Gaining weight, researchers now know, involves your brain, hormones and genes, as well as your stomach.

"You're constantly fighting an uphill battle of food cravings … hungry or not," said Tracey Ledoux, a faculty member at the University of Houston who uses virtual reality to study food cravings and other behavior linked to obesity.

Ultimately, she hopes the tool helps people control their cravings for chocolate, cupcakes, pizza and other diet-busters.

"You hear a lot of judgment about (how) people need to eat better and start exercising," Ledoux said.

That advice is still valid. But as science reshapes what we know about obesity and weight loss, it has become clear that it isn't so simple.