Is Intermittent Fasting A Good Idea For Athletes?

by Mark Wolff

Intermittent fasting has undoubtedly come to the forefront over the past few years, and more and more, I am getting asked by athletes whether I think intermittent fasting is a good idea? One needs to ask, what is your reasoning for doing intermittent fasting? I mostly hear athletes telling me that they want to lose weight, lean out, become more fat efficient, or improve health. I guess the big question is, what is your actual goal? Are you trying to lose weight? Are you trying to improve your previous best marathon or Ironman time, or are you just doing it for health reasons?

Firstly, it’s a fact that intermittent fasting is a form of calorie restriction. So if you can restrict calories by fasting, why can’t you limit calories by eating correctly? When you place your body into a fasted state, you begin playing with leptin and ghrelin hormones. The inability to balance these hormones properly due to overeating or fasting can cause roller-coaster effects that do not serve the intended purpose. If trying to limit the insulin response by placing yourself in a fasted state is the objective, you should know that this can also be done in a non-fasting state.

The main question, though, is, why would you even consider fasting as an active individual or an athlete? 

Weight loss comes from eating correctly, meaning a calorie-restricted diet and proper macronutrient intake to support your unique requirements. Lean muscle gain also comes from eating correctly and ensuring proper macronutrient intake daily. I am just trying to stress how vital nutrition is for fat loss, muscle gain, and maintenance. However, what about energy, immunity, recovery, and performance as an athlete. Intermittent fasting does not support those systems. Quite the opposite, it will hamper the recovery process, limit your energy levels, and place your immune system under additional stress. 

Let me explain to fully understand the consequences of intermittent fasting while trying to be the best possible healthy athlete.


When it comes to recovery, there are three main aspects that I look at, and these are:

  • Rehydration
  • Muscle Protein Synthesis
  • Glycogen Restoration and blood sugar stabilization

If an athlete embarks on intermittent fasting, then rehydration and adequate fluid intake will not be an issue. The main problems will arise regarding muscle protein synthesis and glycogen replenishment. Adequate protein intake for any athlete is essential for recovery. Generally, I work on around 1.4g – 1.7g of protein per kilogram of body weight for endurance athletes. Then, for lean muscle increase or strength athletes, the protein volume can quickly move to 2g/kg body weight and higher. The volume needed will depend on the athletes’ goals and exercise regime. 

Strength work and developing more lean muscle will require a higher protein intake, while maintenance and repair will be lower. 

Take as an example a 70kg athlete who is training on average 90min a day. His goals are body fat loss, increased lean muscle, and strength. More of a combination athlete. In this case, I would advise a protein intake of at least 120-130g of protein per day. In animal nutrition terms, 5 chicken breasts or plant terms 2kg of boiled lentils. That is quite a lot of food in protein terms. Most intermittent fasters deploy a 16:8 window which is quite long. It means squeezing this protein intake into a small amount of time.

However, as we know, when it comes down to protein, the human body can only process and absorb a certain amount of protein every +-3hrs. This is generally in the region of +- 8-11 grams per hour. If you are a plant-based eater, you will expect severe GI distress in consuming a high amount of plant protein in a short window period as fiber content is a lot higher. So, as you can see, it’s not possible to squash large protein portions into fewer meals and expect the correct portion amount for proper muscle protein synthesis to take place. Overeating protein can also trigger elevations in blood sugar, easily leading to weight gain. We also know that consuming a protein with a carbohydrate post-exercise up-regulates amino acid uptake, specifically leucine, a significant trigger for muscle protein synthesis. 

What also allows for proper muscle protein synthesis is a positive nitrogen balance. Nitrogen is the most crucial component of amino acids, which are the building blocks of protein. This means that if you are fasting, the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested, and there is no repair work taking place. It places you in a catabolic or breakdown state. This means limited training adaptations and limited progress. 


The next aspect I want to look at is the energy system, our primary fuel tanks being glycogen and fat. Glycogen is our rocket fuel used at higher intensities, while fat becomes the fuel of choice in lower aerobic zones. 

A hard workout where glycogen depletion takes place will require carbohydrate intake post-exercise to start the glycogen replenishment process and stabilise blood sugar rapidly. This would often result in high muscle damage, meaning protein intake also needs to be considered after exercise. Extending the fasted period a few hours post-exercise will delay the recovery process, leading to fatigue. This will also hamper back-to-back sessions. Then when it does become time to break the fast, how do you know how much to consume in carbohydrates and protein. The body has requirements from a macronutrient perspective and post-exercise stresses, which must be met to ensure a proper and healthy recovery. 

When it comes down to the primary energy system, carbohydrates are, without a doubt, our rocket fuel if you are going to be doing an FTP (Functional Threshold Power) test or an interval or track session. Then how can you expect to achieve the best possible numbers without fuelling the session properly? Pushing those sessions to the best possible effort will ultimately lead to better gains and more fat burning post-exercise by elevating the metabolic rate that much more. It has been scientifically proven that a fuelled session will give better performance numbers. Better performance numbers equate to better training adaptations.

When it boils down to performance gains and being a fitter, faster and stronger athlete, you need to fuel your effort. Fuelling during an exercise session or eating post-exercise recovery meals is not why athletes are overweight. Weight gain comes with overeating and inadequate macronutrient meal construction throughout the whole day and week. 

The Fat Efficient Athlete

To become a fat-efficient athlete, intermittent fasting is not required. Fasted training is a good way of developing this, but that is entirely different from intermittent fasting. A fasted session does require a recovery meal post-exercise, and the volume and type of meal will be determined by the duration and intensity of the exercise session. Failing to recover from an exercise session is, without a doubt, failing the session itself. 

To summarise my views on intermittent fasting and sports performance. They do not go hand in hand. Whether endurance or strength disciplines or a combination of the two, they place a lot of physical stress on the body. This impacts the musculoskeletal system, the immune system, and the energy system. The athlete’s body requires nutrients to fuel, repair, and fortify it constantly. Failing to give the body what it needs will ultimately lead to a lack of performance and potentially illness or injury.

If you want to perform, get stronger, lean out, and get your resilient engine and body, then learn to eat correctly and avoid shortcuts that don’t benefit those goals.

If you are only trying to lose weight or have a medical condition that requires a calorie-restricted diet, then intermittent fasting can play a role. However, if you are an athlete intent on being the best version of yourself, get your nutrition right and stop playing games. 

Mark Wolff is a preventative health specialist with a physiology, chemistry/blood chemistry, exercise physiology, and nutrition background. He has been consulting in this space for more than 25 years, focusing on endurance and strength athletes. Working with professional and amateur athletes in various sports disciplines and people just wanting to change their lifestyles, Mark believes that a person can only reach their full potential when the foundation of health is given the proper focus. Mark places a significant emphasis on recovery, immune system and metabolic health, emotional stability, stress management, and performance.

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Anton Bester February 23, 2022 - 5:09 pm

Good evening Mark. I totally agree with your post. I am a keen and very competitive 58 year old cyclist and can not go without proper fueling before, during and after my sessions. All I am doing is to eat and drink most of the time the healthier options and cutting out sugar and mostly processed foods. I am in general very lean and it is so important to keep my energy level topped up. I know my recovery depends on pre, during and post fueling. It has become a habit and a way of life. NO DIET’S, NO FASTING, Nothing of that! Keep the balance right with Protein, Carbs and Good Fats. If you do it right and be consistent, your body will adapt for the better.
My weight and lean body developed over time with just the plain and simple “EAT RIGHT TO TRAIN RIGHT”.

Mark Wolff February 23, 2022 - 6:19 pm

100% Anton keep up the good work 👌

Sanjid Mallik March 3, 2022 - 6:37 am

This is some good stuff


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