There are so many 'little' issues with this that add up to the bigger picture.
The quality of food isn't what it used to be generally. Intensive farming practices have reduced the quality of a lot of food we eat. Eg fruit and veg grown out of season in polyntunnels isn't comparable nutritionally with that grown naturally and in season. I watched a documentary around 15+ years ago that showed the nutritional value of a chicken breast had reduced since the 50s - including more fat and less protein.
In the past, people ate less because of the cost but the quality of even the most basic natural food was higher.
Breakfast being the most important meal of the day was a marketing campaign by Kelloggs/cereal companies.
Carbs were added to everything, not because we needed that many carbs on our diet, but because it was a cheap and plentiful filler for a lot of foods. It increased profits for the food industry - it wasn't based on our health or nutritional requirements.
People see eating low carb (for example) as a fad diet but eating low fat as common sense. The role of sugar in poor health has been ignored until very recently (aside from dental health) because so much of the food industry and the NHS guidance was based upon a high carb low fat model and fat was demonised. This dietary advice has not changed for years even though there have been studies since that have challenged this.
Research has shown that people who consume more (healthy) fat consume fewer calories/food overall because fat tastes good and fills you up. People are less likely to snack if they consume more fat because their body is satisfied and, yes, blood sugar spikes and crashes don't occur so people dont feel hungry again half an hour later.
There has never been definitive research that concludes 'eating fat makes you fat' but it sounded reasonable so people ran with it. Sugar fucks up your metabolism which is why weightloss is often described as being hormonal - which people scoff at - but all of it is hormonal. What makes you feel hungry, full, satisfied, unsatisfied food wise - all hormones.
UPFs aren't the devils work in that many of us will have grown up eating them without too much ill effect but mainly in lower quantities than are available/consumed now. But because UPFs contain things that aren't really food that our bodies haven't been designed to eat, our bodies don't really know what to do with them and they impact on the normal hormonal functions of recognising hunger and satiety. I know (because I've done it) that I can eat a huge amount of crap and my stomach will be rumbling an hour later. Given what I've just consumed, I shouldn't be hungry - yet I am. I shouldn't have been able to consume that amount in the first place!
People will often post on here talking about wanting to lose weight or eat more healthily. They're doing all the 'right' things - very low fat, high carbs, lots of whole grains, lots of fruit but they can't lose weight. People suggest it's still too many calories - weightloss is all calories in vs calories out! But it's not. It's not the calories that are the issue per se. It's the type of food that is eaten and the effect it has on the body. We are not simple machines. But dietary advice assumes we are.
That's why some people track their blood glucose levels despite not being diabetic because they've understood that spikes in their blood sugar causes issues for them.
People complain that they're always hungry and that there is always food noise - that's generally carb intake. I couldn't eat any less, I have a big bowl of pasta for tea and I'm STARVING by breakfast. i think about food constantly. They assume that, if they replaced the big bowl of pasta they'd be even hungrier but maybe it's the big bowl of pasta that is causing the intense hunger.
I've said before on here that I low carb. I don't do keto because it's too strict for me but I keep my carb intake to below 50g a day in the main. I know that sometimes it goes over. But I don't track or keep a count of everything. I've lost over a stone and a half since January.
I do it because I've tried various things over the years. I could very easily become obese and over been close a couple of times. My calorie intake over this weightloss has been around 1500 a day and I'm a lazy bastard so I'm not in a deficit. I just don't eat UPFs and I've cut down on carbs. But I actually consume more food than when my BMI was 29.
On the days when my carb intake has been higher, I wake up with a growling stomach and hunger pangs. I feel light headed. I'm often bloated. It's unpleasant. But on the days where my carb intake has been very low, I don't experience any of that.
I don't trust a lot of the 'healthy diet' advice because I wonder who it serves best. And I really don't believe that it's us. What I'd really like to see is some proper independent and independently funded research because, as we can all see, there is a lot of conflicting and contradictory advice. I'd like a real understanding of what our bodies actually need (for health) and for the food available and cost to reflect this.