What's My Calorie Deficit — and Why Does It Matter on a GLP-1 Medicine?

Your calorie deficit is the gap between the energy your body needs to maintain its current weight — often called your Total Daily Energy Expenditure (TDEE) — and the calories you actually consume. When that gap is consistently present, your body draws on stored energy, which over time results in weight loss.

How GLP-1 medicines interact with your deficit

Medicines like tirzepatide and semaglutide act on appetite-regulating pathways in the brain and slow gastric emptying, which many people experience as reduced hunger and less preoccupation with food (sometimes called reduced food noise). This often means eating noticeably less without consciously counting every calorie — but it does not mean a deficit is guaranteed or that the size of any deficit stays constant. Appetite effects can vary week to week, especially around dose changes.

What size of deficit is realistic?

General nutritional guidance — not specific to any individual — typically places a moderate, sustainable deficit in the region of 300–600 kcal below maintenance. Deficits that are too large can lead to muscle loss, fatigue, and nutrient gaps, which is particularly worth being aware of when appetite is suppressed. Protein intake becomes especially important in this context: prioritising protein helps protect lean muscle while in a deficit. For guidance tailored to your own situation, speak with your prescriber or a registered dietitian.

To go deeper on how to work out what your deficit should be, see our guide on choosing the right calorie deficit target, or read through our full calorie deficit hub for GLP-1 users for a comprehensive overview.

Tracking matters more than guessing

Because appetite suppression on these medicines can be inconsistent — often stronger in the days immediately after an injection and softer later in the week — many people find their actual intake fluctuates more than they expect. Logging what you eat gives you a real picture of where your deficit sits, rather than an estimate. You can explore how to calculate your own starting figure on our page covering how to work out your calorie deficit.

Knowing your deficit in theory is one thing; seeing it in real time is another. GLP Trackr's AI food photo scan is the fastest way to log what you have actually eaten — point your camera at a meal and the app returns an instant calorie and macronutrient breakdown. No manual searching required, which matters on days when appetite is low and eating feels like an afterthought.

Once your calorie target is set in the app, every meal you log contributes to a running daily total so you can see at a glance whether you are on track, under, or over. The app also tracks protein separately, which is useful given how important it is to protect muscle while in a deficit on a GLP-1 medicine.

Over time, the insights and weekly summaries help you spot patterns — for example, whether your intake dips sharply in the days after an injection and rises later in the week — giving you genuinely useful information to discuss at your next prescriber appointment.

A note: GLP Trackr is an information and tracking tool, not a clinical service. Any decisions about your calorie targets, diet, or medication should be made with your prescriber or a qualified healthcare professional who knows your full history.

Ready to start seeing your deficit in real numbers? Download GLP Trackr on the App Store or Google Play — your first seven days are completely free.