Although only recently gaining traction as an exciting new way to eat or diet, fasting has been part of practice in religions like Islam and Hinduism for thousands of years. Most of us fast incidentally every day in the time between dinner and breakfast the following morning. Intermittent fasting takes this to a new level where we intentionally increase the window of fasting while condensing the window of eating.
The way in which intermittent fasting and its derivatives can be done is very flexible and can be adjusted according to personal preference. In some instances, it can involve just restricting calories as opposed to consuming none at all (no food or drinks apart from water). The body responds differently to eating small amounts of food versus having no food at all, however, there is little evidence to support the benefits of intermittent fasting past creating a calorie deficit for weight loss. Nonetheless, if you're wanting to adopt any of these methods it is beneficial to do so with the advice of an Accredited Practising Dietitian who is qualified to give personalised dietary advice.
The most common methods of intermittent fasting include, but are not limited to:
Alternate Day Fasting - as the name implies, alternates between a 'normal' day of eating and a day of energy restriction amounting to around 500-600 calories (2100 - 2500kj) (1) or complete fasting.
5:2 Method (also whole day fasting) - a little less restrictive than the above, where for five days a week a 'normal' diet is consumed combined with two separate days of fasting or restricting energy to 500-600 calories (2100 - 2500kj).
16:8 Method (or Time Restricted Feeding) - is more than likely the most common as it is far less restrictive than the first two. This protocol is a daily 16hr fast followed by an 8hr eating window. For example, restricting all your meals from 12pm to 8pm while fasting all night and all morning. This method is pretty malleable so people might choose to eat between 8am and 4pm, or any other 8hr window which works for them.
These methods come with varying hormonal and metabolic responses, whether it be due to meal timing, energy consumption or circadian timing, among other factors. For the sake of simplicity and scope of this article, we will say that the most relevant similarity between these methods is that they're restrictive and therefore, almost always create a calorie deficit, which will result in weight loss.
Perhaps the biggest advantage of intermittent fasting is that a large body of scientific literature suggests that intermittent fasting is just as effective at weight loss as daily calorie restriction. (2, 3, 4, 5)
Weight loss is associated with improved plasma lipid concentrations (fat in your blood that may lead to heart disease and stroke), decreased inflammation, lowered blood pressure and improved blood glucose control. It might also improve body composition and performance given the type of sport.
On the topic of body composition, there is also promising evidence to suggest that intermittent fasting may be better at maintaining muscle mass than daily calorie restriction, although more research is needed (6, 7). Put into context, however, if the body is without protein for too long then muscle mass will most likely begin declining, leading to complications in body composition and performance.
Further benefit lies in fasting’s flexibility between methods compared to standard calorie restriction across the day or week. Some people just don't like to spread their meals over the day or simply don't find the time, so intermittent eating provides a structure in which they may delegate time and effort into preparing and consuming some quality food. If food quality remains satisfactory, then there should be no need to count calories as falling into negative energy balance will be quite easy.
Disadvantages & Limitations
Despite showing some body composition and practical benefits, intermittent fasting presents challenges to some groups of people.
For instance, restrictive eating behaviours may cause more harm than good for people with a history of disordered eating. In Australia, eating disorders are more prevalent in athletes than non-athletes (8). Furthermore, infrequent and insufficient energy and carbohydrate intake can compromise performance. (1) If a fasting period clashes with a heavy session, athletes might find they are not sufficiently fuelled nor can they recover optimally for their next session.
Furthermore, if athletes (or anybody for that matter) are given restrictive time guidelines to eat, they may mistake this as an excuse to eat whatever they like, displacing key nutrients with high calorie, nutrient sparse foods. This change in their attitude towards food may lead to an array of physical, mental, social and emotional complications.
Practically speaking, fasting can fit in well for some (as previously mentioned), however may not fit in at all for others. The social element of food does not always align with intermittent fasting, no matter the method chosen. For example, not being able to eat at major social events because of time restricted feeding or a time of fasting may impact the person and people in their life. It can be difficult to adhere to this level of discipline when all your peers are eating, not to mention the annoying and invasive questions that come with the decision.
Some may simply find it hard to resist eating as hunger hormones take over across the day, which can lead to overeating once the fast is broken. Even if overrating a couple of times per week, an overall net positive energy balance could result leaving the individual confused as to why they have gained weight when they think they're doing all the right things to lose it. This can leave them feeling frustrated and begin diet cycling.
Sometimes, the cracks can begin to show when energy intake is restricted. Frustration, irritation and even potential cognitive impairment (especially after bouts of exercise) can be a real side effect of undereating. This can affect the individual internally and socially, as they may unpleasant to be around – nobody likes a “hangry” person!
If you're wanting to lose weight, experimenting with time-bound caloric restriction could be a great option for you. It may need some tweaking before you find a method that works best without dramatically affecting your lifestyle.
Alternatively, there are plenty of other ways to take steps towards weight loss that don’t involve such noticeable changes. Perhaps first try
Decreasing portion sizes
Drinking more water (tip: having a glass before mealtimes can help you eat less food)
Eating high volume, nutrient dense, low calorie foods over the course of the day most of the time
Consuming satiating foods high in lean protein and fibre
As for the scientific evidence on the benefits of intermittent fasting – the jury is still out. Most of the research has been limited to animals, overweight persons or Ramadan (a religious practice which involves fasting from dawn till sunset).
As always, before making any dramatic changes to your diet, consult your GP, a dietitian, and do your own research.
Sports Dietitians Australia. (2017, March 13). HOW INTERMITTENT FASTING AFFECTS YOUR PERFORMANCE. SDA Sports Dietitians Australia.
Rynders, C.A., Thomas, E.A., Zaman, A., et al. (2019) Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients, 11(10), 2442.
Stockman, M., Thomas, D., Burke, J. & Apovian, C.M. (2018). Intermittent Fasting: Is the Wait Worth the Weight? Current Obesity Reports, 7(2), 172-185.
Seimon, R.V., Roekenes, J.A., Zibellini, J., et al. (2015) Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Molecular and Cellular Endocrinology, 418 pt 2, 153-172.
Moro, T., Tinsley, G., Bianco, A., et al. (2016) Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine, 14(1):290.
Varady, K.A. (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obesity Reviews, 12(7), e593-e601.
Byrne, S., Mclean, N. (2002) Elite athletes: effects of the pressure to be thin. Journal of Science and Medicine in Sport, 5(2), 80-94.