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
Weight Loss With Intermittent Fasting — How It Works
Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and fasting. Rather than focusing on what you eat, IF concentrates on when you eat. Evidence suggests intermittent fasting can support weight management and may help create a calorie deficit, making it a popular approach in the UK for sustainable weight loss. This guide explains how IF works, the main methods, and how it compares to other weight loss approaches including prescription medication.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Common Intermittent Fasting Methods and How They Work
Several evidence-based intermittent fasting approaches are used for weight management in the UK:
- 16:8 method: A 16-hour fast followed by an 8-hour eating window. Popular for beginners — many people naturally skip breakfast and eat between midday and 8pm.
- 5:2 diet: Eating normally five days a week, then restricting calories to 500–600 kcal on two non-consecutive days. Research shows this can be as effective as continuous calorie restriction.
- Eat-Stop-Eat: A 24-hour complete fast once or twice weekly. More challenging and requires careful planning to maintain nutrition.
- Alternate-day fasting: Alternating between fasting days and normal eating days. Evidence-based but can be difficult to sustain long-term.
The key mechanism: IF may help reduce total calorie intake and support metabolic flexibility — the body's ability to switch between burning carbohydrates and fat for energy. Use our free BMI calculator to track your baseline weight, then monitor progress over time with our progress tracker.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Does Intermittent Fasting Work for Weight Loss?
Clinical evidence suggests intermittent fasting can support weight loss when combined with a reduced-calorie diet. The NHS recognises IF as one approach to weight management alongside other methods. Results typically show a loss of 0.5–1 kg per week, depending on the total calorie deficit created and individual factors.
Important context: Individual results vary significantly. Some people find IF sustainable and report steady progress; others find the fasting periods difficult or experience hunger that makes adherence challenging. IF does not clinically studied weight loss — success depends on whether the overall calorie intake remains in deficit. Our weight converter can help you track progress across kg, stone, and lbs.
For those seeking additional support, prescription weight loss medication may be combined with IF. GLP-1 medications like Wegovy (semaglutide) or Mounjaro (tirzepatide) are prescription-only treatments that may help reduce appetite and food cravings, potentially making it easier to maintain a calorie deficit during eating windows. Our GLP-1 weight loss calculator can help estimate potential outcomes with medication. Any weight management medication requires a clinical assessment from your GP or a licensed prescriber to confirm suitability.
- 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?
Intermittent fasting is considered safe for most healthy adults. However, it is not recommended for people with a history of eating disorders, pregnant or breastfeeding women, children, or those with certain medical conditions. Always consult your GP before starting IF, especially if you take regular medication or have diabetes.
How much weight can I lose with intermittent fasting?
Weight loss depends on your total calorie intake during eating windows. The NHS recommends losing 0.5–1 kg per week for sustainable results. If IF helps you create this calorie deficit, you can expect similar results to traditional calorie restriction. Individual results vary based on diet quality, activity level, and metabolism.
Can I combine intermittent fasting with weight loss medication?
Yes. Prescription weight loss medications like Mounjaro and Wegovy can be combined with intermittent fasting. These medications may reduce appetite during eating windows, potentially making IF easier to follow. Always inform your prescriber about any fasting plan you're following to ensure the approach is safe for you.
Which intermittent fasting method is most effective?
Research suggests 16:8 and 5:2 methods are roughly equally effective for weight loss when total calorie intake is similar. The "best" method is the one you can sustain long-term. 16:8 suits people who naturally skip breakfast; 5:2 works for those who prefer flexibility across most days.
How is intermittent fasting different from simply eating fewer calories?
Intermittent fasting changes *when* you eat rather than primarily focusing on *what* you eat. The weight loss mechanism is still calorie deficit, but IF may offer advantages: easier adherence for some people, potential metabolic benefits, and simplified meal planning. Evidence suggests IF and traditional calorie restriction produce similar weight loss over time.
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.