Exercise has become one of the main variables that determine the current and future health status of the population (1, 2). In the same way, nutrition is presented as a key in the longevity process and in the physical and metabolic well-being of our society (3, 4). This “new” science aims to analyze the effect of food consumption on humans.
In recent decades, the eating pattern of the world’s population has changed dramatically towards the consumption of nutritionally poor foods lacking in macronutrients and micronutrients essential for human life. The consumption of diets high in fat, sugar and ultra-processed elements has been shown to have an impact on blood glucose levels, blood pressure and cardiovascular risk parameters in healthy adults and athletes (5).
However, what about diets that seek to improve athletic performance? What elements predominate? What diet is best for an athlete?
Numerous studies have confirmed the benefits that exist between a balanced diet, intermittent fasting and a good diet on different health parameters in the world population (6, 7). The Mediterranean diet has been established as one of the diets that most helps to improve the quality of life during adulthood and old age. In addition, it has been shown that it is the type of diet that has the best relationship with cardio-metabolic risk parameters. However, when we talk about performance there are different trends in relation to food. For example, many athletes follow the so-called “paleo diet” which is a type of diet that tries to imitate the diet that our ancestors followed during the Paleolithic era (hence its name). The study by Andikopoulos (2016) showed that following the paleo diet improves fasting blood glucose levels
On the other hand, the ketogenic diet is a type of diet where carbohydrate intake is reduced to the maximum and where the main diet comes from the consumption of proteins and fats. This type of diet has also been shown to be beneficial for athletes by improving the power at which the lactate threshold is reached and its maximum oxygen volume (9). Finally, intermittent fasting is considered a type of diet marked by a time restriction. That is, dividing the 24 hours of the day into two moments (food and fasting) in order to reduce the time of food consumption. In this way, we will be able to accustom the body to spending long periods of time without consuming any food.
This type of nutritional strategy is influenced by the eating pattern that our ancestors followed and what we are genetically prepared for. So that we understand it better, we are going to talk about epigenetics. Epigenetics is the science in charge of studying the alterations that occur in human DNA. In this sense, the human being has not suffered an exacerbated modification of its genetic expression so, in theory, our organism is still prepared to go without food for long periods of time and to carry out physical exercise on a regular basis. That is why, in recent years, we see how people who lead a sedentary life in which they spend most of the time sitting and maintain a nutritionally poor diet develop serious health problems at a very early age. On the contrary, people who lead a lifestyle similar to that of our ancestors and eat in a similar way obtain a clear improvement in metabolic parameters (10-12).
There are different types of intermittent fasting that vary depending on how long you spend fasting or eating. In order to correctly understand this part, we will define the feeding window as the time that is dedicated to eating food and the fasting window as the time when no food can be eaten.
The simplest method of intermittent fasting is 8/16 in which the feeding window is 8 hours and the fasting window is 16 hours. This time distribution is usually very comfortable when intermittent fasting begins because the hours of sleep are used as fasting and the feeding window does not open until approximately 2:00 p.m. The most typical intermittent fasting is to maintain a feeding window from 2:00 p.m. to 10:00 p.m. as it allows for lunch, snack and dinner, and a fasting window from 10:00 p.m. to 2:00 p.m. In addition, this intermittent fasting has already been shown to have benefits in fat% levels (13) in people with obesity. On the other hand, the study by Varady et al. (2013) also showed that following this type of intermittent fasting could reduce weight and % fat in adults with normal weight and overweight without reducing % lean mass (14). That is, the people who followed this eating pattern maintained their muscle mass and only decreased the % fat. Finally, the study by Kroeger et al. (2012) confirmed that following intermittent fasting reduced visceral fat levels and modulated the expression of adipokines such as leptin, which is an appetite regulator (15).
Another method of intermittent fasting is 4/20 in which the feeding window is shortened to 4 hours and the fasting window is 20 hours per day. Understandably, this type of intermittent fasting is much more restrictive and more difficult to apply. It is not recommended to start the intermittent fasting methodology with this type of fasting due to the possible side effects that can occur such as fainting, nausea or weakness. There is currently no scientific evidence on the possible benefits of this very restrictive fast. However, it is worth mentioning that there are studies where the fasting time has been more than 6 days (only drinking water and being fully monitored in the hospital) that have produced improvements in blood pressure levels in hypertensive people (16). This is why fasting can be presented as an alternative to conventional treatments for different metabolic diseases. But what about intermittent fasting in athletes? Will it offer improvements?
One of the first studies that we wanted to mention in relation to intermittent fasting and sports performance is the one carried out by Burke and King (2012) (17). They analyzed how Ramadan (the fast performed by Muslim religionists), which does not allow them to eat throughout the day, could affect athletic performance. The conclusion of this study establishes that physical performance worsens both in training sessions and during the sporting event due to the lack of nutrients. However, a certain type of intermittent fasting pattern can be interesting for athletes depending on its modality. There are modalities where the energy produced by the body comes mostly from anaerobic glycolysis meaning there is a predominance of the use of carbohydrates to produce energy. In this sense, the studies that have been carried out so far show that the follow-up of intermittent fasting is not good for modalities of this type where sprints are carried out or go at a high intensity (18, 19). In this line, studies such as that of Naharundin (2018) and Terada (2019) have also verified that a period of fasting prior to a sports effort compromises sports performance, worsening power and exercise time (20, 21). When it comes to long-term sports, intermittent fasting is not known to improve or worsen any of the brands.
Regarding strength training and intermittent fasting, the results of different scientific studies are not conclusive.
Thus, for example, Tinsley et al. (2019) studied the effect of intermittent fasting on the adaptations of strength training in women and concluded that this type of eating pattern did not modify the adaptations to exercise (22). In the same way, his research group conducted a similar study in men and they obtained very similar results. They compared two groups of men who performed the same training while some performed an intermittent fasting with a 4-hour window of intake and the other had a regular meal. The result of the study concluded that there were neither better nor worse adaptations to exercise in the group practicing intermittent fasting (23).
In view of the existing results, it cannot be confirmed that intermittent fasting is an ideal strategy for athletes of endurance and strength sports. On the other hand, we must be cautious with the studies mentioned since many of them contain methodological errors or simply did not perform intermittent fasting in the best possible way. And, furthermore, as we have been able to verify, several of them used Ramadan as “intermittent fasting”. There is no doubt that more scientific studies are needed to analyze the effect of intermittent fasting in both endurance and strength athletes to obtain a clear conclusion.
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- Zajac A, Poprzecki S, Maszczyk A, Czuba M, Michalczyk M, Zydek G. The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists. Nutrients. 2014;6(7):2493-508.
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- Moro T, Tinsley G, Bianco A, Marcolin G, Pacelli QF, Battaglia G, 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):1-10.
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- Cherif A, Meeusen R, Farooq A, Ryu J, Fenneni MA, Nikolovski Z, et al. Three days of intermittent fasting: repeated-sprint performance decreased by vertical-stiffness impairment. International journal of sports physiology and performance. 2017;12(3):287-94.
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- Terada T, Toghi Eshghi SR, Liubaoerjijin Y, Kennedy M, Myette-Côté E, Fletcher K, et al. Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance. J Sports Med Phys Fit. 2019;59:357-65.
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- Tinsley GM, Forsse JS, Butler NK, Paoli A, Bane AA, La Bounty PM, et al. Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European journal of sport science. 2017;17(2):200-7.