Intermittent Fasting and Athletic Performance
Intermittent fasting (IF) has been gaining popularity in both the general public and in athletics. Making sense of all the information and opinions out there can get confusing considering all the different types of IF, proposed benefits, and the mixed findings of the current research- especially related to exercise performance!
The benefits that have been demonstrated from various types of intermittent fasting include reduced body fat, decreased blood pressure, improved markers for metabolic disorders, and improved sleep, just to name a few. For athletes specifically, the popularity stems from the metabolic shifts that occur during moderate bouts of fasting. Roughly 3-8 hours after a meal (depending on meal composition), the body goes into the post-absorptive phase; this phase lasts 12-18 hours. To maintain blood glucose levels during this phase, glucose is taken from the liver through glycogenolysis. Additionally, the body increases fatty acid oxidation through lipolysis in order to maintain it’s metabolic rate (which surprisingly, the body maintains its metabolic rate during these short-moderate fasting bouts). It’s not until ~36 hours of fasting when the body starts mobilizing more proteins (amino acids) for fuel. Using protein as fuel would lead to decreased lean body mass, which would be undesirable for most individuals, especially athletes. That is why all the popular fasting protocols are short/ moderate bouts of fasting. The metabolic adaptations of increased fatty acid use intrigues many athletes since utilizing more fatty acids as fuel can be a desirable sounding outcome for many athletes, especially endurance athletes, since our fat stores are massive compared to our carbohydrate stores.
For the general population, IF is intriguing due to the positive changes in body composition- decreasing body fat, while often preserving lean body mass. Additionally, studies have demonstrated positive impacts of IF for decreasing measures of metabolic, and cardiometabolic disorders. Animal and human studies have also suggested a potential benefit in the circadian rhythm from intermittent fasting.
Common Types of Intermittent Fasting (IF)
Time restricted feeding: fasting 16-20 hours consecutively each day
Alternate day fasting: 25% intake every other day; other days eat normally
Whole day fasts: 1-2 days per week completely fasting, usually not consecutive
As a whole, research studies on the effects of intermittent fasting and exercise performance are very limited. Weight training, endurance, and high intensity exercise (such as sprints) have all been examined. Different intermittent fasting techniques have been used, and mixed results between studies have been produced. The majority of studies suggest that intermittent fasting has a negative impact on high intensity exercise, some studies show decreased performance for endurance and resistance exercise, while others show no difference with intermittent fasting. The one thing ALL the studies have in common is that NO PERFORMANCE BENEFITS have observed.
Many of the studies on running and intermittent fasting examine the performance impact of athletes taking part in the month-long intermittent fast during Ramadan. During Ramadan, Muslim do not consume food or drink during daylight hours. Interpretation of these finds to the general athletic population, outside of a Ramadan fast, may not be appropropriate since the type of intermittent fasting is unique to many of the other commonly practiced intermittent fasting types. For example, most other intermittent fasting protocols allow for water consumption, and do not restrict intake only during daylight hours; these differences can be huge since dehydration and a disrupted circadian cycle can negatively impact performance, regardless of fasting.
Several studies examined the impacts of time restricted feeding (TRF) on exercise performance, and as mentioned above, no benefits have been shown. However, TRF has gained popularity due to other (non-athletic studies) showing benefits on overall health, including the circadian cycle, and decreased risk of certain health conditions, especially those associated with metabolic syndrome. While TRF has not been demonstrated to have exercise performance benefits, being well rested can certainly be favorable on your ability to perform. The take-away with this, consider getting into a good eating schedule and make sure your eating schedule is not keeping you up at night. Your eating schedule may impact your sleep schedule and work with or against your body’s circadian cycle.
Summary of Intermittent Fasting in Athletics
Improves body composition, decreases body fat
Some IF protocols may positively impact circadian rhythm- in theory that may improve exercise outcome measures
Mixed results on lean body mass- some show it does decrease LBM, others show no impact
Negatively impacts high intensity performance and skill based performance
Very mixed results for resistance and endurance sports- either showing diminished or no difference in performance
NO PERFORMANCE BENEFITS have been observed, for any type of exercise/ sport
Is Intermittent Fasting Appropriate for Athletes?
The current studies on intermittent fasting consistently show that energy intake is either lower (on-average) or comparable to non-intermittent fasting diets. Considering that distance runners have higher energy needs and some with especially high training struggle to meet their total energy needs. For these individuals, the potential consequences of low energy availability that could occur from engaging in intermittent fasting eating patterns would far outweigh any potential benefit that may be achieved.
For runners at a low risk of low energy availability, and therefore do not struggle to meet their energy needs, it is a different story. The evidence on endurance performance is lacking, and therefore, performance benefits have not been observed. However, other health benefits, such as improved body composition, improved circadian rhythms, low risk of certain health conditions, etc have been observed.
The research is much stronger supporting performance benefits of consuming adequate energy, especially carbohydrates during the periexercise period. This can also be said regarding the timing of protein for post exercise recovery. Considering the evidence exists for getting adequate intake near the periexercise period, and research is lacking to show any performance benefit of intermittent fasting, I would generally shy away from athletes using this technique for the purposes of enhancing their athletic performance.
The literature does not support that IF has any performance benefits, and many of the studies have a high amount of drop-out participants, which nicely highlights the difficulty most people face following a diet of these types. If you do not have other underlying reasons for wanting to follow an IF diet (e.g. sleep disturbances, metabolic syndrome, etc), consider skipping intermittent fasting and replacing it with a good eating schedule of balanced meals.
However, if you are still intrigued to try out intermittent fasting, here is what I would suggest:
Avoid eating late at night if you feel it interferes with your sleep cycle; only do this if it will not interfere with you meeting your total energy needs
If a modified/ less-intense intermittent fast interests you, consider getting your 3 meals and any necessary snacks and during-exercise fueling within a ~12 hour time window, to allow for a full overnight fast
Avoid completely if you are at risk of low energy availability/ getting inadequate energy to support your body's needs (including the high needs to support exercise)
Time your workouts to happen during the hours you are eating to maximize the periexercise time frame
Stay hydrated throughout the day
References
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