Intermittent fasting has gained popularity over the last several years. Understanding the mechanism by which the various macronutrients, metabolic parameters, and inflammatory markers are impacted by a fasting state, provides us greater insight into why we’ve seen an increase in this method of eating.  In addition, while the physiology of fasting has been scientifically demonstrated, it’s important to also gain an understanding of the impact on athletic performance and recovery.  Important parameters of intermittent fasting on athletic performance include weight loss, maintenance of muscle mass, and post exercise metabolism due to decreases in glycogen stores, low levels of insulin, increase in hormones responsible for lipolysis (fat breakdown), and an increase in free fatty acids (Paoli 2019, Gieske 2018).




Time-restricted eating or intermittent fasting is defined as an intake of food within a specified eating window that ranges from 3 to 12 hours daily.  Based on the research, the benefits of time restricted eating are observed when food intake is limited to a 6 to 9 hour eating window (Paoli 2019).  This method of eating is considered to have the potential for significant fat loss as it is associated with an increase in fat metabolism (Smith 2017).  Additionally, it has demonstrated improvements in various metabolic parameters such as reduction in total and LDL cholesterol and blood pressure (Malinowski 2019, Paoli 2019).  Other protocols include eating for a 24-hour period, followed by a 24 hour fast repeated 2 to 3 times a week (Malinowski 2019). The benefits of this protocol that have been observed include reducing visceral fat, increasing adiponectin, reduced LDL and leptin concentrations (Malinowski 2019).

A study done on mice compared 2 groups that ate either in an 8-hour window versus a 24-hour window over a 100-day period.  Both groups consumed the same amount of food, which is an important control factor in the study.  With that being said, in contrast to the mice that ate in an open eating window, those mice that ate within the restricted eating window were resistant to the development of obesity as well as other health biomarkers.  In addition, the restricted eating mice maintained a lower respiratory exchange ration which correlates to an increase in fat metabolism (Smith 2017).  With more than just a focus on weight loss, many athletes strive for overall improvement in body composition. The ultimate goal is to not only decrease fat mass, but to also increase muscle mass to improve athletic performance.  As it pertains to intermittent fasting, mixed results are reported regarding the ability to maintain lean body mass, and not negatively impact overall strength and lean body mass (Tinsley 2017).  




Time restricted feeding has been of special interest in sports and athletics because of reports that demonstrate its ability reduce weight and maintain muscle mass (Malinowski 2019).  A study by Moro et al. compared 2 groups of athletes that consumed their daily energy intake in either an 8-hour eating window or a 12-hour eating window.  What the results showed after 8 weeks was a decrease in fat in the time restricted eating group (TRF) compared to the normal diet group. However, the most favorable outcome was that while the TRF group lost more fat, both groups maintained the fat-free mass and muscle area of the arm and thigh. This demonstrates that TRF can reduce fat mass, while maintaining muscle mass (Moro 2016).  In addition to maintaining muscle and improving overall body composition, it is essential to understand the effects of time restricted eating in athletes on physical performance and endurance (Real-Hohn 2018).




While intermittent fasting and time restricted eating have gained popularity, it is important for athletes to appreciate how implementing this change in eating impacts metabolic adaptations, body composition, physical performance and overall endurance, and recovery.  For the majority of athletes, there is an overarching goal to reduce fat mass and improve body composition, while increasing overall lean muscle mass and athletic performance.  Recently it has been demonstrated that intermittent fasting combined with athletic training allows athletes to achieve this desired outcome, and additionally improves various metabolic parameters that have an effect on post exercise metabolism and recovery.


In reviewing the effect of fasting on fat oxidation it’s important to review the methods of fat oxidation.  There are 2 methods to fat oxidation, either through fasting intervals between meals or during aerobic exercise at a VO2 max of 45-55%.  This aspect is important for endurance athletes as the balance between lean body mass and fat mass is necessary to improve and maximize performance. When an athlete employs both of these strategies the question is whether we expect to see an additive effect or not.  Fat is a major fuel source during exercise when in a fasted state. With that being said, exercise intensity plays a critical role in the level of fat metabolism.  An analysis by Vicente-Salar et al. noted that at moderate to high intensity exercise, plasma free fatty acids were higher in the fasting group versus the fed group after 90 minutes of exercise.  Based on these findings the authors don’t support the notion of training while fasting to improve body composition and reduce fat mass. (Vicente-Salar 2015).  Additionally, a study done in wistar rats, compared endurance training for 40 minutes 5 days a week for 6 weeks while fasting to those that did not.  The results demonstrated and increase in the reduction in body weight and reduced fat content in the intermittent fasting group while exercise versus in those that did not exercise. Unfortunately, they did not measure lean fat mass to assess the impact on lean fat mass composition (Moraes 2017).  Fortunately, Tinsley et al. looked at the effects of time restricted eating in young men performing resistance training and its impact on lean fat mass.  The study was broken out into 2 groups, one that did resistance training with a normal diet and one that followed a time restricted diet. Those in the study group, did the restricted eating plan on the 4 days of the week where they were not performing resistance training. The restricted eating parameters consisted of a 4-hour window between 4:00 p.m. and midnight in which participants consumed their daily caloric intake.  Overall the study found that those in the time restricted group consumed an overall 667 fewer calories and a reduced intake of macronutrients across the board.  There were no significant changes in body weight or total body composition in either group over the course of 8 weeks. While this study does not demonstrate a benefit overall in athletic performance in the fasting group a major limitation is that the time-restricted group had no limitation or requirements on foods consumed.  This study also found that the restricted eating did not lead to any reduction in lean soft tissue, although it also hindered any growth observed as a result of resistance training.  Despite the lack of increase in lean tissue overall in the restricted eating group, it was found that muscular strength and endurance were equivalent or better than those following the normal diet (Tinsley 2017).  In order to truly assess the impact on lean tissue growth, there would need to be a standardization of appropriate macronutrient intake to maximize performance and assess a restricted eating state.  A key factor in maintaining athletic performance in a fasted state versus fed state is ensuring the macronutrient composition and energy intake it maintained (Chaouachi 2012).


In assessing the impact of endurance on fasted exercise, a study by Real-Hohn at al. tested the metabolism adaptations and the synergy of high-intensity exercise and intermittent fasting, the researchers found improvements in endurance and energy production.  The reported improvements in endurance were double in the fasting group versus the non-fasting group.  The intermittent fasting group that performed high intensity interval exercise presented significantly lower levels of NAD(P)H. This finding aligns with the notion that a lower level of NAD(P)H results in higher mitochondrial oxidation rate.  An additional measurement included mitochondria respiratory control rate (RCR), which is high in a healthy mitochondrion.  The results demonstrated improve oxygen flux and ATP production in the fasting groups that was statistically significant (p<0.05).


Of additional interest is to not only look at the effects of feeding state on endurance athletes, but its impact on high intensity interval training.  Sprint interval training (SIT) is a common mechanism in performing exercise training to improve VO2max, endurance performance and muscle metabolism.  Rocha da Silva et al. found no significant different when assessing SIT in the fasting versus fed state on parameters of peak, mean, and minimum power (2018). However, when assessing VO2, it was found to be higher, 60 minutes following SIT, in the fasted versus fed state.  Additionally, in the fasting state, respiratory exchange ratio (RER) was higher in the first 30 minutes post exercise, but lower 40-60 minutes post-exercise.  RER was higher across the board for those in the fed state.  This indicates that in the fasted state, fat was the main source of fuel, as opposed to the fed where glucose was the main source with an RER reported closer to 1.  Additionally, based on the findings one can conclude that while there isn’t a conclusive difference in overall performance in the fasting versus fed state, the fasting state stimulated a higher level of fat oxidation that occurred for 30 minutes post exercise (Rocha da Silva 2018).  Based on several reports the mechanism of fat oxidation from the fasted state seems to be most prominent in the post-exercise phase as opposed to during exercise.


It has been suggested the ingesting protein immediately before exercise may have a beneficial effect on post exercise energy expenditure, with an increase in fat oxidation and resting metabolism.  Gieske et al. looked at the effects of casein versus whey protein taken immediately before exercise in the fed state compared to a fasting individual to assess if a difference exists.  Interestingly, the results did identify an appreciable difference between the effects of exercise in the fasted state versus pre-exercise casein protein supplementation.  While this is one factor that demonstrates this difference, it’s important to compare all aspects of the benefits in each eating protocol (Gieske 2018). Also, a systemic review found that in exercise performance that lasts for >60 minutes, pre-exercise feeding enhanced performance as opposed to those in the fasted state (AIrd 2018).

In addition, to understanding the impact on how intermittent fasting effects metabolic parameters, the impact on anaerobic power and exercise performance is critical to know for athletes.  Nashrudin et al. observed the effects of intermittent fasting over a 10-day period to assess the bodies adaptation to the changes in dietary regimen.  They utilized Wingate anaerobic (WT) and prolonged high intensity time to exhaustion cycling test (HIT) to observed the effects of intermittent fasting versus a control group.  This study findings are critical in that it found that intermittent fasting reduced WT at day 2, and by day 4 performance returned to normal state. In addition, time to exhaustion (TTE) was decreased throughout the study period (10 days), but they began to observe a trend of recovery towards the end of the study period.  This is an important finding for athletes, to manage expectation of the impact on performance that will observed initially, and that continuous dietary restrictions through intermittent fasting beyond 10 days are important to see a long-standing effect (Nashrudin 2018).



In addition to its impact on athletic performance as measured by time to exhaustion as well as other parameters, fasting has also demonstrated an impact on inflammatory markers.  Fasting has a positive impact on inflammation as observed through a reduction in NF-Kappa Beta, which could potentially translate into improved recovery in some athletes (Paoli 2019).  An additional study found that pro-inflammatory factors homocysteine, interleukin-6, C-reactive protein were all reduced in 40 healthy participants that fasted intermittently during Ramadan (Malinowski 2019). Oxidative stress markers are also important factors in post exercise recovery, and it is implied that intermittent fasting combined with high intensity interval training demonstrates an additive and synergistic effect on oxidative stress markers (Real-Hohn 2018).


conclusion on INTERMITTENT FASTING in athletic performance


It is well known that nutrition and the impact on exercise metabolism and performance are continuously evolving with additional mechanisms to reach optimal impact.  As fasting exercise has produced an interest in athletes with is various observed benefits, it’s vital to break down the findings and extrapolate across the general population.  The data has clearly demonstrated that in a fasted state our primary source of energy is fat, as we shift from glycogenolysis, with an upregulation in fat oxidation.  Time-restricted eating has ample potential for fat mass loss because of what appears to be an associated up-regulation of fat metabolism. In comparison to maintaining an energy deficit chronically, this approach represents a lifestyle that is likely more sustainable (Smith 2017).  Also, the evidence is clear as it relates to other metabolic parameters, such as total and LDL cholesterol, blood pressure, and glucose levels, in that a benefit is clearly observed.

When assessing the benefits of intermittent fasting specifically on athletic performance and recovery, the data does not exhibit a benefit in this state. The data demonstrated minimal improvements in exercise performance, where most studies concluded equivalency.  This is a positive finding, in that we know there isn’t a negative effect as long as exercise performance was maintained at <60 minutes.  Based on the available research, when comparing endurance exercise versus high intensity interval training (HIIT), no significant differences in performance measures were observed in HIIT.

All in all, intermittent fasting in athletes demonstrates a beneficial impact on metabolic parameters and an increase in lipid oxidation, however, does not demonstrate an improvement in athletic performance and recovery.  It is important to note while the findings did not support improvement in athletic performance, a negative outcome was also not observed. Rather, in a fasted or fed state the results across the data were similar for performance and recovery.

Aird TP, Davies RW, Carson BP. Effects of fasted vs fed‐state exercise on performance and post‐

exercise metabolism: A systematic review and meta‐analysis. Scandinavian Journal of Medicine & Science in Sports. 2018;28(5):1476. Retrieved from: LINK


Chaouachi A, Leiper J, Chtourou H, Aziz A, Chamari K. The effects of Ramadan intermittent fasting on

athletic performance: Recommendations for the maintenance of physical fitness. Journal of

Sports Sciences. 2012;30(Supp 1):S53-S73. Retrieved from: LINK


Da Silva CR, Santana PV, Mendes PC, et al. Metabolic and cardiorespiratory acute responses to fasting

versus feeding during high-intensity interval training. Sport Sciences for Health.

2018;14(2):347. Retrieved from: LINK


Gieske BT, Stecker RA, Smith CR, et al. Metabolic impact of protein feeding prior to moderate-intensity

treadmill exercise in a fasted state: a pilot study. Journal of the International Society of Sports

Nutrition. 2018;(1). Retrieved from: LINK


Malinowski B, Zalewska K, Węsierska A, et al. Intermittent Fasting in Cardiovascular Disorders-An

Overview. Nutrients. 2019;11(3). Retrieved from: LINK


Moraes RCM de, Portari GV, Ferraz ASM, da Silva TEO, Marocolo M. Effects of intermittent fasting and

chronic swimming exercise on body composition and lipid metabolism. Applied Physiology,

Nutrition & Metabolism. 2017;42(12):1341-1346. Retrieved from: LINK


Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal

metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in

resistance-trained males. Journal Of Translational Medicine. 2016;14(1):290. LINK


Naharudin MNB, Yusof A. The effect of 10 days of intermittent fasting on Wingate anaerobic power and

prolonged high-intensity time-to-exhaustion cycling performance. European Journal of Sport Science. 2018;18(5):667-676. Retrieved from: interlibrary loan


Paoli A, Tinsley G, Bianco A, Moro T. The Influence of Meal Frequency and Timing on Health in Humans:

The Role of Fasting. Nutrients. 2019;11(4). Retrieved from: LINK


Real-Hohn A, Navegantes C, Ramos K, et al. The synergism of high-intensity intermittent exercise and

every-other-day intermittent fasting regimen on energy metabolism adaptations includes

hexokinase activity and mitochondrial efficiency. PLoS ONE. 2018;(12). Retrieved from: LINK


Smith ST, LeSarge JC, Lemon PWR. Time-Restricted Eating in Women – A Pilot Study. Western

Undergraduate Research Journal: Health & Natural Sciences. 2017;8(1):1. Retrieved from: LINK


Tinsley GM, Forsse JS, Butler NK, et al. Time-restricted feeding in young men performing resistance

training: A randomized controlled trial. European Journal of Sport Science. 2017;17(2):200-207. Retrieved from: LINK


Vicente-Salar N, Urdampilleta Otegui A, Roche Collado E. Endurance Training in Fasting Conditions:

Biological Adaptations and Body Weight Management. Nutricion Hospitalaria. 2015;32(6):2409-

  1. Retrieved from: LINK