French fries, donuts, and other deep fried or packaged and highly processed foods are often called “carbs” but most of the calories are from high PUFA seed oils, not carbs.

I do think high PUFA seed oils are often an issue there… they cause damage to the liver, and mitochondria, such that people feel worse, and have less energy- which in turn promotes both inactivity, and overeating. There is lots of good data on various species of lab animals getting NAFLD and metabolic diseases when swapping out MUFA and SFA for PUFA.

Also vegetarian/vegan diets are generally associated with better health outcomes, favourable BMI and lower all-cause mortality in meta studies.

I thought all those studies have ppl eating processed meat along with industrial junk food. Its hard to eat vegetarian food like that.

Is it just purely correlation ?

Most vegetarians I know eat tons of heavily processed frankenfoods meant to mimick popular non-vegetarian foods. They also tend to eat lots of sugar and simple grains. The biggest exception has been my Indian coworkers, who tend to either make or source vegetarian meals that respect that vegetable is the root word in vegetarian.

Correlation doesn't imply causation.

People picking a vegetarian diet (which has been the mainstream healthy option for the last 50 years) are probably more health conscious than someone downing 5 mcdonalds per day.