What does a 500 calorie deficit actually do?
Quick answer
By the 1958 Wishnofsky rule, 500 kcal a day for seven days is 3,500 kcal, which that rule equates to one pound of body weight. Hall showed in 2008 that the energy density of the weight lost is not a constant, so the rule is an upper bound rather than a schedule.
Wishnofsky's 1958 paper is where the 3,500 kcal per pound figure comes from - about 7,716 kcal per kilogram. It is genuine arithmetic on an assumption about the composition of the tissue gained or lost, and it is the source of every "500 a day is a pound a week" statement you have read.
Hall's 2008 analysis is the correction that has to travel with it. The energy density of the weight actually lost is not constant: a larger cumulative deficit is required per unit of weight lost by people with more initial body fat, and the 3,500 kcal rule roughly matches only above about 30 kg of initial body fat, overestimating the required deficit below that. Applied over months, a straight-line projection drifts away from what happens.
For the longer view, Hall and colleagues published a rule of thumb in The Lancet in 2011: for an average overweight adult, a sustained change of about 100 kJ a day corresponds to an eventual bodyweight change of about 1 kg - roughly 10 kcal a day per pound - with about half of that reached in about a year and about 95 percent in about three. The word doing the work there is eventual.
This is why the tools on this site return a range and a duration rather than a weight on a date. A calorie deficit is a gap from an estimate that already carries a few hundred kcal of uncertainty, which means the gap itself is uncertain before any biology is involved. A projection presented to the day would be a promise, and this site does not make those.
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