by Mark Wolff

Intermittent fasting has certainly come to the forefront over the past few years and more and more I am getting asked by endurance athletes whether I think intermittent fasting is a good idea?

The question I throw back in return is why do you want to do intermittent fasting? Mostly I hear endurance athletes telling me that they want to lose weight, lean out or become more fat efficient athletes. 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 intermittent fasting is a form of calorie restriction, correct? So if you can restrict calories by fasting then why can’t you restrict calories by eating correctly?
When you place your body into a fasted state you begin playing with hormones specifically leptin and ghrelin. The inability to balance these hormones properly due to either over eating or fasting can cause roller coaster effects which in the end do not land up serving the correct purpose.

The main question though is as an endurance athlete why would you even consider fasting?

Weight loss comes from eating correctly meaning a calorie restricted diet and proper macronutrient intake to support you unique requirements. This could be fat loss, muscle gain or maintenance. However as an endurance athlete what about energy, immunity, recovery and performance. Intermittent fasting will not support that at all. Actually quite the opposite it will hamper the recovery process, limit your energy levels and place your immune system under stress.

Let me further explain so that you fully understand the consequences of intermittent fasting while trying to be the best possible healthy endurance athlete.

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

Muscle Protein Synthesis
Glycogen Restoration and blood sugar stabilization

If an athlete does intermittent fasting then rehydration and adequate fluid intake will not be an issue. The main issues will arise when it comes to muscle protein synthesis and glycogen replenishment. Adequate protein intake for an endurance athlete is important for recovery. Generally I work on around 1.4g – 1.7g of protein per a kilogram of body weight. This will depend on the athletes goals and training. Strength work and developing more lean muscle will require a higher protein intake while maintenance and repair will be on the lower side. Take a 70kg athlete who is training on average 90min a day. His required protein intake would be around 100-120g of protein per a day. This is in animal nutrition terms 4-5 chicken breasts or plant terms 2-3 cups of raw chickpeas. That is quite a lot of food. If the fasting window is quite long it means squeezing this protein intake into a small amount of time. However as we well know when it comes down to protein the human body is only capable of processing and absorbing a certain amount of protein every +-3hrs. This is generally in the region of +- 8-10 grams per hour. So its not possible to squash large protein portions into less meals and expect the correct portion amount for proper muscle protein synthesis to take place. Actually the opposite over eating protein ultimately causes a rise in blood sugar and that can easily lead to weight gain. We also know that consuming a protein with a carbohydrate post exercise up-regulates amino acid uptake specifically leucine which is a 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. What this means is 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 actually the opposite. So you are actually not recovering and triggering decent training adaptations.

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 and used at higher intensities while in lower aerobic zones fat becomes the fuel of choice.
A hard workout where glycogen depletion takes place will require carbohydrate intake post exercise to rapidly start the glycogen replenishment process and stabilise blood sugar. Extending the fasted period a few hours post exercise will delay this from happening and can lead 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 the form of carbohydrates and protein. If you do not ensure proper glycogen replenishment and stabilisation without triggering a roller coaster ride you will generate the the opposite effect from fat burn to fat gain by overeating.

When it comes down to the main 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. Being able to push those sessions to the best possible effort is what ultimately will lead to better gains, as well as more fat burning post exercise by elevating the metabolic rate that much more. It has been scientifically proven many times over that a fuelled session will give better performance numbers.

When it boils down to performance gains and being a fitter faster and stronger athlete then you need to fuel your effort.
Fuelling during an exercise session or eating post exercise recovery meals is not the reason endurance athletes are overweight. Weight gain comes with overeating and inadequate macronutrient meal blends.

The Fat Efficient Athlete
In order to become a fat efficient athlete intermittent fasting is not required. Fasted training is a good way of developing this, but that is completely 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 endurance sport. They do not go hand in hand. Endurance sport replaces a large amount of physical stress on the body. This impacts the musculoskeletal system, the immune system and energy system. The athletes 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 and injury.
If you want to perform, lean out and get your own resilient Lamborghini engine and body then learn how to eat properly and avoid short cuts which don’t benefit that goal. If you are only trying to lose weight or have a medical condition which requires a calorie restricted diet then intermittent fasting can play a roll. However if you are an athlete intent on being the best version of yourself then get your nutrition right and stop playing games.

Mark Wolff is a certified exercise & sports nutritionist, endurance nutrition and physiology expert with over 20 years experience. An endurance multi-sport athlete with a running, triathlon, mountain biking and weight lifting background, he works extensively with professional and amateur athletes in a variety of sports disciplines as well as those just wanting to change their lifestyles. He firmly believes that a person can only reach their full potential when their health and nutrition is given the proper focus. Mark’s focus on nutrition and physiology is not just on training and racing, but he places major emphasis on recovery, immune system health, emotional stability, stress management and performance. Mark is co-founder of 32Gi, a sports nutrition company, focused mainly on health and endurance nutrition. He is also co-founder of Rapid Recover focused on pneumatic compression equipment to improve circulation for recovery and health.

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Siao Singapore March 16, 2020 - 5:04 pm

I take 32gi. Just surfing around and found out you are founder. Wonderful product. Love from Singapore

Mark Wolff March 16, 2020 - 5:04 pm

Yes I am the founder 🙂 thank you for touching base Singapore is a beautiful country I have been there to the Standard Chartered Marathon. 🙏


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