Why energy expenditure falls when you diet, and by how much

For six weeks the scale moved 600–700 g a week. Then it stopped, and as far as you can tell you are eating exactly what you were eating before. The obvious conclusion is that your metabolism has packed up.

It hasn't. But it has changed, in several ways that stack into something you can genuinely feel. Here is what's solidly established, what's only likely, and what gets repeated without support.

What's solidly established

A smaller body costs less to run. Resting metabolic rate — heart, kidneys, brain and liver ticking over while you lie still — accounts for roughly 60 to 70 per cent of daily expenditure in a moderately active person, and it tracks body mass closely. Lose 10 kg and you've shed both fat and some lean tissue, so resting burn typically falls by something in the region of 100 to 150 kcal a day.

On top of that, every step, every flight of stairs and every set of squats is cheaper because you're carrying less. Put the two together and a 10 kg loss can easily mean 200 to 300 kcal a day less. None of that is adaptation. It's arithmetic. The deficit that worked at 100 kg simply isn't a deficit at 90.

There is a further drop on top of the predicted one. When measured expenditure is compared with what you'd expect for the new body size, most people land slightly lower than predicted. The evidence here is consistent: the effect is real and measurable. Its size, though, is far more modest than the internet suggests — usually tens of kilocalories a day, rarely more than 100 to 150. Variation between individuals is wide.

Everyday movement drops, and it drops hard. This is the single biggest culprit behind the calories that seem to vanish. Several weeks into a deficit you walk less, sit more, walk slower, fidget less and wait for the lift instead of taking the stairs. It happens without a decision and usually without you noticing. In some people it's trivial; in others it's a few hundred kilocalories a day. There's a fuller treatment of it in the piece on daily movement outside the gym.

Hunger rises, and it isn't a character flaw. The hormones that regulate appetite shift in a predictable direction: fullness signals weaken, hunger signals strengthen, and food becomes more noticeable and more appealing. The evidence for this is strong and repeatable. The practical result is that intake creeps upwards — a slightly bigger glug of oil, a forgotten handful of something, a looser weekend.

People systematically underestimate what they eat. This isn't lying, it's a measurement problem, and it shows up almost every time reported intake is checked against measured expenditure. The error is often in the range of 20 to 30 per cent, sometimes more, and it tends to grow the longer a diet runs.

Likely, but not settled

How long the adaptation lasts. Whether expenditure returns to the predicted level once weight stabilises, and over what timescale, is genuinely unresolved. Some findings suggest the drop mostly reverses once the deficit ends; others suggest something lingers for years. Probably both, depending on how aggressive the diet was and how much lean mass went with the fat. That's an estimate, not a conclusion.

Diet breaks and refeeds. The idea that periodically eating at maintenance protects expenditure is plausible and popular. The results are mixed — some work supports it, some doesn't. What looks steadier is the psychological side: the diet is easier to stick with. This is poorly resolved so far, so don't build a plan around it.

How much lifting protects your burn. That resistance training plus adequate protein preserves lean mass in a deficit is well supported. How much that translates into saved calories per day is much less clear, because a kilogram of muscle burns only around 13 kcal a day at rest. The real payoff is probably that you stay stronger and more active — which is also the thread running through the piece on strength and long-term health.

What gets reported badly

"You're not losing because you're eating too little." Repeated everywhere, supported by nothing. A body cannot burn less than it takes in and hold its weight steady — that breaks basic physics. If the scale has been flat for weeks, intake and expenditure are equal: intake is higher than you think, expenditure lower than you're assuming, or both.

"Your metabolism slows by 40 per cent." Numbers of that size come from extreme situations or from misreading results. In an ordinary diet we're talking about a total drop measured in the hundreds of kilocalories, and that figure already includes the part caused purely by being a smaller person.

"My metabolism is broken." Permanent damage isn't what shows up in people who have been through a sensible diet. What does show up is duller and more useful: maintaining is harder than losing, and expenditure stays somewhat lower than in someone who was never heavier. That calls for an adjustment, not a diagnosis.

Headlines about a newly found "metabolic switch". Most of those stories describe measurements in cells or animals, not in people running a deficit. How to read that kind of coverage without being taken in is covered in the piece on reading headlines about new research.

What this means for your training and food

• Plan to revise, not to react. For every 5 kg you lose, drop intake by roughly 100 to 150 kcal or add the equivalent in movement. Don't wait for a three-week plateau before doing anything.
• Count steps, not just calories. Set a daily target — 8,000 to 10,000 works for most people — and watch the weekly average. A falling step count is the first visible sign that adaptation is doing its work.
• Keep the deficit moderate. Around 10 to 20 per cent below maintenance, which for most people is 300 to 500 kcal a day, giving 0.5 to 1 per cent of body weight lost per week.
• Protein at 1.6 to 2.2 g per kg of body weight. One of the best-supported numbers in sports nutrition, and it earns its keep most clearly during a deficit.
• Lift two to three times a week, roughly 10 to 20 working sets per muscle group per week. In a deficit the aim is to hold your loads, not to chase records.
• Cardio is a tool, not an obligation. What suits whom, and why, is worked through in the piece on intervals versus steady cardio for fat loss.
• Measure honestly for seven days before blaming your metabolism. Weigh your food, include the weekend, then compare it with what you thought you were eating.

When to see a doctor

Get checked if weight is dropping sharply without you trying, if fatigue persists no matter how much you rest, or if you notice marked cold intolerance, hair loss, missed periods, or a fast or irregular heartbeat. The same applies if food is starting to feel out of control — skipped meals, binge episodes, obsessive counting. And if you're planning a very low intake, or already take medication for thyroid or blood sugar, clear the plan with your doctor first.

In short

Expenditure falls during a diet for three reasons: the body is smaller, non-exercise movement shrinks, and there's an additional but modest adaptation. The first is the largest and most predictable, the second is the sneakiest because you don't notice it happening, and the third is the smallest despite getting the most airtime. The fix isn't dramatic — revise intake on a schedule, watch your step count, keep protein high, and keep lifting when the diet starts to bite.