I have been talking to my mum quite a lot about this subject because neither of us can quite figure out why we weigh a good stone and a half more than we should do, despite exercising and counting calories.
Back in the 70s, we ate pretty much the same way we do now. We've always been a "from scratch" family and ate quite exotic (for the time) food because we were from refugee family.
There are a couple of things we think might be responsible. The first is the low fat, high carb, low red meat ideal that came in the 80s; this meant that a lot of food stuffs were reformulated to have a higher sugar content and we stopped using saturated fats to cook with, going with veg oils instead. The second is the explosion of new dairy products (prior to the 70s, in the days before ski, yoghurt was quite a specialist item).
But moreover, we think there could be changes in the food stuffs themselves as they have been bred or processed to be sweeter. A lot of fruit seems sweeter than it did back in the 70s, so does modern fruit contain more fructose than 70s fruit?
Again, chicken and bacon seems sweeter. And ham, even if you buy it cut from the bone, seems a lot sweeter than it ever did, as though they are doing something to the meat.
I do wonder whether part of this obesity explosion is the sugar or sweetness increasing processes that are layered through the entire food industry in a way the didn't exist in the 70s at all, and it's just creating havoc with people's bodies.