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Writer's pictureDana Eshelman

To Fast or Not to Fast?

Updated: Sep 11

Dana Eshelman, MS, RDN, CSSD

Fasting

In the endurance world, it is well known that carbohydrates fuel our muscles for training, performance, and recovery. New practices are rearing their head in among the endurance community including intermittent fasting (IF), fasted workouts, and glycogen depleted workouts to improve fat oxidation (or use of fat for fuel) during training and racing for mental focus, and weight loss.


FastingĀ is a lifestyle strategy that has been adopted as a therapeutic intervention for the management of several chronic diseases and health in 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) and where there is no fuel coming in during the training session. For example, a fasted workout can be performed first thing in the morning before breakfast.


There is also significant evidenceĀ that exercise improves health markers. However, combining fasting and exercise (aka fasted workouts) is a questionable practice for endurance athletes.Ā 


Research For FastedĀ 

To date, the research on fasted training favors benefits in the male population versus the female populationĀ (more on the sex differences below). The proposed performance benefits of fasted training in endurance exercise are:

  1. Increasing fat oxidation (ability to utilize fat for fuel)

The primary argument with fasted training is that your body learns to use fat for fuel because there are limited glycogen (or carb) sources available. This is true. 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 carbohydrate alone.Ā 


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


Research Against Fasted Training

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

  1. Low energy availability (LEA) or relative energy deficiency in sportĀ (REDs)

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

Low energy availability

  1. Elevated stress response with training

Exercise is a stress in the body. Fasting/ low energy availability is a stress in 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 responsesĀ in the body including poor sleep, difficulty adapting to training load, performance plateau, impaired glucose tolerance and ā€œstubbornā€ fat. More on this below.


  1. Poor performance/low energy

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



Men Versus Women

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 SimsĀ isĀ  creating ripples in this space as she explains, ā€œwomen are not small men.ā€


When women deprive themselves of nutrients during a fasted workout, especially carbohydrate, 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, and 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 II diabetes).Ā 


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 (aka 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 -- 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 ā€œfastā€ into your dietary regimen, you may consider doing an intentional 12-13 hours overnight fast where you stop eating after an early dinner and do not eat again until your pre-training snack in the morning.Ā 


As a performance dietitian, fasted training is notĀ something I recommend to any of my athletes.Ā 


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


If you have specific questions on whether or not fasted training is right for you, please work with a registered sports dietitian and medical professional to monitor the metrics you are trying to adjust.




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