When I was 20 (that's 20 years ago now), I visited America from Australia. I stayed for about 4 months, travelling around the country and visiting friends all over. My diet was similar in content to what it had been in Australia (I ate a lot of junk back then, so definitely processed carbs everywhere) and I was actually getting more exercise in America than I did in Australia. I still came home 15kgs heavier, in just 4 months. And I dropped that 15kgs again in a few months in Australia, on the same junk diet. The only main difference that I'm aware of is that Australia never really got on the HFCS bandwagon the way the US did.

On an unrelated note, my partner put on 20kgs in 3 months while eating the exact same diet, all because her doctor put her on Zoloft. It's now ten years later and we've been on LCHF/Paleo for six years and that weight has refused to budge. It's only finally starting to drop now because we found a holistic doctor who seems to be able to treat the underlying causes.

Processed carbs absolutely made both our lives a LOT more worse. We are both much healthier off processed carbs. But they are just a part of what I feel is a very complex story.