I’m sure you’ve heard this diet being thrown around in the health and fitness world as being a “fat-loss miracle” and other similar buzzwords and phrases. It’s quite trendy right now and has been for the past few years.  

Intermittent fasting (IF) has several variations, but it what it comes down to is fasting a for a predetermined amount of time, followed by a “feeding window” in which you are “allowed” to eat your allotted calories and macronutrients for the day. Probably the most common type of this diet is called the 16/8; most often implemented by those in physique sports such as bodybuilding. This is where your fasting window is 16 hours long, and your feeding window is 8 hours long. 

However, have you ever thought of how this diet could possibly help your performance both in competition and in practice? As more and more research has been released on the diet, scientists are examining the ways in which it not only affects performance directly but also how its metabolic effects play a role on your performance indirectly as well.  

Effects on General Health and Well-Being 

Before we can begin to understand how it affects our performance, let’s dive into how this diet works on our bodies.  

Both human and animal model research has shown that following IF has the ability to reduce the risks of obesity and other metabolic disorders due to its effects on metabolism and biomarkers such as insulin and glucose control [1]. Plus, the great thing about IF is that it poses no additional risk when compared to traditional calorie restriction.  

Probably why this diet works well for so many people has to do with total calorie consumption. When compared to traditional dieting, those who follow IF have been shown to eat fewer calories throughout the day, sometimes eating only one meal a day! [2].  

So for weight loss, it can definitely help those who have trouble with overeating to regulate their calories throughout the day and avoid excessive snacking. But what about for endurance athletes, does any of this play a role and provide us with any tools in improving our performance? Well, it’s a little less clear.  

Effects on Aerobic Performance 

Unfortunately, most of the studies out there on intermittent fasting focus on resistance training and strength sports. Either that or they focus on its general health effects, often in overweight or obese individuals.  

But not to fear! There is hope! We still have data to work with.  

One study that worked with elite judo athletes during Ramadan (a Muslim holiday in which they fast from sunrise to sunset) analyzed maximal aerobic capacity utilizing something called the Multistage Fitness Test [3]. This test, more commonly known as the Beep Test, is utilized by various athletic organizations internationally. It predicts an athletes VO2 max (a staple variable in aerobic/endurance-based activities) by having them run back and forth between two lines before a beep sound is heard. The test becomes more difficult as the beeps get faster. The test ends when the athlete doesn’t reach the line in time before the beep is heard [4].  

Interestingly, the researchers discovered that partaking in intermittent fasting during Ramadan did not cause a detriment to their performance. However, what did occur was a reduction in body weight (an average of 1.8%) and increased levels of perceived fatigue.  

Now, as previously stated body weight reduction most likely has to do with eating fewer meals. The lower the frequency of meals, the more likely the athlete won’t eat as many calories. But the most compelling thing here is the result relating to perceived fatigue. This basically means that the subjects more often than not felt more tired when following this type of eating pattern compared to when they ate a normal diet. However, this happened despite a lack of drop in performance. So we’re honestly quite unsure what aspect of IF causes this phenomenon. 

Here’s where it gets complicated, as other studies have illustrated varying results. In another study examining Ramadan fasting (which is one of the most practical ways to research IF in the scientific literature), professional soccer players had a 16% decrease in the total distance that they could cover during a 12-minute run [5]. Also, another study used a test called the Leger shuttle-run to assess aerobic performance, which is similar to the Beep Test. While performance decreased during the second week of fasting, it returned to normal back during the fourth week [6]. So the early onset of detrimental effects was not that significant when viewed over the grand scheme of things.  

So What Does This All Tell Us? 

From a weight loss perspective, IF appears to have great effects due to the energy deficit created from eating less meals per day. However, there’s still much work to be done with IF and its applications to aerobic and endurance performance. From the data that we have at our disposal right now, it looks like it doesn’t seem to make all that much of a difference which type of diet you follow, as long as it’s sustainable for you and is aligned with the whatever performance goal you have in mind. So experiment on yourself, see if it works, and remember the best diet (and workout plan) is the one an athlete will be most consistent with.

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1. Patterson, R. E., & Sears, D. D. (2017). Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition, 37(1), 371-393. doi:10.1146/annurev-nutr-071816-064634 

2. Carlson O, Martin B, Stote KS, Golden E, Maudsley S, et al. 2007. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. Metabolism 56:1729–34

3. Chaouachi, A., Coutts, A. J., Chamari, K., Wong, D. P., Chaouachi, M., Chtara, M., . . . Amri, M. (2009). Effect of Ramadan Intermittent Fasting on Aerobic and Anaerobic Performance and Perception of Fatigue in Male Elite Judo Athletes. Journal of Strength and Conditioning Research,23(9), 2702-2709. doi:10.1519/jsc.0b013e3181bc17fc 

4. Léger, L.; Lambert, J.; Goulet, A.; Rowan, C.; Dinelle, Y. (June 1984). “[Aerobic capacity of 6 to 17-year-old Quebecois–20 meter shuttle run test with 1 minute stages]”. Journal Canadien des Sciences Appliquées Au Sport. 9 (2): 64–69 

5. Zerguini, Y., Kirkendall, D., Junge, A., & Dvorak, J. (2007). Impact of Ramadan on physical performance in professional soccer players. British Journal of Sports Medicine, 41(6), 398-400. doi:10.1136/bjsm.2006.032037 

6. Kirkendall, D. T., Leiper, J. B., Bartagi, Z., Dvorak, J., & Zerguini, Y. (2008). The influence of Ramadan on physical performance measures in young Muslim footballers. Journal of Sports Sciences, 26(Sup3). doi:10.1080/02640410802422199 

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