The US longevity gap is becoming a chasm.

For decades, life expectancy in the States has lagged behind wealthy nations like Japan, Switzerland and Australia, with Americans consistently dying younger than their peers abroad.

Now, a landmark “population autopsy” analyzing more than 63 million deaths has identified two key factors fueling shorter lifespans nationwide, as experts warn the trend is only getting worse.

In the study, researchers compared the US to 17 peer nations and found that between 1999 and 2022, nearly 12.7 million American deaths could have been avoided if death rates matched those countries.

And the problem isn’t easing up. Annual “excess deaths” in the US more than tripled over the 23-year period, rising from about 346,000 in 1999 to 905,159 in 2022.

The top two factors: rising rates of cardiovascular and metabolic disease, along with a growing number of “deaths of despair,” or those linked to drugs, alcohol and suicide.

Digging deeper, researchers found that circulatory diseases like heart attacks and strokes were the leading driver of excess deaths in nearly every year they studied.

Those deaths actually improved for a time, falling between 1999 and 2009. But while rates continued to decline in other wealthy countries, the trend reversed in the US, climbing sharply from 2009 through 2022.

Breaking the numbers down further, the researchers found that the rise in circulatory disease deaths among adults aged 45 to 64 began a full decade earlier than in older adults.

The findings suggest that long-term, population-wide factors, including higher obesity rates and shifts in diets, may be playing a major role.

“As we look into this, we’re finding, and this paper appropriately points out, that as compared to older adults, the level and amounts of cardiovascular disease and mortality are starting earlier,” Dr. Neil Shah, a cardiologist with Northwell Health, told The Post.

“In order to tackle this, I think as a country, from a policy standpoint, we need to figure out how we are going to get patients into preventive care and preventive screening, so that we can use the medications and the counseling that truly make a difference early on to prevent these later cardiovascular outcomes.”

A similar pattern showed up in diabetes, kidney disease and other metabolic conditions, which saw relatively stable excess deaths through the 2000s before rising sharply after 2010 and continuing through 2022.

By 2022, US death rates from circulatory and metabolic diseases were 1.63 and 2.25 times higher than in other wealthy countries, respectively — and together, these conditions made up more than half of all excess deaths.

“All of this is rather a shared cardiometabolic continuum rather than separate isolated diseases,” said Shah, who was not involved in the new research. “Or in other words, each of these things affects the other.”

While circulatory and metabolic diseases accounted for the largest share of excess deaths over the study period, the fastest-growing category was “deaths of despair,” especially among men.

“‘Deaths of despair’ is an umbrella term for a number of behavioral, psychological and social factors that are driving individuals to some unhealthy behaviors and less than healthy coping strategies that then have health consequences, including death,” explained Dr. Xavier Jimenez, director of consultation psychiatry, addiction medicine and chronic pain at Northwell Long Island Jewish Medical Center.

“This can be a range of things. It could be substance use to check out of their realities. It could be poor eating. It could be poor social decisions, not seeking mental health care, maybe escaping through a variety of other behaviors that then catch up. And that eventually leads to death, unfortunately.”

In the US, deaths from drug poisonings, alcohol-related causes and suicide rose from levels similar to peer countries in 1999 to more than 130,000 excess deaths in 2022 alone.

“Deaths of despair” were the main reason for the widening mortality gap among those under 45.

“This study is remarkable. I, frankly, am not surprised by the findings,” Jimenez said. “Behavioral health concerns that go unaddressed catch up with individuals and contribute to an enormous slice of mortality via overdoses, impulsive behaviors, suicide, violence and accidents.”

Taken together, deaths from cardiometabolic diseases and from drugs, alcohol and suicide accounted for about 24% of the increase in excess deaths over the study period — and for most of the rise among those under 44.

The Covid-19 pandemic also drove a sharp spike in excess deaths in 2020 and 2021, with coronavirus accounting for roughly 1 in 5 excess deaths during that period.

Researchers found that other causes were also pushed higher during the pandemic, including circulatory disease, metabolic conditions, drug poisonings, alcohol-related deaths and suicide.

This, they suggested, may reflect disruptions in medical care, worsening mental health, broader social stressors and in some cases, misclassification of Covid-related deaths as other causes.

“Covid-19 really only jet-fueled the despair that was already being experienced even before that pandemic,” Jimenez said.

“If you remember, we had opioid crises, we had political shifts, economic recessions, and then Covid-19 came and isolated individuals, reduced access to care, created enormous burdens and stress, not to mention the physical effects,” he continued.

“So, it only makes sense that during the pandemic and a few years thereafter, you see it contributing to the despair that was already being felt and accelerated by these processes.”

Looking ahead, Jimenez and Shah advocate for expanding access to routine preventive care, addressing physical, behavioral and psychiatric conditions early before they lead to downstream effects.

“We always say an ounce of prevention is worth a pound of cure,” Shah said.

The most recent data suggests that an American born in 2024 can expect to live to about 79 years old on average — an all-time high since national tracking began in 1900.

But that still lags behind peer countries like Switzerland, where life expectancy is about 84, and Japan, where it’s about 85.