For endurance performance, we’ve often heard ‘leaner is better.’ But is that always right? Yes, leaner can be faster to a point—until fueling for training or health is compromised by additional dieting for fat loss. And you might be surprised just how quickly that happens. The optimized body fat percentages for performance, as a population, are typically 5‐10% for males and 10‐15% for females. That is, those are body compositions common to the elites in endurance sport. 

But these body fat percentages might not always be optimal for health and may not be maintained even by elite athletes year-round. While some rare endurance athletes may perform well and remain healthy at the low end of these body fat percentage ranges, for most maintaining this extreme leanness impacts performance and probably has serious health costs. 

Why additional muscle mass can impact performance

In addition to metabolic costs, mass can impede performance by creating a physical obstacle, aerodynamic drag, or increased rolling resistance—i.e. You have to carry more around. Range of motion and ease of movement can also be impacted by muscle mass itself. That is, running economy, cycling economy, and swimming economy tend to decrease with increasing muscle mass when other factors are held the same.

Of course, some muscular development is required for endurance, but only in areas that directly enhance performance. Muscle is heavy and uses more oxygen and fuel than any other tissue in the body during exercise, so we only want as much of it as is necessary to perform. Endurance athletes also benefit from being light, which is not always the same as “lean,” in order to avoid increased energy demand per unit of distance covered. Indiscriminate hypertrophy (excessive growth of muscles not needed for performance), therefore, can be detrimental.

The precise leanness needed by an individual for optimal performance likely depends mainly on genetics and body composition history. Finding the body fat percentage at which you can achieve your best race times without suffering consequences is a process of leaning out and regularly assessing, but safe starting points are around 12% for males and 18% for females. If you are over 12% body fat as a male or over 18% as a female after a year or more of endurance training, and do not have a long history of carrying significantly higher body fat percentages, getting your body fat level down could guarantee better race times. 

When you should NOT try to lose weight

But what if you have succeeded in previous weight loss? What if you have attempted weight loss off and on throughout the course of your career—or you struggle with yo-yoing? What if you are currently trying to lose weight but can’t make any headway? What if you are suffering from symptoms related to diet burnout or have dealt with disordered eating in the past?

1. Chronic hunger
2. Chronic fatigue
3. Chronically cold
4. Constant thoughts of or concerns surrounding food
5. Dreaming of food during sleep
6. Inability to adhere to any diet plan
7. Decreased libido
8. Depression or anxiety related to food
9. Feeling like you need to restrict intra-workout fueling as your only option to limit calorie consumption

If you answered “yes” to any of the above questions or are experiencing any diet burnout symptoms, it is likely that there may be unforeseen consequences to further attempts at weight loss, and that these consequences may completely erase any potential performance benefit that could have arisen from your physical reduction in body fat.

By far and away the two most common pitfalls I have seen among endurance athletes attempting to lose weight:

1. Cutting carb consumption, and often cutting intra-workout carb consumption the most.  Performance decrements and nighttime binge eating are a common result.
2. Trying to become slightly leaner or lighter than genetics and body history would allow without serious tradeoffs. Depression, anxiety, sleep loss, performance loss, and binge eating are all common and terribly unfortunate results.

Is it worth the tradeoffs?

Before you decide to allow or seek weight loss in endurance sport, you should seriously and honestly consider these questions:

1. How will this affect my psychology? Will I be satisfied with some progress? Is my end goal reasonable? What if I do not get there? How will I feel? If the answer is that you will feel like a failure if you do not get to X on the scale, now is not the time for weight loss. Now is the time to work on your perfectionism.
2. Is this the best time for weight loss? Is there a better time, with less anticipated life or training stress?
3. How will this affect my training quality? Race prep is NOT the time to be flirting with a restrictive diet.
4. Do I have any evidence that indicates I might already be so lean that any restriction or further restriction would come with significant tradeoffs as listed above?
5. Will there be any health tradeoffs? Chronic calorie restriction can lead to reduction in metabolic rate, making future weight loss more challenging.
6. Do I have time to sleep more now? You need more of it if you’re eating less!

How to enter a weight loss phase intelligently

There are only three meaningful ways to become more recovered and less fatigued—and recovery is vital to performance, particularly during periods of high training load:

1. Eat more.
2. Train less, or less intensely.
3. Sleep more.

When you are in a fat loss diet phase, sleep needs are increased by as much as an hour per night to feel similarly well-recovered. In a fat loss phase, the other two options—eating more or training less—are likely not conducive to your goals, and so are probably out. Sleep more it is.

Making the choice to attempt weight loss and the successfulness of that choice, in terms of endurance performance and of health, comes down to choosing the right time and the right goals, as much as it does to having a reasonable diet approach.

If you have decided to tackle a temporary weight loss phase, first and foremost, it should be just that: temporary. Capping weight loss phase duration at 12 weeks, or better yet just 8 weeks for most folks, is a great way to increase the chance of successful weight maintenance after the fat loss diet phase has ended. The less rapid the weight loss the better. And the longer you can maintain your weight between any fat loss diet phases, the better, for both your performance and your psychological and physiological health. Hormones and brain chemicals need time to recover.

Your body will send increasingly strong signals the farther you get from your recent homeostasis and the faster you deviate from it. That is, if you have been heavier or carried more fat recently, you will have signaling happening in your body to return you to that previous place or status. The faster you have moved from where you recently were, the stronger the homeostatic signaling at play.  

Hence, crash diets usually result in a yo-yo effect. All of these cellular and hormonal signals will be more pronounced and more psychologically powerful in endurance athletes, who deal with more cumulative chronic fatigue than virtually any other type of athlete. Targeting less than 1% bodyweight loss per week is advisable for all people, always, and for endurance folks, 0.5% of bodyweight lost per week may be a wiser choice, given all the performance and health tradeoffs at play.

So, once you have decided on a fat loss phase duration and a range for goal rate of weight loss, what comes next?

There are no two ways about it: calories are king when it comes to reducing body weight. To lose weight, you must expend more than you consume. But, in the real world, and more specifically in your endurance sport world, the macronutrient breakdown and timing of consumption plays an enormous role in both your performance and your success. If you have decided it is time to cut weight as an endurance athlete, creating an airtight game plan is as important as the training itself. Hint: It is not going to come from restricted intra-workout fueling!

In our next article, we’ll talk about how to effectively trim weight (when appropriate) while still crushing training and maintaining healthy recovery and overall health.

NEXT ARTICLE: How to Trim Weight While Still Crushing Training

Dr. Alex Harrison, a certified USA Triathlon coach, holds a PhD in Sport Physiology and Performance. He is the author of The RP Diet for Endurance, creator of the RP Endurance Macro Calculator, and has authored and contributed to dozens of articles. When he isn’t pumping out training and nutrition plans in his RV-garage-turned-mobile-office, he can be found on his bike, clinging for dear life to his wife’s wheel.