’t be certain, a new study highlights the damaging role of the latter.

For 40- to 84-year-olds, the US has an annual decline in mortality rates of 1.53 percent, but the UK, for example, experienced a 2.45 percent deline. This is comparable to Switzerland, Norway, Finland, and Australia's rate of decline.

Led by the University of Pennsylvania and the Boston University School of Public Health, the team explain that “deaths of despair”, those linked to the opioid crisis – as well as suicide rates, alcohol, and other drug-linked afflictions – can’t explain “why the rate of mortality improvement in the United States has declined relative to other wealthy countries.”

So they took a look at obesity instead.

Obesity is largely defined by the body mass index (BMI), which is a person’s weight (in kilograms) divided by their height, squared. A BMI of 30 or above classifies someone as obese, with higher categories available for more extreme forms of obesity. According to the US Centers for Disease Control and Prevention (CDC), more than a third of adults (36.5 percent) are obese. So could this all-time high be playing a role in declining improvement rates?

Writing in the Proceedings of the National Academy of Sciences, the team explain that they mined their data from the National Health and Nutrition Examination Survey, or NHANES. This nationwide survey combined interviews and physical examinations to provide a broad but detailed picture of the state of Americans’ health.

Comparing these stats to linked mortality files and calculating the role BMI played in changes of circumstances, the team found that rising BMIs has, in 2011 alone, “reduced life expectancy at age 40 by 0.9 years, and “accounted for 186,000 excess deaths that year.” That’s about 510 obesity-linked deaths per day.

Ultimately, between 1988 and 2011, “rising Max BMI is estimated to have slowed the annual rate of mortality decline during this period by 0.54 percent,” which the team note is “a large amount by international standards.”

It’s fair to point out that BMI is a flawed measure. It’s a metric devised in the 1800s, and it fails to take into account things like muscle mass. That means some athletes, who are otherwise healthy, can be registered as obese.

In any case, no one is debating that being obese is extremely bad for your health. From higher incidences of strokes, cardiovascular diseases, type 2 diabetes, and several types of cancer, being obese is something bets avoid – so why is it so unusually prevalent in the US?

Vox point out that becoming obese in the Land of the Free is, well, incredibly easy. Portions sizes have dramatically increased in a short space of time; breakfast is often a form of dessert. The sugar industry has been spreading disinformation about its products for decades.

Consequently, the number of obese men, women, and children in the US are increasing pretty much year-on-year. Far from being just a medical burden, it’s creating a hefty bill: One that the CDC peg as $147 billion per year (as of 2008), about half the cost of 2017’s record-breaking year of destructive natural disasters.

Yes, the opioid epidemic is a blight too, and one that is also contributing to this reported lack of improvement in mortality rates. Between heroin and prescription painkillers, it’s estimated that at least 150 people die per day of such overdoses, which adds up to around 59,000 a year.

As studies like this show, though, there’s a long way to go with obesity too. It’s a killer, worse than many other causes of mortality- and the solution isn't going to be a simple, straightforward fix.

"High taxes on sugared beverages is a start, more intensive physical education classes in schools, better facilities for physical activities at the workplace may have some marginal effect," lead author Samuel Preston, a professor of sociology at the University of Pennsylvania, told IFLScience.

"My own guess is that the most successful weapon will ultimately prove to be pharmacological."