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 Times For Weight Loss — What Works In The UK
Fasting for weight loss involves eating within set time windows to create a calorie deficit. Popular methods include the 16:8 fast (16 hours fasting, 8 hours eating) and the 5:2 diet (eating normally five days, restricting calories two days). While fasting can support weight management in the UK, evidence shows results vary by individual. For some people, prescription weight loss options may be more effective alongside lifestyle changes. This guide explains how fasting timing works and when to consider additional support from your GP.
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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 Times Support Weight Loss
Fasting works by extending the time between meals, which may help reduce overall calorie intake and support steady weight management in the UK. The most popular fasting method is intermittent fasting (IF), where you eat within a specific window each day.
- 16:8 fasting: Fast for 16 hours, eat within an 8-hour window (e.g., noon to 8pm). Many people find this sustainable long-term.
- 5:2 diet: Eat normally five days per week, restrict calories to 500–600 kcal on two non-consecutive days. NHS research suggests modest weight loss is possible.
- Eat-stop-eat: Complete 24-hour fasts once or twice weekly. More challenging but may suit some individuals.
Safe fasting typically involves a calorie deficit of around 500 kcal per day, which may result in approximately 0.5 kg weight loss per week — aligned with NHS weight loss guidance. However, results depend on what you eat during eating windows. Fasting alone does not clinically studied weight loss if total calorie intake remains too high.
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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.
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Fasting Versus Prescription Weight Loss Options In The UK
Fasting is a free, accessible approach but requires discipline and may not suit everyone. For weight management in the UK, prescription options like Mounjaro (tirzepatide) and Wegovy (semaglutide) offer another route. These GLP-1 medications are prescription-only and available to eligible adults with a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes or high blood pressure.
Many people combine fasting with other lifestyle changes or medication for better results. Your GP can assess whether prescription weight loss treatment is suitable for you, or whether lifestyle changes like fasting, reduced-calorie eating, and increased physical activity are the best first step. Use our free GLP-1 weight loss calculator to explore potential outcomes at different medication doses, and our BMI calculator to check your personal weight health baseline.
- 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 fasting safe for long-term weight loss in the UK?
Fasting is safe for most adults if done correctly — typically 16:8 fasting or 5:2 diets. Always speak to your GP before starting, especially if you have diabetes, take medications, or are pregnant. NHS guidance supports modest calorie deficits through fasting as part of broader weight management.
How long does it take to see weight loss results from fasting?
Most people see modest changes within 2–4 weeks, but results vary. Safe weight loss is 0.5–1 kg per week. Progress depends on your eating habits during eating windows, overall calorie intake, activity level, and individual metabolism. Track your weight weekly using our free progress tracker.
Can I combine fasting with prescription weight loss medication in the UK?
Yes. Many people use fasting alongside GLP-1 medications like Mounjaro or Wegovy for better results. However, you must discuss this with your prescriber, as medication timing and eating windows need coordination. Never start medication without clinical assessment from a licensed healthcare professional.
What should I eat during fasting eating windows?
Balanced, nutritious meals are essential. Focus on protein, vegetables, wholegrains, and healthy fats. Avoid high-calorie, sugary foods during eating windows — they can undo fasting benefits. A reduced-calorie diet combined with fasting typically supports sustainable weight loss better than fasting alone.
Who is eligible for prescription weight loss medication in the UK if fasting isn't working?
Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions, may be eligible for medications like Mounjaro or Wegovy. Eligibility is assessed individually by your GP or a licensed prescriber following clinical review. Access is available through NHS specialist services or private prescribing.
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.