Intermittent Fasting – What’s it all about?


  • Intermittent fasting involved eating within a certain time frame such as 8 hours, and fasting for the remaining 16 hours of the day. For this reason, intermittent fasting is often referred to as ‘time-restricted eating’.
  • Intermittent may be beneficial for those pursuing weight-loss but more research is needed regarding any other potential benefits, especially among athletes.
  • Applying a 12 hour eating window and a 12 hour fasting window may be a more practical and sustainable approach to your eating routine

What is intermittent fasting?

Intermittent fasting (‘IF’) has been described as a diet regimen that cycles between brief periods of fasting, with either no food or significant calorie reduction, and periods of unrestricted eating” (1). This involves consuming all of your food and fluid calories for the day within a certain period of time, although calorie-free liquids can be consumed during fasting.

How is it done?

There are two popular approaches to IF which are outlined below.

  • The 16/8 approach:

Some individuals will follow a 16/8 approach whereby they consume foods within a 8 hour window (eg: between 10am and 6pm or between 12pm and 8pm) and fast for the remaining 16 hours. There are other methods using time-restricted eating with shorter or longer eating windows, making IF flexible in this regard. 

  • The 5/2 approach:

Others will consume very low calorie diets for 2 days of the week and eat ‘normally’ for the other 5 days of the week. On the 2 days of very low calories, females consume just 500 kcal and males consume 600 kcals. The low calorie days can be consecutive or fall on alternate days of the week, depending on the individual’s preference. Often, during a 16/8 approach or similar, individual’s may simply push their first meal of the day to a later time than usual and ensure that they do not consume any more food after the end of the eating window (eg: after 18:00).

Other people will fast for whole days (24 hour fasts) one or two days per week. 

To complement Daniel’s and Brendan’s in depth conversation on the topic, we have compiled a short list of the potential advantages and disadvantages of intermittent fasting. 

Potential advantages:

  • Fat loss – May help people to lose fat mass if this is their goal by narrowing their eating window, thus, reducing their overall calorie intake and creating a calorie deficit. This is particularly evident within overweight and obese population groups (2, 3, 4).
  • Health benefits – Independent of weight loss, there may be some other benefits in relation to metabolic health which including improve insulin sensitivity, but more research is certainly needed in this area. 
  • Flexibility – It may work well for people who do not mind pushing their first meal of the day to a later time or for those who may struggle with late-night snacking which increases their overall calorie intake.
  • Adaptable – An individual does not need a great detail of nutrition knowledge to follow an IF eating style.

Potential disadvantages:

  • Sustainability – Some evidence suggests that when weight was lost following IF, it was regained following resumption of normal eating habits. For some individuals it may be entirely sustainable and suitable and for others, it may not be the correct long-term approach to suit their lifestyle. 
  • Retention of muscle mass – Caution must be aired particularly among athletes to ensure that muscle loss is minimised if following IF for the purpose of body fat. 
  • Athletic performance – More research is needed among athletes to determine any effect of IF on performance in the short and long-term.
  • Compensatory behaviours – People may engage in subconscious compensatory behaviours if reducing their overall food intake. For example, individuals may seek out higher calorie foods to compensate for a reduced intake or may increase their overall energy intake at each meal. 
  • Medical conditions – IF may not be suitable for some people such as those with a current or history of an eating disorder, in pregnancy or when breast-feeding, anyone suffering from diabetes or those who require regular feeding to manage health conditions or medications.
  • Food relationship – The relationship people have with food is hugely important and putting measures or restrictions in place can often be the worst thing people can do and lead to a more dysfunctional relationship with food rather than dealing with underlying issues a person may have.
  • Research – Much of the promising research around metabolic health has currently been carried out in animal and more is needed among humans to draw conclusions.

What does daveynutrition think?

Overall, intermittent fasting appears to reduce total calorie intake by minimising the time-frame in which people have the opportunity to consume calories. Therefore, this approach has worked well for some individuals who have struggled to lose excess bodyfat and benefits have been noted especially among obese and overweight groups. 

However, there are other methods to reduce total calorie intake outside of Intermittent Fasting and one must consider all of the pros and cons of this kind of eating style. As with any new trend or regime, don’t just jump on the bandwagon – do some research and find out from qualified professionals whether the approach has been robustly researched and if the results of research are applicable to you and your goals. 

Should you try it?

Intermittent fasting may not be suitable for those engaged in a high-level of physical activity as it narrows the time-frame in which to obtain all necessary nutrients for fuelling and recovery. It would be important for athletes to discuss any such approach with their Nutritionist to determine what time of the year  this approach may be suitable and how best to minimise negative effects such as loss of muscle mass or reduced energy for training. The sensible and sustainable approach would be to firstly adopt a structured approach of 12 hours fasting and a 12 hour eating window which may be more practical and flexible while still applying a level of structure to your meals. 

Remember – context matters and what works for one person will not necessarily work or be the right approach for another.

Research is ongoing in both animal and human studies in relation to IF and it’s associations with circadian rhythms, athletic population groups and performance, metabolic health and much more, so it is certainly a topic to keep an eye on and an exciting area of nutrition research.

For more on sustainable fat loss and other ways to reduce your overall energy intake in a healthy manner, check out this article. Check out Daniel and Brendan’s chat on the attached vlog for more information on Intermittent Fasting and some of the current, ongoing research on this evolving topic. 

daveynutrition would like to thank Dr. Brendan Egan for this contribution to this vlog and article. Brendan is an Associate Professor of Sports & Exercise Physiology in Dublin City University. Brendan also practices as a Sports Nutritionist and regularly works with elite athletes and teams. Brendan has played sport, namely inter-county football, at a high-level for many years and has carried out numerous research projects within sports and exercise physiology and performance nutrition.


  1. Harvard T.H Chan. (2019). Diet Review: Intermittent Fasting for Weight Loss. Available: Last accessed 01/12/2020.
  2. Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, Hills AP, Wood RE, King NA, Byrne NM, Sainsbury A. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 2015 Dec 15;418:153-72
  3. Headland M, Clifton PM, Carter S, Keogh JB. Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients 2016; 8(6).
  4. Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, Hills AP et al. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol 2015; 418 Pt 2:153-172.
  5. Davis CS, Clarke RE, Coulter SN, Rounsefell KN, Walker RE, Rauch CE et al. Intermittent energy restriction and weight loss: a systematic review. Eur J Clin Nutr 2016; 70(3):292-299.