A GLP-1 medication turns your appetite down for you, so eating less stops feeling like a fight. But “eat less” is an instruction with no target attached until you know less than what.
That “what” has two parts. Your basal metabolic rate (BMR) is the calories your body burns at complete rest just to keep you alive, with your heart beating, your lungs working, and your temperature steady. Your total daily energy expenditure (TDEE) is that BMR plus everything you do on top of it, from a workout to digesting lunch to fidgeting at your desk. That total is the real number you eat above or below each day.
This article covers three things: what those two numbers mean, how to estimate your own, and where a GLP-1 drug fits the math. That last part threads through everything below.
These medications don’t change your TDEE. They make it easy to eat under it, and knowing the number tells you how large a deficit you’re actually running.
What Is BMR, and Why It’s Most of the Calories You Burn
Most of the calories you burn today, you’ll burn lying still. Your BMR, sometimes called resting metabolism, is the single largest line in your energy budget, accounting for about 60 to 70 percent of your total daily burn. Digesting food adds roughly 10 percent more. Everything you actually do, from a workout to walking to the mailbox, makes up the rest.
That surprises people who assume exercise dominates. It doesn’t. The average BMR runs around 1,410 calories (roughly 5,900 kilojoules) a day for women and about 1,696 calories (roughly 7,100 kilojoules) a day for men, according to Cleveland Clinic. One calorie equals 4.184 kilojoules, which is worth keeping handy if your food labels use kilojoules.
Your own number depends mostly on body size, and especially on lean muscle, since muscle burns more energy at rest than fat does. Age and sex shift it too. That muscle point matters later, because losing it during weight loss quietly drops your BMR.
To estimate BMR without a lab, clinicians use the Mifflin-St Jeor equation, the modern standard. In plain terms it takes your weight, height, age, and sex and returns your resting calories:
- Women: (10 x weight in kg) + (6.25 x height in cm) minus (5 x age) minus 161
- Men: the same, but plus 5 instead of minus 161
Clinicians prefer it over the older Harris-Benedict formula because it’s less biased, particularly for people carrying extra weight. Any equation is still an estimate, though, and can be off by a couple hundred calories for one individual. BMR is the base. Add your life on top and you get TDEE.
From BMR to TDEE: The Activity Math, With a Worked Example
Give the math three numbers and one honest answer about how active you are, and you’ll have your maintenance calories. Your TDEE is simply your BMR multiplied by an activity multiplier, a factor that scales up your resting burn to account for how much you move.
The standard multipliers are set, and the trick is picking yours honestly, because most people overrate how active they are.
| Activity level | Multiplier | What it means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1 to 3 days a week |
| Moderately active | 1.55 | Moderate exercise 3 to 5 days a week |
| Very active | 1.725 | Hard exercise 6 to 7 days a week |
| Extra active | 1.9 | A physical job, or hard training twice a day |
Run one real person through it. Take a 40-year-old woman who is 5 feet 6 inches (168 cm) and 180 pounds (82 kg). Her BMR from the female equation works out step by step: 10 times 82 kg is 820, plus 6.25 times 168 cm adds about 1,050, which comes to roughly 1,870. Subtract 5 times her age (200) and then 161, and she lands at about 1,503 calories a day at rest.
She has a desk job, so her multiplier is 1.2. Multiply 1,503 by 1.2 and her TDEE is about 1,804 calories (roughly 7,550 kilojoules) a day. That is her maintenance, the intake that holds her weight steady.
Pick the wrong tier and the whole number moves. Had she called herself “moderately active” (1.55), her estimate would jump to about 2,330 calories, a 500-plus-calorie overshoot that quietly erases any deficit she thinks she’s running. This is why the honest answer matters more than the flattering one.
Subtract about 500 calories a day for weight loss and her target is around 1,300 calories. Hold that figure in mind, because it’s already close to a floor we’ll come back to. And it’s an estimate, not a fixed value. It also won’t hold still once the weight starts coming off, which is where the next section picks up.
How GLP-1 Medication Fits the Energy Equation
A common assumption is that these drugs speed up your metabolism. They mostly don’t. They work on the other side of the equation.
A GLP-1 medication copies a gut hormone your body makes after eating, one that turns down appetite and slows how fast your stomach empties. So you eat less without white-knuckling it. Because the drug acts on how much goes in, not on how much you burn, your TDEE stays the same. What changes is how far under it you naturally land. Appetite drops, intake drops, you settle below your TDEE, and energy balance tips toward loss.
The measured intake numbers show the size of that effect. In a controlled feeding study, semaglutide cut total daily food intake by about 24 percent compared with placebo. In a separate trial, tirzepatide reduced how much people ate at a single free-choice lunch by roughly 658 calories by week six. Those drops are the deficit doing its work.
The brand names sort out like this. Ozempic, Rybelsus, and Wegovy are all semaglutide (the first two for type 2 diabetes, Wegovy for weight management); Mounjaro and Zepbound are both tirzepatide (Mounjaro for diabetes, Zepbound for weight management).
Tirzepatide acts on two gut-hormone signals at once rather than one, which may explain its somewhat larger effect in trials, so it isn’t simply a stronger semaglutide. On the scale, semaglutide produced about 15 percent body-weight loss at 68 weeks in the STEP 1 trial, while tirzepatide reached up to 22.5 percent at 72 weeks in SURMOUNT-1.
The medication supplies the deficit. Your TDEE tells you how big it is.
Why Your TDEE Falls as You Lose Weight
The calorie target that melted weight off in month one quietly stops working, and it isn’t that the drug failed. Your energy needs genuinely shifted down, for three reasons that stack.
First, simple physics. A lighter body needs fewer calories to move and maintain, so your TDEE falls as the scale does. The same daily intake that ran a deficit at 200 pounds runs a smaller one at 175.
Second, adaptive thermogenesis. Your body reads weight loss as a possible famine and dials its resting burn down further than your smaller size alone would predict. The long-term “Biggest Loser” follow-up study showed how far this can go. Contestants’ resting metabolism fell by about 610 calories a day during rapid loss, and six years later it still sat about 704 calories a day below baseline, even after most of the weight had returned.
About 500 of those calories reflected adaptation beyond what their smaller bodies alone could explain. That happens with any weight loss, not just on a GLP-1 drug. The medication doesn’t “break” your metabolism. This is also part of why the body defends a weight range, the set-point idea covered elsewhere in this cluster.
Third, muscle loss. Roughly 25 to 40 percent of the weight lost on GLP-1 therapy can be lean mass rather than fat, and because muscle burns more at rest than fat does, losing it pulls your BMR down another notch. Put the three together and the same plate that worked at the start leaves you at maintenance later. The fix is to recalculate and protect your muscle. Both are covered next.
Setting and Adjusting Your Calorie Target on GLP-1 Medication
Setting a calorie target on semaglutide, tirzepatide, or any GLP-1 medication starts from one number: your TDEE. Four steps turn it into a daily figure you can actually eat to, and the deficit builds from there.
Set it. Take your TDEE and subtract about 500 calories a day. That points you toward roughly one pound of loss a week, since about 3,500 calories is tied to a pound, a rate that CDC and NIDDK guidance calls safe and sustainable.
Hold a floor. This is the GLP-1-specific catch. Appetite suppression can passively push your intake far below your target without you noticing, and lower isn’t better. A joint clinical nutrition advisory flags checking for nutrient shortfalls once intake drops under about 1,200 calories a day for women or 1,800 for men on these drugs, and general low-calorie guidance echoes 1,200 to 1,500 for women and 1,500 to 1,800 for men.
Recall the worked example: that woman’s 1,300-calorie target already sits near this line, so eating still less would work against her.
Protect the number. Aim for 1.2 to 1.6 grams of protein per kilogram of body weight a day, about 80 to 120 grams for many people, plus resistance training on at least two days a week and around 150 minutes of moderate activity weekly.
Dietitians and the same advisory back this combination to blunt muscle loss and defend your BMR. This is where a sensible calorie target on semaglutide or tirzepatide either preserves muscle or quietly spends it.
Recompute. Recalculate your TDEE every 10 to 15 pounds lost, or after a dose change, since the number keeps sliding down. The drug delivers the deficit; the math and the protein are how you make that deficit come off fat, not muscle.
Dosing and whether a medication suits you are clinical decisions for your doctor, including on-label versus off-label use. For the bigger picture on how these medicines support weight management, learn more about GLP-1 weight-loss medication.
FAQ
What’s the difference between BMR and TDEE?
BMR is the calories your body burns at complete rest, just for basic functions like breathing and circulation, and it makes up about 60 to 70 percent of your daily total. TDEE is your BMR multiplied by an activity factor, so it adds in digestion and all your movement. In short, BMR is the base rate, and TDEE is the full daily burn you eat above or below.
How many calories should I eat on a GLP-1 medication like Ozempic or Zepbound?
There’s no universal number. Estimate your TDEE using the Mifflin-St Jeor equation times your activity multiplier, then subtract about 500 calories a day for gradual loss. Stay above roughly 1,200 calories for women or 1,500 to 1,800 for men to avoid nutrient shortfalls and muscle loss, and recalculate as you get lighter, since your needs keep falling.
Does GLP-1 medication slow my metabolism?
Weight loss itself lowers your TDEE, through a lighter body, adaptive thermogenesis, and some muscle loss. That happens with any weight-loss method, not just these drugs, and it isn’t permanent damage to your metabolism. Adequate protein and resistance training protect your muscle and blunt the drop, which is why both belong in the plan from the start.
Is tirzepatide just a stronger semaglutide?
No. Semaglutide acts on a single receptor, the GLP-1 receptor. Tirzepatide is a dual agent, working on both the GIP and GLP-1 receptors, so it engages two gut-hormone systems rather than one. That mechanism is different in kind, and the two haven’t been tested head-to-head, so calling one a stronger version of the other oversimplifies how they work.