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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5:2 Diet for Weight Loss: How It Works in the UK
The 5:2 diet is an intermittent fasting approach where you eat normally five days a week and restrict calories to around 500–600 kcal on two non-consecutive days. Evidence suggests this method can support weight loss and may help with weight management in the UK. It's one of several dietary approaches available, alongside prescription weight loss medication and other lifestyle changes. This guide explains how the 5:2 diet works, what the evidence shows, and whether it might be right for you.
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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 the 5:2 Diet Works
The 5:2 diet works by creating a calorie deficit across the week without requiring daily restriction. On your five "normal" eating days, you eat as you usually would—though most nutritionists recommend keeping portions balanced and avoiding excessive calories. On your two "fasting" days (typically Monday and Thursday, or any two non-consecutive days), you consume around 500 kcal for women or 600 kcal for men.
Why this approach may work:
- Creates an overall weekly calorie deficit (roughly equivalent to eating 300–400 kcal less per day on average)
- Allows flexibility—no strict daily restriction
- May help regulate blood sugar and improve insulin sensitivity
- Some evidence suggests it supports metabolic health
Research published in nutrition journals shows that 5:2 dieters typically lose 3–8% of body weight over 8–12 weeks, with results varying based on overall lifestyle and adherence. Use our free BMI calculator above to check where you currently sit, then track progress with our weight loss progress tracker as you implement dietary changes.
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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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5:2 Diet vs Other Weight Loss Approaches
The 5:2 diet is one option among several evidence-based weight loss strategies. A balanced, reduced-calorie diet with consistent portion control often produces similar results. Some people find intermittent fasting easier to follow because they don't count calories every day, whilst others prefer steady daily restriction. For weight management in the NHS context, your GP may recommend tier 1 or tier 2 lifestyle interventions first—including structured diet and exercise programmes.
If you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea), prescription weight loss options may also be discussed. These medications work alongside lifestyle changes, not instead of them. Talk to your GP about which approach suits your health circumstances and goals.
- 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.
How much weight can I lose on the 5:2 diet?
Evidence suggests people lose 3–8% of body weight over 8–12 weeks, though individual results vary significantly. Weight loss depends on your starting weight, overall calorie intake on non-fasting days, physical activity, and adherence. The NHS recommends losing 0.5–1 kg per week for sustainable results.
Is the 5:2 diet safe for everyone?
The 5:2 diet is generally safe for most healthy adults, but it is NOT suitable for pregnant or breastfeeding women, people with a history of eating disorders, or those with certain medical conditions. Always discuss any new diet with your GP, especially if you take diabetes medication or have other health concerns.
Can I combine the 5:2 diet with prescription weight loss medication?
Yes, prescription weight loss medications like Mounjaro or Wegovy are designed to work alongside lifestyle changes, including dietary modifications. However, any combination should be managed by your prescriber or GP to ensure it's appropriate for your individual health and to avoid nutrient deficiency or excessive restriction.
What should I eat on 5:2 fasting days?
On 500–600 kcal fasting days, focus on protein-rich foods (chicken, fish, eggs, yoghurt), non-starchy vegetables, and whole grains. Protein helps you feel fuller for longer. Avoid sugary foods and excessive fat, which are calorie-dense. Stay hydrated with water, tea, and coffee (without added milk or sugar).
How does the 5:2 diet compare to daily calorie restriction?
Both methods create a calorie deficit and can support weight loss. The 5:2 diet may feel less restrictive for some people because five days are unrestricted, whilst daily calorie cutting requires consistent portion control. Long-term weight loss success depends more on adherence than the specific method chosen.
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.