Ten to fifteen years ago, I watched a documentary of the movie "Jaws" made in 1975. There was a beach boardwalk scene that showed hundreds of extras (probably just regular "Joes" and "Janes" from that area (now probably "Joses" and "Jaunitas")) walking around...everyone was really lean. Not a fatty in sight.
I see old pictures of myself, shirtless, as a kid from about the same time, and I look emaciated compared to the kids of the same age today (and over the past twenty or so years).
I remember when 7-11's "Big Gulp" cola cup size came out in the mid 80's where I lived. I think this was about the same time that high-fructose GMO-corn syrup started saturating the market, a much cheaper alternative than the cola made with pure cane sugar from Florida (of which there used to be a lot of ads on TV). HFCS was far cheaper to manufacture and thus able to sell massive quantities to the zombified public then and much more so now; except that now, the "small" cola is bigger than the "Big Gulp" you could get back in 1985.
It was rare that you saw someone with MS, autism, colitis, type II diabetes, or some other debilitating disease. Breast cancer rates started to soar as well. The sperm count of men started to drop precipitously, especially over the past 10-15 years with all of the ubiquitous WIFI and Cellular transmissions, not to mention the BPA chemical found in plastics, probably going all the way back to that "Big Gulp" cup.
Also, increasing automation, cable and satellite TV, Internet, video games, etc. that helped incentivize a sedentary life style. There's also the proliferation of "fast food" joints that started in the 1970's, a side-effect of "women's liberation", when women were propagandized to believe that being a "house wife" was a burden and that the drudgery of a "9-5" corporate or government job was a far more "empowering" and edifying way to live than raising a healthy family with good values which included mostly nutritious meals.
So, I think it's a combination of technological changes, societal/cultural changes (via social engineering), and environmental changes (some intentionally malicious (GMOs, vaccines, ChemTrails)) and some a by-product of "progress" or a "cost-benefit" analysis where the FDA looked the other way so corporate heads could make a bigger profit to afford another beach front home in Malibu or a nice mansion in the Hamptons.