How Many Calories to Lose a Pound? 3,500 Is a Rule About One Week, and Everyone Uses It as a Rule About a Year
Cut 500 calories a day and you will lose a pound a week. It is the most repeated number in weight loss, it is printed on apps and handouts, and it comes from dividing 3,500 by seven.
Over one week, in someone with a decent amount of fat to lose, it is roughly right. We are going to say that first because most pages about this skip straight to the sneering, and the number does have a real basis: it traces back to a 1958 paper working out the calorie equivalent of gained or lost weight. [4]
The problem is what happens when you run it forward.
The rule, doing its own arithmetic
| Daily deficit | 500 calories |
| Over one week | 3,500 calories → 1.0 lb |
| Over one year | 182,500 calories → 52.1 lb |
| Over five years | → 260 lb, which is where the rule stops describing a person |
Nobody believes the five-year line. But the same straight line is what an app is drawing when it tells you your projected weight, and it is what people are comparing themselves against in month four when the scale has stopped moving.
Why it stops being true
A smaller body costs less to run. As you lose weight, the calories you burn each day fall — so a fixed 500-calorie cut is a smaller and smaller deficit relative to where you now are. Eventually intake and expenditure meet, and weight stops changing.
That is not a theory. A model of this published in The Lancet, built to simulate how expenditure adapts during weight loss, reports that the bodyweight response to a change of energy intake is slow, with a half-time of about one year. [1]
A half-time is worth pausing on, because it is the whole finding. It means that after about a year, you are roughly halfway to wherever that change in intake is eventually going to settle you. Not halfway through losing weight forever — halfway to a destination that exists.
So the rule and the measurement do not disagree about a week. They disagree about the shape of the whole thing. One is a straight line with no end. The other is a curve that flattens.
The same paper reports something that is genuinely encouraging, and gets lost: people carrying more fat lose more weight for the same change in intake — and take longer to reach their new steady weight. [1] The people most often told they have the hardest job have the largest response.
The second problem: not all of a pound is fat
The 3,500 figure is the energy stored in a pound of body fat. But you do not lose pure fat — some of what goes is lean tissue, which holds far less energy per pound.
Working through this properly, a 2008 analysis found the rule “approximately matches” the real energy density for people carrying a lot of fat, and overestimates the deficit needed for leaner people. [2] In other words the rule is calibrated for one kind of body and applied to all of them.
The same analysis notes this is likely part of why men often lose more weight than women on the same deficit — women typically carry more body fat at a similar weight, and more initial fat means more energy needed per pound lost. [2] Which is worth knowing before you conclude something is wrong with you. A pound is a pound; what it is made of is the variable.
And the study everybody misuses
Fourteen contestants from The Biggest Loser were followed for six years. They had lost an average of 129 lb (58.3 kg) during a 30-week competition. Six years later they had regained about 90 lb (41.0 kg), and their resting metabolic rate was still 704 calories a day below where it started. [3]
Adjusting for how much smaller they now were and how much older, the researchers measured a residual slowdown of 499 calories a day that body size alone did not explain. [3] Metabolic adaptation is real and it persisted for years.
Now the part that gets dropped, and it reverses the usual moral. In that same paper, weight regain was not significantly correlated with metabolic adaptation at the end of the competition. [3] And the people who kept the most weight off at six years were the ones with the most metabolic slowing, not the least. [3]
So “my metabolism is broken, that is why I regained” is not what this study found. Read what a correlation of −0.1 at p = 0.75 is saying: nothing.
It is also fourteen people from a television show, losing weight at a rate almost nobody attempts, which is about as far from a general population as a study gets. It shows adaptation is possible and lasting. It does not tell you the size of yours.
What to do with a number that is wrong in a useful direction
Keep it for the week and throw it away for the year. As a way of sizing a change — is this a small adjustment or a large one — 3,500 per pound is fine. As a projection, it is a straight line describing a curve.
Three things follow, and they are the practical part:
— A stall is the expected shape, not a failure. The curve flattening is what the model predicts. Your app’s straight line is the thing that is wrong.
— Give it much longer than you think. A half-time of a year means most of the change is still ahead of you when most people conclude it is not working.
— The deficit has to keep up with the smaller body, which is the honest reason maintenance is harder than losing.
And the levers that still work are unglamorous: protein while dieting, lifting, walking. Foods that burn more than they contain do not exist, and your metabolism did not collapse at 40.
One caveat about the evidence itself. Two of the three sources here share a first author, whose research group has done most of the modelling work in this area. That is normal for a specialised field and it is not a criticism of the work — but a finding resting largely on one group is a weaker thing than the same finding from four, and you should know which one you are being handed.
