Why the weight you defend keeps rising

Researchers do not agree on a single model of how body weight is regulated, and three models are currently in use. The set point model treats weight much as a thermostat treats temperature, with a target that the body returns to. The settling point model holds that weight comes to rest wherever intake and expenditure happen to balance. The dual intervention point model sits between the two, and at present it fits the human evidence best: it describes a zone within which weight can drift with little resistance, bounded by a point above and a point below at which the body begins to push back.C-III-01

In practical terms, what the body defends is a range rather than a single number. The Endocrine Society's scientific statement refers to "the defended level of body-fat mass" for much the same reason.C-III-02

Many people assume that weight loss damages the metabolism, and that the rest of life is spent fighting that damage. In reality, the studies point mostly to appetite.

The metabolic figure comes with an important caveat. The number quoted most often, close to 500 calories a day, comes from a study of 14 people who had regained most of the weight they lost. Four separate studies of people who kept the weight off found resting metabolic rates within a few percent of what their new size would predict.C-III-05

In one study, a year after participants finished a diet, their appetite hormones had still not returned to their starting levels. Leptin, the hormone that signals to the brain how much energy is stored, remained about 35% lower; ghrelin, which drives hunger, remained elevated; and the participants themselves reported being hungrier than they had been before the diet.C-II-04

The clearest evidence that leptin is behind this comes from experiments that replace it. When leptin is given back to people who have lost weight, much of the adaptation reverses.C-II-02 Hunger that persists a year later is that signal still running, not a lapse in resolve.

The two ends of the range are not defended equally. The body resists weight loss far more strongly than it resists weight gain.C-III-07

Over a lifetime, that asymmetry accumulates. A system that pushes back hard against downward movement and only gently against upward movement does not stay where it started. Each period spent at a higher weight is defended more firmly than the return journey, and the range works its way upward, a few pounds at a time.

If the range moves upward when a higher weight is held for long enough, it would be reasonable to hope that it moves back down the same way. The best available test of that idea is not encouraging.

More than a year at a lower weight, maintained by treatment, did not leave a new defended level in place once the treatment was withdrawn. That finding is difficult to reconcile with the hopeful version of the argument.

Duration helps. Among people who had kept weight off, the odds of regaining were substantially lower for those who had maintained the loss for two to five years, and lower still beyond five.C-III-10 The caveat is that these were people for whom maintenance had already worked; the study has no record of those who regained earlier. It therefore says something about that group, and nothing reliable about any one person's chances.

People who keep weight off are highly active. In one study they were expending around 812 calories a day through physical activity and taking roughly 12,100 steps, against about 8,900 for normal-weight controls.C-III-11 It is not a comfortable finding, but it may be the most useful one on this page. Long-term maintenance does not appear to involve the body giving up its defense; it looks more like people staying ahead of that defense, day after day.

If effort on its own tends to lose to this defense over time, if a year at a lower weight does not reset it, and if the range drifts upward across a lifetime, then the useful question is not how to try harder but what acts on the regulatory system itself. Part IV takes up that question, and sets out what the evidence shows each of those options does and does not do.