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Intermittent Fasting - What You Need to Know

Fasting refers to voluntary periods of abstaining from food for health, performance, spiritual, or religious reasons. Intermittent-fasting describes a type of fasting that generally continues for 12 to 24 hours, as opposed to extended periods of fasting that may last from 24 hours to many days (or even weeks). But why would we voluntarily go without food - especially when there are people all over the world forced to go hungry, and breakfast is the most important meal of the day, right?

Fasting has been around for thousands of years and was traditionally used in medicine as a strategy for healing the body. For example, fasting was one of the original methods for treating diabetes in the early 1900’s, long before insulin was discovered [1]. Periods of fasting heavily reduce the human body’s demand for insulin, and in this case, allow sufficient time for a person with diabetes to stabilise their blood glucose levels between periods of feasting. This mechanism is one of the main reasons as to why many people are choosing to implement fasting strategies today – not necessarily because they have diabetes, but because the benefits of having less insulin hanging around are proving to be extremely beneficial for a multitude of health and well-being reasons.

And yes, breakfast is the most important meal of the day – but it doesn’t have to be consumed before 8.30 am in the morning! The word breakfast describes a meal of which breaks your fast (break – fast). However, if you’re guilty of late-night snacking and/or throw down an early breakfast before you rush out the door in the morning, are you truly allowing yourself adequate time between meals to experience an overnight fast? Maybe it’s wrong to even be classifying that first meal of the day as breakfast, when there is no fast to break…

Let’s consider a scenario where one consumes their last meal of the day at 6.30 pm and does not eat again until 10.30 am the following morning (16-hour fast). Depending on the amount of food eaten and the macronutrient composition of the evening meal, it may take the body up to 10 hours to fully digest the food before entering a ‘fasting state’. This state (between the 10 and 16-hour mark) is where all the juicy stuff happens, including but not limited to:

  • Blood insulin levels significantly reduce and the body can access the use of stored body fat [2] (because there is less need for it – we aren’t filling our bodies with glucose that needs to be shuttled into the cells by insulin).

  • Resting energy expenditure increases (i.e., you burn more stored fuel per minute just by sitting around and doing nothing!) [3].

  • Less energy is required for the digestion of food (well, there is no food to digest…) so your body can divert its energy to other physiological processes that it may never otherwise get around to! This involves cellular repair and regeneration, including the removal of toxic waste products and damaged cells (autophagy) [4].

  • Several genes undergo positive changes (gene expression) in response to fasting. Your genes respond to healthy amounts of stress to help you become more resilient. Fasting increase your body’s expression of genes related to longevity and disease-protection, including the prevention of cancer and diabetes [5].

Like any dietary or lifestyle strategy, fasting is a tool for your health toolkit that may or may not be effective, let alone safe, for you. There are many contraindications to fasting (reasons why you should not do it) that must be discussed on an individual-basis with a qualified health professional before introducing fasting into your lifestyle.

Additional Considerations (Thyroid Health)

Alongside the physiological and hormonal changes listed above, intermittent fasting has an impact on your thyroid function (I mean, of course it does!). Intermittent fasting can reduce circulating T3 and normalise T4 (thyroid hormones) [6, 7]. In other words, intermittent fasting induces somewhat of a “mild hypothyroidism” state, and it is this state that many of fasting’s benefits may be attributed to. But what does this mean for people who already have hypothyroidism? Unfortunately, this topic not been researched enough to provide a definitive answer (a common theme in nutritional science). However, what we can rely on with some-what confidence is YOU. If you have hypothyroidism (or even if you don’t), ask yourself the following questions:

  • How long do you naturally go between meals?

  • Do you graze across the day or do you have regimented meal times, and how do you feel trialling each approach?

  • What does hunger feel like do you? Do you eat out of habit, physiological hunger, emotions, cravings, discomfort, dizziness, or some other reason?

  • What is your energy like across the day? What is your energy like before and after meal-times? Do you experience a “slump” 1-2 hours after meals or do you feel stable, irrespective of your food timing and choices?

  • What stage in your health journey are you? Are you aware of your current hormonal balance? Are you undergoing a time of stress or illness? Are you just not feeling so great right now?

You may be able to easily identify how your answers to the questions above will indicate whether intermittent fasting should be implemented as a strategy for you right now or not. Nevertheless, the following point must be emphasised: fasting is a form of stress. Though many forms of stress are healthy for the body by stimulating adaptive mechanisms which allow us to become stronger and more resilient, too much stress (even “healthy” stress) can be harmful. If your body is already undergoing some form of stress (or multiple stressors), fasting may not be right for you – right now. If you’re unsure as to what your answers to the above questions mean? Take them to a qualified health professional with experience in fasting strategies and seek personalised advice.

Practicalities of Intermittent Fasting

So, you’re clear of any contraindications that may prohibit you from fasting and you want to implement this strategy into your daily, weekly or monthly practice, but are not sure exactly how to go about it? Let’s discuss some different intermittent fasting strategies.

1. 16 : 8 Fasting

As described above, this strategy essentially involves a 16-hour fasting block and an 8-hour feasting block. This regimen may be practiced daily or on specific days of the week. Otherwise, it may just happen as a side-effect of improving your diet, lifestyle and/or health. Generally, only water, herbal tea, black tea and/or coffee are consumed during the fasting block and there are no restrictions during the feeding block. This means, you may choose to completely skip a meal (e.g., your first meal of the day). Otherwise, you may choose to condense your normal daily food intake into an 8-hour window. Our hormones are far more powerful when it comes to weight and health, then the over simplified ‘calories-in-calories-out’ hypothesis. Even if you consume the same number of Calories in the 8-hour feasting window as you would over a 24-hour period (not fasting), you’re going to experience the benefits of the fast (including the ramped-up fat-burning). However, people usually find that they cannot comfortably fit their normal food intake into an 8-hour window and naturally choose to eat less.

2. Alternate day fasting

Like the 16:8 method, but stretched out across two days. Generally, this involves a fasting period that lasts for 24-hours, followed by a feasting period of 24-hours (i.e., a normal day of eating). This method may work for those who enjoy the freedom that fasting gives them and like the idea of completely “turning off” the whole food-thing every second day. If done correctly (i.e., you are not eating more than you usually would on the feasting days), this method may help you reach your health or weight-loss goals a little faster than the 16:8 method. However, not consuming food for one whole day on every other day is a far more difficult task then many initially consider it to be. This method may be implemented as a short-term approach under medical supervision but is generally too extreme or unnecessary for most individuals to maintain.

3. 5:2 fasting

This common method of fasting generally involves 5-days of usual/normal eating and 2-days of fasting per week. You can choose whether you want your fasting days to be consecutive (e.g., fast on Wednesday or Thursday) or spaced in between your feasting days (e.g., fast on Wednesday and Saturday). Many fasting experts argue that the longer you fast in one block, the more benefits you will experience (to a point). A popular version of this method is to consume 500 kcal (i.e., very little food) on the fasting days so that you get to chew on something, and somehow – this is still classified as fasting. However, consuming 500 kcal over a 24-hour period (whether as one meal or spread across several snacks) is not the same as consuming 0 kcal over a 24-hour period, particularly if these 500 kcal are coming from carbohydrates or protein. Both carbohydrate and protein stimulate an insulin response, and every time insulin is secreted, (most of) the fasting benefits are put on pause. Also, consuming measly portions here and there might work against you, as you’ll hunger hormones will be sent on a roller-coaster. Alternatively, allowing your body to fully switch over to a fat-burning state (by avoiding food) will produce ketones, which are well-known to have an appetite-suppressing affect.

4. One meal a day

Consider the first strategy (16:8), but stretch out the fasting window to 23 hours and the feasting window to 1 hour. You might finish consuming your meal at 2.00 pm, then not eat again until the following day at 1.00 pm, and so on. At least you still get to eat something everyday, right? This can be a useful strategy for many individuals, particularly if you find yourself pressed for time (you’ll only cook and prepare food once a day as opposed to three or four times!). However, the most difficult thing to get right with this type of fasting is formulating that one meal. This meal is extremely valuable and should contain most of your recommended daily intake (RDI) for the essential vitamins and minerals. Of course, you’re not going to ‘stop working’ if you don’t reach the RDI for every single micronutrient every single day. Butt you do need to balance out your intake of micronutrients across the week such that your body is eventually getting everything it needs to function optimally. This precious meal should also contain adequate healthy fats to keep your energy levels humming along for the next 23-hours. You’ll also need a good dose of high-quality protein and a small amount of complex carbohydrate. In other words, you can’t just get stuck into anything you like for that 1-hour of the day simply because you’ve avoided food for the other 23-hours.

5. Fat-fasting

Fat is the least insulinogenic macronutrient such that you can eat a certain amount of it and not create an insulin demand for your body. That is mainly because, fat does not break down into glucose in your body. Heard of Bulletproof Coffee? Bulletproof coffee is a blend of black coffee, butter and MCT-oil (i.e., a type of fat – like coconut oil) designed by biohacker, Dave Asprey, to provide your body with a source of fuel and micronutrients that suppresses ghrelin (so you can fast for longer), resets leptin (so when you do eat, you’ll be more in tune with your satiety signals) and does not stimulate insulin (you haven’t put your fasting efforts on pause). There are plenty of ways you can include small amounts of fat during your fasts (any type) to help with compliance, improve your mental performance and/or sneak in extra micronutrients – it doesn’t have to be through Bulletproof Coffee.

Once again, before embarking on any diet or lifestyle change, speak to a qualified health professional for personally-tailored advice. You can schedule an appointment with an Ellipse Health Dietitian here. Intermittent fasting can be an extremely beneficial strategy when utilised in a safe and effective manner. However, fasting is not necessarily for everyone and some individuals will find smaller meals on more frequent occasions across the day (or night) to be more beneficial or achievable for them. For those who do not have a medical condition that requires them to eat every few hours, get started by simply becoming aware of your current dietary habits and set your first goal as establishing a decent overnight fast (12-hours) and allowing your body to properly rest and digest before anything else.

By Jessica Turton

This article provides general information from the current scientific evidence base and clinical judgement of the author. It is designed for educational purposes only and should not be substituted for medical advice. The author recommends you seek personally tailored support from a qualified healthcare practitioner before undertaking any major lifestyle change.


[1] Osler W, McCrae T (1920) The principles and practice of medicine. New York, London, D. Appleton and company [2] Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E (2005) Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. The American journal of clinical nutrition 81: 69-73 [3] Zauner C, Schneeweiss B, Kranz A, et al. (2000) Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American journal of clinical nutrition 71: 1511-1515 [4] Alirezaei M, Kemball CC, Flynn CT, Wood MR, Whitton JL, Kiosses WB (2010) Short-term fasting induces profound neuronal autophagy. Autophagy 6: 702-710 [5] Zhu Y, Yan Y, Gius DR, Vassilopoulos A (2013) Metabolic regulation of Sirtuins upon fasting and the implication for cancer. Current opinion in oncology 25: 630-636 [6] Burman KD, Smallridge RC, Jones L, et al. (1980) Glucagon Kinetics in Fasting: Physiological Elevations in Serum 3,5,3′-Triiodothyronine Increase the Metabolic Clearance Rate of Glucagon*. The Journal of Clinical Endocrinology & Metabolism 51: 1158-1165 [7] Buhner SH (2003) The Fasting Path: The Way to Spiritual, Physical, and Emotional Enlightenment. Avery

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