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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16-8 Fasting Weight Loss Results in 1 Week
Intermittent fasting using the 16:8 method (fasting for 16 hours, eating within an 8-hour window) has become a popular weight loss approach in the UK. However, realistic weight loss in just one week is typically modest—usually between 0.5 to 2 kg—depending on your starting weight, diet quality, and overall lifestyle. Most of this initial loss is often water weight rather than fat loss. This guide explains what to expect from 16-8 fasting, how it works, and whether it's a sustainable approach to weight management on the NHS guidance framework.
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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 16-8 Fasting Works for Weight Loss
The 16:8 method involves fasting for 16 hours and consuming all calories within an 8-hour eating window—typically between noon and 8 PM or 10 AM to 6 PM. During the fasting period, you consume only water, tea, or coffee (no milk or sugar). This approach may help with weight management in the UK by naturally reducing daily calorie intake, as you have fewer hours to eat.
Research suggests that intermittent fasting can support modest weight loss when combined with healthy food choices during your eating window. However, the weight loss mechanism is primarily calorie reduction—not something unique to fasting itself. One week of 16:8 fasting typically produces water weight loss initially, as your body depletes glycogen stores. Sustainable fat loss requires a consistent calorie deficit over weeks and months, aligned with NHS weight loss recommendations of 0.5 to 1 kg per week.
- Initial 1-week loss often includes 1–2 kg of water weight
- Actual fat loss is usually much smaller in week one
- Effectiveness depends on what you eat during your 8-hour window
- Combined with balanced nutrition, 16:8 may support gradual weight loss
Clinically BackedWeight Loss
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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.
Is 16-8 Fasting Safe and Sustainable?
The 16:8 method is generally safe for healthy adults but isn't suitable for everyone. People with a history of eating disorders, those taking certain medications, pregnant women, and anyone with medical conditions should consult their GP before starting intermittent fasting. The NHS emphasises that sustainable weight loss uk comes from long-term lifestyle changes, not rapid short-term methods.
For prescription weight loss uk support, if you have a BMI of 30 or above (or 27+ with weight-related health conditions), your GP can refer you to specialist weight management services. Some people combine dietary approaches like 16:8 fasting with other evidence-based strategies. Track your progress using our free progress tracker to monitor what works for your body, and use our BMI calculator to understand your current health context alongside any fasting approach you choose.
- 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.
Will I lose 5 kg in one week with 16-8 fasting?
No. Realistic one-week weight loss is typically 0.5 to 2 kg, mostly water weight. Losing 5 kg in one week would require an unsustainable calorie deficit and is not recommended by the NHS. Sustainable fat loss requires weeks and months of consistent, moderate calorie reduction.
Is the weight loss from 16-8 fasting permanent?
Not automatically. The initial weight loss is largely water. Long-term results depend on whether you maintain healthy eating habits during your 8-hour eating window and sustain the fasting pattern. Many people regain weight if they return to previous eating patterns, so lifestyle sustainability matters most.
How does 16-8 fasting compare to prescription weight loss medication?
16-8 fasting is a dietary approach requiring discipline; prescription medications like GLP-1 injections (Mounjaro or Wegovy) work by suppressing appetite and may support larger weight loss alongside lifestyle changes. Our GLP-1 weight loss calculator can help you explore potential results with medical support if eligible. Both require sustained healthy choices.
Can I do 16-8 fasting if I'm on medication?
Some medications require food with dosing, so fasting may interfere. Always consult your GP before starting 16:8 fasting, especially if taking diabetes medication, blood pressure medication, or other regular treatments. Your GP can advise whether it's safe for your individual situation.
What should I eat during my 8-hour eating window?
Focus on balanced, nutrient-dense foods: lean protein, vegetables, whole grains, and healthy fats. Avoid relying on ultra-processed foods during your window—calories still matter for weight loss. Quality of food directly affects whether 16:8 fasting supports sustainable weight management and overall health.
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.