What a slow week on a GLP-1 means, and what it does not

A week where the scale does not move, or ticks up, is weather. It is not a verdict on you, and it is very rarely a verdict on the medication. This page is about why the number does what it does from week to week, what the trial curves actually look like once you stop staring at the headline figure, and the one thing that is worth reacting to.

What the trial lines look like

The numbers everybody quotes are averages at the end of a long trial. In STEP 1, 1,961 adults took semaglutide 2.4 mg or placebo for 68 weeks; the semaglutide group lost 14.9 percent of their body weight on average, the placebo group 2.4 percent.¹ In SURMOUNT-1, 2,539 adults took tirzepatide for 72 weeks and lost 15.0, 19.5 or 20.9 percent depending on the dose, against 3.1 percent on placebo.³

Spread evenly, 14.9 percent over 68 weeks is about a fifth of a percent a week. For someone of 100 kilograms that is roughly 200 grams a week, which is less than the weight of the water in a large glass. And it is not spread evenly: the average line falls faster early and more slowly later. The two-year STEP 5 trial says it plainly. Weight fell steeply at first, plateaued after roughly week 60, and was then maintained for the rest of the study, ending at 15.2 percent down at week 104.²

So even the average, the smoothest line there is, has a slow end built into it. Your line is one person’s, not an average, and it will be lumpier than that.

Why a week reads flat, or up

Fat leaves slowly and steadily. Almost everything else on the scale moves fast and both ways.

• Water and salt. A salty dinner, a restaurant week, a hot day with less to drink: the body holds or sheds water by the litre, and a litre is a kilogram.
• Glycogen. The stored carbohydrate in your muscles and liver is kept with water around it. A few days of more bread and pasta refill it and the water comes with it. A few days of less, and it drains. Neither is fat.
• The cycle. For many women the week before a period is a week of holding water, and the scale says so, every month, on schedule.
• A slow gut. These medications slow digestion, and constipation is common. What has not left yet weighs something.
• A step-up week. The first days on a higher dose often mean eating very little, then a rebound when the nausea lifts. The scale follows the food in and out, not the fat.
• The scale itself. A different time of day, before or after coffee, with or without shoes, on a soft floor. Half of the mystery drops is this.

Put two of those together in the same week and the number can sit still for seven days while the fat is leaving exactly as it was.

Read the line, not the number

Weigh under the same conditions each time: same scale, first thing, after the bathroom, before breakfast, the same clothes or none. That does not make the number true, it makes it comparable.

Then read it as a line, not a point. If you weigh daily, the number that matters is the average of the week. If you weigh weekly, it is this month against last. A single morning has no meaning at all; it is one frame of a film. Judge the film by the month. Four weeks flat on a steady dose is information. One week is not.

The plateau is coming for everyone

At some point the line will flatten and stay flat. In STEP 5 that happened on average after about week 60, and the weight then held for another year.² That is not the medication wearing off. It is the body arriving at a new weight it can hold on the dose you are on, with the appetite you have. The trial’s authors called the result maintained; most people, in the moment, call it stuck.

It helps to decide in advance what the plateau will mean to you. For most people the honest answer is that holding a weight fifteen or twenty percent lower than where they started, without a fight, is the entire point.

What is actually worth reacting to

• Several weeks flat, on a steady dose, early on. That is a conversation with your prescriber, about where you are in the dose steps and whether it is time for the next one. It is their call, always.
• Weight leaving, but strength too. The joint advisory on nutrition during GLP-1 therapy puts protein at 1.2 to 1.6 grams per kilogram a day during active weight loss and asks for strength training at least three times a week, because protein alone does not keep muscle.⁴ If the scale is fine and the stairs are harder, that is what to fix.
• Weight leaving too fast, with dizziness, weakness or not being able to keep fluids down. That is a call, not a celebration.

What is never the answer to a slow week is skipping meals. A GLP-1 already has the appetite down; eating less than the little you can manage takes protein and strength with it and moves the scale by water, not fat. And what is never the answer is changing the dose or the day yourself. The step up, the step down, the pause: those belong to the person who prescribed it.

Tilly draws your weight as a line over weeks, not as this morning’s number, so a flat week looks like what it is. On a slow week it says so, in one sentence, and moves on to what to eat today. It never tells you what dose to take.