Correlation doesn't mean causation. It could have been the removal of fat, the addition of extra sugar or simply the switch from high fat to high sugar.

I'm this case you are not far off. This is actually part of my field of study. Substituting low fat/high carb diets by high fat (source doesn't even really matter) low carb diets actually reduces the amount of atherosclerosis as well as reducing the risk for diabetes type 2 significantly. Not only that but there are strong indications that we should consume less vegetable oil and more animal fat (including) than we do at the moment. The ratio should be more around 1:5-1:6 instead of the 1:10 people generally eat at the moment. Vegetarians and vegans seem to suffer more from heart disease than healthy individuals who eat small amounts of animal products. Another thing we should abolish completely is processed meats, especially those with added sugar. Also, look up Maynard reactions and Google on the effect the resulting product has on humans.

It's really a complicated field and people claim to have found the answer are lying. But when we moved our food production from the land to factories we made pretty much everything worse for ourselves. Which is an indication about what it wrong with how we eat.

OK I'm rambling now. But anyways, all studies done in the last years indeed point towards refined sugar as an enormous health problem. Not only the increase in body weight though. Skinny people who consume a lot of sugar also develop a lot of obesity related problems too, not all of them ofcourse. But it shows how bad that shit is for you. That and the fact you can live without any sugar at all, since your body is perfectly capable of making any glucose it (mostly the brain) needs.