Did you see the Horizon programme about sugar and fat? It's still available on iPlayer until about the 3rd of March if you haven't seen it. It's an interesting experiment with some interesting conclusions. I know it's a bit of a spoiler, but the final conclusion, that refined foods are the biggest issue, is far from as surprising at it appears to be in the programme. What was interesting is the reason why.
It would appear that it's down to the fat/sugar ratio and the way that tricks our bodies into not self-regulating our intake. In other words, we just keep eating. These refined foods are calorie dense, very pleasing to eat and have the ability to switch off the self-regualting system. That's what makes them dangerous.