The Truth Behind Fasted Training

Understanding the real effects of a restrictive diet on an athletes body during training By Dana Eshelman MS, RDN.

Understanding the real effects of a restrictive diet on an athletes body during training By Dana Eshelman MS, RDN

It is well known that carbohydrates fuel our muscles for training, performance and recovery. But new practices including, intermittent fasting (IF), fasted workouts and glycogen depleted workouts are rearing their head among the endurance community. So why would an athlete deviate from carbs and attempt fasted workouts? The simple answer is to improve fat oxidation (or use of fat for fuel) during training and racing, mental focus and weight loss. But this approach should not be done without serious consideration. 

Fasting is a lifestyle strategy that has been adopted as a therapeutic intervention for the management of several chronic diseases and health among the general population. Fasting is the abstinence of consuming food and beverage for a period of time that can last several hours per day to a few weeks. For the sake of this article, a fasted workout is training in a depleted state (no fuel for 10-14 hours) with no fuel coming in during the training session. For example, a fasted workout can be performed first thing in the morning before breakfast.

Combining fasting and exercise (aka fasted workouts) is a questionable practice for endurance athletes. Below is a breakdown of the pros and cons of fasted training. 

Performance benefits of fasted training in endurance exercise are:


During fasted training, your body learns to use fat for fuel because there are limited glycogen (or carb) sources available. Fat is broken down in the body via lipolysis and oxidized as free fatty acids (FFA). This is beneficial because you have more fat calories stored and you can continue to use these stores at low intensity for longer than if you were to use glycogen or carbohydrates alone. 

Improvement in aerobic endurance capacity (VO2 max) in the general untrained and highly-trained athlete population may be beneficial as part of pre-season preparation.

Many believe hGH only helps with muscle growth; however its main functions are a.) maintaining blood sugar balance by turning the foods you eat into energy and b.) adolescent growth. When looking at the physiology of fasting, hGH is increased in order to maintain blood sugar balance and ensure normal body functions (i.e., heart beating, breathing, brain function) continue to take place.

On the contrary, there are several concerns with fasted training, including:


This suppresses immunity, decreases bone density, increases muscle loss and results in poor recovery, decreases overall performance and can leave the body at high risk for injury.

  • Elevated stress response

Exercise is a stress on the body. Fasting/low energy availability is also a stress on the body. Stress can be a good thing when your body has the tools it needs to mitigate this response; however, when your body is chronically loaded with stress, there is a cascade of biological responses including, poor sleep, difficulty adapting to training load, performance plateau, impaired glucose tolerance and “stubborn” fat. More on this below.

  • Poor performance/low energy

Fasting works (for some) in the low to moderate intensity sessions. But as intensity ramps up (i.e, lifting weights, tempo sessions, interval training, etc.) carbs become the primary fuel source for working muscles.

Men Versus Women

To date, the research on fasted training favors benefits in the male population versus the female population. 

Women have been an understudied group in the world of exercise, nutrition and medicine and there are significant physiological differences between men and women. Dr. Stacy Sims is creating ripples in this space as she explains, “women are not small men.”

When women deprive themselves of nutrients during a fasted workout, especially carbohydrates, there is a significant reduction in the neuropeptide kisspeptin (KP). Kisspeptin is more sensitive in females than in males and is responsible for sex hormones, endocrine function (blood sugar balance, hunger and fullness, body composition), and reproductive function. Reduction in KP increases appetite and decreases sensitivity to insulin, thus leading to elevated blood sugar levels and impaired glucose tolerance (or type 2 diabetes). 

As exercise is added into the picture, the stress hormone cortisol increases. This is a normal response to exercise stimulus (and is actually beneficial), but when your body does not have the resources/nutrition it needs to lower those cortisol levels your fight or flight response remains elevated. High cortisol levels lead to a cascade of biochemical and hormonal imbalances that mirror the reduction in KP including, blood sugar imbalance, weight gain, suppressed immune system, gastrointestinal (GI) distress/inflammatory bowel conditions and fertility complications. Additionally, inadequate nutrition suppresses thyroid function and menstrual regularity.

So, ladies, you are looking at high stress and anxiety, increased appetite and cravings, weight gain, menstrual irregularity, GI disturbance, and impaired performance. This is the exact opposite of why you likely started fasted workouts in the first place.

Is Fasted Training Right for You?

When looking at the research, there is some evidence supporting fasting and glycogen depletion for male athletes in moderation, whereas female athletes should be cautious in adopting this approach. As always, every individual is unique, so it is important to always consider your current regimen, your training goal(s), your end goal(s) and your health.

If you are an athlete that really wants to add a “fast” into your dietary regimen, you may consider doing an overnight fast where you stop eating after an early dinner and do not eat again until your pre-training snack in the morning. 

As a performance dietitian, fasted training is not something I recommend to any of my athletes. 

Big picture with nutrition, performance and health, your body is going to reap optimal training adaptations by fueling enough to support your body. A well fed athlete is best, especially as training ramps up. I encourage athletes to think about how they can add value to their plate to improve both health and performance rather than restricting nutrition.

Dana Eshelman MS, RDN is a performance dietitian that helps endurance athletes find hormone balance and optimize gut health through flexible nutrition so they can level up their performance. Dana is a multisport athlete and runner herself, and understands the demands of endurance training and competing. She strives to simplify nutrition for athletes, improve understanding of how to fuel and hydrate for training and competition, and enhance daily lifestyle habits for life-long success.

IG: @adashofdanardn