In my opinion, that's not going to work. It certainly hasn't worked with cigarettes and smoking. People will continue to buy their daily Mars, they just won't be able to afford the apple-a-day as well.
France, on the other hand, is right down at the bottom of the obesity tables for OECD countries. Some people like to argue that that's all down to a Mediterranean diet consisting of lots of veg and olive oil, but I don't buy that. France is the country with over 350 different types of cheese, remember? The country where people eat croissants for breakfast and where the most common type of regional speciality is a sausage. This is not to say that French people don't eat a healthy diet, but merely to point out that if they do, it's not purely down to the natural bounty or culinary traditions of their homeland.
What I believe makes the difference is that in France, people aspire to eat good food. Not healthy food, or diet food, but food that tastes good. Quality matters, and quality means food that is produced slowly, in the right place and the right season, by people who know what they are doing. And when you eat high quality food that tastes good and satisfies your body as well as your appetite, you don't want to eat too much. Try eating yourself sick on dark chocolate instead of a Mars bar. But in France, you might not even want the chocolate, because the fresh fruit is so delicious. And that rich, cheesy Tartiflette might be packed with calories, but when you've eaten it, you'll feel full and you'll be ready to stop, because your hunger and your appetite both agree that you've had enough. That's not really true of a Big Mac. French people respect their food, and when you respect something, you don't abuse it.
To solve the obesity crisis, governments need to make good food both aspirational and affordable. Telling people peaches are healthy isn't going to make them buy them if they're expensive and taste like water. Saying that chicken nuggets are unhealthy won't stop people eating them, but making sure that actual chicken has more flavour will make them choose that instead.
The developed world is in the process of proving that awareness of health risks doesn't stop people trying to satisfy their appetites. The French have proven that when your appetite is truly satisfied, the health risks aren't there.