Is Intermittent Fasting Good for Athletes?

This time of year, we are inundated with promises of a diet or supplement that can do everything from drop inches from your waist in a matter of days to those that purport to resolve any malady that ails you. These are enticing, but often seem too good to be true. 

What if I were to tell you that there was a diet that could not only help you lose weight, but live longer? Many popular diets make such grandiose claims, but few have provided any evidence to substantiate  this—save for one; long-term caloric restriction. 

From a health standpoint, caloric restriction is essentially the fountain of youth. Caloric restriction has been studied for over 60 years in animals and has consistently proven that consuming a continuously low-calorie diet over many years can extend lifespan. This begs the question, if eating less means living longer then why isn’t everyone doing that? To put it simply; it sucks. 

Cutting one’s calories by 40% is tough. Ask anyone who has been on a strict low-calorie diet and they are unlikely to give it a rave review. Sure, they may have lost weight, but chances are they didn’t end up sticking with it long-term because ofnagging hunger pangs and serious FOMO anytime they saw a slice of pizza. 

Hence, there aren’t any long-term studies on calorie restriction in humans because most participants drop out after a few months of these drastic diets. That said, those in the medical community have begun to wonder if similar benefits could be achieved through an alternative method, intermittent fasting. 

Why Timing Meals Matters

Intermittent fasting (IF) refers to regular periods of little to no caloric intake. IF has become increasingly popular over the past several years with many touting it as the best way to lose weight without the usual rigmarole of counting calories or cutting out entire food groups. 

Although IF may seem like a recent diet craze it is far from a novel approach to eating. In fact, our ancestors ate this way for tens of thousands of years. That was the simple reality of the hunter-gatherer lifestyle in which food was not always readily available. Our ancestors survived never knowing where their next meal would come from, subsisting with regular periods of little or no food intake. 

Today, the vast majority of individuals living in industrialized countries have just the opposite problem; the ability to constantly eat regardless of whether they need to, resulting in excessive calorie consumption. Thus, the question becomes can IF achieve the same benefits as calorie restriction? To answer this question, it is first important to understand what IF actually is.

There are many different approaches to IF, the most popular being the 16:8 method. This style of IF entails abstaining from all food intake for 16-hours per day. For the remaining 8-hours adherents can eat normally with no need to count calories or avoid certain foods. One of the major draws to IF is the fact that you do not have to restrict what or how much you eat, but simply when you eat. 

Whether it’s early in the day or in the evening, there are clear “feeding windows” and “fasting windows.” Of course, that doesn’t mean it’s easy. It can take a while to adjust to ignoring a growling stomach at specific hours. However, for those who can stick with it and adapt, the benefits may be worth it.

When it comes to weight loss the results are clear; IF works. Studies have shown that individuals following an IF diet lost just as much weight as those on a calorie-restricted diet and reported lower hunger levels over time.

But before you start fasting for 16 hours there’s a bit disclaimer. Studies indicate that IF is not a magic bullet for weight loss. That weight loss is relatively short-lived — almost all long-term studies observed that after a year IF participants regain the weight that they lost. Although disappointing, this is not altogether unexpected.

That’s not to say there aren’t other important health benefits to be gained from IF. Reduced insulin resistance, cancer prevention, reduced risk of heart disease, improved cognitive function, reduced risk for Alzheimer’s, decreased inflammation, and the aforementioned increased longevity are all potential perks of IF. 

Researchers theorize that these health benefits are largely derived from the “metabolic switching” that occurs as a result of fasting. This “switch” is essentially the body going from its preferred fuel source of glucose, derived from the body’s storage form of carbohydrates, glycogen, to fatty acids and ketones. This “switch” occurs after approximately 12 to 36 hours of fasting depending on an individual’s nutritional status prior to their fast. That means a 16-hour “fasting window” just squeaks into the lower end of the range. 

The reason relying on fatty acids and ketone bodies elicits so many potential health benefits is still a bit of a question mark; researchers have yet to definitively determine why our bodies seem to fare better health-wise when fueled from fat. Regardless of the how, many are also left wondering whether athletes can reap these benefits on an IF diet without seeing their performance suffer.

Is Fasting Fast?

Before IF became so trendy, some athletes were already trying to navigate periods of fasting with training and competition. For centuries, many religions have included traditions and rituals that incorporate various forms of fasting. 

Of particular interest is the Muslim tradition of Ramadan. For the month-long period of Ramadan, Muslims abstain from all food and beverages from sunup to sundown. Although not a precisely a 16-hour fast, it comes pretty close and because of that, Muslim athletes have served as an ideal study population when considering how fasting can impact athletic performance. The bad news is these studies may leave us with more questions than answers. 

In one study conducted on professional soccer players there was a significant reduction in the players’ endurance, speed, and agility during the month of Ramadan. However, a subsequent study on elite judo athletes found little effect of fasting on aerobic performance or speed. 

Conflicting outcomes such as these are largely representative of the lack of consensus when it comes to the performance impact of fasting during Ramadan. So far, the  body of research on athletes observing Ramadan seems to indicate that fasting has a negative effect on high-intensity sports. 

From a scientific standpoint this makes sense. High-intensity exercise relies predominantly on glycogen for a fuel source because it can be broken down quickly to provide energy for working muscles without the need for oxygen. Fat can also be used as a fuel source but takes much longer to process and requires oxygen. Not exactly an ideal fuel when you’re gasping for air in a dead sprint. Studies have shown slower sprint times for athletes during Ramadan, which researchers speculate is due to depleted glycogen stores and a shift to fat as the predominant fuel source. 

It is important to note that although Ramadan entails a similar structure to IF, they are not one and the same. For one, the time during which individuals fast for Ramadan is based on the rising and setting of the sun and thus provides no flexibility for athletes to schedule their food intake around their training schedule. Just imagine how you would feel going out for a long run in the evening after not eating anything since the early morning hours. 

IF, on the other hand, allows followers to choose when they want their feeding window to be as long as it is consistent day to day. That same evening run would feel very different if your 8-hour feeding window started at noon and thus allowed you to have a meal prior to that run, to fuel during the run, and to have a recovery meal afterwards. Another major factor that must be accounted for is the ability to consume beverages. IF does not restrict the intake of water, tea or coffee during one’s fasting window, which could mitigate some of the negative performance outcomes observed in athletes fasting for Ramadan.

There are few studies that have examined the specific protocol of fasting for 16-hours and eating for 8-hours, and those that have been done show mixed results when it comes to athletic performance. Like the studies on athletes during Ramadan, most preliminary findings show IF to be detrimental to performance in high-intensity exercise. 

However, this negative effect was reduced after a few days fasting, leading researchers to speculate that the body can adapt and experience no ill-effects on performance after this adjustment period. At best, IF seems to provide no performance benefit and at worst could negatively affect endurance exercise.  

Arguably the biggest concern for athletes considering IF is often the motivation behind it, weight loss. That’s because body weight represents more than just fat. Loss of muscle mass is a common outcome of “dieting” and can have negative effects on both health and performance. Calorie restricted diets typically result in 20% to 35% of weight lost from muscle. Not exactly ideal for athletes who train hard to build muscle for strength, power, and endurance. 

Fortunately, this is one area in which IF can help. One study conducted on healthy-weight males who already engaged in strength training found IF subjects were able to lose body fat and still maintain their muscle mass. This is promising, especially for athletes looking to shed weight in their off-season and not have to worry about keeping up with more intense training and competition. 

As millions weigh the different approaches to dieting this year (pun intended), IF likely tops the list considering it was rated the most popular diet of 2021. When it comes to athletes, it seems IF won’t be a boon for performance and is best utilized in conjunction with resistance training for those whose primary goal is reducing body fat. Whether or not IF can deliver when it comes to increased longevity, decreased risk for disease, or weight loss remains to be seen.

 Jamie Sheahan, M.S., R.D. is the Director of Nutrition at The Edge in Burlington where she works closely with athletes to develop custom fueling plans to optimize health and performance. Sheahan is also an adjunct professor of sports nutrition at University of Vermont and an avid runner

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