Jump to: What happens when you fast on Mounjaro | Why three meals work better | Fasting vs three meals comparison | Take home message
We wouldn’t recommend fasting while on Mounjaro, as it can increase the risk of losing lean tissue, such as muscle and bone, alongside fat, and increase the risk of developing nutrient deficiencies.
This is because fasting can limit the amount of energy, nutrients, and protein that we consume each day, providing insufficient nourishment for our bodies.
While we need to be in a calorie deficit to lose fat, losing weight slowly can help reduce the loss of lean tissue and the risk of deficiencies.
Mounjaro reduces our energy intake by ~30%. If we then skip meals and fast while eating smaller portions, we’ll likely not get enough protein, energy, and nutrients in our meals to support healthy, sustainable weight loss.
Important safety information: Mounjaro (tirzepatide) is a prescription-only medication for treating type 2 diabetes and managing obesity. This article is for informational purposes only. Always consult with your healthcare provider before starting any new medication or making significant changes to your eating patterns, particularly if you have had your gallbladder removed or have other health conditions.
What happens when you fast on Mounjaro?
Mounjaro works by mimicking GLP-1, a hormone that slows digestion and reduces appetite. Clinical trials have shown that this leads to a 30-40% reduction in food intake compared to baseline.
If we then skip meals and fast while eating smaller portions, we’ll unlikely get enough protein, energy, and nutrients in our meals to support healthy, sustainable weight loss.
Protein intake drops below recommended levels
Research indicates that to preserve muscle mass during weight loss, it’s recommended to consume 1.2-1.6 grams of protein per kilogram of body weight per day2. For an 80kg person, this translates to 96-128 grams of protein per day.
It’s also important to spread protein intake out across the day. Studies show muscle protein synthesis (muscle growth) peaks after consuming 20-30 grams of protein and remains elevated for several hours before dropping ahead of our next meal5.
If you’re eating one or two meals a day on Mounjaro, you won’t be stimulating sufficient muscle growth and repair to maintain muscle mass while losing weight.
Increased muscle loss
A 2024 meta-analysis of GLP-1 medications found that ~25% of total weight loss results from lean mass rather than fat, likely due to insufficient protein intake and resistance training among participants in these studies1.
Lean mass includes muscle, bone and organ tissue. Maintaining muscle mass is crucial for our health as it provides stability for our body, promotes independence as we age, and helps to maintain stable blood sugar levels by burning and storing excess sugar from our diet.
Research published in September 2024, examining body composition changes with GLP-1 medications, found that higher protein intake (>1.3g/kg/day), combined with resistance exercise, reduced lean mass loss to 15-20% of total weight lost, compared to 40% or more without these interventions1.
Increased risk of nutrient deficiencies
Studies examining rapid weight loss show that severe calorie restriction leads to deficiencies in4:
- Iron
- Calcium
- Vitamin D
- B vitamins (particularly B12)
- Magnesium
- Potassium
These deficiencies can develop within weeks and manifest as fatigue, hair loss, frequent illness and poor concentration. Over time, these deficiencies can lead to severe health complications.
A 2024 systematic review of weight loss therapies found that restricting eating patterns while on weight-loss medication increases the risk of these deficiencies compared to regular meal patterns2.
Key points:
- Mounjaro reduces appetite by 30-40%, making it easy to undereat when combined with fasting
- Protein requirements for muscle preservation are 1.2-1.6g/kg body weight a day
- Research shows 25% of weight loss on GLP-1s comes from lean mass, likely due to lack of protein intake and exercise
- Eating too little can increase the risk of nutrient deficiencies
Why we recommend eating 3 balanced meals a day
At Second Nature, we’ve worked with the NHS for over 8 years, supporting people to lose weight and reverse their type 2 diabetes.
Our NHS-trusted approach was designed to help people lose weight sustainably, while allowing them time to develop the healthy habits necessary to maintain weight loss in the long term.
Protein distribution throughout the day
Our body is like a house that’s constantly being renovated. It’s continuously recycling amino acids (the building blocks of protein) to redistribute around the body for various functions, like our immune system5.
When we go too long without eating protein from food (like if we decided to fast at regular intervals), we force our body to recycle more amino acids from our existing tissues for its essential functions.
But, when we consume protein more regularly across the day, we’re providing our body with the building blocks it needs for repair, growth, and other essential functions.
Protein becomes even more important when we’re losing weight, as the body will use protein for energy if we’re not giving it enough energy from fat or carbohydrate.
So, we recommend consuming a serving of protein the size of an open hand at breakfast, lunch, and dinner.
If you snack, we also recommend including a protein source, like Greek yoghurt with berries, or tinned fish on rye crackers.
Meeting nutritional requirements
Eating 3 meals provides 3 opportunities to consume a variety of nutrients that support the body’s essential functions.
Breakfast may provide calcium and vitamin D from dairy products. Lunch brings iron from meat or legumes. Dinner adds omega-3 fatty acids from oily fish.
Enjoying this variety becomes more challenging when we limit the number of meals we’re eating each day by opting to practice any form of intermittent fasting.
Building sustainable habits
Clinical trial data show that weight gain is likely when you stop weight-loss medication unless you’ve built sustainable habits to manage your eating habits without the support of the medication3.
Our preliminary research on people who completed our behaviour change programme alongside GLP-1 medication showed weight regain of only 3.2% at five months after stopping, compared to 6% or more in studies without structured habit support.
Key points:
- Eating 3 main meals a day ensures we’re eating enough protein, energy, and nutrients to support sustainable weight loss
- It’s essential to lose weight slowly and build healthy habits if you want to keep the weight off in the long term
Comparing fasting and three meals on Mounjaro
| Factor |
Fasting (16:8 or OMAD) |
Three balanced meals |
| Protein intake |
Difficult to consume enough protein with 1-2 meals a day |
Achievable with 30-40g per meal across three meals |
| Protein distribution |
One or two stimuli for muscle protein synthesis a day, longer time in protein breakdown |
Three separate stimuli throughout the day, less time in protein breakdown |
| Muscle loss |
Research shows lean mass loss a bigger risk with fasting approaches |
Will be minimised with enough protein in 3 meals |
| Nutrient intake |
Higher risk of deficiencies in iron, calcium, vitamin D, B vitamins |
Easier to meet requirements with three opportunities for variety |
| After stopping medication |
No skills developed for managing normal appetite |
Skills built for maintaining weight with regular eating |
How to structure meals on Mounjaro
Each meal should contain:
- Protein (¼ plate): Palm-sized portion, 30-40g per meal
- Vegetables (½ plate): Leafy greens, broccoli, peppers, courgettes, tomatoes
- Complex carbohydrates (¼ plate): Fist-sized portion of brown rice, quinoa, sweet potato, beans
- Healthy fats: Thumb-sized portion of avocado, 1 tbsp olive oil, small handful of nuts
Protein sources per meal
To reach 30-40g protein:
- 2-3 eggs plus Greek yoghurt
- Small chicken breast (100-120g)
- Palm-sized fish fillet
- 200g Greek yoghurt (high-protein varieties)
- 150g tofu plus legumes
When appetite is very low
There will be times on weight-loss medication when you won’t feel like eating at all, but this is the impact of the medication masking your body’s needs.
On days when your appetite is very low, prioritise meals that are easier to digest, like chili with beef mince, stews and casseroles with meat that falls apart, or white fish as your protein source.
You can also make soups with diced meat, fish, or boiled eggs, and experiment with smoothies with added protein powder.
It’s essential to maintain the habit of eating 3 balanced meals a day to support sustainable weight loss in the long term.
The goal isn’t to lose weight as fast as possible, but to look ahead 5 years from now and be confident you’ll be able to maintain the positive progress you’ve made.
Key points:
- Structure meals with ¼ protein, ½ vegetables, ¼ complex carbohydrates, plus healthy fats
- Each meal should contain 30-40g protein from sources like eggs, meat, fish, Greek yoghurt, tofu
- On days when you don’t feel that hungry, prioritise meals that are easier to consume and digest
Frequently asked questions
Is it safe to fast whilst taking Mounjaro?
Fasting whilst taking Mounjaro increases the risk of inadequate protein intake, accelerated muscle loss and nutrient deficiencies.
Research shows GLP-1 medications already reduce appetite by 30-40%, making it easy to undereat when combined with time-restricted eating. We recommend three balanced meals daily to ensure adequate nutrition.
How much protein do I need while taking Mounjaro?
Aim for 1.2-1.6 grams of protein per kilogram of body weight daily2. For an 80kg person, this means 96-128 grams daily.
Distribute this across three meals, targeting 30-40 grams at each meal. Research shows that this distribution supports muscle protein synthesis more effectively than consuming all protein in one or two meals5.
Will fasting speed up my weight loss on Mounjaro?
Fasting may initially appear to accelerate weight loss, but research shows much of this rapid loss comes from water and muscle rather than fat4.
It’s better to lose weight slowly and develop the healthy habits to keep the weight off in the long term than focusing purely on speed of weight loss now.
How do I prevent muscle loss whilst taking Mounjaro?
If you lose weight slowly, eat enough protein, and do some form of resistance training, you can prevent muscle loss while losing weight on Mounjaro1.
What should I do if I have no appetite on Mounjaro?
Prioritise meals that are easier to consume and digest like soups, stews, and other one pot recipes.
Take home message
Retry
Research consistently shows that adequate protein intake distributed across three meals daily, combined with resistance exercise, provides the best outcomes for muscle preservation during weight loss with GLP-1 medications1,5.
While Mounjaro makes fasting feel easy due to its impact on lowering hunger, this combination increases the risk of inadequate protein intake and accelerated muscle loss.
Eating 3 balanced meals a day rich in protein encourages sustainable weight loss and avoids the negative effects of rapid weight loss due to very low-calorie intake.
Second Nature's Mounjaro and Wegovy programmes
Second Nature provides Mounjaro or Wegovy as part of our Mounjaro and Wegovy weight-loss programmes.
Why choose Second Nature over other medication providers, assuming you're eligible?
Because peace of mind matters.
We've had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro and Wegovy) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We're here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK's most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
References
- Neeland IJ, Linge J, Birkenfeld AL. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes Obes Metab. 2024;26(Suppl 4):16-27.
- Kokura Y, Ueshima J, Saino Y, Maeda K. Enhanced protein intake on maintaining muscle mass, strength, and physical function in adults with overweight/obesity: A systematic review and meta-analysis. Clin Nutr ESPEN. 2024;63:417-426.
- Apovian C, Yerevanian A, Dushay J. Preserving lean body mass in patients taking GLP-1 for weight loss. Mass General Advances in Motion. 2025 June.
- Jaime K, Mank V. Risks associated with excessive weight loss. StatPearls. Updated 2024 Feb.
- Cava E, Yeat NC, Mittendorfer B. Preserving healthy muscle during weight loss. Adv Nutr. 2017;8(3):511-519.
