Intermittent fasting with working out is an increasingly popular approach for those looking to improve body composition, boost metabolic health, and maintain an active lifestyle. By cycling between defined eating and fasting periods — such as the widely used 16:8 or 5:2 protocols — alongside a structured exercise routine, many people aim to maximise the benefits of both strategies. However, combining fasting with physical activity requires careful planning around nutrition, workout timing, and individual health needs. This guide covers the evidence, practical strategies, safety considerations, and who should seek professional advice before getting started.

How Intermittent Fasting Affects Exercise Performance

Fasting increases fat oxidation and raises noradrenaline and growth hormone, but depletes liver glycogen, which can impair high-intensity exercise performance and requires adequate protein intake to prevent muscle loss.

Intermittent fasting (IF) involves cycling between defined periods of eating and fasting, with popular protocols including the 16:8 method (16 hours fasting, 8 hours eating) and the 5:2 approach. When combined with regular exercise, fasting alters several physiological processes that can influence how the body performs and recovers during physical activity.

During a fasted state, liver glycogen — which helps maintain blood glucose between meals — is progressively depleted. Muscle glycogen is relatively preserved unless prior carbohydrate intake or training load was already low. As a result, the body shifts towards greater reliance on fat oxidation for energy. This metabolic shift may support body composition goals and could benefit lower-intensity endurance training over time, but it does not reliably translate into improved race or competition performance. Activities requiring explosive power or sustained high-intensity effort — such as sprinting or heavy resistance training — may be impaired when glycogen availability is low.

Hormonal changes also play a role. Fasting is associated with rises in noradrenaline and growth hormone, both of which support fat mobilisation.^[4] However, these hormonal changes do not by themselves prevent muscle loss; sufficient protein and overall energy intake remain the primary determinants of lean mass preservation. Elevated cortisol — a stress hormone that rises during prolonged fasting — can have catabolic effects on muscle tissue if fasting periods are excessively long or combined with very high training volumes.^[5]

Individual responses to fasting vary considerably. Factors such as fitness level, fasting duration, training intensity, and overall dietary quality all influence adaptation. Many people find that after an initial adjustment period, their tolerance of exercise in a fasted state improves, particularly for low-to-moderate intensity activities. However, this adaptation is not universal, and performance during high-intensity or competitive sessions may remain affected.

Best Times to Work Out During a Fasting Window

Training towards the end of the fasting window or within the eating window is optimal; high-intensity sessions should ideally be performed one to four hours after a balanced meal containing carbohydrate and protein.

Timing your workouts strategically within an intermittent fasting schedule can make a meaningful difference to both performance and recovery. There is no single universally optimal time, but understanding the physiological context of your fasting window can help guide your decision.

Working out towards the end of the fasting window is a popular approach. At this point, fat oxidation is elevated, which may support body composition goals. Training just before your eating window opens also means you can consume a post-exercise meal relatively soon afterwards, supporting muscle protein synthesis and glycogen replenishment — both important for recovery.

Training early in the morning in a fasted state (for example, after an overnight fast) is widely practised and generally well-tolerated for low-to-moderate intensity sessions such as:

• Brisk walking or jogging

• Yoga or Pilates

• Light resistance training or bodyweight circuits

For higher-intensity sessions — such as HIIT, heavy weightlifting, or competitive sport — it is generally advisable to train within your eating window, ideally one to four hours after a balanced meal containing carbohydrate and protein, adjusted for individual tolerance. This ensures adequate fuel availability and reduces the risk of dizziness, poor concentration, or injury associated with exercising in a significantly depleted state.

Evening training during a fasting window (for those following a morning eating window) may suit some individuals, though it can limit opportunities for post-exercise nutrition and may affect sleep quality in some people. Individual tolerance varies, and this approach is not inherently unsuitable — personal preference and consistency should guide your timing choices.

Important note for people with diabetes: If you take insulin or a sulfonylurea (such as gliclazide or glipizide), fasted exercise carries a significant risk of hypoglycaemia (low blood sugar).^[6][17] You should check your blood glucose before, during, and after exercise, always carry fast-acting carbohydrate (such as glucose tablets or a sugary drink), and seek an individualised plan from your diabetes care team before combining fasting with regular physical activity. Further guidance is available from Diabetes UK.

Nutrition and Hydration Guidance for Active Fasting

Prioritise protein (1.2–2.0 g/kg/day), complex carbohydrates, and healthy fats within your eating window, and consume a combined protein and carbohydrate meal within one to two hours of exercise for recovery.

Nutrition quality during your eating window becomes especially important when combining intermittent fasting with working out. Because the window for food intake is restricted, every meal should be nutritionally dense to support energy levels, muscle repair, and overall health.

Key nutritional priorities include:

• Protein: For most active adults, a daily protein intake of approximately 1.2–2.0 g per kg of body weight is appropriate to support muscle protein synthesis and minimise muscle breakdown. Intakes towards the higher end of this range may be beneficial during periods of energy restriction or heavy training to help preserve lean mass. Good sources include lean meats, fish, eggs, dairy, legumes, and tofu.

• Carbohydrates: Do not neglect carbohydrates, particularly around training sessions. Complex carbohydrates such as oats, brown rice, sweet potato, and wholegrain bread help replenish glycogen stores and sustain energy.

• Healthy fats: Include sources such as avocado, nuts, seeds, and oily fish to support hormonal function and reduce inflammation.

A well-structured post-workout meal consumed within one to two hours of exercise — ideally combining protein and carbohydrates — is particularly beneficial for recovery when training in a fasted or semi-fasted state.

An important safety note on longer sessions: For exercise lasting more than 60–90 minutes at moderate-to-high intensity, consuming carbohydrate during the session may be necessary for both safety and performance, even if this means breaking your fast.^[16] In this context, maintaining safe blood glucose levels and supporting performance should take priority over adhering to a fasting window.

Hydration is equally important and is often overlooked during fasting periods. Water, plain sparkling water, black coffee, and unsweetened herbal teas are generally considered acceptable during fasting windows. Modest caffeine intake from coffee or tea is unlikely to cause significant dehydration in habitual users.^[7] Electrolyte balance — particularly sodium, potassium, and magnesium — can be disrupted during prolonged fasting combined with sweating, so prioritise electrolyte-rich foods such as bananas, leafy greens, and nuts within your eating window. For long or particularly intense sessions, an electrolyte supplement may be appropriate; however, if carbohydrate is also needed for safety or performance, a carbohydrate-containing sports drink is preferable. Refer to NHS guidance on hydration for further information. Always consult a GP, registered dietitian, or sports nutrition professional if you are unsure about your individual nutritional needs.

If you suspect a side effect from any medicine or medical device, report it via the MHRA Yellow Card Scheme (available at yellowcard.mhra.gov.uk).

Potential Risks and Who Should Take Extra Care

People with diabetes, eating disorders, cardiovascular conditions, pregnancy, or frailty should consult a GP before combining fasting with exercise, as risks include hypoglycaemia, muscle loss, and relative energy deficiency in sport (RED-S).

Whilst intermittent fasting with working out is safe for many healthy adults, it is not appropriate for everyone. Certain groups should exercise caution or seek professional guidance before combining fasting with regular physical activity.

Those who should consult their GP or a registered healthcare professional before starting include:

• People with type 1 or type 2 diabetes, particularly those taking insulin or sulfonylureas, due to the significant risk of hypoglycaemia during fasting and exercise^[6][17]

• Individuals with a history of disordered eating or eating disorders

• Pregnant or breastfeeding women, who have increased nutritional requirements

• Those with cardiovascular conditions, kidney disease, or other chronic health conditions

• Children, adolescents, and older adults, who have specific nutritional needs

• People who are underweight (BMI below 18.5) or who have frailty

• Those with menstrual irregularities or a history of relative energy deficiency in sport (RED-S)

Common side effects when beginning intermittent fasting alongside exercise include fatigue, headaches, irritability, difficulty concentrating, and reduced exercise tolerance. These symptoms are often transient and may improve as the body adapts. However, persistent symptoms warrant medical review.

More serious risks include muscle loss if protein intake is insufficient, overtraining if exercise volume is too high relative to caloric intake, and disordered eating patterns that may develop in susceptible individuals. There is also a risk of relative energy deficiency in sport (RED-S), a condition recognised in the IOC consensus statement, in which chronically low energy availability impairs health and performance across multiple body systems.^[8][9]

Call 999 or go to your nearest A&E immediately if you experience: chest pain, severe shortness of breath, collapse or fainting, stroke symptoms (such as facial drooping, arm weakness, or speech difficulties), or a severe hypoglycaemic episode that does not resolve with treatment. These symptoms require urgent medical assessment and should never be attributed to fasting without proper clinical evaluation.

Seek prompt non-emergency medical advice if you experience heart palpitations, persistent dizziness, or significant unintentional weight loss.

What the Current Evidence Says About Fasting and Fitness

Evidence suggests time-restricted eating combined with resistance training can reduce body fat whilst preserving lean mass, but effect sizes are modest and benefits for endurance performance remain inconsistent.

The scientific evidence on intermittent fasting combined with exercise is growing, though it remains an evolving field. Many studies are limited by small sample sizes, short durations, and varied methodologies, so findings should be interpreted with appropriate caution.

A number of studies suggest that time-restricted eating (a form of IF) combined with resistance training can support reductions in body fat whilst preserving lean muscle mass, particularly in recreationally active adults. A frequently cited study by Moro et al. (2016), published in the Journal of Translational Medicine, found that an 8-hour time-restricted eating protocol in resistance-trained men led to reductions in fat mass without significant loss of muscle mass compared to a control group. More recent systematic reviews and meta-analyses (including Tinsley et al.) have broadly supported these findings for body composition, though effect sizes are modest and results are not consistent across all populations. Some research has also noted potential reductions in anabolic hormones such as testosterone and IGF-1 with prolonged time-restricted eating, which warrants further investigation.^[10][11]

For endurance performance, the evidence is more mixed. Some research indicates that fasted aerobic training enhances fat oxidation capacity over time, which may benefit long-distance athletes.^[16] However, there is limited evidence that this translates into improved race performance or superior outcomes compared to fed training when total caloric and macronutrient intake is matched.

NICE guidance on obesity and weight management acknowledges time-restricted eating as one dietary approach that some individuals may find helpful, though it does not specifically endorse IF for athletic performance.^[14] Readers should refer to the current NICE obesity guideline for the most up-to-date recommendations. The British Dietetic Association (BDA) advises that IF can be a valid dietary strategy for some people but emphasises that it is not superior to other balanced dietary approaches when total energy intake is equivalent.^[15] Further high-quality, long-term randomised controlled trials are needed before definitive clinical recommendations can be made.

Practical Tips for Combining Fasting With Regular Exercise

Start with a shorter fasting window, schedule demanding workouts within your eating window, prioritise recovery nutrition, and seek advice from a registered dietitian if symptoms persist beyond two to three weeks.

Successfully integrating intermittent fasting with working out requires thoughtful planning, patience, and a willingness to adapt your approach based on how your body responds. The following practical guidance can help you get started safely and sustainably.

Start gradually. If you are new to intermittent fasting, avoid making dramatic changes to your eating and training schedule simultaneously. Begin with a shorter fasting window (such as 12 hours) and gradually extend it as your body adapts. During the first one to two weeks, consider reducing your training intensity or volume and re-progressing as your tolerance improves.

Plan your meals around your training. Whenever possible, schedule your most demanding workouts to coincide with your eating window. Ensure your pre-workout meal includes a balance of protein and complex carbohydrates, and prioritise a recovery meal or snack within one to two hours of finishing exercise.

Listen to your body. Performance dips and fatigue are common in the early stages. However, if you consistently feel unwell, dizzy, or unable to complete your usual sessions after two to three weeks, reassess your fasting protocol or seek advice from a GP, sports dietitian, or qualified fitness professional.

Additional practical tips:

• Keep a simple food and training diary to identify patterns and optimise timing

• Avoid fasting windows beyond 18–20 hours if training volume is high, and do not fast for more than 24 hours whilst training unless under clinical supervision

• Prioritise sleep, as poor sleep amplifies the hormonal stress of both fasting and exercise

• Stay well hydrated throughout the day, not just during workouts — refer to NHS guidance on water and hydration

• In hot weather, take extra care to hydrate adequately and avoid training during peak heat to reduce the risk of heat exhaustion

• Pause fasting and reduce training if you are unwell; resume only when you have recovered

• Consider working with a registered dietitian (find one via the BDA website at bda.uk.com) for personalised guidance

Remember that the best dietary and exercise strategy is one that is sustainable, enjoyable, and compatible with your overall health. Intermittent fasting with working out can be an effective approach for many people, but it is not a one-size-fits-all solution.

Frequently Asked Questions