How did we get so fat? Part of the answer is easy; most of the answer is not.

The easy part first: Blame our primeval ancestors who learned the hard way to fend off starvation by hunting down and gorging themselves on fat-laden prey. Thanks to them, we are hardwired through our genes to crave fatty, energy-packed foods.

Fast-forward through the eons and we still like to consume whopping platters of mega-calories. Pile on the carbohydrates and don't forget sugar and alcohol, too.

Herein lies a big part of the origin of today's obesity crisis: Unlike our cave-dwelling ancestors, we don't burn up energy running cross-country to catch dinner.

Instead, our food is abundantly more plentiful and is produced, prepared and presented to us in gigantic high-calorie portions far beyond what humans need to survive and function. It proves once again that you really can't fool Mother Nature. We may have drastically altered our food environment, but our genes have stayed the same.

Unlike our parents and their parents before them, we have thrown the age-old formula of “energy in = energy out” far out of balance. Biologically, this is why so many of us are overweight or obese.

We need to tell America in big bold letters:

Energy In = Energy Burned

How do we know the formula works? Calories are energy; we can measure how we use, abuse and burn them.

The details are in the data.

In the first study of its kind, researchers at Harvard late last year found that the gap between the energy our kids take in and what they burn off is more like an abyss.

The study, which we sponsored, shows that America's overweight teens consumed an average of 700 to 1,000 calories more than required each day over a 10-year period. This energy gap resulted in an average of 58 extra pounds for overweight teens.²³

It's not just the heaviest kids who are falling into the energy gap—it's all of them. Over a 10-year period, both children and teens, on average, consumed 110 to 165 more calories than they burned each day. For all teenagers, that means an excess of 10 pounds, on average, per person. And they carry their overeating habits along with them into adulthood and parenting.²⁴

A generation ago, a standard day's energy intake for American adults was 2,234 calories. Today we're up to an average of 2,757 calories a day, a jump of nearly 20 percent. And pay attention to this detail: Most of that comes from lots more fats and oils (up 63 percent), grains (up 43 percent) and sugar (up 19 percent).²⁵

Do we work it off? No way. A quarter of all adults report no leisure time physical activity at all—no gardening, calisthenics, walking for exercise—nothing.²⁶

Among kids, 10 percent say they don't take part in any moderate or vigorous activity at all. More of them are spending three or more hours a day watching TV (37.2 percent) than spending any time in daily physical education classes at school (33 percent).²⁷

Do the math. To lose a pound a week, you need to burn off 3,500 calories. That's 500 calories a day. Huge numbers of us, however, are heading in the opposite direction by adding, not subtracting extra calories. Only fat can follow.

So, we ask, why don't we stick to the energy in = energy burned formula?

This question is much tougher to answer. Coming into play is a confounding, twisted complex of genetics, biology, socioeconomic and environmental dynamics, commercial and cultural environments, and, above all, the awesome influence on children of the promotion of branded food and beverage products in the marketplace.²⁸

In other words, if we're going to have any chance to reverse this epidemic and save our kids' future, it is not enough to tell them to put down the Twinkies and fries, turn off the TV and go play ball. It's going to take a lot more than that. We are going to have to weave together environment, economics and individual behavior in healthful instead of harmful ways.

Until recently, much of the evidence about obesity's consequences has been sketchy, anecdotal and not substantial enough to act on.

That's not good enough to foster fundamental, wide-scale change. We're finally learning what we need to know about childhood obesity. Factors that we now know for certain are driving the epidemic among children include:

• Individual choice and behavior are important. True, a quarter of all teens drink as many as four cans of sweetened beverages a day, the 600-calorie equivalent of adding an extra meal.²⁹ And they don't burn it off. On average, kids spend an amazing 6.71 nonschool hours a day in front of a TV, video game or computer, watching movies, and listening to music.³⁰ Sitting there, they'll opt for junk food every time. I know. I've been a working mom.
• The world our kids live in plays a big role, too. Only 10 percent of kids walk to school, the rest are driven in. Too many kids can't find safe places to walk, bike or play. Most schools have cut out daily physical education. And too many schools have relied on junk-food vending machines and cafeterias serving cheap, high-fat foods like pizza and fries to make up for budget gaps. So much for energy in = energy burned.
• Where families live and what they earn make a difference. If your family resides in a depressed neighborhood and is struggling to make ends meet, you are more likely to be overweight or obese. Between 1971 and 2004, the rate of overweight among 16-year-olds from families living just above the poverty level surged a shocking 233 percent. The list of causes is an invitation for achievable prevention: These kids habitually skip breakfast, have poor food choices at home, drink lots of sweetened beverages, and have limited access to supermarkets with healthy foods.³¹
• The food and beverage industries are big power drivers. They spend more than $10 billion a year marketing high-calorie food packed with fat, sugar and salt to kids ages 2 to 11, still young enough to be forming their own food preferences. Not coincidentally, children in this age group are most strongly influenced by food product advertising.³²
• We've turned into a 24/7 fast-food, junk-food nation. One-fourth of all Americans eat fast food at least once a day. One-fourth of all vegetables eaten in the United States are french fries.³³ Americans consume 16 pounds of potato chips per capita a year. ³⁴ That's 38,912 calories. To burn them off, an obese 130-pound 10-year-old girl would have to walk briskly for about 115 hours a year. And her parents' Venti Strawberries & Crème Frappuccino with whipped cream from Starbucks is 770 calories.
• Race, class, ethnicity and gender matter. More than 40 percent of all African-American teenagers are overweight or at risk of becoming overweight; nearly 24 percent are obese.³⁵ African-American and Hispanic women, the mothers of the highest-risk kids, are at higher risk themselves for overweight and obesity than white women. Mexican-American men have higher obesity prevalence rates than white and black men. Women with low incomes are 50 percent more likely to be obese than women with higher incomes.³⁶ About one-half of adult Pima Indians have diabetes; 95 percent of them are obese.³⁷ Increasingly, Pima youth are diagnosed with “adult diabetes” before age 20 and increasingly are likely to suffer kidney failure by the time they reach their mid-50s.³⁸

What does it all add up to? In 1970 less than 5 percent of all children and adolescents were obese.³⁹ Today childhood obesity prevalence rates are rising so radically that in comparison America's business-as-usual ways of countering dangerous health threats seem stuck in place.

These kids grow up. At least 25 percent of all adults in 42 states are now obese.⁴⁰ To reverse the epidemic, we have to begin with the children, and it's going to take all our kids and all the adults to do it.

If this page were a wall, the handwriting would be all over it.

By now most of us would agree that obesity is a menacing juggernaut that is adversely affecting people of all ages. Physically, medically and psychologically, the epidemic is changing the kind of people we are in ways we have never experienced before.

If our strategy to reverse the epidemic is to succeed, we need to thoroughly understand exactly what is happening. We do that the old-fashioned way—by measurement.

For perspective, here's how the Institute of Medicine reports the speed of the obesity epidemic's spread, based on data collected between 1963 and 2004.⁴¹ As a physician, I am so shocked by the acceleration that it takes my breath away.

• For children ages 2 to 5, the obesity rate nearly tripled from 5 percent to 14 percent.
• For children ages 6 to 11, the rate jumped almost fivefold, 4 percent to 19 percent.
• For children ages 12 to 19, the rate increased from 5 percent to 17 percent.

America's adolescents are now the most obese teenagers in the world. One study comparing teens in the United States with teens in 15 European countries and Israel found that no one else's kids come close.⁴²

At the core of our concern: The medical, psychosocial and financial consequences of obesity are threatening the country's public and private capacities to contain the epidemic over time. In fact, for more young people than we can yet fathom, their fates as adults may be sealed already.

Medically, overweight and obese children are at much higher risk for terribly debilitating chronic conditions like type 2 diabetes and high blood pressure that just a short time ago were considered adult illnesses.⁴³

An obese toddler already is trapped in a spiral of escalating risk. If obese at age 4, he or she has a 20 percent chance of being an obese adult. An obese teenager's risk of becoming an obese adult is as high as 80 percent.⁴⁴ If you are still obese in your 20s, the chance of premature death becomes very real and very high, with current odds running 50 to 100 percent against you.⁴⁵

Indeed, death from obesity and diet-related factors may occur as much as 20 years sooner than normal—sooner, even, than your own parents—because the medical realities keep getting worse the older you get.

Researchers analyzing government health care data on adults discovered a 79 percent increase in the number of obesity-related cases of diabetes and a 29 percent increase in obesity-related high blood pressure.⁴⁶

Besides diabetes, serious illnesses related to obesity may include many of the top 10 causes of death: cardiovascular disease; stroke; colon, kidney and breast cancers; plus musculoskeletal disorders and gall bladder disease.

It's as if millions of obese kids are having their medical charts for adult chronic care prepared in advance, just waiting for them to come of age and mature into obese and sickly seniors.

Psychosocially, what hits obese kids especially hard is that their quality of life is severely compromised by their condition. Anxiety, depression, more missed days at school and low self-esteem are routine parts of each day.⁴⁷ They report elevated levels of sadness, loneliness and nervousness.⁴⁸

At school they tend to function less well academically and socially. Unable to keep up with their peers, they report being teased, punched and bullied, even becoming bullies themselves.⁴⁹ The psychosocial scars may last a lifetime. Overweight adolescents are less likely to marry as adults than their average-weight peers; obese adolescents have lower household income as adults than non-obese adolescents.⁵⁰

Financially, the prospects are foreboding.

RAND researchers predict that obesity will disable up to 22 percent more adults in the coming years, with as many as 25 percent more people entering nursing homes by 2020 at a huge cost. They estimate one of every five health care dollars spent by older members of our families will be to treat conditions related to their obesity.⁵¹

Another intriguing RAND study concluded that if you are obese and manage to survive middle age, your later years are likely to be miserable.

If you are an obese 70-year-old, for example, you can expect to live for maybe 14 more years. But you are likely to be seriously disabled, with a 40 percent chance that you will need help bathing, dressing, using the toilet, getting in and out of a chair, and, ironically, even eating.⁵²

The cost of your care will be just as wretched, running about a quarter of a million in 2006 dollars. You can expect Medicare to pick up about $150,000 of that, leaving you on the hook for at least $100,000.⁵³

But we'll all be on the hook as the true cost of obesity to society continues to grow.

Federal officials publicly put the yearly obesity-related medical expenses and lost productivity at between $99 billion and $117 billion. However, internal CDC documents posted on the Internet set the cost much higher, at “over $200 billion annually.”⁵⁴ And that's before the current crop of 13 million overweight and obese kids even reach adulthood.

This is frightening. Many of these kids may never escape the corrosive health, psychosocial and economic costs of their obesity.

But is childhood obesity a true epidemic?

Here is this physician's diagnosis:

Childhood obesity is an epidemic unlike others. Though not a virus, it is virulent. Though not infectious, it is spreading rapidly. Though treatable, it resists treatment.

It is an epidemic about complex social and human behaviors. How we bring it under control will be as much about the art and practice of social change as about the science and medicine of behavioral change.

The questions we ask about this epidemic and the answers we find will hold the power to alter the course of America's health history—as it occurs.

The needless loss of countless lives will bring unspeakable suffering to the next generation coming of age and an immeasurable forfeit of human potential.

Our public and private treasuries will be drained of resources badly needed for other national priorities.

Health programs for the elderly, the disabled and the poor will be destabilized.

The country's health care financing and delivery systems, already in precarious condition, will be overcome by the sheer weight of the health and medical needs of the obese.

The good news is that the work's already begun.