How Intermittent Fasting Affects Your Performance

intermittent fasting

Intermittent fasting diets have grown in popularity over recent years; touted to promote weight loss, superior fat ‘burning’, increase insulin sensitivity, reduce inflammation, increase growth hormone secretion and a myriad of other claims of metabolic and performance benefits. However, many of these claims are anecdotal at best and most lack scientific support.

To add further confusion, as the popularity of fasting diets swells, so does the range of ways to fast (or avoid eating). Some intermittent fasting diets promote fasting between certain hours – e.g. only eating between noon and 6pm or fasting all day followed by a large meal at night. Others promote days of flexible eating interspersed by strict fasting days, while more extreme options encourage fasting for several consecutive days, even weeks at a time.


Under the Microscope – The 5:2 Diet

The 5:2 Diet is one example of a popular intermittent fasting plan. The diet allows you to follow a ‘normal’ healthy pattern of eating for 5 days (feasting) followed by 2 days of ‘fasting’. On fasting days, energy intake is restricted to around 500-600 calories (~2100-2500kJ) per day. To put that into food terms, a total day of eating for a day could be something like:

  • Breakfast – Spinach and egg white omelette + latte
  • Lunch – Salad with mixed greens, tomato and capsicum with small tin of tuna
  • Dinner – Bowl of broth-style soup (e.g. chicken and vegetable)
  • Snacks – Vegie sticks or punnet of berries

Fasting days can be consecutive or split over the week. The daily calorie allowance can be eaten over various combinations of one, two, or three meals plus snacks, depending on personal and practical preferences. There are also variations of the 5:2 Diet, including 6:1 (restricting only 1 day per week), 4:3 (restricting 3 days per week) or ADF (alternate day fasting).


Potential benefits of intermittent fasting

  • Promotes choosing high quality and nutritious foods such as vegetables, small portions of lean meats, fish or eggs, and soups to help you ‘fill up’ within calorie restriction
  • ‘Feasting’ days allow you to eat favourite foods which can help avoid feelings of missing out that commonly occur with other diets


Downsides of intermittent fasting

  • Potential for over-consumption of total calories on feasting days due to deprivation mentality and hunger after fasting
  • Concentration and energy levels may be reduced on fasting days
  • Possible loss of muscle mass if quality protein foods are not included throughout the day
  • Potential to compromise performance if training on a fasting day due to low consumption of fuelling carbohydrate
  • Potential negative impacts on recovery (failure to adequately replace glycogen stores or promote muscle repair)



Whether there is a small reduction of energy intake over consecutive days or a substantial reduction over a few fasting days, any eating plan that leads to energy restriction over a week will result in weight loss. However, whether this is sustainable for the long term is questionable, especially in athletes where performance and recovery must be prioritised.

Fasting can compromise your performance as a result of insufficient energy and carbohydrate intake for speed or endurance, as well as reduced concentration and/or skill. There is also a high risk of poor recovery if you are training on fasting days, as meals and snacks are unlikely to be able to meet nutrition recovery targets. There is also the potential for rebound weight gain as a result of over-restriction, if not sustained for the long term.

If you’re trying to optimise performance and recovery in your training program, this approach may not be the best option for you. However, if you are interested in knowing more about how intermittent fasting could fit your personal situation, get in touch with an Accredited Sports Dietitian to discuss your needs further.