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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Intermittent Fasting and Weight Loss: What the Evidence Shows
Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and fasting, and some people use it as a weight management strategy in the UK. Research shows that intermittent fasting can support weight loss when combined with a reduced-calorie diet, though it works similarly to standard calorie restriction overall. This guide explains how intermittent fasting works, what the NHS says about it, and how it compares to other weight loss approaches.
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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 Intermittent Fasting Works for Weight Loss
Intermittent fasting works primarily through calorie reduction. By limiting your eating window, you typically consume fewer calories overall, which creates the deficit needed for weight loss. Common IF patterns in the UK include:
- 16:8 (Time-Restricted Eating) — fasting for 16 hours, eating within an 8-hour window (e.g., midday to 8pm)
- 5:2 Diet — eating normally five days per week, restricting calories to 500–600 on two non-consecutive days
- Eat-Stop-Eat — complete 24-hour fasts once or twice weekly
- Alternate-Day Fasting — alternating between fasting days and regular eating days
During fasting periods, your body uses stored energy (glycogen and fat), though this metabolic shift is modest. The key to weight loss success is maintaining a calorie deficit over time — IF simply helps some people achieve that deficit more easily by reducing eating occasions.
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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.
Intermittent Fasting vs Other Weight Management Approaches
Clinical research shows that intermittent fasting produces similar weight loss to regular calorie restriction when total calorie intake is equal. A 2020 meta-analysis found no significant advantage of IF over continuous calorie reduction. The NHS indicates that sustainable weight loss of 0.5–1 kg per week is healthiest, and this applies whether you use IF or traditional dieting.
For some people, IF works well because it reduces the number of eating decisions per day. For others, hunger during fasting windows makes it difficult to sustain. Success depends on individual preference and lifestyle. If you're considering IF alongside prescription weight loss medication like GLP-1 injections, discuss timing with your prescriber — fasting may affect medication absorption or appetite signalling.
- 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 scientifically proven for weight loss?
Yes, intermittent fasting can support weight loss, but clinical trials show it works mainly through calorie reduction, not metabolic clinically supported. When total calories are matched, IF produces similar results to regular dieting. Success depends on whether the eating pattern helps you maintain a calorie deficit consistently.
How quickly will I lose weight with intermittent fasting?
Weight loss with IF follows the standard rate: approximately 0.5–1 kg per week with a 500 kcal daily deficit, according to NHS guidance. Individual results vary based on your calorie deficit, metabolism, and consistency. Use our free BMI calculator and progress tracker to monitor your results over time.
Can I combine intermittent fasting with prescription weight loss medication?
You can, but discuss timing with your GP or prescriber first. Medications like Mounjaro or Wegovy work with your appetite hormones, and fasting periods may interact with how these work. Your prescriber can advise on safe combinations and optimal timing for your specific situation.
Who should avoid intermittent fasting?
Intermittent fasting is not suitable for pregnant or breastfeeding women, people with a history of eating disorders, those taking certain diabetes medications, or anyone with blood sugar regulation issues. Always consult your GP before starting IF, especially if you have existing health conditions.
Does intermittent fasting work better than other diets?
No — research shows IF produces similar weight loss to standard calorie-restricted diets. The "best" diet is one you can stick to consistently. Some people find IF easier because it reduces eating occasions; others prefer regular meals. What matters most is maintaining a calorie deficit and making sustainable lifestyle changes.
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.