Two old models, both incomplete
- Set point: the brain actively defends one level of body fat. This fits the biology of hunger and metabolism, but it can't explain how so many people's weight rose since the 1980s. If the set point were fixed, that shouldn't happen.
- Settling point: weight drifts to wherever your food environment and activity push it. This explains the environmental side, but not why genetics matter so much.
- Newer models try to combine both: there are upper and lower limits where biology pushes back hard, and between them your weight follows your environment and habits.
How hard does the body push back?
Beyond the expected drop from being smaller, metabolism slows by an extra amount when you diet. That extra averages about 120 kcal a day and takes more than two weeks to appear, with big differences between people. It's real and it's modest. The extreme cases (like the Biggest Loser follow-up) came from extreme weight loss.
In practice the bigger push-back is usually hunger and moving less without noticing, not resting metabolism.
Both papers are reviews that pull together controlled feeding, observational and genetic studies, mostly in people with overweight or obesity. Neither is a single trial. They describe how the system works, not a number to plan around.
What this means for you
You are not stuck at a weight, and you are not entirely free either. Your habits, food environment, sleep and activity decide where you land within your range. Lose weight and you'll have to defend the new weight on purpose for a while. That's what maintenance is for.
What to do
- Expect push-back: more hunger, less fidgeting. Plan for it instead of fighting it with willpower.
- Keep daily steps up while dieting. Unplanned activity is one of the first things to drop.
- Change your environment: what food is in the house, what's easy to grab. It moves the needle more than motivation does.
- Treat maintenance as a phase with its own plan, not the end of the diet.
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