Rachel Boutagy

Rachel is an Exercise Scientist, Fitness Trainer, Wife and Mum to two beautiful daughters. Having been qualified in exercise science for over 25 years, she's been fortunate to work alongside some of the leading authorities in the fitness industry. Rachel has a wealth of knowledge in health and nutrition, through her significant amount of evidence based research she has conducted throughout the years.

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The science behind fasting and if it's right for you.

The science behind fasting and if it's right for you.
Fasting has become pretty popular over the last couple of years. One of those reasons could be that the 'fasting diet' is far less complicated than most others. We ask Dr Tony Boutagy the facts about fasting and the different ways to do it.

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We recently caught up with Tony Boutagy, PhD, ND, AEP, ESSAM, to ask if fasting is the answer to our health problems and if so, how do we actually do it...

Dr Boutagy, why is fasting beneficial? 

Fasting promotes metabolic flexibility, where metabolism can ‘switch’ back and forth between from glucose metabolism to fat.

Fasting is a term used to describe a variety of eating patterns in which no or few calories are consumed for time periods that can range from 12 hours to several days, on a recurring basis.

Fasting causes unique physiological responses of multiple major organ systems, including the musculoskeletal system, to the onset of the metabolic switch, which is, the point of negative energy balance at which liver glycogen stores are depleted and fatty acids are mobilized (typically beyond 12 hours after cessation of food intake).

Emerging findings suggest that the metabolic switch from glucose to fatty acid-derived ketones represents an evolutionarily conserved trigger point that shifts metabolism from lipid/cholesterol synthesis and fat storage to mobilization of fat through fatty acid oxidation and fatty acid-derived ketones, which serve to preserve muscle mass and function.

Thus, fasting regimens that induce the metabolic switch have the potential to improve body composition in overweight individuals.

Moreover, fasting regimens also induce the coordinated activation of signalling pathways that optimize physiological function, enhance performance, and slow aging and disease processes.

Fasting regimes have been shown to improve diseases of aging, cardiovascular disease, insulin resistance, reduce inflammation, decrease body weight, and improve neurodegenerative diseases.

There seems to be lots of ways to fast? Which is the right way?

There are actually 4 type of fasting categories.

1. TRE - Time-Restricted Eating

Time-restricted eating (TRE) is a pattern of eating that attempts to apply circadian rhythm physiology to our eat/fast cycle.

TRE has two features (1) encourages consumption of all our daily food within an 8-12 hour period and (2) the dinner would be consumed early in the evening, preferably whilst still light.

This pattern of a condensed eating window would therefore have a fasting period of 12-16 hours, with the exception of water. This might help lower blood glucose and insulin levels, reduce several risk markers for cardiovascular disease, decrease total energy intake and reduce body mass.

There is a minor increase in circulating ketones but the effect on autophagy at this time is not known. There is human data to show reduction in breast cancer reoccurrence.

2. TAD - Twice-a-Day Feeding

Similar to TRE, TAD is a way of arranging eating to occur early in the day and later in the evening, known as Twice-a-Day Feeding (TAD).

TAD has been shown in mice to modify circadian autophagy and glucose/lipid metabolism that correlate with feeding-driven changes in circulating insulin. TAD feeding decreases adiposity and, unlike caloric restriction, enhances muscle mass.

TAD feeding drives energy expenditure, lowers lipid levels, suppresses gluconeogenesis, and prevents age/obesity-associated metabolic defects. Studies suggest that consuming two meals a day without caloric restriction could prevent the metabolic syndrome.

3. IF - Intermittent Fasting

IF is a pattern of eating that encourages a complete abstinence of calories either on two days of the week or on alternate days.

IF has been modified to include two to three days of markedly reduced calories (less than 800 calories per day) that attempt to mimic fasting physiology. Examples include the 5:2 diet and the Deficit Day System.

4. FMD - Fasting Mimicking Diet

The Fasting Mimicking Diet (FMD) was developed by Valter Longo in an attempt to mimic the benefits of fasting without actually completely abstaining from food.

FMD recommends that for 5 consecutive days per month you would consume around 800-900 calories per day, roughly 10% of energy intake from protein, 40% from carbohydrate and 50% from fat.

The rest of the 25 days in the month would see a Mediterranean diet being consumed. This 5/25 cycle would be performed for 3 consecutive months to see the benefits, which include:

  • increases in autophagy in multiple tissues
  • improvements in autoimmune disease, and
  • a wide range of age-related disorders including diabetes, cardiovascular disease, cancers and neurological disorders such as Alzheimer's disease Parkinson's disease and stroke.

Fasting is defined as the complete abstinence of calories for 24 or more hours. Normally performed for 1 to 3 days.

If 24 hours, fasting can be routinely performed twice per week with high compliance. If performing a prolonged fast of several days, this is normally done one to three times per year and should be medically supervised.

How do we incorporate fasting into our lifestyle?

My practical recommendation for incorporating fasting regimes into one’s lifestyle is to follow these steps:

  • Transition to a diet that would be considered carbohydrate restricted, that is, less than 150g/day of high-fibre, nutrient dense, minimally processed carbohydrates. The reduction in carbohydrates would be replaced by dietary fat;
  • Commence time-restricted eating, starting with a fasting period of 12 hours and progressing to 16 hours over 4-6 weeks for at least 5 days of the week;
  • Incorporate steady-state cardiovascular training into the fasting period, starting with 20 minutes and progressing up to an hour (or even 90 minutes, if time allows);
  • Avoid intense or prolonged exercise the day prior to the first 24 hour fast;
  • Now you are reading to try your first 24-hour fast.
  • Eat dinner as early as is realistically or practically possible, ideally around 5pm. Some light exercise, like walking, would be recommended after dinner;
  • In the morning, perform your easy cardiovascular training and consume some black coffee;
  • Have green tea and black coffee when you would normally drink them, or when you would eat or snack or when you feel a hunger pang (which normally only last 15 minutes, so tell yourself it won’t last long);
  • Break your fast with a meal that includes 25-50 grams of quality protein, and a serve of minimally processed high fibre carbohydrates, up to 50 grams. Eat slowly and don’t over consume. Your stomach will have shrunk, so it is easy to gorge and feel unwell!

What about training? Will my performance be affected?

Eventually, you will adapt to your habitual exercise in a fasted state.

Moderate cardiovascular training is advised initially. Then your quality session, either weights or HIT can be programmed to occur just prior to breaking the fast so that the hormonal and autophagic response is targeted towards the tissues and systems just trained. Although unstudied at this stage, it is mechanistically sound.

A recent review by Mark Mattson has argued that exercise during the fasted state is a normative feature and state from an evolutionary perspective, that humans could only have survived if they could hunt for many hours in the absence of food.


Renowned sports scientist Dr Tony Boutagy, has a PhD in exercise and sports science from Charles Darwin University in Australia and is an Exercise Physiologist. Having conducted over 50,000 training sessions in his career that has spanned 25 years, Tony is regarded as one of the premier personal trainers in the country.


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