How a Calorie Deficit Works — and What Changes on GLP-1
Your body needs a certain number of calories each day to maintain its current weight — often called your total daily energy expenditure (TDEE). When you consistently eat less than that, your body draws on stored energy, and weight loss follows over time. The size of the deficit influences the pace of loss, but bigger is not always better: too large a deficit can lead to fatigue, muscle loss, and difficulty sustaining the approach.
Why GLP-1 medicines change the picture
Mounjaro (tirzepatide) and Wegovy (semaglutide) act on appetite-regulating pathways, and many people find they feel full on noticeably less food. This can make a moderate calorie deficit feel effortless in a way it might not have before — but it also means it is easy to eat too little without realising, which can compromise protein intake and energy levels.
Sensible general targets
Most general nutritional guidance suggests a deficit of roughly 500 calories a day as a starting point for steady, sustainable loss — though individual needs vary widely based on starting weight, height, age, activity level, and health history. Calorie needs also differ between men and women, largely due to differences in average body composition and muscle mass, which affect how many calories the body burns at rest. There is no single figure that fits everyone. For a deeper look at what deficit size might suit your situation, see our guide to choosing a good calorie deficit, and for practical eating strategies, explore our calorie deficit diet guide.
Protein and food quality within the deficit
On a GLP-1 journey, protein is especially important. Eating enough protein within your calorie budget helps preserve muscle as weight comes off and keeps you feeling satisfied on smaller portions. Prioritising whole foods — lean proteins, vegetables, wholegrains — helps you hit nutritional needs even when total intake is lower than before. For the full picture on how this fits into a calorie deficit, visit our calorie deficit hub.
average weekly weight change in the first 16 weeks of Mounjaro & Wegovy use*
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Tracking Your Calorie Deficit with GLP Trackr
Knowing your deficit in theory is one thing; hitting it consistently is another — especially when appetite fluctuates week to week on a GLP-1 medicine. GLP Trackr's AI food photo scanner makes logging effortless: point your camera at a meal and get an instant calorie and protein breakdown, no manual searching needed. On days when appetite is low and meals are small, that speed matters.
Once you have your food logged, GLP Trackr lets you set a daily calorie target and a protein goal, so you can see at a glance whether you are hitting your deficit without going so low that protein suffers. The app's weekly insights can also surface patterns — for example, whether your food intake, energy, or weight are moving in step with each other — giving you something concrete to discuss at your next prescriber appointment.
GLP Trackr also includes a barcode scanner and manual food search alongside the photo scan, and a library of GLP-1-friendly recipes and meal plans built around protein-rich, gentle-on-the-stomach options — useful when reduced appetite makes meal planning feel harder than usual.
Snap your meals, hit your targets, and watch the data build into a clear picture of your progress. Download GLP Trackr on the App Store or Google Play and explore it free for your first 7 days.
General calorie guidance is educational and not a substitute for personalised advice. Your prescriber or doctor can help you work out targets that are right for your health history and current treatment — always follow their guidance first.
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Frequently asked questions.
General guidance often suggests a deficit of around 500 calories a day for steady loss, but the right amount varies by body size, activity, age, and health. There is no universal figure — your prescriber or doctor can help you find a target that suits your individual circumstances.
Calorie needs differ between men and women, largely because of differences in average muscle mass and body composition, which affect how many calories the body burns at rest. The principle — eating less than you expend — is the same, but the numbers will vary from person to person regardless of sex.
Start by estimating your total daily energy expenditure (TDEE) based on your weight, height, age, and activity level, then subtract a moderate amount — often around 500 calories — to create a deficit. Online TDEE calculators can give a rough starting point; your healthcare professional can advise on what is appropriate for you.
Very large deficits can lead to fatigue, muscle loss, and inadequate nutrition, which is a particular risk on GLP-1 medicines where appetite can drop sharply. Always follow the guidance of your prescriber or doctor — they can help ensure your intake stays safe and sustainable while you are on treatment.
GLP Trackr lets you log meals via AI photo scan, barcode, or search, set a daily calorie and protein target, and see your intake patterns over time. This makes it easy to stay within your deficit consistently and brings useful data to your prescriber appointments.