Intermittent fasting (IFast) refers to the idea of abstaining from eating food for a certain period during a day and consuming calories ab lib during the eating hours. The idea of IFast and its potential benefits has been around in a variety of ways for most of human history, especially in the setting of religion (1–3). The reported benefits of IFast include weight loss, reductions in blood pressure, and improvement in markers of metabolic disease risk (1,4–7). IFast has garnered more public attention with recent endorsements by celebrities and athletes, and it has been featured in numerous magazines, such as Time, Men’s Health, and Cosmopolitan. As the popularity of IFast increases, it is important for the health care physician to be able to counsel athletes on the effects of the IFast diet on athletic performance.
There are numerous definitions that encompass the term “intermittent fasting” which range from restriction of caloric intake during designated hours of the day (time-restricted eating) to partial or complete restriction of caloric intake for multiple days of the week (alternate day fasting or whole day fasting) (8). The 16/8 diet consists of fasting for 16 consecutive hours and eating ad libitum for 8 h. Other time-restricted eating protocols call for 20 h of fasting and 4 h of eating ad libitum. The 5:2 diet is an alternate day fasting protocol where one alternates days with no calorie consumption and days consisting of food and liquid consumption ad libitum. There are fasting regimens that also fall somewhere in between 16/8 and 5:2, including 18/6 and every other day fasting (9). Fasting also is a key component in many religions, one notable example is Muslims fasting during Ramadan, a month-long period when no food or liquid is consumed during daylight hours. Studies on Ramadan have helped clinicians get a better understanding of IFast and sports.
Several studies and reviews have looked at how the body reacts to fasting (8,10,11). The first phase in fasting consists of the postabsorptive phase, which begins around 3 to 8 h into fasting (depending on the makeup and quantity of the last meal) and lasts about 12 to 18 h. During this period, the blood glucose level is maintained by glycogenolysis from the liver. To keep up with the body's metabolic rate (which appears to remain the same despite fasting), whole-body lipolysis and fat oxidation increases. Thus, there is an increase in free fatty acids in the blood, which becomes a source of fuel for muscles (12). In regard to protein metabolism during IFast, Soeters et al. (13) found that short-term ADF did not alter whole body protein. Energy used in the first 2 to 3 d of fasting appears to come from glycogen and fat stores, as protein catabolism does not increase until 36 h into fasting (8).
High-intensity exercise, such as cycling to exhaustion or sprinting, is dependent on carbohydrate availability, thus there is concern about IFast and high-intensity exercise. Several studies have looked at the effect of fasting on performance in these glycogen-dependent exercises, and the majority of these studies have been done in athletes practicing Ramadan (12,14). In 2012, Chaouachi et al. reviewed the studies on fasting during Ramadan and athletic performance, in particular sprint performance. Results from these studies were not homogeneous in their findings but most studies show a small decrease in mean sprint performance that did not meet statistical significance. Since the review by Chaouachi, multiple newer studies have looked at repeated sprints during fasting during Ramadan (15,16). Aziz et al. (15) looked at sprint times before, during, and after Ramadan in a cross-over study. Sprint times throughout a 60-min period of intermittent sprint exercise were significantly slower in the fasting period, even controlling for preexercise conditions like nutrition, sleep, and training loads. Cherif et al. looked at repeated sprints during a 3-d Ramadan-like fast (16). They found that after a 3-d IFast, the fasting group had impaired speed and power.
Outside of observational studies examining the effects of fasting during Ramadan, there are only a few studies examining the effects of IFast on high-intensity exercises. These non-Ramadan-associated studies have mostly been done in cyclists and varied in the types of fasting, but these studies do show a similar loss in performance compared with nonfasting particularly in measures, such as peak power output and time to exhaustion (17,18). In 2018, Naharudin et al. (14) looked at IFast effects on Wingate anaerobic power and cycling time to exhaustion. The fasting group omitted lunch. The fasting group had reduced Wingate performance (absolute peak power output) on day 2 of testing but the performance restored to baseline by day 4 and onward. Time to exhaustion in high-intensity cycling was reduced during the 10-d period of the study with a reduction of the effects size at the end of the 10 d. They concluded that the IFast must exceed 4 to 10 d to ensure that the initial detrimental effects on performance are regained.
In 2019, Cornford and Metcalfe (19) studied the effects IFast and 2000 m rowing time trial performances. IFast in this study was defined as skipping breakfast (which mimics a 16:8 IFast regimen). It was a randomized and counterbalanced crossover study with ten competitive rowers (eight females). Cornford and Metcalfe found an impairment of rowing time trial performance in the IFast period when compared with the breakfast eating phase. In a similar study, Clayton et al. (20) found that omitting breakfast might be an effective means of reducing daily energy intake (energy intake 19% ± 5% greater in breakfast group) in cyclists but it impaired performance later in evening cycling tests.
In 1993, Aragon-Vargas (11) did a review of the literature on fasting from 24 h to 4 d and its effects on endurance exercise. He found a negative effect of fasting on endurance in all but one study. Since that time, a majority of the literature has looked at IFast only in the context of overnight fasts. In 2018, Aird et al. (21) did a systemic review and meta-analysis of the effects of fasting on performance. They looked at fasting of greater than 8 h and excluded Ramadan-based studies to avoid confounding factors of sleep and reduced caloric intake. Of the studies looking at aerobic exercise, 43 of 46 evaluated only overnight fasting while the other three looked at fasts from 27 h to 44 h. Outcomes were varied with 4 (57%) of 7 studies of exercise <60 min showed no difference between fasting and fed states. Similarly with >60 min of aerobic activity, 6 (46%) of 13 studies showed no difference between the two groups.
In 2015, Clayton et al. (20) looked at the effect of skipping breakfast on exercise performance. Athletes completed a 30-min cycling exercise at ~60% V˙O2peak followed by a 30-min maximal cycling performance test the evening of breakfast omission. They found that athletes who skipped breakfast had greater energy intake at lunch than those who did not skip breakfast and performed a lower workload over a 30-min performance test that evening.
Most studies have looked at short-term fasting and performance, but most studies do not look at effects of long-term IFast. As cited earlier, it may take more than 10 d for the body to adapt to the IFast. Most long-term fasting studies looking at performance have been done in animals. In 2018, Marosi et al. (22) assigned male mice to ad libitum feeding or alternate-day food (ADF) deprivation and put them on a treadmill daily for 1 month. They performed a run to exhaustion endurance test after 1 month. They found that mice with ADF deprivation ran significantly further and for longer periods than ad libitum feeding group. They concluded that the training effect for the 1 month was enhanced when mice ate an ADF diet. They hypothesized that this was due to the switch from carbohydrates to fat as a fuel source.
The effects of prolonged IFast in nonhumans may not translate to humans. Zerguini et al. (23) looked at Algerian football players during Ramadan and found that endurance was negatively affected during the IFast, with a 16% decrease in endurance capacity. However, Kirkendall et al. (24) found no effect of IFast during Ramadan on endurance in Tunesian football players. In 2010, Aziz et al. (25) found that moderately trained men who were fasting for Ramadan had a small decrease in distance ran during a 30-min timed trail. Multiple other studies on the effects of IFast during Ramadan and endurance have shown a mix in results as well of decreased effect or no effect (26–28). None of the studies show a performance benefit from IFast.
IFast with a reduction of energy intake and body weight could lead to reductions in lean body mass and strength and, therefore, theoretically, a reduction in the ability to perform resistance training (29,30). In 2017, Tinsley et al. (31) looked at the effects of time-restricted eating (20:4) on resistance training. They looked at healthy active men without a consistent resistance training regimen and randomized them to an IFast program of 4 h of eating and 20 h of fasting for 4 d·wk−1. They found that after initiating a resistance program 3 d·wk−1 for 8 wk, total body composition was not affected and cross-sectional area of muscle was not negatively affected.
In a similarly designed study, Moro et al. (32) looked at the IFast (16:8) program and its effects on maximal strength. Unlike the study by Tinsley et al., participants were not naïve to resistance training programs. Healthy males were randomly assigned to IFast versus normal diet. Both groups performed a resistance training program for 8 wk. After 8 wk, the IFast group had a decrease in fat mass compared with the normal diet group (despite similar kilocalories consumed) but muscle area of the arm and thigh and maximal strength were maintained.
Only a few studies have looked at the effect of fasting during Ramadan on resistance training. In 2011, Trabelski et al. (29) looked at 16 Turkish bodybuilders during Ramadan and found that there was no difference in body mass or body composition between fasted and fed states. In 2007, Karli et al. (33) looked at Turkish male elite power athletes and found no negative effect on power output as long as they maintained daily energy intake and sleep hours.
Studies evaluating the effects of IFast and athletic performance are varied in the definition of fasting and, thus, not surprisingly, they are varied in their outcomes. There is a paucity of research regarding common specific IFast protocols like 5/2 (5 d fasting and 2 d ad lib eating) or 16:8 (16 h of fasting and 8 h of eating). Most of the conclusions made about fasting and performance have been extrapolated from fasting during Ramadan data or overnight fasts that were <12 h. Unlike 5:2 and 16/8 fasting regimens, fasting during Ramadan focuses on no food or liquid during daylight hours, confounding performance evaluations with changes in sleep and concern for dehydration. In this review, we looked at the data available for various fasting protocols, including during Ramadan and how it effects three different types of exercise; high intensity, endurance, and resistance.
Multiple studies have looked at sprinters fasting during Ramadan and found a negative effect of fasting on sprint times. Other studies have looked at IFast and cycling times and found reduced performance. Interestingly, the effect of IFast was diminished after a few days of fasting. These results imply that the body may undergo adaptation changes that allow recovery of performance.
Most of the IFast research has focused on endurance training. Many of these studies looked at overnight fasting and Ramadan, while only a few looked at fasting regimens that more closely mimicked IFast. In recent studies, outcomes have been variable with some studies showing negative effects and others showing no effect, but no studies showing beneficial effects. This variation in results of negative and no effect is likely due to a variety of the type of athletes tested, fasting protocols tested, trial protocols, and the studies' outcomes measures. One study in mice suggested that alternate-day food deprivation, closest to the 5:2 diet, led to an improvement in endurance performance. This has not been replicated in humans but would be an important next step in studying long-term IFast in athletes.
For resistance training, one would expect an increase in loss of lean tissue. Two recent studies looking at IFast and resistance found minimal negative effects of IFast in resistance training. This was similarly concluded in athletes observing fasting during Ramadan. Again, these results were only looked at during a short period of IFast and further adaptation in the body might occur with longer fasts. The study by Moro et al. did not support this finding as they found a decrease in testosterone and IGF-1 at 8 wk in the IFast group, which may lead to a negative effect on performance in the long term (32).
Lastly, there is a paucity of data regarding using IFast to rapidly drop weight in sports that have weight classes. Despite prior studies showing negative effects of rapid weight loss on performance, competitors in different sports continue the practice (34). Aloui et al. (34) review rapid weight loss during Ramadan. Of note, weight loss methods in the studies reviewed did not include specific IFast regimens. Multiple studies showed a decrease in performance during Ramadan with rapid weight loss. However, in the context of Ramadan, results are confounded by the fact that athletes are unable to restore their fluid deficits after weigh in. Regardless, as IFast is a long-term weight loss regimen, it is unclear if it could be used for a rapid weight loss strategy. Further research needs to be done on the effects of using IFast for “making weight” on sport performance.
In conclusion, studies on athletic performance during IFast are varied, and results have not been uniform, except in the fact that none of the studies showed an improvement in any type of athletic performance. As more and more athletes look to adopt an IFast diet for its benefits of weight loss, sports medicine physicians and other health professionals need to be aware of the negative or neutral effects on athletic performance. Further studies should be done to look at specific long-term fasting regimens and their effects on performance.
The authors declare no conflict of interest and do not have any financial disclosure.
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