The threshold worth watching is 1% of body weight per week, sustained over several weeks. At 100 kg that is 1 kg a week. Above that, the share of lean tissue in what you are losing tends to rise.

One fast week means nothing. Three or four in a row means something.

Why speed costs you

Two mechanisms, and they compound.

The first is energetic. The larger the deficit, the less energy the body has for maintaining tissue it is not using. Muscle is expensive, and in a large deficit it becomes the obvious economy.

The second is practical. Losing weight very fast on these medications almost always means eating very little in total — and when total intake is low, protein intake is low with it. So the deficit is at its largest exactly when the input that protects muscle is at its smallest.

Working out your actual rate

Take your weight now, your weight three or four weeks ago, and:

rate = (weight then − weight now) ÷ weight then ÷ weeks × 100

At 96 kg now against 100 kg four weeks ago: 4 ÷ 100 ÷ 4 × 100 = 1% a week. Right at the threshold.

Two cautions. Use several weeks, not one — daily weight moves by a kilogram or more for reasons that have nothing to do with fat. And do not apply this to your first month, where a large part of the drop is water rather than tissue.

Our muscle check calculates this for you and weighs it against your protein and training, which is more useful than the rate alone.

What to change, in order

1. Protein first. This is almost always the largest gap and it is the one entirely within your control. Get your target and count for three days honestly. Fast loss with low protein is the combination that costs the most lean mass, and it is extremely common.

2. Resistance training. Two sessions a week. In a large deficit this matters more, not less — it is the only thing giving the body a reason to keep muscle it is otherwise inclined to shed.

3. Eat a little more. This is genuinely counterintuitive on a weight-loss medication, and it is often correct. Adding a protein-dense item to the meal where your appetite is best does not undo the deficit. It narrows it slightly, which is the point.

4. Talk to your prescriber. If it continues, or if you are struggling to eat at all, this belongs with them. Dose reductions and titration pauses are real and reasonable options — and they are clinical decisions. Do not make them because of a calculator, including ours.

When it is not just about muscle

Some things make this a medical question rather than a body-composition one. If you are unable to keep food or fluids down, feeling faint or unwell, losing weight far faster than anyone expected, or finding you cannot eat at all rather than simply eating less — contact your prescriber rather than adjusting your protein.

The same applies if you have a history of an eating disorder and rapid loss is affecting how you think about food. That is worth raising early rather than late.

Keeping perspective

Fast loss is not an emergency and it does not undo the benefit of the weight you have lost. It is a signal that the composition of the loss is probably worse than it needs to be, and that is fixable from wherever you are — the levers work the same at any rate.

Getting to the same destination more slowly usually arrives with more muscle intact. That is the whole argument.