To eat or not to eat? Intermittent Fasting for Athletic Performance and Health

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Intermittent fasting, or the concept of restricted eating for extended periods of time, has dated back centuries.  Many cultures and religions incorporate some form of fasting into their lifestyle.  The medical community started to investigate intermittent fasts for health benefits around 1915 and in recent years the concept has gained traction in the media.  Researchers found that intermittent fasting aids in increasing weight loss, decreasing blood pressure, reducing the risk of metabolic diseases, and increasing longevity.  Whether individuals choose to practice intermittent fasting for health or religious reasons, they have a variant of different protocols to choose from.  Some options include: 16 hours of fasting followed by 8 hours of food ad libitum (16/8), alternating days of fasting with eating ad libitum (5:2), or the Muslim fast during Ramadan (1 month with no food or liquid consumed during daylight hours).  The Muslim fast during Ramadan has gained a significant amount of media attention when multiple professional athletes revealed that they practice this religious tradition.  This begs the question that if professional athletes can practice intermittent fasting and still successfully compete, is there a performance advantage to fasting? 

Athletic Performance 

intermittent fasting for sport performance

When seeking to identify whether or not intermittent fasting improves athletic performance, we must first realize that different types of exercise (e.g., strength/power, sprint, or endurance) have different fuel requirements.  Thus, we will examine the efficacy of intermittent fasting for each exercise type.  Strength and power-based activities (e.g., gymnasts, powerlifters, high jumpers, sprinters) that are usually explosive in nature, require maximal muscle contractions.  These contractions are fueled by carbohydrates.  Thus, training in a fasted state, with no glucose from recently consumed food and likely depleted glycogen stores, will not be advantageous for power and strength based explosive sports. 

As we travel further down the continuum of exercise types, we arrive at sports that require intermittent anaerobic or sprint efforts.  Examples of these types of sports may include: soccer, football, basketball, and field hockey.  Aziz et al. investigated the impact of a Ramadan fast on the intermittent sprint performance of soccer players.  The study was well designed, controlling for pre-exercise dietary intake, sleep and training load.  The researchers found that the Ramadan fast was not effective at enhancing intermittent sprint performance.  Another study looked at the implications of a three days of Ramadan fasting on the biomechanics of repeated sprint performance.  They found that fasting resulted in a decrease in speed and power due to a decrease in vertical stiffness.  This suggests that the decrease in performance due to fasting may be mitigated by an increase in strength.

Endurance athletes, on the contrary, may benefit from alternate day feeding.  A study by Marosi et al. found that one month of alternate day feeding produced mice that ran a further distance and longer duration than mice that were fed ab libitum.  Keep in mind that the study was conducted with mice so these results may not translate into humans.  In theory, the body’s low blood glucose and glycogen levels will cause the body to turn to fat oxidation for energy.  While fat oxidation is unable to fuel high intensity exercise or maximal muscle contractions, it is able to supply the body with energy for long, slow, endurance exercise.  In this way, free fatty acids become the primary fuel for muscles.  Thus, it is plausible that intermittent fasting may be a viable option for individuals participating in long duration, low intensity activities.   

Health Benefits 

health benefits of intermittent fasting

Even though intermittent fasting may not provide a performance advantage for every type of athlete, individuals may wish to try this dieting style in the off season in search of potential health benefits.  Intermittent fasting has been shown to improve metabolic flexibility, improve stress resistance, increase longevity, and reduce the risk of various diseases.  Your body switches back and forth between two primary sources for fuel, glucose and fat.  Glucose is primarily used after meals (since you obtain most of your glucose from food consumption) and for high intensity activity.  On the contrary, fatty acids are the primary fuel source when you have not eaten for a while or when you are performing low intensity activities.  Since the majority of people eat three meals and a few snacks each day, there is rarely a time during waking hours that we are without food.  Thus, the body learns to efficiently utilize glucose as fuel and relies on this as the primary source of energy for the body.  By practicing intermittent fasting the body can practice utilizing fatty acids as fuel.  This decreases the body’s reliance on glucose and increases your metabolic flexibility.  Intermittent fasting also aids in the cellular adaptations that increase mitochondrial stress resistance, antioxidant defenses and autophagy (cell death).  In the long run, these cellular adaptations lead to reduced inflammation, reduced blood pressure, increased longevity and an improved resilience to disease.

Food for Thought and Practical Applications 

intermittent fasting

When deciding if you want to give intermittent fasting a try there are a few things you should know.  Since most people are used to three meals per day, switching to intermittent fasting will require an adjustment period.  During the first month of intermittent fasting it is common to have increased hunger, irritability, reduced concentration and reduced high-intensity exercise performance.  After approximately one month these side effects should dissipate.  In order to experience as few side effects as possible, individuals should ease into intermittent fasting.  For example, if the goal is to increase the amount of time each day spent fasted, you could start with not eating after 7pm and pushing breakfast back to 9am.  This would result in 14 hours of fasting.  After a few weeks you could increase the time spent fasting by not eating after 6pm, then 5pm, and so forth until the desired amount of fasting time is achieved.  If you prefer the 5:2 intermittent fasting protocol, then your end goal would be to eat normally 5 days/week then practice restricted eating (500-600 calories) for the other 2 days. You can ease into this protocol by limiting yourself to 1000 calories 1 day/week for a month.  Then implement a 1000 calorie diet on 2 days/week.  The next step would be to gradually reduce the calories on the dieting days.  In this way, people can ease themselves into whatever protocol they choose and reduce the likelihood of experiencing negative side effects.  In summary, while intermittent fasting will not help athletes gain a competitive edge on their competition, there are plenty of health benefits associated with this dieting style. 


Author: Dr. Colleen Gulick