Intermittent Fasting for Athletes: What Does the Research Say?

Intermittent fasting is an increasingly common way of eating popularized over the last couple of years by a variety of people from medical doctors to internet gurus. In essence, intermittent fasting means restricting when you consume food—your eating window—to a set number of hours. The eating window most commonly is eight hours, with individuals eating their first meal at midday and their last meal at 8 pm, therefore fasting for 16 hours.

Fasting has garnered a lot of interest over the years since research in animals has shown that those consuming fewer calories (caloric restriction) tended to live longer. Recent research suggests that even in the absence of caloric restriction, regular or semi-regular periods of lower food intake (i.e., a fast) can have some important health benefits.

#Fasting has garnered interest since research in animals has shown that those consuming fewer calories tended to live longer, says @craig100m Share on X

The research on intermittent fasting is somewhat scarce, with intervention studies both low in number and participants. Nevertheless, there are some promising early findings, including behaviors associated with enhanced weight loss in some studies and improved metabolic markers in others. Interestingly, these improvements occur even when calorie intake matches non-intermittent fasting behaviors, suggesting that the improvements are not due solely to consuming fewer calories.

Insights from Athletes Observing Ramadan

Of course, athletes are always looking for the next edge they can gain over their competitors, and intermittent fasting potentially represents such an edge. As you might expect given the paucity of research on intermittent fasting in non-athletes, research in elite athletes is essentially non-existent, which makes it difficult to draw firm conclusions.

Fortunately, there is a group of people that acts as natural participants in an experiment, allowing us to understand the impact of intermittent fasting on sporting performance. Every year, millions of Muslims observe Ramadan, a religious period in which people are unable to consume food or liquid during daylight hours—from dawn to sunset.

Clearly, this doesn’t act as a perfect proxy for intermittent fasting. Usually, intermittent fasters eat later (or, in some cases, earlier) in the day, and are free to consume fluids at any point. During Ramadan, observers can’t consume liquid, which skews the results slightly, and they often eat overnight, which can negatively impact sleep. Nevertheless, there are some lessons we can learn.

So, what does the science say about performance during Ramadan? Some studies found that performance declines. In a group of professional soccer players, speed, agility, dribbling speed, and endurance were reduced during Ramadan. Similarly, just three days of intermittent fasting during Ramadan significantly reduced speed and power ability. A 2009 review found that a number of studies across different groups of athletes reported various decreases in performance. It seems clear that the altered eating patterns of Ramadan pose a significant problem for athletes.

But drawing conclusions from these studies and attempting to apply their findings to intermittent fasting is difficult. For a start, those who fast during Ramadan tend to consume far fewer calories than normal, which by itself will reduce performance, and often they suffer disturbances to their circadian rhythm caused by overnight eating. They also can’t consume any fluid in the day, which again limits performance.

A more recent review concluded that, as long as athletes maintain their total intake of calories and micronutrients and their typical sleep quality, they likely would not see any negative effects on performance. This is similar to the International Society of Sports Nutrition’s position stand on nutrient timing: total energy and nutrient intake are what’s important. For most people most of the time, the timing of these factors is less important provided they consume enough food and fluid.

Intermittent Fasting and Adaptation

In fact, more recent research suggests that, for some athletes, periods of time with less—or even no—energy intake may help enhance performance. An important training adaptation for endurance athletes is the production of new mitochondria, called mitochondrial biogenesis. When endurance athletes train, their bodies detect when energy stores are getting low, which releases several signaling factors including AMPK. These signaling factors, in turn, stimulate many of the positive adaptations to endurance exercise, including mitochondrial biogenesis.

As such, training in a low energy state is associated with enhanced training adaptations in some specific measures. It’s worth noting, however, that these studies often use a low carbohydrate diet as opposed to completely fasted training, as would be the case in intermittent fasting.

If you’re involved in resistance training, a recent review suggests that carbohydrate restriction—often induced by intermittent fasting—may reduce subsequent muscle hypertrophy as well as the ability to undertake lengthy training sessions, which are common for elite athletes.

So periods of lower carbohydrate intake, which we might consider as a proxy (although not a perfect one) for intermittent fasting, may enhance or reduce training adaptations, depending on your overall training goal.

As such, it’s difficult to state objectively whether intermittent fasting is a good or bad idea for athletes since it depends on many factors.

If you train in the morning, intermittent #fasting may compromise training adaptations, performance, says @craig100m. Share on X

For example, if you train first thing in the morning, but don’t eat anything until the late afternoon, you compromise your training adaptations. Similarly, not eating before a high-intensity exercise, such as sprints and resistance training, also will likely reduce training performance and, in turn, hamper competition performance. If you train later in the day, however, there doesn’t seem to be any harm in intermittent fasting as long as you consume sufficient energy and nutrients in your pre- and post-training meals.

The only downside is when it comes to protein intake. While by far and away the most important factor with protein is total daily intake, some evidence suggests that eating 20-40g every 3-4 hours provides a small advantage regarding muscle protein synthesis, which could affect muscle recovery and growth. And though the impact of this may be small, it could make all the difference for elite athletes.

Key Takeaways

In summary, we can conclude that:

1. 1. It’s most important to consume sufficient energy and nutrients to support performance. If you can consume the energy, nutrients, and protein you need from your meals—regardless of whether they come from one meal or ten—you’re most of the way there.
   2. Of course, elite athletes tend to have greater nutrition requirements than “normal” people. When I was competing in bobsleigh, I weighed 95kg and trained for 4-5 hours daily. As such, I had to consume around 4000 calories per day, which included 200g of protein. Consuming all this in one or two meals, or a short time window, would have been hugely impractical and likely impossible due to feelings of fullness.
   3. An important consideration is that even when high-level athletes follow free eating (meaning eating that’s not time restricted) patterns, research suggests that they don’t consume enough energy. As a result, we can speculate that consuming more energy with fewer meals during intermittent fasting is unlikely to occur due to the huge volumes of food required.
   4. It also seems like a very good idea to consume energy, protein, and nutrients before, and in most cases for elite athletes, during training sessions. This may not be possible during intermittent fasting, depending on the eating window and the time of day the athlete trains.
   5. You definitely have to consume energy, protein, and nutrients after training (although likely not immediately after). This might be possible with intermittent fasting depending on both the training time and eating window.
   6. If you have more than one training session per day—as many elite athletes do—intermittent fasting could make it harder to consume food both before your first session and after your second session.

Some evidence suggests that regular intakes of protein spaced throughout the day are better than larger intakes consumed less often. While the impact might be small, it could make the difference for elite athletes, and intermittent fasting would preclude this.

Concluding Thoughts

Given that high-level athletes require much more nutrition than non-elite athletes and must spread out these nutrients across the day when they have multiple daily sessions, this time-restricted approach seems to be a sub-optimal option. However for recreational athletes who train for shorter durations and with less intensity—and therefore have lower energy demands—intermittent fasting may be appropriate, especially for weight management.

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