For each day over more than a month I tracked my weight and food energy in an effort to empirically discover the basis of a strategy for achieving my ideal weight.
I found that weight in pounds, measured right after waking up, is proportional to the calories consumed the day before, with the calories per pound randomly distributed around almost exactly 10 (with repeatability, measured as the standard deviation, of 1.4). Some research into how many calories are used with varying kinds of exercise showed that this relationship tracks closely with the energy spent on a full day of sleep as a function of weight.
This made the strategy simple: daily consume only the amount of calories needed to maintain my ideal weight, calculated by multiplying 10 by that weight. To improve my chances of not exceeding that weight, I wouldn't consume any more than that; and to avoid getting too much underweight, I would consume no less than 8.6 (10 minus 1.4) times the weight.
I couldn't help but compare what I was learning about myself with what I had learned about consumption of resources by humanity as a whole. The calories needed to maintain ideal weight seemed to correspond to what I had derived as the "minimum ecological footprint," the amount of resources provided by other species that is required for stable basic survival where the resources are reliably available (as became the case globally, on average, about fifteen hundred years ago). The lower value of calories I was aiming for corresponded to the footprint for a hungry state of being, with uncertain availability of resources, which I had calculated as 80% of the "minimum" and was the starting point for idealized groups of people driving historical population and consumption change since the start of civilization.
It is tempting to try making a comparison between being overweight and consuming more resources than is healthy for the world. As we are able to consume more stuff besides food, we are also able to consume more food. Our life expectancy, which tracks with footprint much like happiness (gaining less and less as we consume more), begins to decrease as we become more overweight, implying that doing so overwhelms our inner ecosystem just as increasing our footprint eventually overwhelms the external ecosystems that support us. I have long hypothesized that there is an upper limit to happiness, beyond which we cannot go without self-destructing, and it's not a great stretch to expect that obesity might have a role in this given that heart disease is the top killer in the affluent U.S.
My personal motivation for losing weight is tied to the health risks of not doing so, just as my motivation for downsizing is tied to my awareness of how consuming more stuff is contributing to global extinction. It amounts to a selection of personal limits, much as half of the idealized groups in my reconstruction of world history (one-sixth of the population) chose to consume only one-fourth of the renewable resources available in a healthy world while the other groups chose to consume everything.
I am a latecomer to all of this. Many others have experienced a similar awakening of a desire to live within healthy limits, with common reaction to growing evidence of the alternative's imminent failure. Although we are far beyond the ability to succeed on a global scale, I share the goal to nurture that desire as much as possible, for as long as possible, and with as many people as might choose to share in it.