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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Fasting Diets For Weight Loss: What Works in the UK
Fasting diets are increasingly popular for weight management in the UK, with intermittent fasting and time-restricted eating gaining scientific attention. These approaches involve eating within specific time windows or reducing calorie intake on certain days. Evidence suggests they can support weight loss when combined with overall healthy eating habits, though results vary between individuals. This guide covers how fasting diets work, safety considerations, and how they fit within broader weight management strategies. As always, consult your GP before making significant dietary changes, especially if you have existing health conditions.
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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 Fasting Diets Support Weight Loss
Fasting diets work through calorie restriction — by eating within a shorter time window, many people naturally consume fewer calories overall. Popular approaches include the 16:8 method (16 hours fasting, 8-hour eating window), the 5:2 diet (eating normally 5 days, restricting to 500–600 kcal on 2 days), and eat-stop-eat (24-hour fasts). Research shows intermittent fasting can produce similar weight loss results to standard calorie restriction — approximately 0.5 kg per week when combined with a healthy diet.
The mechanism is straightforward: fasting creates a calorie deficit without requiring constant calorie counting. Some evidence suggests fasting may improve insulin sensitivity and reduce blood sugar spikes, though individual responses vary. However, fasting is not inherently superior to other dietary approaches — sustainable weight loss depends on choosing an eating pattern you can maintain long-term alongside increased physical activity.
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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.
Safety, Suitability, and When to Seek GP Advice
Fasting diets are generally safe for most healthy adults, but not suitable for everyone. Avoid fasting if you are: pregnant or breastfeeding, have a history of eating disorders, take certain diabetes medications, or have type 1 diabetes. Fasting may cause initial side effects including fatigue, irritability, and difficulty concentrating — these often improve as your body adapts.
For sustainable weight management in the UK, your GP can assess whether fasting fits your individual health profile. If you're overweight with weight-related health conditions, your GP may also discuss NHS weight loss programmes or, in some cases, prescription weight loss options alongside lifestyle changes. Use our free BMI calculator above to understand your current weight status, then discuss personalised approaches with your GP.
- 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 better than other diets for weight loss?
Evidence suggests intermittent fasting produces similar weight loss to standard calorie restriction — approximately 0.5 kg per week. Success depends on finding an eating pattern you can sustain long-term. Some people find fasting easier to follow than daily calorie counting, while others prefer traditional dieting. Individual results vary significantly based on overall diet quality and physical activity.
Can I combine fasting with prescription weight loss medication?
Yes, dietary changes and prescription medications work together. If prescribed weight loss medication through NHS services, your healthcare team will provide specific guidance on combining fasting with your treatment. Always inform your prescriber about your fasting plans to ensure safe, effective weight management without conflicting approaches.
What should I eat during my eating window?
Focus on nutritious, whole foods: vegetables, lean proteins, wholegrains, and healthy fats. Avoid using your eating window to consume excessive calories from processed foods — weight loss still requires overall calorie balance. Quality nutrition during eating periods supports better energy and sustainable results.
How long does it take to see weight loss results from fasting?
Initial weight loss typically appears within 1–2 weeks, though much of this is water loss. Sustained fat loss usually becomes noticeable after 4–6 weeks when combined with a balanced diet. The NHS recommends losing 0.5–1 kg per week for sustainable, healthy weight loss.
Should I consult my GP before starting a fasting diet?
Yes, especially if you have existing health conditions, take medications, or have a history of disordered eating. Your GP can assess whether fasting suits your individual health profile and recommend alternatives if needed. This ensures your weight loss approach is safe and effective for your circumstances.
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.