Ali McGowan, MS, RD, LDN, MTI Contributor
Disclaimer: The information presented herein is for educational purposes only and is not meant to be a substitute for medical advice.
What is Intermittent Fasting?
Intermittent fasting (IF) is a type of eating pattern that alternates between periods of voluntary fasting and eating. IF does not specify what or how much to eat, and may or may not include calorie restriction. The most common approaches to IF include:
- Alternate day fasting (ADF): Alternations between 24 hours of fasting and 24 hours of eating,
- Time-restricted eating (TRE): Typically a 16-hour fast followed by an 8-hour ad libitum (i.e., as desired) eating window,
- 5:2 diet: A regular eating pattern for five days followed by a calorie restricted (<600 calories) eating pattern on the remaining two days, and
- The Eat-Stop-Eat method: A 24-hour fast one to two times per week, with a normal eating pattern on the remaining days.
Where does the science stand?
While intermittent fasting is an ancient practice, it has gained scientific attention in recent years due to its proposed health benefits on health and training outcomes. 1 Given the importance of nutrition in optimizing athletic performance, concerns regarding the effects of IF in athletes exist.
To-date, a number of randomized controlled trials (RCTs) have sought to explain the effects of IF on athletic performance and metabolic outcomes on male and female adults of various health and fitness levels. Here, we look at three recent RCTs and one randomized crossover study that examine the effects of IF on trained, untrained, lean, and obese individuals in a variety of endurance and strength-based activities. It’s important to note that the research on IF continues to evolve and the following information reflects what the science currently shows. Further studies are needed to elucidate these findings.
A 2019 study: Time-restricted feeding plus resistance training in active females: a randomized trial2
- Type: 8-week long randomized, placebo-controlled study
- Objectives: To compare the physiological and performance effects of TRE with or without supplementation of β-hydroxy β-methylbutyrate (HMB) during fasting periods to a control diet (CD) during progressive resistance training (RT) in healthy non-pregnant females. Participants were between the ages of 18 and 30, had prior RT experience, and a body fat percentage of 15-33%. HMB is a metabolite of the amino acid leucine, which has been shown to stimulate muscle protein synthesis, the metabolic process of building muscle. Because TRE includes prolonged periods without eating, it has been hypothesized that consuming amino acids or their metabolites during fasting periods may help preserve lean muscle mass.
- Participants were randomly assigned to 1 of 3 study groups:
- CD plus placebo (CD): Consumed breakfast upon waking and ate subsequent meals at self-selected intervals throughout the day
- TRE plus placebo (TRE): Ate between 12 p.m. and 8 p.m. daily
- TRE plus HMB (TREHMB): Ate between 12 p.m. and 8 p.m. daily
- Training: All groups completed 8 weeks of supervised RT, which included alternating upper- and lower-body sessions during which participants trained to momentary muscular exhaustion. Fasted participants shifted their eating window one hour earlier on training days to ensure training sessions did not take place in the fasted state.
- Participants were randomly assigned to 1 of 3 study groups:
- Results: Fat-free mass, muscular strength and endurance (assessed via leg press and bench press), and muscle thickness in the elbow flexor and knee flexor increased in all groups without statistically significant differences between groups. No significant changes in resting energy expenditure or average cortisol concentrations were observed in any group.
- Fat-free mass: CD: +2%; TRE: +2%; TREHMB: +3%; p=0.0001.
- Muscle thickness of elbow flexor: CD: +5%; TRE: +5%; TREHMB: +9%; p=0.001.
- Muscle thickness of knee flexor: CD: +8%; TRE: +7%; TREHMB: +4%; p=0.0001.
- Limitations: All participants were instructed to consume 25 grams of a provided whey supplement (Elite 100% Whey) immediately after training sessions to achieve a protein intake of > 1.4 g/kg/day, which may have differed from typical protein intake. Dietary guidance during the course of the study was minimal.
- Conclusions: A TRE program in which all calories were consumed in 7.5 hours demonstrates similar gains in fat-free mass, skeletal muscle hypertrophy, and muscular performance as a CD with daily feedings spread across 13 hours when energy and protein intake are matched. The present study suggests no detrimental effects of TRE on RT adaptations. This study also suggests no added benefit of HMB on body composition and performance outcomes in exercising and/or energy-restricted individuals. Thus, TRE does not appear to pose any superior or detrimental effects on fat-free mass, skeletal muscle hypertrophy, and muscular performance compared to a control diet when energy and calories are equal.
A 2020 study: Moro T, Tinsley G, Longo G, et al. Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial3
- Type: 4-week long parallel randomized controlled trial
- Objectives: To investigate the effects of TRE with a daily 16-hour fast and 8-hour feeding window in a group of young (ages 19.3 ± 0.1 years) male elite cyclists with at least three seasons of experience on an elite cycling team. Due to the intensity of training sessions that exceed 90 minutes, endurance athletes often experience cycles of physiological stress that may perturb the immune system and promote inflammation. The authors hypothesized that TRE would reduce inflammatory markers and fat mass without affecting physical performance.
- Methods: Participants were randomly assigned to a TRE group or a standard diet group (ND)
- TRE: Consumed breakfast, lunch, and dinner (BLD) between 10 a.m. and 6 p.m.
- ND: Consumed BLD between 7 a.m. and 9 p.m.
- Diet: All athletes received the same 7-day diet plan set to 4,800 kcal. This plan considered data from participants’ food diaries and specific energy and macronutrient requirements for athletes, and closely resembled their baseline intake.
- Training: Training sessions consisted mainly of long outings at a mild-medium steady state pace, including 500 + 50 kilometers divided into six sessions per week that took place within the feeding window (10 a.m. to 6 p.m.).
- Body composition: After four weeks, the TRE group experienced a statistically significant decreases in total body mass (-2%, p=0.03) and fat mass percentage (-1.1%, p=0.01) with no significant changes the ND group.
- Performance: No significant differences were observed between groups in any of the performance test outcomes including peak power output (PPO), VO2 max, CO2 production, and heart rate. The TRE group increased their PPO to bodyweight ratio by 4% due to weight loss (p=0.04), while the ND group decreased this by 2% (p=0.44, not statistically significant).
- Hormonal response: Insulin-like growth factor (IGF-1), a hormone that regulates growth hormone, significantly decreased only in the TRE group (-12%, p=0.03), while no significant change was observed in the ND group. Similarly, free testosterone decreased in both groups but was significantly lower from baseline only in the TRE group (-27% p=0.01).
- Immune function: The percentage of neutrophils (a type of white blood cell) significantly decreased by ~12% in both groups (p<0.05), while lymphocytes increased by ~34% in the TRE group (p=0.0004) and ~27% in the ND group (p=0.001).
- Limitations: This study included a small sample size (16 participants) and short intervention period (4 weeks).
- Conclusions: In this study, TRE allowed for maintenance of fat-free mass and reductions of fat mass in elite cyclists and, as a result, increases in PPO to bodyweight ratio. This study suggests that TRE may reduce anabolic hormones (IGF-1, testosterone) without affecting lean mass or endurance performance, and may exert a protective effect on the immune response to exercise training by decreasing the reduction in white blood cell count compared to a normal feeding pattern. Thus, TRE may be beneficial to highly trained endurance athletes who seek to marginally improve their performance and mitigate immune disturbances. However, these effects are slight and thus may not be applicable to the general athlete.
A 2020 study: Stratton MT, Tinsley GM, Alesi MG, et al. Four Weeks of Time-Restricted Feeding Combined with Resistance Training Does Not Differentially Influence Measures of Body Composition, Muscle Performance, Resting Energy Expenditure, and Blood Biomarkers4
- Study type: 4-week long randomized controlled trial
- Objectives: To examine the effects of four weeks of 16/8 TRE versus a normal meal distribution (ND) with a matched degree of energy restriction and protein intake combined with RT on measures of body composition, muscle performance, resting energy expenditure, and blood biomarkers on 26 recreationally active males who train for peak performance seasons.
- Methods: Participants were randomized to TRE or ND groups.
- TRE: Consumed total daily calories in an 8-hour window (12-8 p.m. or 1-9 p.m.)
- ND: No time-based mealtime restrictions
- Diet: Both groups were prescribed a 25% calorie deficit and consumed 1.8 g protein/kg/day to preserve lean body mass. The remaining calories were dispersed between carbohydrate and fat at the discretion of the participants.
- Training: Participants underwent four consecutive weeks of a periodized RT program consisting of full-body routines three times per week. All training sessions occurred between 3 and 8 p.m. during the feeding window.
- Body composition: Both groups experienced statistically significant and similar decreases in body mass (~-2 kg, p=0.001), fat mass (~-1.5 kg, p<0.001), and body fat percentage (~-1.5 percentage points, p<0.01).
- Performance measures: Participants in both groups increased leg press one-repetition maximum (LP1RM) and bench press one-repetition maximum (BP1RM) to a significant degree (p<0.001) independent of their prescribed dietary intervention. Despite muscular strength improvements, muscular endurance did not improve in either group, however these results may be attributed to the training regimen rather than the dietary intervention.
- Hormonal responses: Unlike the study by Moro et al., the present study found no significant decreases in serum testosterone in either group.
- Limitations: This study included a short intervention period (4 weeks) and carbohydrate and fat intakes were at the discretion of the participants.
- Conclusions: Short-term TRE does not elicit more favorable alterations in body composition or muscular strength when compared to ND in a hypocaloric state when dietary protein is elevated and a strenuous RT program is in place.
A 2021 study: Correia JM, Santos I, Pezarat-Correia P, Minderico C, Schoenfeld BJ, Mendonca GV. Effects of Time-Restricted Feeding on Supramaximal Exercise Performance and Body Composition: A Randomized and Counterbalanced Crossover Study in Healthy Men5
- Study type: 10-week long randomized and counterbalanced crossover study
- Objectives: To investigate the effects of long-term TRE without calorie restriction (CR) on Wingate (WnT) performance of well-trained, physically active healthy male physical education students, and to determine if the effect of TRE on WnT performance are associated with alterations in fat mass and fat-free mass.
- Methods: 12 healthy male physical education students (ages 2.4 + 2.8 years, BMI 24.2 + 2.0 kg/m2) who averaged nine hours of physical activity/week and had > three years of continued experience and current participation in thrice weekly power-sports training were randomly assigned to TRE (in which they ate as desired between 1-9 p.m.) and non-TRE dietary interventions. All participants completed a four-day dietary record prior to the start of the study and continued their habitual training throughout the duration of the study. Participants served as their own controls, meaning that they adhered to their assigned dietary regimen for four weeks, completed a two-week washout period in which they adhered to no specific diet, then completed the opposite dietary regimen for an additional four weeks. Baseline dietary records were obtained prior to each dietary intervention, and body composition and WnT performance were determined at baseline and after four weeks of both dietary interventions.
- Results: Both dietary interventions were equally effective in increasing fat-free mass after four weeks. Absolute mean power increased from pre- to post-four weeks of TRE (p=0.04), but no significant changes were observed in the non-TRE intervention. No significant changes in WnT peak power or fatigue index were observed after four weeks in either intervention. Participants improved their total work time by more than one second after TRE.
- Limitations: This study includes a small sample size (12 participants) and diet was not controlled for in terms of calories or macronutrients. Moreover, this study population included young, well-trained male students, thus these findings may not be applicable to more general populations.
- Conclusions: Compared to four weeks of a non-TRE calorie restriction, four weeks of TRE demonstrates improvements in absolute mean WnT power and total work time, but does not elicit any superior changes in fat-free mass, WnT peak power, or fatigue index. For certain athletes, improvements in total work time may make the difference between qualifying for or winning events. However, these findings may not be generalizable to the average athlete.
An important note to consider: Numerous studies have observed benefits of TRE on weight loss, reductions in blood pressure, and cardiometabolic benefits including reduced insulin and inflammatory markers, however the majority of these studies did not include an exercise intervention.7 Thus, the information provided here only includes research studies that observed the effects of TRE alongside training interventions to examine the effects of fasting on physical performance in humans. This post also excludes studies on rodent models as their findings may not be applicable to human athletes. Additional studies that observe exercise in the fasted state are required to elucidate the aforementioned findings.
Anecdotal evidence: My personal experience with intermittent fasting for about five years included a similar 16:8 eating style where I would train fasted (anywhere between 6:30 and 9 a.m.) and eat my first meal around 12 p.m. My training sessions typically included four sessions per week, including a mix of resistance training and conditioning workouts utilizing barbells and kettlebells, as well as yoga and natural movement roughly once per week. Throughout this time, I completed nine Tough Mudder races, the Spartan Beast at Killington, and maintained a regular habit of walking daily to meet my daily step goal of 10,000 steps. I experienced no ill effects while training in the fasted state as long as I ate adequately the day prior with an emphasis on a well-rounded dinner. Since engaging in more high-intense training (i.e., CrossFit) and adopting a more normal eating pattern in the past year, I have noticed that spacing my meals throughout the day has led me to increase my muscle mass and to not feel as though I need to “make up” calories towards the latter half of the day. It’s likely these body composition changes are related to the demands of CrossFit and subsequent increases in hunger and food intake, and less related to a timed eating window. Interestingly, I did feel a bit stronger, leaner, and more cognitively engaged during and after my training sessions when I intermittent fasted. It’s difficult to determine if these effects are related to my training style and the escape that working out offered me during that time in my life (working a full-time job in a field that didn’t feel like the right fit for me) versus now (a Registered Dietitian doing what I love).
Here’s what other individuals I train with have to say about their experience with intermittent fasting/TRE:
- Ryan, a 40-year-old fitness studio owner, father of 3, Spartan Ultra, and Level 3 MovNat certified trainer who has been intermittent fasting for 10+ years: I discovered fasting by accident when I was working a full-time job and didn’t eat before an evening CrossFit class. I was apprehensive about my performance, but ended up PRing my lifts by 5% that day. I left that class feeling charged up and haven’t lifted in the fed state since. I feel lighter and sharper when I’ve fasted, especially during other forms of training, like running and sprinting at the sand dunes at 6 p.m. in the fasted state.
- Jamie, a 54-year-old with experience as an aerialist in the circus who now trains for menopause with heaving lifting, sprints, and Spartan races: I typically follow a 16:8 eating window, and a bare minimum of 14:10: I train fasted and love it. I feel stronger, I can lift heavier, run longer, and faster, and my recovery is flawless. I trained for the Spartan Beast at Killington, which included 3+ hours of work on a weekly basis, fasted. I follow a mostly carnivore eating style and can’t imagine doing IF without a decent amount of fat.
- Lisa, a 49-year-old business owner and mom of two who has been working out consistently since 2018: I followed TRE for about a year and would typically eat my meals between 12-8 p.m. At that time, I trained fasted in the morning and still do. While I wouldn’t say I noticed a huge difference in my performance while fasting, I notice that working out at night and having a meal earlier in the day doesn’t feel as good. I don’t adhere to a certain eating style, but I’ve focused on eating more whole foods, which sort of started with IF. While I don’t follow IF now, I’ve found that it has allowed me to pay attention to my hunger cues more than simply eating by the clock.
The aforementioned studies suggest that:
- TRE does not impair gains in fat-free mass, skeletal muscle hypertrophy, or muscular performance when energy and protein intake are matched in resistance trained females and elite cyclists.
- TRE may be a successful regimen to adopt during training phases if an athlete’s goal is to maintain lean mass or muscular strength while reducing fat mass.
- TRE may exert a small protective effect on the immune response to exercise training in endurance athletes by decreasing the reduction in white blood cell count compared to a normal feeding pattern.
- TRE may improve absolute mean WnT power and total work time by more than one second, which may make the difference between qualifying for or winning events.
- TRE may require an adoption phase longer than 1 week in order for the body to adjust. I.e., individuals may feel worse for the first 1-2 weeks after adhering to TRE.
- It’s likely that IF/TRE affects individuals differently based on age, sex, training style, life stressors and demands, and dietary eating patterns, among other factors.
The take-home message: The current literature shows TRE may yield fractional gains in performance outcomes that may be applicable to elite athletes who seek to get 1% better, but may not have any significant effects for the general athlete, especially if fasting creates or exacerbates physiological or mental stress that impede performance or quality of life. IF/TRE may affect individuals differently depending on various factors listed above and may be worth experimenting, however it may take the body 1 to 2 weeks to adjust to an TRE pattern.
- Individuals should ensure quality lifestyle practices like adequate sleep, hydration, and a balanced diet before fasting, as well as a positive relationship with food.
- Fasting may not be appropriate for those who have a history of eating disorders, disordered eating, or restriction.
- Fasting is not recommended for those who are pregnant or trying to conceive.
- Fasting does not justify “free for all” eating, and total daily calories and macronutrient ratios matter.
- For some individuals, eating in a timed window may cause digestive issues due to an increased volume of food at mealtimes. Thus, some athletes may adjust better to a 10-hour eating window and 14-hour fast to allow additional time for digestion.
- It may take individuals anywhere between 1 to 2 weeks to adjust to IF and individuals may feel worse during this adjustment phase.
- None of the content provided herein is meant to be a substitute for medical advice and individuals are encouraged to consult with their doctor or Registered Dietitian before attempting IF.
Ali McGowan is a Registered Dietitian and Master of Nutrition Science who seeks to break down complex nutrition science into digestible content that inspires long-lasting behavior change. An avid obstacle course racer and lover of all things fitness, Ali is passionate about sports/performance nutrition, gut health, preventive nutrition, and healthy, sustainable weight loss. Follow her on Instagram at @sproutoutloud.
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- Levy E, Chu T. Intermittent Fasting and Its Effects on Athletic Performance: A Review. Curr Sports Med Rep. 2019;18(7):266-269. doi:10.1249/JSR.0000000000000614
- Tinsley et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. doi:10.1093/ajcn/nqz126
- Moro T, Tinsley G, Longo G, et al. Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial. J Int Soc Sports Nutr. 2020;17(1):65. Published 2020 Dec 11. doi:10.1186/s12970-020-00396-z
- Stratton MT, Tinsley GM, Alesi MG, et al. Four Weeks of Time-Restricted Feeding Combined with Resistance Training Does Not Differentially Influence Measures of Body Composition, Muscle Performance, Resting Energy Expenditure, and Blood Biomarkers. Nutrients. 2020;12(4):1126. Published 2020 Apr 17. doi:10.3390/nu12041126
- Correia JM, Santos I, Pezarat-Correia P, Minderico C, Schoenfeld BJ, Mendonca GV. Effects of Time-Restricted Feeding on Supramaximal Exercise Performance and Body Composition: A Randomized and Counterbalanced Crossover Study in Healthy Men. Int J Environ Res Public Health. 2021;18(14):7227. Published 2021 Jul 6. doi:10.3390/ijerph18147227
- Williamson E, Moore DR. A Muscle-Centric Perspective on Intermittent Fasting: A Suboptimal Dietary Strategy for Supporting Muscle Protein Remodeling and Muscle Mass?. Front Nutr. 2021;8:640621. Published 2021 Jun 9. doi:10.3389/fnut.2021.640621
- Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017;37:371-393. doi:10.1146/annurev-nutr-071816-064634