What is it?
Intermittent fasting (IF) (or time restricted feeding, as it’s commonly referred to in studies) relates to any diet that cyclically restricts energy intake for a predetermined period of time (1), and is most often used as a tool for weight loss (2). It doesn’t detail particular foods to be eaten, rather it specifies when you should eat them. In this sense, IF is not technically a typical diet but more an eating pattern. All IF approaches by greatly limiting calorie intake, have the same thing in common, they all result in less calories consumed than expended.
Which protocol should you use?
You should use the protocol that is easiest for you to follow and is suited to your current lifestyle and training demands.
There are several different IF protocols. Examples are as follows:
Alternate-Day Training: The involves fasting every other day. For example, eat on Monday, fast on Tuesday, eat on Wednesday etc. Another variation allows for 500 calories to be consumed on fasting days. This protocol has been used frequently in obesity research; and as you could imagine; people get very hungry on it but do lose weight successfully if they stick at it.
Eat, Stop, Eat: This involves a single 24-36 hour fast, either once or twice per week with other days set up according to your goal. This can be fasting from dinner one day, to dinner the next, which amounts to a 24-hour fast. The same can be done from breakfast to breakfast, or lunch to lunch.
16/8 Method: Otherwise termed the ‘Leangains’ approach, the 16/8 method involves a 16 hour fast (including sleep) followed by an 8-hour window of eating (whereby you consume all of your daily calories), which usually brackets training. This should be repeated daily.
The Warrior Diet (20/4): This is another variation of a window of eating protocol which involves fasting for 20 hours, and then a 4-hour window for eating. In essence, this involves eating small amounts of raw fruits and vegetables throughout the day, then eating all of your calories in one huge meal at night. This should be repeated daily.
5:2 Diet: The 5:2 diet originates from an evidence-based background. It involves 5 days of the week with normal calorie-controlled eating (typically at maintenance calories), and 2 very low-calorie days (generally restricted to 500-600 calories, however, can be adjusted to suit your personal goals).
It’s important to note that the fasting window allows for water, coffee and other zero-calorie beverages to be consumed, but anything containing calories should be restricted.
What are the Benefits of IF?
There are several benefits of IF. The main benefits include the following:
Weight loss: IF can help you to lose body weight (body fat), without consciously reducing daily calorie intake (2,3). By essentially skipping meals and therefore eating less per day, by default, IF can result in a reduction in daily calorie intake. A review in 2014 found that IF can lead to 3-8% weight loss over a 3-24 week period, which is significantly more than the majority of weight loss studies (3).
Blood sugar and insulin levels: Intermittent fasting can improve insulin sensitivity, reduce fasting blood sugar by 3-6% and lower fasting insulin levels by 20-31%, which can help to protect against the development of type 2 diabetes (3)
Inflammation: Fasting has also been shown to significantly decrease markers of inflammation in the body and therefore may help to protect against the development of many chronic diseases (4,5,6).
Cardiovascular risk: IF may lower levels of “bad” LDL cholesterol and triglycerides in the blood, inflammatory markers, blood sugar and insulin sensitivity – which all increase the risk of heart disease, therefore IF can have a cardio protective effect (3,7).
It’s important to be mindful that some of these benefits are based on animal studies therefore may not be truly applicable to humans. In a similar fashion, proposed benefits may be specific to the particular protocol looked at in the study and therefore may not be generalisable.
Who Should NOT do it?
The three scenarios where you should not follow an IF protocol include:
1) when you are pregnant
2) if you are underweight (BMI <18.5) or have an eating disorder (e.g anorexia) and
3) it simply just doesn’t suit you and your lifestyle (8).
St-Onge, M. et al. (2017) "Meal Timing and Frequency: Implications for Cardiovascular Disease Prevention: A Scientific Statement From the American Heart Association", Circulation, 135(9). doi: 10.1161/cir.0000000000000476.
Johnstone, A. (2014) "Fasting for weight loss: an effective strategy or latest dieting trend?", International Journal of Obesity, 39(5), pp. 727-733. doi: 10.1038/ijo.2014.214.
Barnosky, A. et al. (2014) "Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings", Translational Research, 164(4), pp. 302-311. doi: 10.1016/j.trsl.2014.05.013.
Faris, “. et al. (2012) "Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects", Nutrition Research, 32(12), pp. 947-955. doi: 10.1016/j.nutres.2012.06.021.
Aksungar, F., Topkaya, A. and Akyildiz, M. (2007) "Interleukin-6, C-Reactive Protein and Biochemical Parameters during Prolonged Intermittent Fasting", Annals of Nutrition and Metabolism, 51(1), pp. 88-95. doi: 10.1159/000100954.
Johnson, J. et al. (2007) "Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma", Free Radical Biology and Medicine, 42(5), pp. 665-674. doi: 10.1016/j.freeradbiomed.2006.12.005.
Azevedo, F., Ikeoka, D. and Caramelli, B. (2013) "Effects of intermittent fasting on metabolism in men", Revista da Associação Médica Brasileira, 59(2), pp. 167-173. doi: 10.1016/j.ramb.2012.09.003.
Fung, J. and Eenfeldt, A. (2019). Intermittent Fasting for Beginners – The Complete Guide – Diet Doctor. [online] Diet Doctor. Available at: https://www.dietdoctor.com/intermittent-fasting.