GLP-1 Weight Loss: What to Measure Before the First Injection

A man comes to see us eight months after starting weight-loss injections. He has lost fourteen kilograms, he is pleased, and he has every reason to be. We ask what was measured before he started; the answer is weight. What has been measured since — weight, three times. That is the whole file. No ApoB, no documented blood pressure, no fasting insulin, no body composition, not one number on strength or function, and nobody has asked what happens on the day he stops.

The gap is not in the drug. It is in everything around it. A medication that changes body composition, the lipid profile, blood pressure and the whole metabolic state is tracked, in most people, with one instrument — a bathroom scale. Three questions fall through the cracks: what the loss is made of, whether the metabolic gain arrived, and what the plan is for after.

Whether the injections work is no longer the question

Semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) act on the gut's hormonal system: agonists of GLP-1, and in tirzepatide's case GIP as well. They change satiety and the rate at which the stomach empties. The registered indications are not identical across these preparations — some are registered for type 2 diabetes and some for the treatment of obesity — so it is worth establishing what exactly is approved for the preparation in question. In the major trial programmes, average weight loss ran to double digits in percentage terms, a magnitude that until then had mostly required bariatric surgery. Earlier drugs produced far smaller losses, several withdrawn over safety. The effect size is real, it has held up in further studies and real-world data, and there is no serious professional dispute about it.

It is worth noticing what happened inside those trials and does not happen in the clinic. There, far more than weight was measured: lipids, blood pressure, glycaemic markers, liver enzymes, in some studies body composition by imaging. The evidence we rely on was built from precisely the measurements that vanish between research and treatment room.

What remains open is a different question: what gets measured around the treatment, and what is concluded from it. Weight loss is a broad biological event — fat mass, muscle mass, blood pressure, lipids, insulin sensitivity, liver enzymes, sometimes bone density. Following one and assuming the rest moved the same way is an assumption, not a finding.

Weight is the wrong metric. Body composition is the right one

Any rapid weight loss is a mixture of fat and lean tissue, and the ratio is not fixed. In studies where body composition was measured by imaging, a considerable share of the weight lost came from lean tissue rather than fat. This is not unique to the injections; rapid loss through dieting or after bariatric surgery looks much the same. But the larger and faster the loss, the larger that share. It can probably be reduced: high protein intake and consistent resistance training throughout treatment are the intervention with the best evidence base today. That only happens when it is planned explicitly. A drug that suppresses appetite also suppresses protein intake, at exactly the moment it matters most.

The practical implication is simple, and worth stating outright because almost nobody says it on the day treatment begins. Protein at every meal, throughout, even when there is no hunger and especially then — with appetite suppressed, protein is the first thing to leave the plate. And resistance training two to three times a week, not walking in its place. Walking is good for almost everything else; it does not preserve muscle during a steep caloric deficit.

Past fifty this is no longer a nuance. Muscle mass declines with age regardless, and muscle is the tissue that decides what the seventies look like — stability, rising from a chair, a metabolic reserve that absorbs glucose. So monitoring should include grip strength and a simple functional test alongside body composition, not just the number on the scale. And what is actually supposed to fall is visceral fat — which waist circumference reflects better than total weight does.

A ten-kilogram loss can be two entirely different things. Without a body composition measurement there is no way to know which one happened.

The metabolic gain has to be verified, not assumed

What justifies long-term treatment is not the number on the scale but what that number was supposed to bring with it. So measure those things themselves. ApoB counts the atherogenic particles and shows whether that burden has actually come down. Blood pressure usually falls with weight, but only structured measurement shows by how much, and whether medications need adjusting. HbA1c and fasting insulin tell you whether insulin sensitivity improved, not merely whether glucose stayed in range. Liver enzymes give a picture of fatty liver, which is very common in this population, and the FIB-4 index, calculated from age, enzymes and platelet count, estimates the risk of advanced fibrosis. Weight loss remains the best-established intervention known for fatty liver.

The inverse matters just as much. If after six months the weight is down but ApoB has not moved, blood pressure is unchanged, and muscle mass has fallen more than expected — that is a reason to reconsider, not to carry on as routine. It is precisely the decision that cannot be made without structured measurement, and without a baseline set before the first injection.

The exit question is decided before the first injection

This is the part almost never discussed in the clinic, and the most consequential of all. When treatment stops, the weight comes back in most cases — much of it within the first year. That is not personal failure, and not a lapse of discipline. It is physiology returning to itself once you remove what was holding it: appetite switches back on, energy expenditure is lower because the body is smaller, and weight drifts back toward where it came from. Anyone presenting this as a surprise has not explained the treatment properly.

So decide in advance which of two things this is. A multi-year treatment, in the same sense as a blood pressure or lipid medication — or a bridge across a defined period, after which something else has to hold the result. Both answers are legitimate. Starting without answering is not. Four questions worth discussing before the start, not after:

• If this is multi-year treatment — who monitors it, on which markers, how often?
• If it is a bridge — to where, exactly? What in habits, training and diet holds the result afterwards?
• What muscle mass and grip strength are we entering with, and what is the floor?
• What happens if the metabolic side does not improve while the weight falls?

These answers change how the treatment is run from day one, not from the day you decide to stop. Someone planning a bridge builds the muscle and the habits while appetite is suppressed and it is comparatively easy — not afterwards, when the hunger is back.

What the evidence does not yet say

Honesty requires marking where the evidence is strong and where it is thin. The cardiovascular outcome evidence — fewer heart attacks and strokes, not merely better numbers on a blood test — comes mainly from trials in people carrying excess weight who also had established heart disease or high metabolic risk. There it is convincing. That benefit cannot be transferred automatically to a metabolically healthy person taking the drug for appearance, and should not be presented as though it had been measured in people like them.

Three further reservations. The long-term data are short relative to a treatment that may run a decade or more. The twenty-year muscle question stays open: whether someone who loses weight at fifty along with muscle mass arrives at seventy with a lower functional reserve. There are no data on it, and anyone claiming otherwise is guessing. And bone density during rapid weight loss is not monitored nearly enough, particularly in postmenopausal women.

Then the side effects. Most are gastrointestinal: nausea, vomiting, constipation or diarrhoea. They usually settle with time, though for some people they are the reason for stopping. Slowed gastric emptying demands attention before anaesthesia, rapid weight loss raises the risk of gallstones, and anyone on diabetes or blood pressure medication may need dose adjustments along the way. There are also situations in which these drugs are not suitable at all, among them a personal or family history of medullary thyroid carcinoma, MEN2 syndrome, and pregnancy. None of this argues against the treatment. It is why the treatment needs an accompanying physician, not just a prescription.

What to measure, and when

This is the sensible minimum for anyone starting treatment, in any setting. The baseline is the critical part: after the first injection you can no longer know where you started from.

What we do, and what we do not

We do not prescribe these medications outside their registered indications. The indications in Israel, and the extent of coverage through the health funds (kupot holim) or through supplementary insurance (bituach mashlim), vary between preparations and between plans, and are updated from time to time — worth clarifying the current position with your own fund and your treating physician, rather than relying on a forum. Those with a medical indication get the prescription from the physician treating them.

The role we fill is the one almost always missing: a complete baseline before the start, and structured follow-up after it — body composition, strength, ApoB, the metabolic and liver profile — alongside the prescribing physician, and including the exit question. If you are about to start, or already on treatment, you can build a monitoring framework or simply book a conversation and work out what to measure before the next injection. A baseline can only be set once, and that window closes quickly.