Why the scale goes quiet

Body weight is not fat. It is fat plus muscle plus bone plus organs plus everything currently inside you plus a lot of water. Glycogen, the carbohydrate your liver and muscles hold in reserve, is stored with three to four parts water attached (Kreitzman et al., 1992). So a bigger carb weekend, a salty meal, a new gym routine or a few constipated days can each park a kilo or two on the scale and keep it there for days, while fat is quietly coming off underneath.

There is also a weekly rhythm to it. When researchers tracked daily weights in 80 adults across up to 330 days, weight rose from Saturday and fell from Tuesday, in almost everyone. Their conclusion was that weekend-to-weekday swings should be treated as normal rather than as weight gain (Orsama et al., 2014). If you weigh on a Monday and compare it to the previous Friday, you are measuring the day of the week as much as anything else.

This is the same mechanism behind the extra weight that shows up after a week away. We wrote about that here, and the maths is worth reading before you conclude anything from a single morning.

The gap between logged and eaten

This is the most common measurable contributor and the one people take hardest, so it is worth being precise about what the research actually found. Researchers studied adults who reported eating under 1,200 kcal a day and were not losing weight. Their total energy expenditure and resting metabolic rate came back within 5% of predicted, so a slow metabolism was ruled out. What they found instead was that actual intake had been under-reported by an average of 47%, and physical activity over-reported by 51% (Lichtman et al., 1992).

Read that as a measurement finding, because that is what it is. The same gap turns up in nutrition researchers and dietitians when they log their own food. Human beings are poor at estimating portions by eye, and the energy in oil, milk in tea, sauces and the bits eaten while cooking is close to invisible. Nobody is lying. Three days of weighing everything is usually enough to see where the number really sits.

What happens to your body as weight comes off

Two things shift, and both are normal. Resting energy use falls by more than the smaller body alone accounts for. Six years after a large weight loss, resting metabolic rate was 704 kcal a day below baseline, of which about 499 kcal a day could not be explained by changes in body composition or age (Fothergill et al., 2016). That was an extreme case of very rapid loss, so treat the size of the number as the top end rather than as your number.

Appetite hormones shift too, and they stay shifted. A year after an average 13.5 kg loss, leptin, ghrelin, peptide YY and reported hunger still had not returned to their pre-loss levels (Sumithran et al., 2011). This is why maintaining a loss takes ongoing effort and why the effort is not a moral test. Your body is doing exactly what bodies do.

Lean tissue is part of that arithmetic, and it is the part you have some say over. Protein and resistance training are the two levers with the most evidence behind them. In a randomised trial where people ate in a deficit alongside hard training, the higher-protein group gained lean mass and lost more fat than the lower-protein group (Longland et al., 2016). If you want the band for your own weight, the protein target calculator works it out.

Sleep sits in the same picture. In a crossover trial, 10 adults did the same 14-day energy restriction on 8.5 and then 5.5 hours of sleep. Total weight lost was similar, but the proportion lost as fat fell by 55% and the loss of fat-free mass rose by 60% on the short sleep, with more hunger reported (Nedeltcheva et al., 2010). Ten people is a small trial, so hold it loosely, but it is a reason not to write sleep off as irrelevant.

If you are on Mounjaro or Wegovy

The trial curves are worth seeing, because they set a realistic expectation. In STEP 1, semaglutide 2.4 mg produced a mean weight change of -14.9% at week 68, against -2.4% on placebo (Wilding et al., 2021). In SURMOUNT-1, tirzepatide produced -15.0%, -19.5% and -20.9% at 5 mg, 10 mg and 15 mg by week 72, against -3.1% on placebo (Jastreboff et al., 2022). In both, the steepest part of the curve is early, during and just after dose escalation, and the line gets shallower the further out you go. A slowing rate later in treatment is what the data does, not a sign that the medicine has stopped working for you.

Those are group averages over large trials, and individual results vary a lot around a mean. They are context for what you are seeing, not a prediction of what you will see.

How to read your own numbers from here

• Weigh at the same point each morning, after the loo, before food or drink, and compare seven-day averages rather than single days.
• Take one non-scale measure weekly. Waist at the navel is the easiest. If the waist is moving and the scale is not, water is the more likely of the two explanations.
• Weigh everything you eat for three days, once, including oils, drinks and the bits eaten standing up in the kitchen. You are collecting a number, not a confession.
• Change one thing at a time and give it three weeks, because that is roughly how long it takes for a seven-day average to say anything useful.

If you want the weekly averages worked out for you rather than doing it on paper, Healthcount tracks it free.

This page is information, not medical advice, and nothing on it is a diagnosis. It cannot tell you whether your weight loss has genuinely stalled. Anything about doses, tapering, stopping or side effects is a conversation with your prescriber or GP. If you have severe stomach pain that will not ease, vomiting that stops you keeping fluids down, or rapid swelling with breathlessness, get urgent medical help: call 999, or contact NHS 111 if you are not sure.