I'd like to see a comparison of the proportion of processed chemicals that were in junk foods of that era vs now. I'd bet there is a lot more now # shelf life trumps overall health in the eyes of these food companies.
Whilst sedentary lifestyles don't help these days, I'd bet the junk food of the 80s and 90s were of a higher quality than today!
Case in point. Rustlers didn't start selling their crap until 1999....
Based on what I've read on processed food, one of the biggest problems is actually what's not in it - rather than the chemicals which are in it (which is mostly an unnatural mixture of fat and carbohydrates). Like you say - shelf life is one of the biggest selling points of processed food, and here in the US Fibre is the number 1 enemy of the food industry, so they take it out.
The food industry spent years trying to convince everybody that fibre was an unnecessary thing that 'got in the way' of making foods that lasted more than a day or two (like actual real food). But they didn't realise/care that our digestive systems have evolved over millions of years with fibre in our diet (fruit and veg). Turns out all that 'roughage' changes how food traverses the digestive tract so that the nutrients can be absorbed properly. Lack of fibre (fruit and veg) is now shown to be one of the driving causes of bowel cancer.
There were a number of 'experiments' performed back in the 80s and 90s, some of which were performed to try and develop artificial fats the common one was trans fat, there was another absolutely awful bloody stupid thing that resulted in a product called "Olestra" which was an attempt at making an artificial fat that you couldn't digest - that had zero calories but the same mouth feel as real dietary fat, and it could be used for cooking.
The biggest problem with Olestra, was that if you ate a lot of it - it inhibits the bodies ability to absorb other nutrients through the colon (presumably because it messes with the absorbant cellular linings) but the biggest side effect was - anal leakage
it suffered terrible press and was banned in most countries - but now here in the good ole US of A, you can still buy it in some cheap 'low fat' products.
Whilst trans fats, are mostly banned (not in the uk suprisingly) the food industry found away to get around the stigma by using something slightly different called, "mono dyglicerides of fatty acids" or "trans fatts acids" essentially - there are artificial fats used as additives in things like cheap pies, pastries, cake mixes, and especially cheap spreads (I can't believe it's not butter). They're bad because there's no metabolic process in the body that can metabolise or burn a trans fat molecule. They exist because it allows the food industry to use cheap oils (like vegetable oil) to make the above types of products, which would require fats which are solids at room temperature, because the process of 'hydrogenation' it takes a liquid fat at room temperature and converts it into a solid. (this makes it cheaper than using real fat such as lard or butter)
Today, there's growing evidence coming out of new research, that's showing a link between emulsifiers used in fast food and obesity related illnesses. Emulsifiers are used so that you can have water and oils/fats in a creamy consistent product that doesn't separate (if you put oil and water in a glass they separate). Emulsifiers allow both to mix (like cheap ice cream which is basically just oil, water and sugar) but guess where else you find emulsifiers - washing detergent.
It turns out from the research, that emulsifiers in food is upsetting the bacteria we have in our guts and the lining of the colon, along with causing inflammation and again - possibly contributing to bowel cancer.
