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 Water Fast Weight Loss: What the Evidence Shows
A 3-day water fast — consuming only water for 72 hours — is an extreme approach to weight loss that circulates on social media, but it's not recommended by the NHS or supported by long-term weight management research. While you may lose weight initially, most of it will be water and glycogen rather than fat, and the weight typically returns when you resume eating. This guide explains why water fasting isn't an effective weight loss strategy and shares evidence-based alternatives that actually work for sustainable weight management 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.
Why Water Fasting Doesn't Work Long-Term
A 3-day water fast creates a severe calorie deficit, leading to gradual weight management — typically 2–3 kg in three days. However, most of this loss is water and muscle glycogen, not fat tissue. Once you resume eating, your body rehydrates and replenishes glycogen stores, causing the weight to return quickly. This cycle — known as yo-yo dieting — can actually be counterproductive to long-term weight management.
The NHS and NICE recommend a sustainable weight loss rate of 0.5–1 kg per week through a moderate calorie deficit (around 500 kcal daily) combined with increased physical activity. Extended fasting can trigger:
- Muscle loss (your body breaks down protein when carbohydrate is depleted)
- Metabolic slowdown (your resting metabolic rate decreases)
- Nutrient deficiencies (no vitamins, minerals, or protein intake)
- Rebound hunger and overeating after fasting ends
For sustainable weight management UK services recommend balanced approaches rather than extreme restrictions.
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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.
Evidence-Based Alternatives for Sustainable Weight Loss
Instead of water fasting, the NHS recommends focusing on nhs weight loss strategies: a balanced diet with a moderate calorie deficit, increased activity, and behavioural changes. If you're struggling to lose weight uk through diet and exercise alone, your GP can discuss prescription options. For adults with a BMI of 30 or above (or 27+ with weight-related health conditions), weight loss medication nhs services may be available through specialist tier 3 services.
GLP-1 medications like Wegovy are licensed for weight loss injections uk and work alongside lifestyle changes — not as a replacement for them. Try our free BMI calculator to check your health category, and use our GLP-1 weight loss calculator to explore what sustainable weight loss might look like over time.
- 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 water fast?
Yes, temporarily — you'll likely lose 2–3 kg in three days, but mostly water and glycogen, not fat. The weight returns when you eat normally again, making it ineffective long-term and potentially harmful to metabolism.
Is a 3-day water fast safe?
No. Extended water-only fasting can cause dizziness, fatigue, muscle loss, nutrient deficiencies, and electrolyte imbalances. If you're considering any extreme diet, discuss it with your GP first.
What does the NHS recommend for weight loss?
The NHS recommends a calorie deficit of around 500 kcal daily (losing 0.5–1 kg per week) combined with increased physical activity and behavioural changes. For eligible adults, prescription weight loss medication is available through specialist services.
What is the difference between water fasting and intermittent fasting?
Intermittent fasting involves eating during set windows (e.g., 16:8) with normal meals during eating periods. Water fasting means no calories at all for extended periods. Intermittent fasting is more moderate and allows nutrient intake.
Can weight loss medication help if diet and exercise aren't working?
Yes. GLP-1 medications like Wegovy are prescription-only and available to eligible UK adults. They work by reducing appetite and are used alongside lifestyle changes, not instead of them. Your GP can assess if you qualify.
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.