Is the 5:2 diet good for you?
The 5:2 diet involves eating a lower-carbohydrate Mediterranean-style diet for five days a week (your normal days) and consuming only 800 calories for two non-consecutive days of your choice (your fasting days).
This calorie limit has increased from the original 500 calories recommended in earlier versions of the diet.
For most UK adults requiring 2,000-3,000 calories daily, this represents a significant reduction to about one-third of normal intake on fasting days.
On low-calorie days, the 5:2 recommends:
- Choose busier weekdays when you’re not socialising to reduce food temptations
- Focus on protein, healthy fats, fibre, and minimal carbohydrates to maintain fullness
- Use calorie-tracking apps like MyFitnessPal to ensure you stay within limits
While popularised recently, the concept of intermittent caloric restriction has been studied since the 1980s, with particularly promising results for metabolic health markers under controlled research settings.
The 5:2 diet aligns with many evidence-based nutrition principles by recommending a Mediterranean-style eating pattern that prioritises whole foods, essential nutrients, and doesn’t unnecessarily restrict healthy fats. This nutrition approach is similar to the nutrition guidelines at Second Nature.
The 5:2 diet encourages the consumption of:
- Non-starchy vegetables (leafy greens, broccoli)
- Oily fish (salmon, mackerel) – important sources of omega-3 fatty acids
- Quality protein sources (chicken, eggs, tofu)
- Full-fat dairy products (British research shows these can support weight management)
- Olive oil (4-5 tablespoons daily on normal days)
- Nuts and seeds (a small handful, about 30g daily)
- Legumes (beans, lentils, chickpeas)
- Moderate amounts of high-fibre carbohydrates and whole grains
- Homemade soups (particularly helpful for fasting days)
The 5:2 diet discourages:
- Added sugars (including those hidden in processed foods)
- Refined carbohydrates (white bread, breakfast cereals, white rice)
- Ultra-processed foods (which make up 56% of the average UK diet)
- Large portions of starchy carbohydrates
- Processed vegetable oils and margarines
UK-based research has demonstrated this nutritional approach can deliver meaningful health benefits. Various forms of intermittent fasting, including the 5:2 method, alternate-day fasting, and time-restricted eating, have been shown to support weight loss, improve blood glucose regulation, and reduce risk factors for cardiovascular disease – all significant health concerns in the UK.
What are the downsides of the 5:2 diet?
As with many dietary approaches that impose strict rules and caloric restrictions, the 5:2 diet can trigger psychological challenges that may undermine long-term success.
Many UK dieters report falling into an ‘all-or-nothing’ mindset when they deviate from the plan. This often leads to the ‘what-the-hell-effect’ – a psychological phenomenon where a minor deviation triggers abandonment of the entire approach.
For example, if you consume 1,000 calories on a fasting day instead of 800, you might feel you’ve ‘failed’ completely.
Conversely, some people develop a counterproductive mindset where they overindulge on normal days, thinking their fasting days will ‘cancel out’ these excesses, creating a cycle that undermines overall progress.
Reducing to 800 calories for two days weekly presents significant practical challenges for many. The typical UK adult requires 2,000-3,500 calories daily, depending on activity levels, gender, and individual metabolism.
Such dramatic restriction, even temporarily, can cause:
- Difficulty concentrating at work
- Increased irritability and mood changes
- Headaches and fatigue
- Strong food cravings
- Disrupted sleep patterns
Women may need to apply more caution
Women, particularly those of reproductive age, appear more physiologically sensitive to severe caloric restriction. Research shows that dramatic calorie reduction can disrupt normal hormonal function, potentially leading to:
- Menstrual irregularities or amenorrhea
- Reduced fertility
- Hormonal imbalances
- Increased stress hormone production
These risks are particularly relevant for women who are already physically active, have a history of hormonal issues, or are in perimenopause.
Second Nature offers an evidence-based alternative if you’re looking for a sustainable approach to weight loss and health without severe restrictions.
Rather than imposing rigid rules, Second Nature provides flexible nutrition guidelines and personalised support from registered dietitians and nutritionists.
With over 280,000 successful members across the UK, Second Nature’s approach focuses on sustainable habit changes rather than temporary calorie restriction.
Over half of the NHS Health and Care Boards now use Second Nature services.
Try our NHS-trusted weight loss calculator to see what results you could achieve through gradual, sustainable changes.
Let’s examine the scientific evidence behind the 5:2 diet’s benefits and limitations to help you make an informed decision.
1) The 5:2 diet can support modest weight loss
Scientific interest in intermittent calorie restriction dates back to the 1980s, with early UK research showing promising effects on weight management and metabolic health markers.
A rigorous randomised controlled trial conducted with 300 UK participants compared three approaches:
- 5:2 diet with group support
- Self-directed 5:2 diet
- Standard brief advice on healthy eating (control group)
After six weeks, the results showed:
- 5:2 with group support: 2.3kg average weight loss
- Self-directed 5:2: 1.5kg average weight loss
- Control group: 1kg average weight loss
This suggests the supported 5:2 approach was initially more than twice as effective as standard advice. However, by the 12-month mark, these differences had narrowed considerably:
- 5:2 with group support: 2.6kg average weight loss
- Self-directed 5:2: 1.9kg average weight loss
- Control group: 1.8kg average weight loss
This diminishing advantage was largely due to adherence challenges. By the one-year mark, only 45-56% of participants were still following the 5:2 approach, comparable to other dietary interventions.
One important finding was that more participants in the 5:2 group achieved clinically significant weight loss (defined as ≥5% of starting weight): 28% in the supported 5:2 group compared to 15% in the control group.
A comprehensive meta-analysis of 5:2 diet studies found the approach produces modest but variable results, ranging from 1.7% to nearly 8% of body weight. This wide range suggests individual factors significantly influence outcomes.
Key points:
- Research on the 5:2 diet shows it typically supports modest weight loss of 2-3kg over 12 months
- The 5:2 approach is approximately 9% more effective than standard dietary advice, but results vary considerably between individuals
- Professional support significantly improves outcomes, with supported 5:2 dieters nearly twice as likely to achieve clinically meaningful weight loss
- By 12 months, fewer than half of participants typically maintain the 5:2 pattern, highlighting sustainability challenges
- Individual factors such as baseline metabolism, activity levels, and psychological approach to fasting days heavily influence results
2) The 5:2 diet may lower your risk of chronic disease
Beyond weight management, the 5:2 diet shows particularly promising effects on metabolic health markers that affect chronic disease risk – an important consideration in the UK, where conditions like type 2 diabetes affect over 4.5 million people.
A well-designed randomised controlled trial with 74 UK patients living with non-alcoholic fatty liver disease (NAFLD) compared three dietary approaches:
- 5:2 intermittent calorie restriction
- Low-carbohydrate diet
- Low-fat diet
After 12 weeks, liver fat reduction (a crucial marker for metabolic health) was:
- 5:2 diet: 50.9% reduction
- Low-carbohydrate diet: 53.1% reduction
- Low-fat diet: 16.8% reduction
Both the 5:2 and low-carb approaches significantly outperformed the low-fat diet in reducing liver fat, weight, insulin resistance, and blood glucose levels. This is particularly relevant as NAFLD affects approximately 25% of UK adults and is a significant risk factor for developing type 2 diabetes.
A comprehensive review of five clinical trials using the 5:2 approach found consistent improvements in:
- Fasting blood glucose (average reduction: 5-10%)
- Insulin sensitivity (average improvement: 10-20%)
- Blood triglycerides (average reduction: 10-15%)
- Blood pressure (average reduction: 3-5%)
These improvements occurred even in cases where weight loss was modest, suggesting the fasting pattern itself may offer metabolic benefits beyond calorie reduction.
For context, even small improvements in these markers can significantly reduce chronic disease risk. A 5% reduction in fasting blood glucose can lower diabetes progression risk by up to 30%, according to UK Diabetes Prevention Programme data.
However, most studies on the 5:2 diet have followed participants for 6 months or less, limiting our understanding of long-term effects.
Ongoing research at several UK universities is investigating whether these benefits persist beyond 12 months and how they compare to consistent daily calorie reduction.
Key points:
- The 5:2 diet shows impressive results for liver fat reduction (approximately 50%), comparable to low-carb diets and significantly better than low-fat approaches
- Multiple metabolic health markers improve with the 5:2 pattern, including blood glucose, insulin sensitivity, and blood lipids
- These benefits appear even when weight loss is modest, suggesting the fasting pattern itself may have independent metabolic effects
- Most studies are relatively short-term (3-6 months), and longer research is needed to confirm lasting benefits
- Individual health conditions and medication usage may affect outcomes, making medical supervision advisable for those with existing health conditions
3) Why 5:2 diets may not be sustainable for many people
While the 5:2 diet shows promising short-term results, UK research indicates significant challenges with long-term adherence – the crucial factor for lasting health benefits.
The same randomised controlled trial of 300 UK adults mentioned earlier tracked adherence over 12 months:
- After 6 weeks: 86% of participants maintained the 5:2 pattern with support
- After 6 months: This dropped to 69%
- After 12 months: Only 45% were still following the approach
Similar patterns emerged in the self-directed group (87%, 77%, and 56%, respectively). This decline in adherence explains why the initial weight loss advantages narrowed over time.
A comparative study comparing intermittent fasting, Mediterranean, or paleo approaches found only 54% maintained fasting patterns after 12 months, no better than other dietary approaches.
Participants who discontinued the 5:2 diet commonly reported:
- Difficulty scheduling fasting days around work and social commitments
- Hunger and irritability affecting work performance
- Gradual reduction in motivation as initial results plateaued
- Difficulty maintaining the pattern during stressful life periods
Potential health considerations beyond weight loss
Research on severe calorie restriction (though not specifically 5:2 diets) raises additional concerns about certain health markers when dramatically reducing calories:
A study in post-menopausal women found that while severe calorie restriction doubled weight loss compared to moderate restriction, it also significantly reduced hip bone mineral density – an important consideration for long-term fracture risk in the UK’s ageing population.
Pre-menopausal women appear particularly vulnerable to hormonal disruption from severe energy restriction. A clinical investigation of women consuming 800-1200 calories daily (similar to 5:2 fasting days) found significant reductions in reproductive hormones:
- 13.5% reduction in follicle-stimulating hormone (FSH)
- 38.5% reduction in luteinising hormone (LH)
These hormonal changes exceeded normal cycle variations and potentially impact fertility and menstrual regularity. While this study examined continuous rather than intermittent restriction, it suggests women should approach any severe calorie restriction with caution.
Key points:
- Long-term adherence is challenging, with fewer than half of participants maintaining the 5:2 pattern after one year
- Practical difficulties with scheduling, hunger management, and social disruption are common reasons for discontinuation
- Women should be particularly cautious, as severe calorie restriction may impact reproductive hormones and bone density
- Monitoring for menstrual irregularities and consulting healthcare providers about potential hormonal impacts is advisable
- The approach may be better suited as a short-term intervention (3-6 months) followed by transition to a more sustainable eating pattern
Take home message
The 5:2 diet reflects a common pattern in dietary approaches: promising early results that gradually diminish as adherence challenges emerge.
Research consistently shows that while the first 3-6 months typically show encouraging outcomes, maintaining these benefits beyond 12 months proves challenging for most people.
Sustainable weight loss requires focusing on the bigger picture, developing a balanced relationship with food rather than imposing rigid restrictions that trigger psychological resistance.
Psychologically, the 5:2 approach may inadvertently reinforce preoccupation with food. When you limit yourself to 800 calories on fasting days, you’re likely to think about food more, not less; a phenomenon psychologists call the ‘white bear effect.’
Telling yourself you can’t have something often makes it more mentally prominent, potentially intensifying cravings.
For many people, a consistent approach that focuses on food quality rather than severe restriction offers greater long-term success. This allows for developing sustainable habits that don’t require counting every calorie or navigating dramatic energy fluctuations.
If you’ve tried fasting or other restrictive diets without lasting success, a more holistic approach that removes rigid rules while emphasising nutritious whole foods might better serve your long-term health.
Second Nature’s programme focuses on sustainable habit changes supported by registered dietitians and nutritionists, helping you develop a healthier relationship with food while achieving your weight and health goals.
Second Nature’s approach to sustainable weight loss
Second Nature has worked with the NHS for over 7 years, providing weight-loss programmes across the UK with a focus on scientific evidence, patient safety, and long-term results.
Our programme offers:
- Personalised guidance from UK-registered nutritionists and dietitians
- Evidence-based meal plans that don’t require calorie counting
- Strategies for developing sustainable eating habits
- Support for navigating social situations without compromising progress
- Tools for addressing emotional eating and food relationships
Want to experience a more sustainable approach to healthy eating and weight loss? Try our NHS-trusted Second Nature meal plan and discover how evidence-based nutrition can support your health without severe restriction.
