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.

Every year, the average American eats as much as 33 pounds of cheese. That's up to 60,000 calories and 3,100 grams of saturated fat. So why do we eat so much cheese? Mainly it's because the government is in cahoots with the processed food industry. And instead of responding in earnest to the health crisis, they've spent the past 30 years getting people to eat more. This is the story of how we ended up doing just that.

The fast-food industry has moved into the grocery store, so you no longer have to go to a fast-food chain to find problematic foods.

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.

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.

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...

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.