If they exercise more they will just get more hungry. (I've been there, I know.)

The most important factor to consider if someone wants to change its weight is the diet. Not 'diet' in the sense of restraining yourself from eating, but rather what you eat.

Why we get FAT by Gary Taubes http://www.youtube.com/watch?v=bTUspjZG-wc

Unfortunately for you, I don't think there's a startup idea there. Or perhaps make a specialized restaurant/grocery? It's really hard to find meal ready to eat that aren't full of carbohydrates.