The trials that produced every weight loss percentage you have read about these drugs ran them on top of a prescribed diet, and both the Wegovy and Zepbound labels state what that diet was: a reduced calorie diet of approximately 500 kcal a day below estimated needs, plus physical activity counselling with a recommended minimum of 150 minutes a week. The 14.9% and the 20.9% are what the drugs added to that, not what they did instead of it.
The drug also does much of the work of hitting that deficit for you. In a 20 week trial, adults on semaglutide 2.4 mg ate 35% less at a free access lunch than those on placebo, 1736 kJ against 2676 kJ, which is about 415 kcal against 640 kcal at a single meal (PMID 33269530).
- The pivotal trials prescribed roughly a 500 kcal/day deficit and at least 150 minutes of activity a week alongside the drug.
- One trial in the Wegovy label went further, with an 8 week low calorie phase at 1000 to 1200 kcal/day, then 1200 to 1800 kcal/day for 60 weeks.
- A 500 kcal daily deficit held perfectly for 68 weeks would predict around 31 kg of loss. STEP 1 averaged 15.3 kg, so the arithmetic overpredicts by roughly double.
- Resting energy expenditure falls during weight loss by more than body composition alone explains, and in one cohort that fall was no larger with semaglutide than with lifestyle change alone.
- Eating far below the deficit is the common failure mode on these drugs, not eating above it.
What the trials actually prescribed
Both labels describe the lifestyle intervention in their clinical studies sections, and the wording is almost identical across the two drugs.
| Trial set | Diet prescribed | Activity prescribed |
|---|---|---|
| Wegovy injection trials (several of the STEP studies) | Reduced calorie diet, approximately 500 kcal/day deficit | Minimum 150 min/week |
| One Wegovy trial with an intensive behavioural programme | 8 weeks at 1000 to 1200 kcal/day total intake, then 60 weeks at 1200 to 1800 kcal/day | 100 min/week rising to 200 min/week |
| Zepbound trials (the two pooled weight reduction trials in the label) | Reduced calorie diet, approximately 500 kcal/day deficit | Recommended minimum 150 min/week |
Two things follow from that table. The deficit is defined relative to your own energy needs rather than as a fixed number of calories, so there is no single answer in kcal that applies to everyone. And where a trial did set absolute intake targets, they were set by the protocol and supported by counselling, not chosen by participants.
Turning a 500 kcal deficit into your own number
A deficit needs a baseline. The equation most calculators use for that baseline is Mifflin St Jeor, derived from indirect calorimetry in 498 adults and still the standard reference for predicted resting energy expenditure (PMID 2305711):
- Men: resting energy expenditure = 10 x weight in kg + 6.25 x height in cm - 5 x age in years + 5
- Women: resting energy expenditure = 10 x weight in kg + 6.25 x height in cm - 5 x age in years - 161
That gives resting expenditure, not daily needs. Total daily expenditure is the resting figure multiplied by an activity factor, typically somewhere between 1.2 for a sedentary week and 1.55 for moderate activity, and the trial style target is that total minus about 500.
You can run this by hand or use the GLP-1 weight loss calculator, which does the same arithmetic and then projects what the trial percentages would mean for your starting weight. Disclosure: that calculator is ours, and it is doing the published equation rather than anything proprietary, so a pencil and the formula above will reach the same answer.
| If your daily maintenance estimate is | Trial style target at minus 500 kcal |
|---|---|
| 1900 kcal | 1400 kcal |
| 2100 kcal | 1600 kcal |
| 2300 kcal | 1800 kcal |
| 2500 kcal | 2000 kcal |
| 2700 kcal | 2200 kcal |
Why the arithmetic overpromises
Run the deficit forward and the numbers do not match the trials. A 500 kcal daily deficit sustained across the 68 weeks of STEP 1 is about 238,000 kcal, which at the textbook 7700 kcal per kilogram of fat would be roughly 31 kg. STEP 1 participants on semaglutide 2.4 mg averaged 15.3 kg (PMID 33567185).
The gap is not evidence that the deficit maths is useless. It is evidence that a deficit calculated once, against a body that is getting smaller, stops being a 500 kcal deficit. Maintenance needs fall as weight falls, nobody holds a prescribed intake perfectly for sixteen months, and expenditure adjusts.
On that last point there is a measurement worth knowing. A 12 month cohort study that used indirect calorimetry found resting energy expenditure fell in both a semaglutide group and a lifestyle only group, with part of the fall exceeding what changes in fat mass, fat free mass, age and sex would predict. The adaptive component was around 210 kcal a day in the semaglutide group, and it was not significantly different from the lifestyle group (PMID 42640861). Weight loss does this, in other words, rather than the drug doing it.
The practical reading is to treat any calculated target as a starting hypothesis to revise every month or two, not a fixed quota for the year.
The risk is eating too little, not too much
This is the part that separates calorie planning on a GLP-1 from calorie planning without one. These drugs cut hunger hard: the same trial that measured a 35% drop in test meal intake also recorded lower hunger and prospective food consumption, higher fullness and satiety, and better reported control of eating. Hitting a 500 kcal deficit stops being the difficult part, and sliding several hundred kcal below it without noticing becomes easy.
Two consequences are documented. About a quarter of the weight lost on these drugs is lean mass rather than fat, which is covered in detail in muscle loss on a GLP-1, and very low intakes make that share worse rather than better. And in the weeks after a dose step, nausea can suppress intake further still, on top of the drug effect, which is a different situation from a deliberate deficit.
No trial of these drugs tested eating as little as appetite allows. The results everyone quotes were produced at about 500 kcal below needs with counselling attached, which makes that the only intake level the published percentages actually describe.
What to prioritise inside the target
The trials standardised the deficit, not the composition of the food, so there is less high quality evidence here than the confident advice online suggests. What the body composition literature does support is a general direction rather than a prescription: when total intake is low and lean tissue is coming off alongside fat, protein and resistance training are the two levers with evidence behind them, and the resistance training case is laid out in the review discussed in our muscle loss article.
Specific gram targets are a question for a prescriber or a dietitian who knows your kidney function, your medications and your starting point. Anyone publishing a single number for everyone is going past what the trials measured.
FAQ
How many calories should you eat on a GLP-1?
The trials behind the approved labels prescribed a reduced calorie diet of approximately 500 kcal a day below estimated needs, together with at least 150 minutes of activity a week. Because it is defined against your own maintenance level, there is no single kcal figure that applies to everyone.
Is there an official GLP-1 calorie calculator?
No. What exists is the deficit the trials used, approximately 500 kcal/day, and published equations such as Mifflin St Jeor for estimating the maintenance level to subtract it from. Any calculator, including ours, is combining those two published pieces.
Why am I losing less than the calorie maths predicts?
A 500 kcal daily deficit held perfectly across 68 weeks would predict roughly 31 kg, while STEP 1 averaged 15.3 kg. Maintenance needs fall as body weight falls, resting expenditure drops by somewhat more than body composition predicts, and nobody holds a prescribed intake exactly for sixteen months.
Can you eat too little on a GLP-1?
It is the more common problem. These drugs lower hunger substantially, with one trial measuring 35% less intake at a test meal, so intake can drift well below the planned deficit. Very low intakes worsen the share of weight lost as lean tissue rather than fat.
Do you need to count calories at all if the drug suppresses appetite?
The trials did not test leaving intake unmanaged. Every published percentage came from a protocol with a prescribed deficit and counselling attached, so the honest answer is that nobody knows what those results look like without it.
Does the drug slow your metabolism?
In the one cohort that measured resting energy expenditure directly, it fell during weight loss by more than body composition changes predicted, by around 210 kcal a day, but the effect was no greater with semaglutide than with lifestyle change alone. That points to weight loss rather than the drug.
Sources
- WEGOVY (semaglutide) prescribing information, sections 6.1 Clinical Trials Experience and 14 Clinical Studies. Novo Nordisk, via DailyMed.
- ZEPBOUND (tirzepatide) prescribing information, sections 6.1 Adverse Reactions, 12.2 Pharmacodynamics and 14.1 Clinical Studies. Eli Lilly and Company, via DailyMed.
- Friedrichsen M, Breitschaft A, Tadayon S, et al. The effect of semaglutide 2.4 mg once weekly on energy intake, appetite, control of eating, and gastric emptying in adults with obesity. *Diabetes Obes Metab*. 2021;23(3):754-762. PMID: 33269530
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. *Am J Clin Nutr*. 1990;51(2):241-247. PMID: 2305711
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). *N Engl J Med*. 2021;384(11):989-1002. PMID: 33567185
- Filippi-Arriaga F, Comas M, Prats A, et al. Longitudinal Changes in Body Composition, Adaptive Thermogenesis, and Muscle Strength in Patients with Obesity Treated with Semaglutide. *Obes Facts*. 2026. PMID: 42640861
This article is for information only and is not medical advice. Dosing, suitability and any change to treatment are decisions for your prescriber. Calorie and protein targets are decisions for your prescriber or a registered dietitian.