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  • Writer's pictureAmie

Intermittent fasting for the everyday athlete

Today’s blog is to give the facts about IF and it’s appropriateness for an athletic population. What I hope to do is present the facts and then offer my opinion based on my knowledge and training, so please keep your mind open as you read. 

What is IF?

Intermittent Fasting (IF) is an alternate method to reduce total calorie intake rather than following traditional dieting practices by dropping calories out across the day. 

There are many forms of IF however the most common are:

· Fasting for 14-16 hours and eating for 8-10 hours (usually from 10am/12pm-8pm) 

· Fasting for 20 hours and eating for 4 hours  

What are the positives of IF?

  • It has been clinically proven to decrease insulin resistance, total cholesterol, and liver functioning tests (LFTs) in obese patients;

  • Increases whole body lipolysis and fat oxidation (basically burning more fat at rest);

  • Lack of appetite during fasting time - as soon as you break the fast your body triggers the hormonal release of ghrelin which stimulates hunger. 

  • Body weight loss occurs. However due to limited research it has not been determined whether this is both muscle mass and body fat. Some research suggests both fat and muscle will be lost because of the way protein is metabolised.  

  • It provides the gut with complete rest. People with different types of inflammatory bowel disease find IF gives the gut rest to help process their food. This in turn decreases diarrhoea and bloating that is a result of their IBD. I have a few clients that have trialled this and have found it works well. Please be mindful this is not to say every single person with gut issues should follow IF. It is merely another option

  • Increased long term success with alternate day fasting which is fasting for 16 hours one day and then eating a traditional diet the following day. 

What are the Negatives? 

  • Definite initial decrease in lean mass;

  • Some research suggests IF decreases resting energy expenditure (REE – known as your metabolic rate) following a 16 hour fast. REE was acutally higher in a 20 hour fast and the normal 8-10 hour fast; 

  • Most research so far has been completed in an obese population who do not exercise and whose current diet plans would consist of an excessive intake of low nutritious foods. By eating less in general these types of subjects, insulin resistance, total cholesterol and LFTs should all decrease along with positive changes to their body composition;

  • People’s compliance long term; 

  • Research shows a decrease in repeated sprint efforts following IF. (See below paragraph);

  • Glycogen depletion (low intake of carbohydrates) going into HIIT training sessions. 

Should IF be used for an athletic/exercising population?

There is quite an excessive amount of scientific evidence about protein synthesis and its metabolism. We know scientifically our lean mass need to be fed in 30-40g lots every 3-4 hours (higher range for master’s athletes). So by fasting for 14-16 hours, muscle will not be fed, so surely lean mass will decrease as a result.  

Additionally, if you are a person that trains in the morning and then fast until midday, there is no protein to promote muscle mass repair and growth, nor or any additional glycogen (stored form of carbohydrates) to help fuel the muscle repair process.

The lack of glycogen will not only affect the muscle repair process but will also effect refuelling the energy of the muscle cell. If you train with HIIT or strength training and do not replenish the glycogen stores post-training, this can lead to overeating in the 8-9 hours of the refeeding time. If the calorie intake as a result does not match the expenditure, then the person will not see body FAT loss.

From a performance point of view, there have been a number of studies completed in athletes that follow Ramadan IF for religious beliefs. In this population there was a decrease in performance for repeated sprint efforts following the traditional regular eating pattern versus IF. 

So putting this information into the everyday athlete’s perspective for those who:

Train in a sport/exercise that requires your heart rate to increase above 70% of your max HR and follow IF, then it is possible you will not have enough muscle glycogen stored for training.

SO What will happen if you do not have enough muscle glycogen? 

Your liver cannot break fat down fast enough in order to create the glycogen (energy) needed for training, so instead the liver will break muscle mass down instead and use this for fuel. So as a result your performance will decrease! 

So what should YOU do? 

Be wary of personal testimonies. They may have seen a change in training or loss in body weight by following IF protocol, but try and consider the whole picture. Before following IF, was this person training consistently? Were they actually eating protein regularly, or even eating around training? If they train in the afternoon and now consume more food around it, is that what is helping them? Are they losing both body fat and muscle mass at the same time? Just the a conscientious consumer of what you read and hear online! 

Is this just the next FAD?

Look at it from a long term perspective: is this something you are going to be able follow for the rest of your life? Is it something you can incorporate into your social and family environment? If you stop following IF for a few days or for a couple of weeks as a result of a holiday or family event, will this decrease your compliance to follow a more restrictive based plan? May this create or resort to a disordered eating pattern and a negative relationship with food? Just because you feel like everyone else is doing it around you does not mean YOU should. After all: this is not your friend’s or family’s diet and lifestyle, it is yours.

So in summary, my take on IF for my clients is that it is not a good option if you care about performance and retaining lean mass. The only time I would incorporate it in for an athletic nature would be to follow it once a week, on a day you DO NOT exercise in order to help bring your calorie intake down. This is something I use currently with some clients to help reduce body fat, however still focusing on eating protein regularly during the 8 hours of intake. 

If you need help figuring out what is best for YOU, contact a sports dietitian and have a discussion about if IF is the right option.


Tinsley, Grant M. MS, CSCS1; Gann, Joshua G. MS1; La Bounty, Paul M. PhD, CSCS2

Intermittent fasting and their effects on body composition: Implications for weight restricted sports. Strength & Conditioning Journal: October 2015 - Volume 37 - Issue 5 - p 60–71

Desgorces F, Moinard C, Chennaoui M, Toussain T, Petibois C. Development of a specific index to detect malnutrition in athletes: Validity in weight class or intermittent fasted athletes. 2016

The Acute Effect of Intermittent Fasting on Resting Energy Expenditure in College-Aged Males Ruth A. Stauffer1 , Cory T. Beaumont1 , Tania S. Flink2 1Department of Health and Physical Education, Edinboro University, Edinboro, PA USA, 2Department of Sport and Exercise Science, Gannon University, Erie, PA, USA

Beelen M, Burke L, Gibala L, Van Loon, L. Nutritional Strategies to Promote Postexercise Recovery International Journal of Sport Nutrition and Exercise Metabolism, 20, 2010, 515-532 © 2010 Human Kinetics, Inc.

Slater G, Phillips S, 2013. Nutrition guidelines for strength sports: sprinting, weightlifting, throwing events, and bodybuilding Food, Nutrition and Sports Performance III. Pages 75-86

Thomas DT Erdman KA Burke LM L. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. 48(3):543–568, MAR 2016

If anyone else is interested in reading good evidenced based practice - authors Louise Burke and Gary Slater are fantastic! Louise Burke in particular has done quite a lot of research into fax oxidation and metabolism.

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