You aren't going to find dramatic changes in 1980. The human body is remarkably adaptable and many health problems accumulate over years or possibly even generations (epigenetics). Obesity implies an impaired metabolism, which takes years (used to be decades) to develop. Moreover, health problems have been epidemic in our society pre 1980 (just with less dramatic consequences). I think you are going to really limit yourself with a 1980 cutoff, and that it is more likely that the same factors just got worse around that time.
This doesn't ring true. Epigenetics may take a lot of time, but memetics don't. Social changes can produce sharp changes in diet very quickly. Many epidemics predate 1980, but obesity, by the numbers, does not appear to.
Yes, social changes could produce dramatic changes. But social changes themselves are usually very slow, and I know of few exceptions to this in the context of health. Normally people blame obesity on social aspects (like exercise) which 1) have changed very gradually and 2) don't matter anyways (explaining that would make this a much longer discussion). If someone could come up with an example of just 1 social change that dramatically shifted in 1980 I would be very open to the idea. The only thing I know of is weight-training- the obesity numbers used are normally just based on BMI- they don't take into account that muscle isn't fat.
Before 1980, 1/3 of Americans were overweight. Every overweight person has the same metabolic problem as the obese, just to a lesser extent. The fact that 1/3 of Americans are now obese 20 years later is what I would expect due to a gradual decline of American health that hit a tipping point for obesity around 1980.
