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It’s our favorite vice and our true national pastime: Here in the land of plenty, Americans love to overindulge.

We eat too much, drink too much, snack too much, sit too much — then rely on the marvels of modern medicine to repair our battered bodies. And that makes us easy prey for a brand-new infection like the novel coronavirus.

“This is on us,” Nicole Saphier, MD, told The Post. “We left ourselves vulnerable to this epidemic. Our health-care system would not be so overwhelmed right now if it was not already overwhelmed with our preventable chronic illnesses.”

About 75 percent of the $2.2 trillion we spend on health care each year goes to treat chronic illness, the Centers for Disease Control estimates.

“And 40 to 70 percent of those illnesses could be prevented,” Saphier said.

Take cardiovascular disease and stroke. In 2018 the United States shelled out $329 billion to treat them, the American Heart Association calculated. About 80 percent of those cases — $263 billion worth — were caused by poor diet, lack of exercise, alcohol use and obesity. And in any given year, those conditions alone cause upwards of 300,000 premature deaths in the United States.

“Obesity is linked to diabetes, high blood pressure and stroke,” said Saphier, a radiologist at New York’s Memorial Sloan Kettering Cancer Center and author of “Make America Healthy Again” (HarperCollins), a new book that examines our floundering health-care system.

“All of those conditions render us susceptible to infectious illnesses — just what we are seeing with COVID-19,” Saphier says.

Since the novel coronavirus emerged in Wuhan, China, five months ago, researchers have raced to understand how the new virus attacks the human body — and what makes it a killer.

“In the beginning, when we looked at Chinese data, it seemed that elderly men were the ones dying,” Saphier said. “The data indicated that mortality would increase with one underlying condition. The death rate in Wuhan was attributed to China’s high air pollution and high smoking rates.”

That led many experts to hope that Americans would be able to brush off the new illness with few fatalities. “Our air pollution is less than China’s, and our smoking rate is much lower,” Saphier explained.

But Saphier’s work as a cancer specialist had taught her that Americans’ battle with chronic illness has had insidious effects on our overall health. She suspected that a whole different slate of risk factors could come into play if COVID-19 reached the United States.

“I feared that the fact we lead the world in obesity could put us at risk,” she said. “And unfortunately I think the data is showing that suspicion to have been correct.”

Two new studies from separate teams of researchers at New York University, both published this month, pointed to obesity — not lung disease or cancer — as a major risk factor for COVID-19 hospitalizations of American patients.

The virus poses acute danger to seniors, the studies confirmed. Older patients tend to have poorer lung function, making age itself a risk factor.

But obese patients under age 60 were almost twice as likely to be hospitalized for COVID-19 as those of normal weight.

“About [89 percent] of the [12,192] people who have died in New York of COVID-19 had chronic illness as well,” Saphier said, referring to data reported on Thursday.

Almost all of those comorbidities, including hypertension, diabetes and coronary artery disease, are linked to lifestyle factors, particularly obesity.

I feared that the fact we lead the world in obesity could put us at risk [to COVID-19]. - Dr. Nicole Saphier

High blood pressure, for example, was diagnosed in 57 percent of the New Yorkers who have lost their lives to COVID, and high cholesterol in 20 percent of cases — all in a state with a 28 percent obesity rate, one of the lowest in the nation.

While New York may be the COVID-19 epicenter, “look at Louisiana, which is also struggling,” Saphier said. “Not because of population density, not because holding Mardi Gras was a bad idea — but simply due to the demographics of that state.”

Louisiana was No. 4 in the United Health Foundation’s 2019 ranking of the most overweight states, with a 37 percent obesity rate — and 97 percent of the Pelican State’s [1,213] coronavirus deaths have occurred in patients with chronic illnesses, according to Saphier.

That includes 60 percent with hypertension, 38 percent with diabetes and 22 percent with morbid obesity. Many of the victims suffered more than one of those conditions.

“The data seems to correlate with the higher percentage of obesity there,” Saphier said.

Hidden within the comorbidity data is a clue to the source of the racial disparities that have been seen in the nation’s COVID-19 deaths, Saphier noted.

African Americans make up 18 percent of those killed by the disease in New York state, even though they are only 9 percent of the state’s population. “It’s sick. It’s troubling. It’s wrong,” Mayor de Blasio said of the data last week.

The difference is even starker elsewhere. Black Louisianans account for 59 percent of the state’s COVID deaths — but make up just 33 percent of its population. In Chicago, African Americans make up 29 percent of the population but have suffered 70 percent of pandemic deaths.

The latest CDC data reveal that the obesity rate nationwide is 25 percent higher among black Americans as compared to whites. The diabetes rate among African Americans is 66 percent higher, the hypertension rate 49 percent higher.

“The racial disparity is real,” Saphier said.

“But right now, all Americans are vulnerable to COVID-19 because of our lifestyle. We don’t have a COVID-19 problem so much as we have a problem with preventable chronic illnesses.”

Those conditions contributed to an alarming trend in the last decade.

For more than a century, life expectancy for the average American made steady progress. Longevity in the United States gradually lengthened from 47 years in 1900 to a high of 78.9 years in 2014.

But then, for three years in a row, that progress halted — and began to reverse, falling to 78.6 years by 2017.

The shift was small, yet momentous: It was “the longest multiyear decline in life expectancy since the years 1915 to 1918, when the decrease was mostly attributable to World War I and the devastating 1918 inﬂuenza pandemic,” Saphier writes in her book.

Her own profession must shoulder some of the blame, she says.

“It’s very easy to sound like the bad guy,” she admitted. “Doctors are often afraid to call out problems like obesity because we don’t want to be considered prejudiced or mean, or to be accused of ‘fat shaming.’ ”

The food industry, too, prods us to give in to our worst impulses.

“We have made it so convenient to eat unhealthily,” Saphier said. “And it’s not that we’re lazy — it’s that we’re working so much and are so busy, even our kids have packed schedules. With home-cooked meals declining, that is when you see more takeout, more fast food.”

A CDC study found that from 2013 to 2016, nearly 40 percent of Americans admitted to eating fast food on any given day — high-salt, high-fat, high-carbohydrate products that contribute mightily to the conditions that cause early death for hundreds of thousands of us every year.

“And now here we are, being told to stay home and do nothing but watch Netflix all day,” Saphier said of the state and local orders meant to blunt the impact of the coronavirus.

“My weakness through this has been pretzel chips with guacamole — I get it!” she said. “But that’s the problem that got us here, being sedentary and eating too much. And not only are we eating too much, we’re eating crap.”

An analysis of data from 68,000 fitness trackers in March found that physical activity dropped 39 percent or more as shutdown orders began to take hold, Evidation Health reported. Add our junk-food binges to that steep drop in exercise, and these weeks behind closed doors could shoot our obesity rates even higher.

Then there’s the booze. With no commute in the morning and plenty of angst to soothe, any hour is now happy hour for millions of Americans. In California, Bay Area residents boosted their alcohol consumption by 42 percent as soon as they started sheltering in place, the San Francisco Chronicle reported. Liquor sales at online retailer Wine.com quadrupled.

It’s a bad idea in the midst of a global pandemic: Alcohol suppresses the immune system.

“People drink because it mellows them,” Dr. Shannon Sovndal told Fox News last week. “Well, alcohol has a similar effect on the immune system, making it slow and lethargic.” One 2014 study found a sharp reduction in two types of disease-fighting white blood cells in the hours after a drinking session.

“So we could find ourselves perpetuating this vicious circle,” Saphier said, if our efforts to slow the spread and flatten the curve end up worsening the bad health habits that exposed us to harm in the first place.

In the coming months, as Americans grapple with a presidential campaign as well as the continuing fallout of the coronavirus crisis, our national health-care system will be a matter of endless debate.

“America is sick, and we are all to blame,” Saphier said. “But I think we have proven two things these last few weeks: that our great medical innovations save lives, and that Americans are capable of taking great personal responsibility, in the form of social distancing, to protect each other,” she added.

“Our personal responsibility will get us through this crisis. But only a new level of lifestyle responsibility will help us in future.”

DEADLY FACTORS

In New York state, 89 percent of COVID-19 deaths occurred in people with one or more chronic conditions, or comorbidities. Along with age, these are the top 10 risk factors for coronavirus deaths in New York through Tuesday, April 14. Because many victims had multiple comorbidities, the percentages will total more than 100 percent.