Intermittent Fasting and Sports Performance

Possible Benefits of Intermittent Fasting in Endurance Athletes

Studies on IF in endurance athletes have often been conducted during the religious fasting period (R-IF) [60,61,63,64], with few studies investigating the effects of time-restrictive eating (16:8) on endurance performance and health-related effects [62,65]. Fasting diets may alter metabolic pathways in the body by acting as a potential physiological stimulus for ketogenesis [155], regulating metabolic, hormonal and inflammatory responses [61], and stimulating mitochondrial biogenesis and suppressing mTOR activity [155], and regulating body composition [62,65].

Energy restriction/fasting for more than 12 to 16 h leads to a metabolic switch in basic energy fuels from carbohydrates to fats, resulting in metabolic ketosis, the same as the ketogenic diets [155]. These KD-like alterations in substrate uses are believed to serve as an inductor for fat oxidation, and a preservative for muscle mass and function [156].

The effect of fasting diets on muscle cells is generally known to be similar to aerobic exercise, including stimulation of mitochondrial biogenesis and suppression of mTOR activity [157]. However, the main mechanism on fasting diets is driven by fatty acid metabolism and peroxisome proliferator-activated receptor delta (PPAR-d), instead of Ca2+, which is known to be effective in aerobic exercise [155]. Although the main mechanism on muscle cells differs between exercise and fasting diets, research findings sugges<sup>t</sup> that application of a fasting diet along with exercise could switch cellular metabolism from glucose to ketone bodies [156], thereby inducing ketone utilization, which might, in turn, trigger mitochondrial biogenesis and preserve muscle mass [158]. Although the potential benefits of IF on mitochondrial biogenesis and mTOR activity appear promising, no study has investigated these metabolic interactions in endurance athletes adhering to IF.

The impact of R-IF on hormonal, metabolic and inflammatory responses is a lessstudied point in terms of IF diets. In a study on middle-distance runners, Chennaoui et al. [61] examined the effects on R-IF on the hormonal, metabolic and inflammatory responses in a pre–post-test study design. Researchers applied a maximal aerobic velocity test 5 days before, 7 and 21 days after Ramadan. No change was observed in the testosterone/cortisol ratio during the RIF trial. A significant rise was reported in IL-6, adrenaline, and noradrenaline concentrations after the RIF; however, all parameters returned to baseline levels 7 days after exercise [61]. More work is needed to interpret these results effectively.

Another aspect of IF is its impact on the body composition of endurance athletes. Studies on endurance athletes and TRE (16:8) revealed that TRE caused a meaningful decrease in BW and body fat percentage in endurance athletes [62,65]. Moro et al. [62] claimed that although VO2max and endurance performance did not change after a 4 week TRE, a meaningful rise in the peak power output/BW ratio was due to the BW loss. However, another study showed a decrease in TT performance ( −25%) and no improvement in running efficiency after R-IF in well-trained middle-distance runners [65]. Taking these studies into account, although IF may provide some benefits by decreasing BW and body fat percentage, we cannot assume that it positively affects endurance performance.

#### Risks to Be Considered When Applying Fasting Diets

Potential risks of IF diets are reduced endurance capacity [60], increased fatigue [61,63], altered sleep habits (i.e., delayed bedtime, decreased sleep time) [61,63,64], and dehydration [159] in endurance athletes.

Studies on IF diets and endurance capacity and performance-related parameters have produced conflicting results in endurance athletes [60,62,64]. Both R-IF and TRE studies on endurance athletes stated that IF diets had no influence on the aerobic capacity, determined by VO2max [60,62,64]. Additionally, one study on TT performance and R-IF in well-trained middle-distance runners showed that R-IF caused a decrease in TT performance [60]. However, another study determining the impact of the CHO mouth rising technique on 10 km TT performance declared that the CHO mouth rising technique provided benefits by increasing 10 km TT performance [64]. For TRE and endurance performance, Moro et al. [62] revealed that a 4 week TRE had no impact on endurance performance. As for evaluating performance-related parameters, several researchers investigated the exerciseinduced fatigue, blood lactate, glucose, and insulin concentrations in endurance athletes [61, 63–65]. Exercise-induced fatigue, as determined by the Fatigue score [61] and the Rated Perceived Exertion (RPE) Scale [63], increased after a maximum aerobic speed test and an intensive endurance training, while it decreased significantly in an R-IF trial applying mouth rising during a 10 km TT performance [64]. One TRE study also showed that blood lactate, glucose and insulin concentrations did not alter during an incremental test [65]. We know that endurance exercise lasts more than an incremental test duration. Therefore, although blood parameters were well-maintained during an incremental test, we cannot interpret the study as the parameters will be preserved during prolonged strenuous exercise. Since there are few studies on endurance performance and IF, further studies should be conducted with an exercise protocol similar to races and competitions, including all performance-related parameters.

One study assessed the effect of R-IF on cognitive function in a non-randomized, repeated-measures, experimental design manner [63]. No difference was observed in cognitive performance, measured using reaction time and mean latency times on simple and complex tasks during Ramadan in trained male cyclists. Therefore, the implementation of IF diets to increase endurance capacity, improve performance-related parameters or cognitive performance does not appear to be a well-approved strategy. On the other hand, it would be wrong to refer the IF diet as a detrimental strategy due to the controversial findings of studies. Further, a review of the role of R-IF in sports performance, which included well-controlled studies, reported that although R-IF generally affected athletic performance with a few declines in physical fitness at a modest level, including perceived exertion, feelings of fatigue, and mood fluctuations, these negative effects may not cause a decrease in sports performance [160]. Furthermore, while prolonged fasting has detrimental effects on endurance performance by decreasing endurance time and causing carbohydrate depletion, hyperthermia, and severe dehydration [161–164], IF causes preventable adverse effects on performance [160].

An important factor among the difficulties that IF can cause is the alterations in sleeping habits of endurance athletes who practice R-IF [61,63,64]. During R-IF, in contrary to other IF diets, sleeping periods alter due to the difference of fasting/feeding cycle, thereby disturbing the circadian sleep/waking rhythm [160]. These changes may trigger general fatigue, mood, and mental and physical performance in endurance athletes. A study on 8 middle-distance athletes who maintained training during Ramadan revealed that R-IF affected physical performance by disturbing sleeping habits, creating energy deficiency, and fatigue [61]. Another study on cyclists showed a significant reduction in the duration of deep and REM sleep two weeks after starting R-IF, although total sleep time was unchanged [63]. On the other hand, a study on adolescent cyclists also reported no change in total sleep time following R-IF [64]. As sleep is one of the major components for maintaining metabolic health and performance [165], during Ramadan IF, the sleep cycle of endurance athletes should be carefully monitored and effective sleep strategies should be developed for this period. Further, in order to determine the effects of Ramadan IF on sleep patterns more accurately, more objective sleep measurements should be applied.

Another adverse effect of R-IF on endurance athletes is the deterioration of hydration conditions before, during and after exercise [159]. Starting competition in euhydrated state is one of the key factors for greater performance [166]. Further, providing adequate fluid ingestion during exercise, especially prolonged strenuous training, has a major impact on body fluid homeostasis. Although glycogen breakdown provides an average of 1.2 L water [155], it is still not enough to meet the body fluid need during the marathon, especially in hot weather conditions [156]. Therefore, fasting due to lack of water/fluid consumption can create adverse health problems beyond performance detriments [146]. Although TRE diets allow the consumption of water and unsweetened coffee and tea, R-IF has restrictive rules that forbid the consumption of anything during the fasting state [127]. Therefore, the water balance and fluid strategy of endurance athletes should be carefully planned, especially for endurance athletes applying R-IF diets.

The adverse effects of the IF diets also vary according to the weather conditions during fasting, training severity, training load, and training level of athletes [159]. These factors and, more importantly, endurance athletes' ability to cope with these metabolic changes determines how their sports performance will be during Ramadan. Evidence suggests that the performance success of athletes following an IF diet depends on their energy availability and macro and micronutrient intake, as well as training load and sleep length and quality [167]. Chennaoui et al. [61] suggested that athletes struggling with R-IF can reduce the negative effects of IF by reducing their training load and taking daytime naps.

Taking all studies into account, the efficiency of IF to improve exercise capacity and performance-related parameters still remains uncertain. Therefore, as we consistently repeat in the review, more work is needed before recommending these diets, especially in hot environments or during intense training periods. Since many Muslim athletes follow a

month-long R-IF diet for religious reasons, even if there is a major competition or tournament [160], we need to develop effective strategies to maintain endurance performance and inhibit any decrease in endurance capacity during Ramadan.
