What I found, over four years of research and reporting, was a conscious effort — taking place in labs and marketing meetings and grocery-store aisles — to get people hooked on foods that are convenient and inexpensive.

They may have salt, sugar, and fat on their side, but we, ultimately, have the power to make choices. After all, we decide what to buy. We decide how much to eat.

Health messages are simply overwhelmed, in volume and in effectiveness, by junk-food ads that often deploy celebrities or cartoon characters to great effect. We may know that eating fruits and vegetables is good for us, but the preponderance of the signals we get - and especially the signals children get - push us in the direction of junk food.

Some of the largest companies are now using brain scans to study how we react neurologically to certain foods, especially to sugar. They've discovered that the brain lights up for sugar the same way it does for cocaine...

In a key--but commonly overlooked--aspect of obesity, weight gain can be caused by the slightest increases in consumption, if it continues day in and day out.

Each year, food companies use an amount of salt that is every bit as staggering as it sounds: 5 billion pounds.

As I spoke with scientists about the way fat behaves, I couldn't resist drawing an analogy to the realm of narcotics. If sugar is the methamphetamine of processed food ingredients, with its high-speed, blunt assault on our brains, then fat is the opiate, a smooth operator whose effects are less obvious but no less powerful.