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Your Quick Guide to Intermittent Fasting

Fasting, also referred to as Intermittent Fasting (IF) and Time Restricted Feeding (TRF), is simply the manipulation of eating and/or fasting windows. There are many forms of IF from the popular 16:8 (fasting for 16 hours and eating for eight hours), 5:2 (restricting energy on two days of the week) and alternate day fasting.

The benefits of IF stem from going for longer periods without eating. Glucose (carbohydrate) and fatty acids (fat) are the main sources of energy for body cells. In the fed state glucose is used preferentially while fatty acids are stored. In the fasted state however, circulating glucose levels are low, so too are insulin levels, both of which promote the oxidation (burning) of fatty acids. You may have heard that our brain needs glucose to function, but that’s not the full truth. The liver can convert fatty acids to ketone bodies which become the new source of energy for the brain and other cells.

The benefits of IF on this metabolic switch / flexibility (ability to burn fat and glucose effectively) have been extensively researched. The benefits include:

  • Improvements in blood glucose (blood sugar) control and insulin resistance

  • Improvements in lipid profile

  • Reduction in blood pressure

  • Improvements in endurance training

  • Reductions in body fat, especially central adiposity

  • Down-regulation of inflammation

  • Improved mitochondrial health (mitochondria are the powerhouse of all cells and are where fat burning takes place)

  • DNA repair

  • Possible inhibition of cancer cell growth

  • Possible delayed onset of Alzheimer’s disease and Parkinson’s disease

What Constitutes Fasting?

Believe it or not, you’re already practicing a form of fasting. The period between breakfast and lunch is a fast. The period between dinner and breakfast is also a fast. The first step in conditioning the body to fast safely and comfortably is being able to increase the meal-to-meal window. Periods of four to five hours without eating between meals or snacks is this goal. Once this can be done comfortably attention can be directed toward the ‘overnight fast’. This is the fasting that takes place overnight, between the last bite on one day and the first mouthful the next day.

For digestive ease and to strengthen appetite control, in my practice I recommend a minimum 12 hour overnight fast. It’s typically very practical and for most, a realistic place to start. It means fasting from 7.30pm until 7.30am the next day and finishing dinner by 7.30pm, for example. As the fasting muscle strengthens and 12 hours overnight becomes a breeze it’s possible to look at extending the overnight fast to 13, 14 and even 16 hours. Remembering that the length of the eating window matters and should be the difference between 24 hours of the length of the overnight fast.

Overnight fasts of longer than 12 hours don’t need to be practiced every day of the week. As outlined below, for some people and in some circumstances a longer fast isn’t advised. So, please keep reading.

What Should the Fast be Broken With?

Breaking the fast with a nutrient dense meal is as important to achieving the benefits of IF as the actual fasting length. It’s certainly crucial to avoiding what I refer to as ‘rebound eating’ later in the day. ‘Rebound eating’ is common and it’s essentially the over consumption of calories later in the day resulting from when not enough is eaten at the first meal or when the duration of the overnight fast has been extended beyond the individual’s state of readiness.

Research shows that eating more with the first meal and less with the final meal of the day can make a significant difference to obesity, fasting triglycerides, insulin, and glucose. In summary, it’s both the content and timing of your first meal that make it the most important meal of the day!

For inspiration on what to break the fast with I recommend checking out my breakfast recipes or The Complete Smoothie Guide for Active Women.

What About Fasting and Training?

It’s not unhealthy or dangerous to exercise at a light to moderate intensity for 20 – 60 minutes before having breakfast. It’s referred to as fasted training and the benefits of such training include increasing aerobic capacity and supporting fat utilisation. For those new to fasted training, particularly females, I recommend starting with shorter sessions (20 – 45 minutes) and less intense sessions.

When training fasted, especially for aerobic sessions greater than 60 minutes, some fuelling may have to happen during the session and refuelling must happen in the 60 minutes post training, regardless of the overnight fasting goals. In other words, fasting should not be done at the expense of training recovery. This means fasting on a rest day or timing training so the post training meal can be had at the end of the overnight fast is required.

For some people higher intensity sessions such as HIIT, strength or interval training can also be done in the fasted state. These sessions require greater carbohydrate utilisation and tend to support a goal of muscle growth, which is why for others, performance and recovery may be enhanced when food is consumed prior. A pre-training snack such as two medjool dates or a small banana could make all the difference to the quality of the session.

Is Fasting Appropriate for Everyone?

Never have we as humans had such ready access to food as we do today. Our hunter gathering ancestors certainly weren’t eating three meals and multiple snacks each day, neither were our great grandparents. Our ancestors adapted to being able to expend large amounts of energy during periods of fasting. For most adults, it’s considered very safe and natural to begin extending meal to meal windows and even progressing to an overnight fast of 12 hours.

In saying this, most of the research on IF has been done on men. It means there are certainly groups in which going beyond 12 hours should be avoided or only occur under the guidance of a health professional and they apply mainly to women. Including:

  • Pregnant and breastfeeding women

  • Women with low reproductive hormones or amenorrhea

  • Teenage women

  • Those experiencing periods of high stress and/or adrenal dysfunction

  • Those with a history of eating disorders

  • Women with less than 18% body fat

  • Those with thyroid dysfunction

  • Athletes with a high training load

Considerations for Applying IF

As already discussed, it’s critical that a step-by-step approach is taken to implementing IF. To support in the process here are a few additional suggestions to help make it an easier, more enjoyable, and healthier process:

  • Hydrate really well during periods of fasting. It can even help to introduce natural electrolytes like fresh lemon juice and a pinch of salt

  • Ensure meals being eaten (not just the first meal of the day) are nutrient dense. Focus on protein and fibre rich non starchy vegetables for satiety as well as healthy fats

  • Break the fast by 10am. To achieve a 16 hour fast that would mean eating dinner by 6pm the night before

  • Extend the fast on rest days or days with minimal training, especially to begin with

  • Get adequate sleep. Fasting is going to feel near impossible for those that are stressed or sleep deprived

  • Stop if you feel unwell, if the menstrual cycle becomes irregular or if there are other signs of stress, thyroid or reproductive hormone imbalances

A Word on Caffeine, Milk and Sweeteners

Pure caffeine is allowed in the fasted period, but in clinic I find this leads to temptation for milks and non-nutritive sweeteners. To achieve a pure fast having calories during the fasted period is not considered allowed. Whether its cow’s milk, sweetened almond milk, coconut milk or most other milks, milk contains calories which means it wouldn’t be suited to a pure fast. In saying that, for the purpose of fat utilisation, very small amounts of unsweetened almond or coconut milk during the fast may still allow some to achieve results.

Stevia, erythritol and xylitol are all examples of non-calorie (non-nutritive) sweeteners which leads some to consider them to be appropriate during a pure fast. However, even though calorie free, the sweet nature of these ingredients triggers specific receptors on the tongue and initiates an insulin response. This isn’t ideal as an insulin response is to be avoided in order for the many benefits of TRF and IF to be gained. If you crave sweetness, allow some of these non-calorie sweeteners in the diet but have them during the eating window rather than the fasting period.

If you are including non-calorie sweeteners or milks in the fasted period and aren’t achieving the desired results, it’s possibly a sign that you’re ready to do without them and need to consider a pure fast.

If you want to learn more about how to tailor your eating / fasting windows and are interested in learning more about developing metabolic flexibility, then I would love to work with you! You can learn more about working with me here or via a complimentary Discovery Call which you can book here.


de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease [published correction appears in N Engl J Med. 2020 Jan 16;382(3):298] [published correction appears in N Engl J Med. 2020 Mar 5;382(10):978]. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136

Jakubowicz D, Barnea M, Wainstein J, Froy O. High caloric intake at breakfast vs. dinner differentially influences weight loss of overweight and obese women. Obesity (Silver Spring). 2013;21(12):2504-2512. doi:10.1002/oby.20460


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