Intermittent Fasting and Running: Compatible or Dangerous?

Intermittent fasting has become one of the most popular nutritional trends of recent years. Millions of people practice it to manage their weight, improve their metabolic health or simplify their eating habits (World Athletics) (WHO). But if you're a runner, the question is inevitable: can I combine intermittent fasting with running without my performance suffering?

The short answer is: it depends. It depends on your level, the type of training, your goals and how you implement it. In this guide we'll analyze the available scientific evidence, the most suitable protocols for runners and, above all, when it makes sense and when it's better to avoid it. If you're looking for a more general guide on eating for running, check out our nutrition guide for runners (ACSM).

1. What is intermittent fasting

Intermittent fasting is not a diet in the traditional sense. It doesn't tell you what to eat, but when to eat. It involves alternating fasting periods (where no calories are consumed) with normal eating periods. During the fast you can drink water, black coffee, tea and unsweetened infusions without milk.

There are several protocols, and each one has its own implications for runners:

The 16:8 protocol (the most popular)

This involves fasting for 16 hours and eating within an 8-hour window. For example, if your last meal is at 8:00 PM, you wouldn't eat until 12:00 PM the next day. It's the most studied protocol and the easiest to maintain long term. Many people practice it without realizing simply by skipping breakfast.

For runners, the 16:8 is the most compatible with training because it allows you to schedule your running session within or right at the edge of the eating window, making post-exercise recovery easier.

The 5:2 protocol

In this protocol you eat normally 5 days a week and on the other 2 days (non-consecutive) you reduce your intake to 500-600 calories. For example, you eat normally Monday through Friday except Tuesday and Thursday, when you drastically cut calories.

For runners, the 5:2 is more complicated to manage because the severe restriction days must coincide with rest days or very light activity. Training on just 500 calories is a guaranteed recipe for poor performance and fatigue.

Eat-Stop-Eat (24-hour fast)

This involves a complete 24-hour fast once or twice a week. For example, having dinner on Monday at 8:00 PM and not eating anything until dinner on Tuesday at 8:00 PM. It's the most aggressive protocol of the three and the least recommended for runners who train regularly.

A 24-hour fast significantly depletes liver glycogen stores and can compromise training quality for 24-48 hours afterward. It only makes sense during periods of very low training load and for runners with previous fasting experience (Mayo Clinic).

2. Potential benefits for runners

Research on intermittent fasting and endurance sport has grown considerably over the past decade. These are the benefits that the evidence supports, with varying degrees of strength:

Improved fat oxidation

This is probably the most consistent and relevant benefit for runners. Training in a fasted state forces the body to rely more on fat stores as an energy source, since liver glycogen levels are partially depleted after overnight sleep. Studies show that runners who regularly train fasted (easy sessions) improve their ability to oxidize fat at moderate intensities, a parameter known as "fat max."

This adaptation is especially interesting for long-distance runners (half marathon, marathon and ultra trail), where the ability to use fat as fuel can make the difference in the final stages of the race, when glycogen stores run out.

Metabolic flexibility

Metabolic flexibility is your body's ability to efficiently switch between using carbohydrates and fats as fuel depending on exercise intensity. A metabolically flexible runner can use fats at moderate paces and quickly switch to glycogen when intensity increases. Intermittent fasting, combined with periodized training, can improve this flexibility over time.

Weight control and body composition

Intermittent fasting can facilitate weight management simply because you reduce the window of time available to eat, which often leads to lower total caloric intake. For runners looking to optimize their power-to-weight ratio without resorting to complicated restrictive diets, the 16:8 can be a practical tool.

However, it's important to understand that intermittent fasting has no magical fat-burning properties beyond the caloric restriction it may create. If you eat exactly the same calories in 8 hours as in 16, the effect on weight will be similar.

Autophagy: cellular cleanup

Autophagy is a natural cellular recycling process where the body removes damaged components and reuses them. Fasting activates this mechanism, and some researchers suggest it may contribute to longevity and reduced risk of chronic disease. However, most autophagy and fasting research has been conducted in animal models, and directly extrapolating to humans (let alone runners) requires caution.

Insulin sensitivity

Intermittent fasting can improve insulin sensitivity, meaning your body handles carbohydrates better when you consume them. This is relevant for overall metabolic health and can facilitate better glycogen replenishment after training. Studies show significant improvements in people with insulin resistance or excess weight, while in already healthy and active individuals the benefit is more modest.

3. Risks and downsides

Understanding the risks is just as important as knowing the benefits. For many runners, the disadvantages of intermittent fasting outweigh the advantages, especially if not implemented correctly.

Risk of muscle loss

When you train fasted and glycogen stores are low, your body may turn to muscle amino acids as an energy source through a process called gluconeogenesis. This is especially concerning for intense or long fasted sessions. The evidence suggests the risk is low for short, low-intensity sessions but increases significantly with the duration and intensity of exercise.

To minimize this risk, make sure you consume enough protein (1.4-1.8 g/kg/day) distributed across your eating window meals, and avoid intense workouts in a prolonged fasted state (PubMed).

Reduced performance at high intensity

Intervals, hill repeats, tempo runs and speed sessions primarily depend on muscle and liver glycogen as fuel. Performing these workouts fasted limits glycogen availability and reduces your ability to sustain high intensities. Multiple studies have shown that performance in high-intensity exercise (above the anaerobic threshold) decreases significantly in a fasted state.

In practice, this means your interval times will be slower, perceived effort will be higher and overall training quality will decrease. For a runner looking to improve their 10K or marathon time, compromising the quality of key sessions is too high a price to pay.

RED-S and specific risk for women

Relative Energy Deficiency in Sport (RED-S) occurs when energy intake is insufficient to cover the demands of exercise and basic bodily functions. Intermittent fasting, by restricting the eating window, can make it harder to consume enough calories, especially for runners with high training volumes.

Women are particularly vulnerable. Caloric restriction and energy stress can disrupt the hypothalamic-pituitary-ovarian axis, causing menstrual irregularities, amenorrhea, decreased bone density and increased risk of stress fractures. If you're a woman and notice changes in your menstrual cycle when starting intermittent fasting, it's a warning sign you shouldn't ignore.

Hormonal disruption

Beyond the reproductive system, prolonged fasting can increase cortisol levels (the stress hormone), especially when combined with high-intensity training. Chronically elevated cortisol interferes with muscle recovery, disrupts sleep and can weaken the immune system. For a runner already accumulating physical stress from training, adding the stress of fasting can be counterproductive.

Thyroid hormone T3 can also be affected by prolonged energy restrictions, reducing basal metabolic rate. This is an adaptive response from the body that can make weight management harder in the long run, precisely the opposite of what many people seek from fasting.

Social and psychological challenges

We can't ignore the social and psychological component. Intermittent fasting can generate anxiety around food, excessive rigidity with schedules and social conflicts when mealtimes don't align with those of family or friends. For people with a tendency toward eating disorders, intermittent fasting can be a dangerous trigger disguised as a healthy habit.

4. Training fasted: yes or no

This is the most practical question runners considering intermittent fasting ask themselves. The answer depends on the type of training session.

Low-intensity sessions fasted: YES

Easy runs (heart rate zone 1-2), under 60-75 minutes, are safe and potentially beneficial when fasted. At these intensities, your body can meet energy demand predominantly with fatty acids, without relying heavily on glycogen. Evidence shows that this type of fasted training improves fat oxidation and mitochondrial adaptations over time.

Recommendations for fasted easy runs:

• Hydrate well: Drink 300-500 ml of water when you wake up. You can have black coffee (caffeine improves fat mobilization).
• Stay in zone 1-2: If you feel the need to run faster, your body is asking for glycogen. Slow down.
• Maximum duration of 60-75 minutes: Beyond this time, the risk of muscle catabolism increases.
• Eat afterward: The post-workout meal is key. Protein + carbohydrates within 60 minutes.

High-intensity sessions fasted: NO

Intervals, fartleks, tempo runs, hill repeats and any session requiring intensities above the aerobic threshold need glycogen as the primary fuel. Performing these sessions fasted reduces training quality, increases perceived effort and limits the adaptations you're seeking from that type of work.

The evidence is clear: for quality sessions, eat beforehand. A light breakfast rich in carbohydrates 2-3 hours before, or a quickly absorbed snack 30-60 minutes before, makes a significant difference in your ability to perform at high intensity.

Long runs fasted: WITH NUANCES

Full fasted long runs (over 90 minutes) are not recommended for most runners. However, there's an interesting intermediate strategy called "train low" or "training with low glycogen availability." It involves starting the long run fasted to stimulate metabolic adaptations, but carrying nutrition (gels, bars) to use when needed, generally from minute 60-75 onward.

This strategy is used by elite runners during specific preparation phases, but it requires experience and a good ability to read your body's signals. It's not recommended for beginners.

5. Fasting protocols for runners

If after weighing the benefits and risks you decide to try intermittent fasting, here's how to implement the most practical protocols for runners.

16:8 for runners (the most recommended)

The 16:8 protocol is the most compatible with running training because it offers enough flexibility to place the eating window around your key sessions.

Option A: You train in the early morning

• 06:00-07:00: Easy run fasted (water + coffee allowed)
• 08:00: First meal (hearty breakfast with protein + carbs + healthy fats)
• 12:00-13:00: Full lunch
• 16:00: Last meal/snack (window closes)
• 16:00-08:00: Fasting period (16 hours)

Option B: You train at midday or in the afternoon

• 11:00-12:00: First meal (brunch/lunch)
• 14:00-16:00: Training (with prior fuel)
• 16:30: Post-workout recovery (shake or snack with protein)
• 19:00: Dinner (last meal, window closes)
• 19:00-11:00: Fasting period (16 hours)

Option B is ideal for intense sessions because it allows you to eat before training, ensuring good glycogen availability.

5:2 adapted for runners

If you prefer the 5:2 protocol, the key is to sync the restriction days with rest days or active recovery. Never do a 500-600 calorie day when you have a quality workout scheduled.

• Restriction days: Always coinciding with complete rest days or light walking
• Normal days: Eat according to your training needs, no time restrictions
• Never two consecutive restriction days
• The 500-600 calories on restriction days: Prioritize protein (30-40g) and vegetables to maintain satiety

Pre- and post-training nutrition within fasting

Regardless of the protocol you choose, nutrition around training should be a priority. If you train within your eating window, make sure you eat something 2-3 hours before (easily digestible carbohydrates) and recover with protein + carbohydrates afterward. To learn more about how to plan carb loading before competitions, read our marathon carb loading guide.

If you train during the fasting phase (for example, a morning run before your first meal), the post-workout meal becomes the most important meal of the day. Don't neglect it: include at least 20-30g of protein, 1-1.5g of carbohydrates per kg of body weight and plenty of fluids.

6. Who should avoid fasting

Intermittent fasting isn't for everyone. There are situations where the risks clearly outweigh the benefits and the recommendation is to avoid it:

• Women with menstrual irregularities: If your cycle is already irregular, adding energy restriction can make things worse. Prioritize energy availability and consult an endocrinologist or sports gynecologist.
• Runners who are underweight or have a BMI below 18.5: If you already struggle to maintain a healthy weight, restricting the eating window will make it harder. You need to eat more, not less.
• Under 18: Adolescents are in full development and have high nutritional needs. Intermittent fasting can interfere with growth, bone maturation and hormonal development.
• High-intensity training blocks: During weeks of peak volume or intensity in your preparation (for example, the 4-6 weeks before a marathon), your body needs maximum energy availability to absorb training and recover. This is not the time to experiment with food restriction.
• Race week: Never practice intermittent fasting the week before a competition. You need to load glycogen, rest well and arrive at the start line with full energy stores. Drop fasting at least 5-7 days before.
• History of eating disorders: If you've had anorexia, bulimia or another eating disorder, intermittent fasting may reactivate harmful patterns. Avoid it and work with a mental health professional if in doubt.
• Pregnant or breastfeeding women: Energy needs are increased and it's not the time for restrictions.
• People with type 1 diabetes or taking hypoglycemic medication: The risk of severe hypoglycemia is too high. Only under strict medical supervision.

7. How to start: 4-week plan

If you've evaluated the risks, you're not in any of the at-risk groups and you want to try intermittent fasting, here's a gradual introduction plan. The key is not to jump from 0 to 16 hours of fasting overnight.

8. Fasting and race performance

There is one absolute rule that every runner who practices intermittent fasting must respect: never race fasted. Not a 5K, not a half marathon, and certainly not a marathon. On race day you need every gram of available glycogen.

Never run a race fasted

Competition is the time to put all your accumulated work into practice. Racing fasted means voluntarily limiting your performance: you'll have less glycogen available, higher perceived effort and a greater risk of "bonking." No matter how good you feel training fasted; racing is a different level of intensity.

Plan a pre-race breakfast rich in carbohydrates, previously tested in training, 2-3 hours before the start. Check our complete race day nutrition guide to learn exactly what to eat and when.

Periodize fasting: base phase only

Intermittent fasting makes more sense during the base or pre-season phase of your training plan, when volume is moderate, intensity is low and the focus is on building aerobic fitness. During this phase, the benefits of improving fat oxidation and metabolic flexibility are more relevant, and the risk of compromising training quality is lower.

When you enter more specific preparation phases (interval blocks, tempos, tapering), drop the fasting and prioritize maximum energy availability. Your body needs all the fuel possible to absorb training loads and to arrive at race day in optimal condition.

Annual periodization example

Training phase	Intermittent fasting	Rationale
Pre-season / Base	Possible (16:8)	Moderate volume, low intensity, focus on aerobic adaptations
Build	Reduce (14:10 or drop)	Intensity increases, you need more fuel
Specific / Intervals	Not recommended	Quality sessions require maximum glycogen availability
Tapering	No	Glycogen loading and optimal recovery are the priority
Race week	Never	Full stores, perfect sleep, zero metabolic stress
Post-race recovery	No (first 2 weeks)	Your body needs to repair, not restrict

Frequently Asked Questions

Can I run fasted without losing muscle?

Yes, as long as the session is low intensity and under 60-75 minutes. At these intensities, the body predominantly uses fatty acids as fuel, preserving muscle mass. Significant muscle loss only occurs with very prolonged fasts (over 24 hours) combined with intense workouts and insufficient protein intake. Make sure you consume at least 1.4 g of protein per kg of body weight per day, distributed across the meals in your eating window.

What is the best fasting protocol for runners?

The 16:8 protocol is the most practical and well-studied for runners. It allows you to train in the morning while fasted (easy sessions) and schedule intense sessions within the eating window. It's flexible enough to fit most schedules and gentle enough not to compromise recovery if nutrition during the window is adequate.

Does intermittent fasting improve race performance?

Not directly. Intermittent fasting can improve metabolic flexibility and fat oxidation at low intensities, which is useful in ultra-distance events. However, for races from 5K to marathon, where you rely on glycogen as the primary fuel, fasting does not improve performance. In fact, if you don't plan nutrition around training well, it can hurt it. Where it can help indirectly is in managing body weight, if that's a goal.

Can female runners do intermittent fasting?

With extra caution. Women are more sensitive to caloric restriction and energy deficit. Intermittent fasting can disrupt reproductive hormones, cause menstrual irregularities and increase the risk of RED-S (Relative Energy Deficiency in Sport). If you decide to try it, opt for gentler protocols (12:12 or 14:10 instead of 16:8) and closely monitor your menstrual cycle. If you notice any irregularity, stop fasting and consult a professional.

Can I do intermittent fasting during race week?

No. The week before a competition is the worst time to practice intermittent fasting. Your absolute priority should be loading muscle and liver glycogen, sleeping well and arriving at the start line with full energy stores. Drop fasting at least 5-7 days before the race and eat normally, with special emphasis on carbohydrates in the 2-3 days before.

Do I need supplements if I do intermittent fasting and run?

Not necessarily, but you need to be more conscious of your nutritional intake. By reducing the eating window, you have fewer hours to cover your calorie, protein, vitamin and mineral needs. Pay special attention to iron, vitamin D, magnesium and total protein. If you struggle to get enough nutrients in fewer hours, a post-workout recovery shake with protein and carbohydrates can be helpful. Read our supplements guide for runners for more details.

How long does it take for the body to adapt to intermittent fasting?

Most runners need between 2 and 4 weeks to adapt to intermittent fasting. During the first 1-2 weeks it's normal to feel hungrier than usual, some fatigue and a slight dip in training performance. These sensations usually diminish as the body improves its fat oxidation capacity and adjusts to the new schedule. If after 4 weeks you still feel significantly worse than before, fasting is probably not for you, and that's perfectly fine.