Many of the factors that contribute to weight gain make it hard to succeed at losing weight and keeping it off. Actually, it’s not hard to lose weight initially—follow any crash diet for a week or two and you’ll lose some weight, mostly due to water loss. But it’s very hard to keep the lost weight off over time. That’s why 80 to 95 percent of dieters regain their lost weight. The key is to make improvements in eating and physical activity a permanent part of your life. Unfortunately, many dieters experience “behavioral fatigue,” whereby they grow weary of dietary and exercise changes that cease to yield benefits after the first six to nine months and thus revert to old eating patterns.
Making matters more difficult is the fact that the body (via a “feedback loop” between the brain and various bodily systems) tends to defend itself against the loss of body fat—an evolutionary trait that helped early humans survive when food was scarce. This triggers a host of metabolic changes through which the body compensates by reducing its metabolic rate and conserving energy (that is, burning less fat and fewer calories) and increasing hunger (usually for fattening foods). These adaptations occur most strongly in people who have been obese for some time and in those genetically programmed to have a “thrifty” metabolism, and they stack the deck against maintaining weight loss, especially in an environment that encourages overeating. For many people, one way to help counter such biological factors that limit weight loss and promote regain is to increase exercise.
Here’s one way it works. A very rough “rule” is that you need a 3,500-calorie deficit (by eating less, exercising more, or both) to lose a pound of body weight. But it’s not just a matter of simple arithmetic. The body will adapt to the calorie deficit and weight loss, notably by lowering its metabolic rate and favoring less-energetic behavior—making it harder to continue losing weight or even maintain a lower weight. In that case, it would take more than a 3,500-calorie deficit (from your pre-diet calorie intake) to lose another pound, since your body is running on fewer calories. And since your smaller body burns fewer calories, you’ll always need to consume fewer calories than you did at your heavier weight in order to maintain the weight loss. All this increases the likelihood of weight regain.
See also: Maintaining Weight Loss: The Hard Part.