Two sides of one ledger
Think of your body as keeping a daily ledger. On one side is energy in: the calories you eat and drink. On the other is energy out: the calories you burn. Whatever does not balance gets stored — mostly as fat — or drawn down from stores. This is the foundation of energy homeostasis, the body's effort to keep its energy reserves roughly constant over time.
Energy out is not one number. It splits into three big pieces: the energy you burn just staying alive at rest, the energy spent digesting food, and the energy of physical activity. The biggest piece, surprisingly, is the first one.
The resting engine: basal metabolic rate
Even lying perfectly still, your heart beats, your kidneys filter, your brain hums, and every cell maintains its membranes. The energy this baseline costs is your basal metabolic rate (BMR). For most adults it accounts for 60–70% of total daily energy use. Organs like the liver, brain, heart, and kidneys are metabolically expensive even though they are small.
A second contributor is thermogenesis — the production of body heat. Some of it is the obligatory cost of digesting and processing food; some is adaptive, turned up or down by hormones (especially thyroid hormone) and by cold. We will return to heat production when we meet brown fat.
Total daily energy expenditure (a typical sedentary adult) BMR (resting, ~65%) ████████████████ ~1500 kcal Diet-induced thermogenesis ██ ~200 kcal (~10%) Physical activity █████ ~500 kcal (~25%) --------------------------------------------------------- Total ~2200 kcal Note: change weight and BMR changes too — a smaller body burns fewer resting calories, which is why loss plateaus.
Why weight resists change
Here is the key idea of the whole track. Body weight behaves like a regulated variable, similar to body temperature. The body seems to defend a set point — a target level of energy stores — using negative feedback. Eat less for a while and the body lowers its resting burn and ramps up hunger to pull you back. Overeat and it does the opposite. This is the core of set-point theory.