The Japanese* have no obesity while most other industrial people in the world do. The only sub-populations in the US that are not getting obese are the yuppies and my gay friends**. Neither is a demographic group. They just don't eat much.
I have stumbled on my first hypothesis in the realm of obesity, largely based on my personal experience and listening to other people at Weight Watcher meetings. The issue is craving. I think food is getting tastier (not better...tastier). Food evolves.
There are four foods I have eaten all my life that I now crave. Example: Skippy peanut butter. This is a commercial product as opposed to an uncooked fruit or a steamed vegetable. I can eat nearly unlimited quantities of it. Each of my four food cravings can put weight on me in a few days, plenty of weight in a week. I crave these four fattening foods and keep them out of the house.
So what is my hypothesis? 1) Most humans have a few very narrow food taste modalities that are impossible to resist, these modalities are nearly addictive. We call food addiction cravings. 2) There has been a massive increase in the number of commercial foods on the market since 1960 when imports opened world markets. This vast array of food products have moved their specific taste modalities in directions that emphasize specific customer craving. The market is big enough to respond to small changes and the market has found individual cravings to satisfy.
I am not talking about the standard Bogey-men: sugar, salt, butter or other fats. Humans have had these foods and tastes for centuries, in abundance. I'm talking about something else, something more interesting something more subtle. I'm talking about new commercial products that evolved to either have, or omit, some ingredient that makes the particular food irresistible to some group of people. Evolved to focus on a taste modality that some people crave.
I'm talking about a range of narrow tastes that have evolved in the commercial food market either consciously or unconsciously as retail food products have shifted over the past 45 years to hone in on individual popular tastes. Skippy got me, something else may have gotten you.
Since I am not inclined toward conspiracy theories, I am inclined to think that market selection has been the driving force. It is a morally neutral Darwinian selection process. People are buying more of the Skippy jars that they crave and Unilever product managers know the particular ingredients to increase or decrease to stimulate that craving.
Summary: Food evolves in the market. Commercial evolution occurs in our broad market; we get survival and expansion by selection. The market has created narrow specialized retail food ingredients (probably 50 or more) that have honed in on narrow segments of consumer cravings. Our individual taste cravings force the foods to evolve to suiteach of our peculiarities.