What actually changes your weight
Step 1 of 6. The question people arrive with is usually "what do I actually have to do — is it the carbs, the timing, the sugar?" The honest answer is simpler and less exciting than any of those.
Energy balance, in plain words
Your body stores energy mostly as fat. Over a period of weeks — not days — body mass changes when the energy you take in and the energy you use differ. Take in less than you use for long enough and stored energy is drawn down; take in more and it is stored. That is the whole mechanism, and no guideline or trial disputes it. It is graded A on this site for that reason alone: the grade applies to this one claim, not to any particular number.
The energy in food is counted in kilocalories. The figures printed on every label in the EU come from a fixed convention — a gram of carbohydrate or protein counts as four kilocalories, a gram of fat as nine, a gram of fibre as two. That convention is a labelling rule, not a model of how much your body absorbs, which is one of several reasons the arithmetic never comes out exact.
Why it is still not bookkeeping
"Calories in, calories out" is true and useless in equal measure, because both sides of the ledger move in ways you cannot see:
- You cannot measure what you eat. People underestimate their intake — substantially, and most of all when they believe they are eating little. A later step on this path covers the measurement study that showed this.
- Expenditure falls as you lose weight. A smaller body costs less to run, so a deficit that was real at the start quietly shrinks. Some further adaptation on top of that is real too, but in the better-controlled studies it is heterogeneous and often small — not the "broken metabolism" of popular telling.
- The scale measures water first. Day-to-day changes of a kilogram or two are salt, glycogen and gut contents, not fat. The path "Something isn't working" has a page just for this.
- The old rule of thumb overstates the result. The idea that a fixed energy deficit always produces a fixed amount of weight loss ignores that the energy cost of each kilogram lost depends on how much fat you start with and rises as the loss proceeds.
None of this means the model is wrong. It means your own numbers are estimates that the scale corrects over weeks.
What a deficit is — and is not
A deficit is a sustained average: less energy in than out, on balance, across a week and then the next one. It is not a daily rule you pass or fail. A day above your target followed by an ordinary day is still a deficit; a week of "compensating" by skipping meals usually is not, because it does not last.
Physics forbids no food. A pattern that excludes a food group works when it makes the deficit easier to keep, not because the excluded food had a special property.
What is not required
- A named diet. The next steps show that, at a year, the pattern you can keep matters more than which one it is.
- A particular timing. Eating windows and meal counts are tools for keeping a deficit; they are not the deficit.
- Cutting a food group. Same.
- Willpower as the plan. A plan that needs willpower every day is a plan that ends.
The general adult ranges for macronutrients are wide — fat, carbohydrate and protein can each vary across a broad band of your energy and still describe a healthy diet. They are ranges for a healthy population, not a prescription for losing weight, and they are the frame inside which every named pattern on this site sits.
The honest limit
Everything above is a population model. The formulas in the next step were fitted to groups of people and predict your expenditure with a margin of error that can be ten percent or more. That is not a flaw to fix with a smarter app; it is the reason the method works the other way round: start from the estimate, then let what the scale does over two or three weeks correct it.