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
3 Day Juice Fast Weight Loss — What You Need to Know
A 3-day juice fast may result in gradual weight management, but most of this is water weight and will often return once you resume normal eating. While juice fasting is a popular quick-fix approach, NHS guidance and weight management research suggest that sustainable, long-term weight loss comes from creating a modest calorie deficit combined with physical activity. This guide explains how juice fasts work, their realistic outcomes, and more evidence-based approaches to achieving lasting weight loss in the UK.
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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.
How Juice Fasting Works and What the Research Shows
A juice fast typically involves consuming only freshly pressed or store-bought fruit and vegetable juices for 1–3 days, with no solid food. The idea is that juices provide nutrients whilst dramatically reducing calorie intake—often to 500–1000 kcal per day compared to a typical 2000+ kcal daily diet.
- Rapid initial weight loss: Most weight lost during a 3-day juice fast is water and glycogen (stored carbohydrate), not body fat. This weight usually returns within days of returning to normal eating.
- Low satiety: Liquid calories do not trigger fullness signals as effectively as solid food, making it easy to feel hungry despite low calorie intake.
- Nutrient gaps: Juice fasts lack protein and healthy fats, which can lead to muscle loss, fatigue, and poor recovery from exercise.
- NHS perspective: The NHS does not recommend very-low-calorie approaches like juice fasts as a primary weight management strategy. Instead, it recommends gradual, sustainable changes.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Sustainable Alternatives to Juice Fasting
For genuine, lasting weight loss in the UK, evidence supports a calorie deficit of around 500 kcal per day, which typically results in a loss of 0.5–1 kg per week. This can be achieved through balanced dietary changes and increased physical activity—approaches supported by NHS weight management guidance. Many people also find that prescription weight loss medication, alongside diet and exercise, provides meaningful support. Our GLP-1 weight loss calculator can help you explore realistic weight loss projections with evidence-based approaches. For personalised advice on the best approach for your situation, speak with your GP or a registered dietitian.
- 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.
Will I lose weight on a 3-day juice fast?
Yes, most people lose weight rapidly—typically 1–3 kg in 3 days—but this is predominantly water weight and glycogen depletion, not body fat. Once you resume normal eating, this weight usually returns within a few days, making juice fasting ineffective for lasting results.
Is juice fasting safe?
Short-term juice fasting is generally safe for most people, but it can cause fatigue, headaches, and muscle loss due to very low calorie and protein intake. People with diabetes, eating disorders, or certain health conditions should avoid juice fasting. Always consult your GP before starting any restrictive diet.
Why does the NHS not recommend juice fasting?
The NHS recommends weight management approaches that are sustainable long-term and preserve muscle mass. Juice fasts are too restrictive, lack protein, and typically result in weight regain. The NHS favours a moderate calorie deficit (around 500 kcal/day) combined with physical activity.
What's a better alternative to juice fasting?
A balanced, reduced-calorie diet with whole foods, adequate protein, and regular physical activity is far more effective for lasting weight loss. Aiming for 0.5–1 kg per week loss is sustainable and supported by NHS guidelines. Your GP can also discuss prescription weight loss options if appropriate.
Can I use a juice fast as a "kickstart" before a proper diet?
Short-term juice fasts don't provide a lasting metabolic advantage and may increase muscle loss. Instead, start with evidence-based weight loss strategies immediately. Use our BMI calculator to assess your starting point and our progress tracker to monitor sustainable, long-term progress.
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.