Intermittent fasting for obesity treatment has emerged as a popular dietary approach that focuses on when you eat rather than what you eat. This pattern alternates between periods of eating and voluntary fasting, potentially helping individuals achieve weight loss through reduced calorie intake and metabolic changes. Whilst evidence suggests intermittent fasting can produce clinically meaningful weight loss comparable to traditional calorie restriction, it is not suitable for everyone and requires careful consideration of individual health circumstances. This article examines the evidence, methods, safety considerations, and how intermittent fasting can be integrated within NHS weight management support.

What Is Intermittent Fasting and How Does It Work for Weight Loss?

Intermittent fasting (IF) is a dietary pattern that alternates between periods of eating and voluntary fasting on a regular schedule. Unlike traditional calorie-restricted diets that focus on what you eat, intermittent fasting primarily addresses when you eat. This approach has gained considerable attention as a potential strategy for obesity treatment, though it should be distinguished from therapeutic fasting protocols used in clinical settings.

The primary mechanism by which intermittent fasting facilitates weight loss is through reduced overall energy (calorie) intake. By limiting the time available for eating, many people naturally consume fewer calories without the need for meticulous calorie counting. This energy deficit is the principal driver of weight reduction with intermittent fasting, as with other dietary approaches.

During extended fasting periods, the body progressively depletes its glycogen stores and increases fat oxidation as an energy source. The timing and extent of these metabolic shifts vary considerably between individuals and depend on factors such as activity level, baseline diet, and metabolic health. Whilst some proponents suggest intermittent fasting triggers ketosis (a metabolic state where the body uses fat-derived ketones for fuel), substantial ketosis typically requires longer fasting periods or very low carbohydrate intake and does not occur uniformly across all intermittent fasting protocols.

Beyond energy restriction, intermittent fasting may influence metabolic pathways, including improvements in insulin sensitivity in some individuals. Research into other proposed mechanisms—such as cellular repair processes (autophagy) and reductions in systemic inflammation—remains an area of active investigation. However, human evidence for these effects at the fasting durations typical of common intermittent fasting patterns is limited and inconsistent. Any metabolic benefits beyond those attributable to weight loss itself require further clarification through long-term studies.

It is important to note that whilst intermittent fasting shows promise, it is not a universal solution for obesity. Individual responses vary considerably, and the approach requires careful consideration of personal circumstances, existing health conditions, and lifestyle factors. The effectiveness of any weight management strategy ultimately depends on sustainability and adherence over time.

Evidence for Intermittent Fasting in Obesity Management

The evidence base for intermittent fasting in obesity treatment has expanded considerably in recent years, though it remains an evolving area of research. Systematic reviews and meta-analyses have demonstrated that intermittent fasting can produce clinically meaningful weight loss, with studies typically reporting reductions of 3–8% of initial body weight over periods of 8–24 weeks. These results appear comparable to traditional continuous calorie restriction approaches, suggesting intermittent fasting represents a viable alternative rather than a superior method for weight loss.

Studies examining metabolic outcomes have shown mixed findings. Some trials have demonstrated improvements in insulin resistance, with reductions in fasting glucose and HbA1c levels in individuals with prediabetes or type 2 diabetes, though results are not consistent across all studies. Similarly, changes in lipid profiles—including reductions in triglycerides and LDL cholesterol—have been observed in some research, but findings vary considerably depending on the fasting protocol, baseline characteristics, and study design. It remains unclear whether any metabolic improvements are independent of weight loss or simply reflect the benefits of energy restriction and weight reduction.

The current evidence has important limitations. Many studies have been relatively short-term, with few extending beyond 12 months, making it difficult to assess long-term sustainability and weight maintenance. Sample sizes have often been modest, and there is considerable heterogeneity in fasting protocols (time-restricted eating, alternate-day fasting, 5:2 diet), making direct comparisons challenging. Furthermore, dropout rates in intermittent fasting trials can be substantial, highlighting adherence difficulties for some individuals.

NICE guidance on obesity management (CG189) emphasises individualised approaches to dietary intervention within a multicomponent weight management programme. Whilst intermittent fasting is not specifically endorsed in current NICE guidelines, the evidence suggests it may be considered as one option for appropriately selected patients. Healthcare professionals should discuss the available evidence transparently, acknowledging both potential benefits and uncertainties, to support informed decision-making. Further information on evidence-based weight management can be found in NICE CG189 (Obesity: identification, assessment and management) and the British Dietetic Association's Food Fact Sheet on Intermittent Fasting.

Different Intermittent Fasting Methods for Obesity Treatment

Several distinct intermittent fasting protocols have been studied for obesity management, each with different fasting and eating windows. Understanding these variations helps patients and clinicians select the most appropriate and sustainable approach for individual circumstances.

Time-restricted eating (TRE) is perhaps the most accessible form of intermittent fasting. This method involves limiting food intake to a specific window each day, typically 6–10 hours, with fasting for the remaining 14–18 hours. The 16:8 protocol (16 hours fasting, 8 hours eating) is particularly popular. For example, an individual might eat only between 12:00 and 20:00 daily. Emerging evidence suggests that earlier time-restricted eating—finishing meals earlier in the day rather than late evening—may offer additional metabolic benefits, though research in this area is still evolving. This approach can be easier to maintain long-term compared to more restrictive protocols.

The 5:2 diet involves eating normally for five days per week whilst restricting calorie intake to approximately 500–600 calories on two non-consecutive days. This intermittent energy restriction allows for greater flexibility, as individuals can choose which days to restrict based on their schedule. The non-fasting days do not require calorie counting, though sensible portion control and balanced nutrition remain important. The NHS and British Dietetic Association provide resources on implementing the 5:2 approach safely.

Alternate-day fasting (ADF) represents a more intensive approach, alternating between fasting days and unrestricted eating days. Modified alternate-day fasting, which allows approximately 25% of usual energy intake (around 500 calories) on fasting days, is more practicable and better tolerated than complete fasting. Whilst some research suggests modified ADF may produce initial weight loss, adherence can be challenging, and it may not be suitable for individuals with demanding work schedules, those prone to disordered eating patterns, or people with certain medical conditions.

Extended fasting protocols (24–48 hours or longer) are occasionally discussed but are generally not recommended for routine obesity management outside specialist clinical supervision. These approaches carry greater risks, including dehydration, electrolyte disturbances, and—in vulnerable individuals—refeeding syndrome. Extended fasting offers no clear advantage over less restrictive methods for most individuals seeking sustainable weight loss and should not be undertaken without medical oversight.

Safety Considerations and Who Should Avoid Intermittent Fasting

Whilst intermittent fasting appears generally safe for healthy adults, several important safety considerations must be addressed before commencing this dietary approach. Certain populations should avoid intermittent fasting entirely or pursue it only under close medical supervision.

Individuals who should not undertake intermittent fasting include:

• Children and adolescents under 18 years, whose nutritional needs for growth and development require regular, balanced meals

• Pregnant or breastfeeding women, who have increased nutritional requirements

• Individuals with a history of eating disorders, as fasting patterns may trigger or exacerbate disordered eating behaviours

• People with type 1 diabetes or those with type 2 diabetes taking insulin or sulphonylureas, due to significant hypoglycaemia risk

• Individuals who are underweight (BMI <18.5 kg/m²) or have a history of malnutrition

Those requiring medical consultation before starting intermittent fasting include:

• Patients with type 2 diabetes taking SGLT2 inhibitors (such as empagliflozin, dapagliflozin, or canagliflozin), as fasting and reduced carbohydrate intake can increase the risk of euglycaemic diabetic ketoacidosis—a serious condition. Specialist advice is essential before commencing intermittent fasting in this group

• Patients taking other medications that require food intake, have specific timing requirements, or affect blood glucose levels

• Individuals with chronic kidney disease, cardiovascular disease, or other significant comorbidities

• People with a history of gallstones, as rapid weight loss may increase risk

• Older adults, particularly those at risk of sarcopenia or frailty

Common adverse effects reported during intermittent fasting include hunger, irritability, difficulty concentrating, fatigue, headaches, constipation, and occasionally reflux or sleep disturbance, particularly during the initial adaptation period. These symptoms often settle after a few weeks as metabolic adaptation occurs. Maintaining adequate hydration, ensuring sufficient fibre intake during eating windows, and balancing electrolytes can help minimise these effects. However, persistent or severe symptoms warrant medical review.

Patients should contact their GP if they experience:

• Severe or persistent dizziness, particularly on standing

• Symptoms of hypoglycaemia (tremor, sweating, confusion, palpitations)

• Significant fatigue affecting daily functioning

• Excessive or rapid weight loss (more than 1 kg per week for several consecutive weeks)

• Development of obsessive thoughts about food or eating

• Symptoms that may suggest ketoacidosis in people with diabetes (nausea, vomiting, abdominal pain, unusual tiredness, fruity breath odour)

Healthcare professionals should conduct a thorough medical history and medication review before supporting patients in commencing intermittent fasting, ensuring appropriate monitoring arrangements are in place. Guidance on managing diabetes and fasting can be found in NICE NG28 (Type 2 diabetes in adults: management). If you experience a suspected side effect from any medicine, you can report it via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Combining Intermittent Fasting with NHS Weight Management Support

Intermittent fasting should ideally be considered as one component within a comprehensive, multicomponent weight management programme rather than as a standalone intervention. The NHS offers various tiers of weight management support that can complement intermittent fasting approaches for individuals with obesity.

NHS Tier 1 and 2 services provide universal and targeted weight management support, including resources such as the NHS Better Health Weight Loss Plan, which offers evidence-based guidance on healthy eating, physical activity, and behaviour change. Individuals considering intermittent fasting can benefit from these resources to ensure nutritional adequacy during eating windows and to develop sustainable lifestyle habits. Local authority public health teams often commission community weight management programmes that provide group support, which can enhance adherence and motivation.

For individuals with more complex needs or higher degrees of obesity who have not achieved adequate weight loss through lifestyle interventions alone, NHS Tier 3 specialist weight management services may be appropriate. Access criteria vary by local integrated care system (ICS), but typically include adults with a BMI ≥40 kg/m², or ≥35 kg/m² with significant obesity-related comorbidities such as type 2 diabetes, hypertension, or obstructive sleep apnoea. For some minority ethnic groups (including people of South Asian, Chinese, African-Caribbean, or Black African family background), thresholds may be 2.5 kg/m² lower. Patients and clinicians should check local referral criteria with their ICS. These specialist services typically include dietitians, psychologists, physiotherapists, and physicians who can provide personalised guidance on integrating intermittent fasting safely within an individualised treatment plan. Specialist dietetic input is particularly valuable for ensuring nutritional adequacy and addressing any micronutrient deficiencies that may arise.

Physical activity remains a crucial component of obesity management and should be maintained during intermittent fasting. The UK Chief Medical Officers' guidelines recommend that adults undertake at least 150 minutes of moderate-intensity aerobic activity (or 75 minutes of vigorous-intensity activity) each week, along with muscle-strengthening activities on at least two days per week. Older adults should also incorporate activities to improve balance. Patients should be advised that exercise can be performed during fasting periods, though some individuals may prefer to schedule more intensive sessions during eating windows. Resistance training is particularly important to preserve lean muscle mass during weight loss.

Behavioural support is essential for long-term success. Cognitive behavioural therapy techniques can help individuals identify triggers for overeating, develop coping strategies, and address emotional eating patterns. NHS Talking Therapies services may be appropriate for individuals whose eating behaviours are significantly influenced by psychological factors such as anxiety or depression.

For individuals with severe obesity (typically BMI ≥40 kg/m², or ≥35 kg/m² with significant comorbidities) who have not responded adequately to comprehensive lifestyle interventions, pharmacotherapy or bariatric surgery may be considered according to NICE guidance. Weight management medicines such as semaglutide 2.4 mg (Wegovy) or orlistat are available within specialist services for eligible patients, as outlined in NICE technology appraisals and clinical guidelines. Intermittent fasting is not a substitute for these evidence-based interventions when clinically indicated, though it may be incorporated as part of pre-operative preparation or post-operative maintenance strategies under specialist supervision.

Regular monitoring and follow-up are essential components of any weight management programme. Patients should be encouraged to track their weight, eating patterns, and any adverse effects, with periodic review by their healthcare team to assess progress, address challenges, and modify the approach as needed. This ongoing support significantly enhances the likelihood of sustained weight loss and long-term weight maintenance. Further information can be found in NICE CG189 (Obesity: identification, assessment and management), NICE PH53 (Weight management: lifestyle services for overweight or obese adults), and on the NHS Better Health website.

Frequently Asked Questions