The Institute of Medicine, the Centers for Disease Control and HBO just released a film, “Weight of the Nation,” warning us of the latest epidemic to sweep the country: obesity. Today, two-thirds of American adults are overweight or obese, along with one-third of children. The tally has doubled since 1988. And this, despite $40 billion spent in 2007 alone for diet aids.

Obesity is a serious problem. It can cause chronic illness, disability and death: The IOM pegs the cost at $190.2 billion a year.

Are we suffering from a national collapse of willpower, as some would have us believe? To strengthen our resolve, the IOM recommends: incorporation of physical exercise into daily life; more healthy food and beverage options; better “messaging” further efforts by health care providers, employers and schools to keep us trim.

That will not be enough, however. If we want to slim down, we must restrain, and redirect, the commercial forces that profit from stimulating our appetites and reducing the need for any physical activity. There is no free lunch.

After all, we are up against survival instincts that evolved over millions of years: food and hydration are essential to all life.

Humans developed over millennia when starvation was a constant threat (as it remains today in some places). So we are hard-wired to eat beyond satiation, because the hunting or gathering might be bad tomorrow. Hormones trigger thirst as soon as our fluid balance begins to drop. Fat, sugar and salt stimulate the appetite and trigger the pleasure centers in our brains, perhaps because fat is the most concentrated form of energy, sugar is the quickest form of energy, and a small amount of salt is essential to life, but not readily available on the ancient plains.

Moreover, new research has identified hormonal changes triggered by weight loss even when people remain heavy: after a diet, anti-starvation mechanisms kick in. One study in the New England Journal of Medicine found that a year after a 30-pound weight loss by obese dieters, their ghrelin (the “hunger hormone”) remained about 20 percent above pre-diet levels, while peptide YY, which suppresses hunger, and leptin, which suppresses hunger and increases metabolism, remained abnormally low.

Nor have humans historically lacked for exercise. As the Bible says, “By the sweat of your brow will you obtain your food.” (Remember — that was a curse.) Manual labor was the lot even of most Americans until very recently.

To those of us of a certain age, it is clear why Americans started putting on pounds in the 1970s. Until then, most children walked or took school buses to school, because it was the only way to get there. There were fewer cars: More people lived in cities and close-in suburbs where they walked, or took public transportation. Many labored in factories. Women who could, stayed home and prepared three meals a day for their families (including lunchboxes).

In becoming sedentary and obese, we are only doing what comes naturally. But in today's environment, the adaptations that allowed us to survive now work against us.

Americans today are victims of their own success. In our free enterprise system, huge corporations profit mightily from developing and marketing inexpensive foods, deliberately loaded with the fat, salt and sugar that speak directly to our brains, and making them available 24/7. Other companies introduce convenient devices that render physical exertion superfluous. Such economic activity helps our economy grow.

Education, exhortation, even more healthy options, will not be enough to conquer obesity. We must change our environment. Consider smoking, a health threat as dire as obesity. (And we don't need tobacco to live.) To reduce smoking, America needed a full-court press — restrictions on advertising, particularly to children, reduced availability, warning labels, high tobacco taxes, smoking bans in public places — along with education.

We are going to need all that and more to control our intake of unhealthy food. Increasing exercise will be just as difficult — not everyone can take off an hour a day to exercise. Over time, we must reinvent our built environment so that we can walk, bike or take public transportation to reach more destinations.

We must redirect producers toward healthier products.

The resistance will be enormous, maybe overwhelming. The food industry alone is huge — $1.5 trillion in revenues in 2011— and powerful. Companies will attack any new regulations as the ruination of free enterprise. Republicans will decry the nanny state; liberals argue the poor can't afford fresh produce; we all believe that we should be free to eat whatever we want whenever we want it. And, of course, we must eat to live.

But too much fat, salt and sugar, together with too little exercise, will kill us before our time.

There is no free lunch.

Caroline Poplin is a physician and lawyer.