Intermittent fasting (IF) gained its popularity in 2012 and with further published research growing in the area, peoples interest in the topic continues to rise. Originally research claims stemmed from animal studies and only recently has human studies began to develop. 

 Circadian Rhythm or otherwise known as ‘Time Restricted Eating’ (TRE) has recently gained interest, with a similar motive to intermittent fasting. The idea of TRE is to follow our natural ‘circadian rhythm’, which involves eating at times our body is primed to eat and digest and to stop/reduce eating at times we are supposed to be at rest or recovery i.e. using stored energy.

The real question is – can these two forms of eating promote “sustainable weight loss, improve our energy levels and benefit our overall health”? 


Intermittent fasting aims to initiate a ‘fasting and feeding’ time period. 

 The most common forms of Intermittent Fasting involve; 

  • Alternate day fasting– one day fasted (<25% of total energy) and one day, normal feeding 

  • Time-restricted fasting– commonly an 8-hour feeding window and 16-hours fasted

  • The 5:2 diet– 2 days involves <25% total daily energy needs and 5 days normal feeding

Time restricted eating is about paying attention to the number of hours you are fasting within a 24-hour window. The concept is based primarily off following your own circadian rhythm, meaning there are times in the day where our body is more metabolically active and times for resting and repairing, i.e. we are more primed to digest towards the morning and middle of the day, than towards the evening, where our bodies are ready for rest. 

Theory & Claims 

  • Increased weight loss

  • Improved biomarkers i.e. insulin sensitivity, blood pressure & total cholesterol

  • Reduced appetite 

  • Reduces chronic disease risk

  • Reduces inflammation  


Yes, we do enter alternate metabolic phases, meaning during a prolonged fasted period our body relies less on glucose and more so on ketones as our primary energy fuel. This involves the breakdown of stored fat mass, resulting in reduced appetite due to the presence of ketones. Yet when alternate day fasting was compared to a ‘calorie restriction weight loss plan’, in a 2017 randomised control trial, results showed intermittent fasting did not appear superior and both diets achieved equally same results in weight loss (6.0% vs 5.3% respectively) and improved biomarkers (i.e. insulin sensitivity). 

What are the health risks of fasting?

  • whilst weight loss and certain biomarkers have improved in certain studies, results were short-term, therefore it is advantageous that future research examines long-term changes in metabolic improvements and body weight management. 

  • when fasting (like alternate diets) is implemented without the proper guidance of a qualified health professional, especially for specific populations and or days where calorie restriction is necessitated, individuals are at risk of lacking key nutrients and food groups, leading to further complications, predominantly irritability and lack of motivation long-term. 



We know late night eating is associated with a higher risk of obesity, metabolic syndrome and poor heart health. Following our ‘circadian rhythm’ can be an effective method, with some small studies showing overweight adults who fasted for more than 14 hours overnight promoted reduced total calorie intake, weight loss, increased fullness towards the evening and improved sleeping patterns. 

 Improving the quality of your diet, monitoring portion control and being energy smart in conjunction with daily movement will help you work towards your weight loss/maintenance goals. If you wish to pursue a form of fasting, we would suggest seeking out an Accredited Dietitian who can ensure you’re achieving complete energy and nutrition, suitable to your lifestyle. The fundamentals of understanding ‘how and when’ you fast and how you break up your meals matters!

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