What the trial protocols used
Every large GLP-1 weight-management trial paired the drug with a lifestyle prescription, and the diet half of that prescription was written as a deficit relative to each person's own estimated needs.
| Protocol | Dietary instruction |
|---|---|
| STEP 1 (semaglutide 2.4 mg, published 2021) | Reduced-calorie diet at about a 500 kcal/day deficit, plus roughly 150 minutes of activity per week |
| SURMOUNT-1 (tirzepatide, published 2022) | Same structure: about a 500 kcal/day deficit and at least 150 minutes of activity per week |
| SCALE (liraglutide 3.0 mg) | About a 500 kcal/day deficit with activity counseling |
| NHLBI obesity guidelines (1998) | A 500 to 1,000 kcal/day deficit, described as producing roughly 1 to 2 pounds of loss per week |
Two things follow from that table. First, the number is personal, because 500 below maintenance for a 5'2" woman is a very different figure than 500 below maintenance for a 6'1" man. Second, none of these protocols asked for heroics. A 500 calorie deficit is a modest instruction, and the medication was expected to do the work of making that instruction easy to follow rather than pushing intake far below it.
Worth noting that Ozempic and Wegovy are the same molecule at different approved doses and indications, and the diet protocols above come from the weight-management trials. If you're taking semaglutide for type 2 diabetes, your calorie and carbohydrate targets are a conversation with the prescriber managing your glucose.
How low is too low
The 2013 AHA/ACC/TOS obesity guideline draws a line that gets almost no attention in GLP-1 content. Intake under roughly 800 kcal/day is a very-low-calorie diet, and that guideline treats a VLCD as something to run under medical supervision, not on a phone app and good intentions. The 1,200 to 1,500 and 1,500 to 1,800 kcal/day ranges in the same document exist because that's where a deficit stays large enough to matter and small enough to stay safe for most people.
A few reasons the floor is real. Rapid weight loss is a known risk factor for gallstones, and cholelithiasis appears in the labeling for these medications. Very low intake sustained for weeks tends to bring fatigue, feeling cold, lightheadedness when you stand up, and hair shedding that shows up two to four months later. Constipation is one of the most commonly reported side effects of GLP-1 medications, and eating almost nothing makes it worse rather than better, since fiber comes from food volume you're no longer eating.
One more thing that sounds obvious and gets ignored anyway: don't treat a 600 calorie day as a bonus you can spend later. The goal is a steady deficit.
The shot-day dip, and why weekly beats daily
The pattern people describe over and over is that appetite suppression is strongest in the first day or three after an injection and eases before the next one. A week of logs can look like two very low days, one mediocre day, and four fairly normal ones. If you judge that week by Wednesday alone you'll conclude you're starving. If you judge it by Sunday alone you'll conclude the medication stopped working.
The seven-day average settles that argument. Take total calories for the week and divide by seven. That number is closer to what your body responded to, and it's the one worth comparing against your target.
Titration weeks deserve their own note. Stepping up to a higher dose often resets the dip, so a week that had been running near target can suddenly drop by several hundred calories a day. That's exactly the sort of thing to mention to your prescriber, because dose timing and dose increases are theirs to adjust. Don't change your own dose based on what your food log says.
Practically, this means your food log and your dose dates need to live in the same place. A line that reads "Tuesday, 1,020 kcal, 48g protein" is worth a lot more when you can also see that Tuesday was the day after your injection.
Protein and fiber get crowded out first
When total food drops, the things that go first are usually the things that take effort to eat. Chicken breast, lentils, Greek yogurt, and vegetables all require chewing and time. A latte doesn't.
The DXA body composition substudy in STEP 1 found participants lost both fat mass and lean body mass, with fat making up the larger share of what came off. That's the expected result of weight loss in general, and it's the reason protein keeps coming up. The range I use across this site during energy restriction is 1.2 to 1.6 grams per kilogram of body weight per day, which sits above the IOM's RDA of 0.8 g/kg and below the 1.4 to 2.0 g/kg the ISSN position stand recommends for people training regularly. For a 180 lb person, about 82 kg, that's roughly 98 to 131 grams a day, which is a real amount of food to fit into a day when you're not hungry. It also means the 40 g of chicken you pushed around the plate is worth logging as 40 g.
Fiber has published targets too. The Institute of Medicine adequate intakes are 25 g/day for adult women and 38 g/day for adult men under 50. Most people miss those on a normal appetite, let alone a suppressed one.
Micronutrients thin out the same way. Iron, calcium, potassium, magnesium, and B12 all ride along with food volume, and there's less of it. Whether you need a supplement is a question for your prescriber, who has your labs.
Signs you might be eating too little
None of these are diagnoses. They're patterns worth writing down and taking to an appointment.
- Your seven-day average sits several hundred calories under the target you were given, week after week.
- You're losing faster than the roughly 1 to 2 pounds a week the NHLBI guidance describes, sustained over a month or more.
- You skip entire meals on dose days and only notice when you look at the log.
- Protein keeps landing under 60 grams even on days you eat normally.
- Strength is dropping in the gym at weights and rep ranges you handled a month ago.
- Dizziness on standing, feeling cold all the time, or unusual hair shedding.
Bring that list to your appointment, in writing, with dates. Dose changes, skipped doses, and supplements are all decisions for the person who prescribed the medication, and the manufacturer's instructions cover the rest.
Setting a number you can adjust
If you want a starting estimate before your appointment, the arithmetic in the trial protocols is simple enough. Estimate your maintenance calories, subtract 500, then check the result against the floor described above. If subtracting 500 puts you under 1,200, the subtraction is wrong for your body and the target needs to come from a clinician rather than a formula.
Then treat that number as a hypothesis. Log for two or three weeks, watch the weight trend rather than any individual weigh-in (daily weight swings two or three pounds on water alone), and adjust once with real data instead of five times on nerves. A Tuesday morning that reads two pounds heavier than Monday is telling you about salt and sleep.
Targets should also move as you go. The maintenance number for a body 25 pounds lighter is lower than it was at the start, and the deficit that fit month one may not fit month six. That's a check-in item.