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
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How To Fast For Weight Loss
Intermittent fasting is an eating pattern that cycles between periods of eating and fasting, and evidence suggests it may support weight loss when combined with healthy food choices. Fasting works by creating a calorie deficit over time, helping your body use stored energy. This guide explains how fasting works, safe approaches based on NHS and clinical evidence, and how it compares to other weight management strategies 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.
Safe Fasting Methods And How They Work
Several fasting approaches have been studied for weight management UK. The most popular include:
- 16:8 intermittent fasting — eating within an 8-hour window and fasting for 16 hours daily. Research shows this can create a modest calorie deficit without counting calories.
- 5:2 diet — eating normally five days per week and restricting calories to around 500–600 kcal on two non-consecutive days. Clinical evidence supports similar weight loss to standard calorie restriction.
- Eat-stop-eat — complete 24-hour fasts once or twice weekly. This requires medical supervision if you have existing health conditions.
The key principle is creating a calorie deficit. The NHS notes that to lose approximately 0.5–1 kg per week, you need a deficit of around 500 kcal daily. Fasting can help achieve this, but results depend on what you eat during eating windows — choosing whole foods, vegetables, and lean proteins matters significantly.
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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.
Fasting And NHS Weight Loss Guidance
The NHS weight loss approach recommends sustainable, long-term strategies rather than extreme restriction. While intermittent fasting is recognised as a valid option, the NHS emphasises that fasting works best when combined with healthy eating habits and increased physical activity. Very low calorie diets (under 800 kcal daily) should only be followed under medical supervision. Fasting alone may lead to muscle loss if you don't maintain adequate protein intake during eating periods. For adults with bmi weight loss concerns, using our free BMI calculator helps you understand your current health status and track changes over time. If you're considering fasting alongside other strategies like glp-1 weight loss medications, your GP can advise whether combining approaches is appropriate for your individual health situation.
- 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 intermittent fasting safe for everyone?
Intermittent fasting is generally safe for most adults but not recommended if you're pregnant, breastfeeding, have a history of eating disorders, or take medications requiring food intake (like certain diabetes medicines). Always consult your GP before starting, especially if you have existing health conditions or take regular medication.
How much weight can you lose by fasting?
Results vary significantly between individuals. Clinical evidence suggests intermittent fasting produces similar weight loss to standard calorie restriction — approximately 0.5–1 kg per week when combined with healthy eating. The NHS recommends this rate as sustainable. Your individual results depend on what you eat during eating windows and your overall activity level.
Can you drink water and tea while fasting?
Yes. During fasting periods, water is essential and encouraged. Unsweetened tea, coffee, and black coffee are generally fine. Avoid adding milk, sugar, or calorie-containing supplements, as these break your fast. Staying hydrated is important for managing hunger and supporting your metabolism.
Does fasting affect metabolism long-term?
Research shows intermittent fasting doesn't significantly damage your metabolism when done properly. Short-term fasting may slightly lower metabolic rate, but this reverses when you return to normal eating. Maintaining adequate protein and staying physically active helps preserve muscle mass during fasting periods.
How does fasting compare to weight loss medication?
Fasting creates a calorie deficit through eating timing, while prescription weight loss medications like GLP-1 injections reduce appetite and support feeling fuller. Some people combine fasting with medication under medical guidance. Your GP can assess which approach — or combination — suits your individual health needs and goals.
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.