A public health emergency is shortening our lives and supersizing our health-care costs. Here's everything you need to know:

How serious is the crisis?

Americans are collectively gaining weight at an alarming rate, with the average adult weighing about 15 pounds more than 20 years ago. In defining obesity, nutritionists focus on body mass index: A healthy person has a BMI of 18.5 to 24, while an obese person has a BMI above 30. Today, about 40 percent of U.S. adults and nearly 20 percent of children are obese, a surge reflected worldwide, with more people now overweight or obese (1.9 billion) than underfed (800 million). In 1985, no state in the U.S. had an obesity rate above 15 percent; now every state's is higher than 20. Excess weight is a major factor driving runaway medical costs. Obesity-related diabetes alone "will break the bank of our health-care system," said University of Colorado nutrition expert James Hill. "Obesity may be the toughest social issue that we have ever faced."

What's making us fat?

Simple: eating too much and exercising too little. Despite constant debate over which dietary villain to blame — fat, carbs, sodium, sugar — obesity is primarily a problem of calorie intake. The average adult is eating about 300 more calories per day than in the 1970s. In 2015, for the first time, Americans spent more money eating away from home than they did on groceries, and research indicates people eat 20 to 40 percent more calories at restaurants, where portion sizes have quadrupled since the 1950s, according to the Centers for Disease Control. American calories are also largely "empty" ones — highly processed foods such as chips, white bread, and sugary cereals account for almost 60 percent of U.S. calorie consumption. Those foods, plus artificial sweeteners, may disrupt nerve signals between the gut and the brain, causing us to keep eating long after we've had enough. As Yale University neuroscientist Dana Small explains, it's as if "the brain doesn't really know the food is even there."

Who's most at risk?

Children, and it's getting worse. "Addressing childhood obesity is like playing whack-a-mole," said Harvard nutritionist Erica Kenney. Kids are spending more and more time indoors looking at screens, where they're bombarded with advertisements for unhealthy foods. Black children see twice as many soda and candy ads as white kids do, which is just one reason why childhood obesity is racially skewed: A 2017 study showed that about 26 percent of Hispanic youth and 22 percent of black youth were obese, compared with 14 percent for white youth and 11 per­cent for Asian youth. Children are gaining weight faster than adults, with the obesity rate for U.S. kids ages 5 to 9 projected to hit 26 percent by 2030.

What's wrong with weight gain?

Having a high BMI doesn't necessarily mean a person is unhealthy. Still, the correlations are strong: As obesity surged over the past three decades, U.S. diabetes rates tripled, and now more than 100 million adults have diabetes or pre-diabetes. Research suggests that obese people are between 1.5 and 2 times more likely to die of heart disease. Other illnesses linked to obesity include high blood pressure, arthritis, Alzheimer's disease, gallbladder disease, sleep apnea, sexual dysfunction, and at least 13 types of cancer. Excess body fat is associated with higher insulin levels, which may spur cell growth and, in turn, increase the probability of cancer; the American Cancer Society believes excess weight is linked to about 7 percent of cancer deaths, and obesity will soon overtake smoking as the top preventable cause of cancer.

How does it affect health-care costs?

Obesity adds between $147 billion and $210 billion to annual U.S. health-care expenses, increasing an average adult's medical costs by 42 percent — an estimated $200,000 over a lifetime. This burden hits low-income households hardest, creating a vicious cycle of poverty leading to poor dietary habits leading to costly weight-related illness. Expenses range from basics like insulin, which is used to treat diabetes and costs up to $900 a month, to the increased need for expensive interventions such as heart surgery. Medicaid now covers bariatric surgery, because doctors believe a roughly $25,000 stomach-shrinking procedure is cheaper than a lifetime of chronic-disease treatments.

What else can be done?

Dieting is a $66 billion industry, but studies show the vast majority of dieters quickly gain back the weight they lose — and sometimes more. Broader lifestyle interventions are required. In Amsterdam, overweight kids are referred to a nurse who helps them and their families identify healthier habits. Between 2012 and 2015, Amsterdam's population of overweight youth dropped 12 percent. Japan, the least obese country, is even more paternalistic, requiring overweight adults to attend diet classes. Americans bristle at any government intervention in their eating habits, such as "sin taxes" on sugary beverages. Yet when Philadelphia imposed such a tax in 2017, it saw a 38 percent drop in sales. In the meantime, obesity is brutally pricey from the crib to the grave: A wider cemetery plot, reinforced hearse, and plus-size ­casket can add $3,000 to the cost of a funeral.

The pain of fat shaming

The comedian Bill Maher enraged many of his liberal fans last month when he said, "Fat shaming doesn't need to end — it needs to make a comeback." He argued that as Americans grow more and more obese, we're letting them off the hook in the name of politically correct "body positivity." But overweight people say there's been no lessening of the scorn they encounter almost every day. That's most painfully true for teens. In a 2016 survey, 85 percent of adolescents reported seeing overweight classmates mocked in gym class. Advocates say the response to obesity needs to recognize that being overweight isn't a failure of character, and many people who've tried every diet in the book can't make a dent in their waistlines. A 2015 study found that obese women stand only a 0.8 percent chance of returning to normal weight. Yet while fat shaming is mean-spirited and unproductive, it's impossible to discuss obesity without talking about necessary lifestyle changes. "Obesity isn't a disease of willpower, it's a biological problem," said researcher George Bray. "Genes load the gun, and environment pulls the trigger."