Here's my take.
1. Amount of processed foods and GMO foods. This was to keep up with demand for third world countries eating like 1st world countries. But also all the globalization of fast food restaurants and such into various parts of the world.
2. Another reason for processed foods was corporate greed. How can they make food cheaper and tastes better? This can be seen with increased usage of ingredients like High Fructose Corn Syrup as a sugar substitute.
3. Technology. In the 80's we actually had to move around a lot to go and pickup the phone(landline), change TV channels, walk more(no e-scooters and such), etc.
4. Less feminism and body positive movements accepting fatties as being OK in society. Even slightly overweight women were shamed back then and couldn't get into any type of magazine, tv, radio, etc.
5. Internet and Social Media has people moving less and being more lazy. They play video games and chat online rather than moving to see people and doing real life things.
6. Soils that have been used up and robbed of mineral content leading to less effective Fruits/vegetables that can provide the proper nourishment for parts of our bodies(liver, kidney, etc) to regulate us effectively.
7. Stronger and different pesticides, fertilizers, steroids(for cattle and chicken) used in our foods which has an effect on our bodies but how we age as well.
8. Possible links to vaccines given now that weren't given before that can also affect us in the long term.
You can see some of these changes in people that live in various parts of the world. For example Italians generally consume a lot of carbs in the form of bread, pasta, cheese, yet a lot of them are slimmer than Americans still. This has to do with the way they process their foods but also they tend to move more.
Certain EE countries where women eat a healthy amount but most of the foods they eat there are not processed and rich in minerals since they still have good fertile soil.
Also take older generation of women above age 45+ and some of them look as young as 35yo women because they didn't grow up on all this GMO and processed foods that altered them and made them prematurely age.
I mean you got both guys and girls getting grey hair as early as low 30's now. This was unheard of back in the 80's.