How Endurance Athletes Can Decide If Intermittent Fasting Is For You

By Chris Carmichael,
Founder and Head Coach of CTS

Intermittent fasting (IF) is a popular dietary and lifestyle choice for a lot of people, for a variety of reasons and in pursuit of a range of different goals. It can be effective for accomplishing some goals, and ineffective or detrimental in other areas, meaning there is no singular answer to whether it is better than unrestricted eating. For endurance athletes, decisions about whether IF is a good idea for you depend on what you’re trying to accomplish.

Definitions

First thing, we should define what we’re talking about. The most common methods for intermittent fasting are:

• 5:2 – Eat normally for five days and restrict energy intake to about 600 calories for two days, although the two days don’t need to be sequential.
• Alternate Day Fasting – Eat normally every other day and restrict energy intake to about 600 calories on fasting days. Some people reduce intake even further on fasting days.
• 16:8 – Consume all your daily calories within an 8-hour window and fast the other 16. Often, this means extending an overnight fast by skipping breakfast, but it can be done by ceasing caloric intake after an early dinner.

The 16:8 method, or any version that restricts caloric intake to a set of hours within the day but allows for feeding on all days, is sometimes referred to as “time-restricted eating” to differentiate it from intermittent fasting.

Weight loss

As a weight loss strategy, intermittent fasting can be effective, but food choices and portion sizes still matter. This point was driven home by a new study that got some media attention earlier this week.

Dr. Ethan Weiss’s study in JAMA Internal Medicine 12-week randomized clinical trial that included 116 men and women (40% women) with a mean age of 46 and Body Mass Index between 27-43 (between 25-30 is considered overweight, >30 is considered obese). The Time Restricted Eating (TRE) group could eat anything and any amount they wanted between noon and 8pm. The control group, which was the Consistent Meal Timing (CMT) group, ate three meals (breakfast, lunch, dinner) and could snack between meals. Both groups were free to make any food choices they wanted and eat as much food as they wanted; the intervention in the study was just the timing of feedings, not what they ate or how much.

After 12 weeks, both groups lost weight, but not very much (.94kg for TRE, .68kg for CMT). The weight loss was statistically significant for TRE, but not for CMT, and the difference in weight loss between the groups was not statistically significant. Similarly, there were no significant differences in secondary measures taken from an in-person cohort (could travel to UCSF), including fat mass, lean mass, fasting insulin, fasting glucose, hemoglobin A1c levels, estimated energy intake, total energy expenditure, and resting energy expenditure. In terms of weight loss and metabolic markers, manipulating the timing of feeding alone didn’t have a meaningful effect.

The results of Dr. Weiss’s study add support to a growing body of research that shows intermittent fasting and time restricted eating are similarly effective and not necessarily any better than daily caloric restriction for achieving weight loss. As such, if TRF or IF are a sustainable or convenient way for you to create a caloric deficit, and weight loss is your primary goal, then it could be a good option. Read on, however, for considerations that may affect your training.

Training in a fasted state

For many athletes, the ideas of intermittent fasting and fasted training–or training with low carbohydrate availability–go hand in hand. Indeed, a 16:8 IF lifestyle is conducive to training in a fasted state if you line up your training schedule such that you are working out at the end of your fasting window. In practical terms, you could do this with early morning workouts and a feeding window of 8am-4pm, during which time you eat for post-workout recovery and to replenish energy stores in preparation for the next fasting period (4pm-8am).

The scenario above is essentially a “sleep high, train low” method to train with limited carbohydrate availability. Increasing fat oxidation, or become ‘fat adapted’, is the typical motive for training with low carbohydrate availability, and I’ll cover that in more detail in the next section. Right now, though, we should talk about the intersection of fasted training and an IF lifestyle.

Training in a fasted state is best done as a component of training, rather than a consistent feature of training. It may fit well during a period focused on aerobic endurance, when rides are longer and lower intensity and there are fewer days that include high-intensity interval work. When it comes time for interval workouts at or above lactate threshold, or group rides and events featuring the full range of intensities, training with high carbohydrate availability is necessary. Could you train in the afternoons, in the scenario above, to achieve this? Yes, but you then have to consider the recovery time available between workouts and the complications this introduces to your training, work, and personal schedules.

From a coaching perspective, I find that athletes do best when they can train at a consistent time of day that works within their schedule. Occasionally manipulating the timing of their eating to train with high or low carbohydrate availability is more sustainable than manipulating the timing of their workouts to align with a time-restricted eating lifestyle. Put another way, long-term IF compromises training more than occasionally implementing a low-carbohydrate availability training strategy.

Fat Adaptation

Increasing fat oxidation and reducing reliance on stored carbohydrate is another potential goal for intermittent fasting, whether in the time-restricted eating format or the whole day fasting format. Training in a fasted state can be part of this strategy, but it is important to note this is one of the big reasons IF is popular with sedentary populations.

For endurance athletes, the issue of using dietary manipulation to maximize fat oxidation is best viewed through the lens of extremely low-carbohydrate lifestyles (Ketogenic diet). Keto is not the same as IF (although many people combine them). The reason I mention Keto here is that recent work from Louise Burke has again shown that increasing fat oxidation doesn’t necessarily translate to improved performance for endurance athletes, even in a “train low, compete high” scenario for carbohydrate availability. And Coach Jason Koop provided a good explanation for why “fat adaptation” is a relatively low priority for most endurance athletes.

Lean Muscle Mass

The age-related loss of muscle mass is an ongoing concern for athletes over the age of 50. In younger populations, a combination of exercise and TRF has been shown to reduce fat mass and overall weight without loss of muscle mass, provided people are consuming enough total calories to support their activity level and sufficient protein. At the same time, it has been shown that older athletes have a higher daily protein requirement than younger athletes in order to maintain muscle mass. And regardless of age, it has been shown that consuming protein throughout the day is better than concentrating protein intake on fewer, larger servings.

When you put all of this together, one concern for masters athletes is that intermittent fasting or time-restricted eating hasten sarcopenia by reducing total daily protein intake and/or the frequency of protein intake. Athletes over 50 are already fighting an uphill battle to build muscle or keep from losing it; intermittent fasting may make it even more challenging.

Health/Longevity

Intermittent fasting has been a feature of religious and spiritual practices for thousands of years, and more recently intermittent fasting and caloric restriction have been investigated as methods for increased longevity, reduced cardiovascular disease risk, improved mental health, and increased insulin sensitivity. In the grand scheme of things, slight but chronic underfeeding appears to be better for human health than chronic overfeeding, even before adding in the effects of food choices (i.e. whole foods better than highly processed foods).

Improved cardiorespiratory fitness imparts many, if not all, of the same health and longevity benefits as IF. Could there be additive effects of being both fit and maintaining an IF lifestyle? Perhaps, but my point is that athletes are already taking great steps toward increasing vigor and vitality, so it is hard to say athletes are missing out on health and longevity benefits by not following an IF lifestyle.

Summary

A few years ago I wrote a more scathing view of intermittent fasting, and people who criticized that article noted that I had been one-sided by not referencing enough of IF’s potential benefits. I have tried to be more even handed this time around, because IF can be effective depending on the goal you are pursuing. As a coach who works with amateur and elite athletes, I find that it is rarely a useful or beneficial strategy for maximizing endurance performance.

References and Reading

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Burd, Nicholas A., et al. “Dietary Protein Quantity, Quality, and Exercise Are Key to Healthy Living: A Muscle-Centric Perspective Across the Lifespan.” Frontiers in Nutrition, vol. 6, 2019, doi:10.3389/fnut.2019.00083.

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Doering TM, Jenkins DG, Reaburn PR, Borges NR, Hohmann E, and Phillips SM. Lower Integrated Muscle Protein Synthesis in Masters Compared with Younger Athletes. Med Sci Sports Exerc 48: 1613-1618, 2016.

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Moon, Shinje, et al. “Beneficial Effects of Time-Restricted Eating on Metabolic Diseases: A Systemic Review and Meta-Analysis.” Nutrients, vol. 12, no. 5, 2020, p. 1267., doi:10.3390/nu12051267.

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Tinsley, Grant M. MS, CSCS1; Gann, Joshua G. MS1; La Bounty, Paul M. PhD, CSCS2. Intermittent fasting and their effects on body composition: Implications for weight restricted sports. Strength & Conditioning Journal: October 2015 – Volume 37 – Issue 5 – p 60–71 https://journals.lww.com/nsca-scj/fulltext/2015/10000/Intermittent_Fasting_Programs_and_Their_Effects_on.9.aspx

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