The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
Why Does Weight Loss Cause Fatigue?
Feeling unusually tired whilst trying to lose weight is more common than you might think. Weight loss fatigue can occur for several reasons — from calorie deficits and nutritional changes to hormonal shifts and increased activity levels. Understanding what's happening in your body helps you manage energy levels effectively and maintain your weight management approach safely. Whether you're following NHS weight loss guidance, adjusting your diet, or taking prescription weight loss medication, knowing how to support your energy is important for sustainable results.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Common Causes of Fatigue During Weight Loss
Calorie deficit: Weight loss requires a calorie deficit, typically around 500 kcal per day for safe, sustainable loss of approximately 0.5 kg per week. This deficit means your body has less energy available, which can feel like tiredness, especially in the first few weeks.
Nutrient deficiencies: Rapid or poorly planned weight loss may lack adequate protein, iron, B vitamins, or electrolytes — all critical for energy production. A balanced approach ensures you get essential nutrients whilst in a calorie deficit.
Sleep quality: Weight loss efforts sometimes disrupt sleep patterns, and poor sleep directly causes fatigue. Hormonal changes during weight loss can also affect sleep.
Metabolic adaptation: Your body adjusts to reduced calorie intake by slowing metabolism slightly, which can contribute to low energy feelings.
Medication effects: If using prescription weight loss medication like Mounjaro or Wegovy as part of UK weight management programmes, fatigue may occur as your body adjusts, though this typically improves over time.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
How to Manage Weight Loss Fatigue Effectively
Eat enough protein: Aim for 1.2–1.6 g per kg of body weight daily — protein preserves muscle during weight loss and supports sustained energy.
Include complex carbohydrates: Whole grains, oats, and root vegetables provide steady glucose for energy, unlike refined carbs which cause spikes and crashes.
Stay hydrated: Even mild dehydration causes fatigue. Drink around 6–8 glasses of water daily, more if exercising.
Don't cut calories too drastically: Severe restriction intensifies fatigue. The NHS recommends a moderate deficit of 500 kcal daily, not extreme restriction.
Prioritise sleep: Aim for 7–9 hours nightly. Better sleep supports weight loss hormones and energy levels.
Exercise moderately: Intense exercise on a low-calorie diet worsens fatigue. Mix strength training with gentle activity like walking.
Track your progress using our free progress tracker to see how your energy changes as your body adapts — improvements often come within 2–3 weeks.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is fatigue normal when losing weight?
Yes, mild fatigue is common, especially in the first 2–4 weeks as your body adjusts to a calorie deficit. If fatigue is severe or persists beyond a month, contact your GP to rule out nutrient deficiencies or underlying conditions.
Does Wegovy or Mounjaro cause fatigue?
Fatigue is not listed as a common side effect of either medication in clinical trials. However, some people report low energy during initial adjustment. This usually improves within weeks. If fatigue persists, discuss with your prescriber.
How much protein do I need during weight loss?
The NHS and sports nutrition research recommend 1.2–1.6 g protein per kg of body weight daily during weight loss. Use our BMI calculator to check your current weight, then calculate your protein target — this preserves muscle and energy.
Can I exercise if I'm feeling fatigued from weight loss?
Yes, but modify intensity. Light to moderate activity like walking or swimming supports weight loss without worsening fatigue. Avoid intense exercise when severely tired — rest days are part of effective weight management.
When should I see my GP about weight loss fatigue?
Contact your GP if fatigue is severe, lasts more than 4 weeks, or is accompanied by other symptoms like persistent headaches, dizziness, or muscle weakness. These may indicate nutrient deficiency or an underlying health condition requiring assessment.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.