Maintenance calories: how to adjust the number in your first weeks

They are the calories you burn in a whole day. The biggest part is what your body burns at rest. On top of that comes what you burn training, what you burn moving around the rest of the day and what it costs to digest your food^[1]. If you eat the same as you burn, on average your weight doesn’t change.

The calorie calculator gives you an estimate of that number. How it works it out and why it gives you a range is explained on its page.

For resting expenditure, the calculator uses the Mifflin-St Jeor equation. A 2005 systematic review compared the four equations most used in clinical practice, in adults with and without obesity: Mifflin-St Jeor came within 10% of measured expenditure in more people than any other, and had the narrowest error range. Even so, the authors warn that it has noteworthy errors when applied to an individual, and that older adults and ethnic minorities living in the US were underrepresented in the studies^[2].

In a study of 150 adults in Scotland, fat-free mass (muscle, organs, bone and water) explained 63% of the differences in resting expenditure between people. Fat explained 6% and age 2%, and 26% was left unexplained^[3]. The calculator asks for your weight, height, age and sex, but not how much muscle you have: two people who weigh the same can burn different amounts.

How much you move outside the gym can also change with what you eat, and not in the same way for everyone. In another study, 16 volunteers without obesity ate 1000 kcal a day above their maintenance for 8 weeks. Two thirds of the rise in their expenditure came from activity that isn’t exercise, such as fidgeting, holding posture or everyday tasks. Each of them raised it by a different amount, and those differences explained why some stored up to 10 times more fat than others^[4]. It’s a small study, and what it measures is the response to overeating. The calculator captures that activity with four levels, so here too it’s an approximation.

Then there’s what we write down. In 10 people with obesity who said they couldn’t lose weight eating under 1200 kcal a day, expenditure was within 5% of what was predicted for their body. What failed was the log: on average they recorded 47% less than they ate and 51% more activity than they did^[5]. It’s a handful of people with a specific history, but it leaves a useful clue: if the scale doesn’t match your numbers, check how you log first.

Weight goes up and down even if you eat the same. In 80 adults who weighed themselves daily, weight was highest on Sunday and Monday and dropped during the week^[6]. In another study, of 1421 people in a European programme to maintain weight they had lost, that swing within the week was 0.35% of body weight^[7]: for someone weighing 80 kilos, almost 300 grams that come and go.

So a single day tells you little. Since weight follows a weekly cycle, what makes sense is to compare whole weeks: the average of one week’s weigh-ins against the next week’s.

The first few days are even more misleading. Your body stores glycogen, the carbohydrate reserve in your muscles and liver, with 3 or 4 parts water for every part of glycogen. In 11 people on a very low calorie diet, that glycogen was already being used up by day 4^[8]. What the scale drops at the start of a diet is partly that water, so it’s no use for working out your maintenance.

Eat the calculator’s number consistently and compare the weekly averages from the second week on. If they hold steady for several weeks, that number is close to your maintenance. If they go up, you’re eating above it; if they go down, below it.

Don’t expect an exact conversion from kilos to calories. The rule of 3500 kcal per pound of body weight (about 7700 per kilo) only comes close to the prediction in people with more than 30 kg of fat; with less fat, it overestimates the deficit needed to lose a kilo^[9]. The number is corrected by watching how your weight responds, and there’s no general figure backed by studies for how many calories to add or remove at each adjustment: that’s decided case by case, depending on how your weekly averages move.

When you lose weight, your expenditure drops by more than the weight you’ve lost explains. In a study of 41 people, 18 with obesity and 23 who had never had it, keeping weight 10% or more below usual reduced daily expenditure by an average of 6 kcal per kilo of fat-free mass in those who had never had obesity and 8 in those who had. Keeping it 10% above raised it by an average of 9 and 8 kcal per kilo, respectively^[11]. These are group averages, with big differences from one person to another, so they can’t be used to work out your own case.

That’s why your number isn’t corrected just once. In their review of natural bodybuilding contest preparation, Helms and colleagues warn that the calories you start with will need to change as you lose weight and your body adapts^[10]. Trexler and colleagues frame it as a stepwise process: when weight loss stalls, you adjust what you eat or how much you move again^[1]. After losing or gaining weight, you check it again the same way as at the start, by comparing weeks. If you’re losing fat, losing fat without losing muscle covers how fast to go.