Fasting means different things to different people. There are many ways to fast ranging from Intermittent Fasting (typically between 16 hours and 3 days depending who you ask), OMAD (one meal a day) up to full-on extended fasts (3 days up to weeks, or even months).
So here I’ll try and put some of my understanding of fasting, and especially some of the different protocols.
One of the confusing things you’ll see is a difference in the use of notation. In some fasting protocols, you will see something like 5:2 – which is a weekly protocol and means, 5 days of normal eating with 2 days of fasting. But in other protocols, you will see e.g. 16:8 – which rather than days refers to hours. 16 hours of fasting and an 8 hour “eating window”. Given the numbers involved, it should generally be self-evident what is being referred to – they will either add up to 7 if days or 24 if hours.
Tracking your fasts
It’s important to track your fasts, and for that there are many apps. I personally use Zero on my Android phone. Zero lets you define the fasting length, enter your start time and then alerts you when you’re done. You can also edit the fast once started, so it’s very flexible.
Now I’ve already started using jargon so before you get confused, let’s break it down!
Eating Window – a term typically applied to the period starting from when we break our fast through to when we last eat before the next fast begins.
An eating window could typically be described as anywhere from 1 hour to 8 hours on a fasting day. See below 16:8 and OMAD.
Although I mention “OMAD” – which means One Meal A Day – it does not necessarily mean one meal / one sitting. You may find you spread out your eating over a longer time window in order to meet your body’s nutrient requirements. Hence OMAD could potentially be anywhere from a 1hr to a 4hr eating window, so OMAD is a generic term, and more specifically you should think about OMAD as being anywhere from a 23:1 fast (23 hrs fasting, 1hr eating window) to a 20:4 fast (20 hrs fasting, 4hr eating window).
Fasting protocols – a fasting protocol is simply any scheduled fast above and beyond the “regular” fast that we automatically do overnight while we sleep. Examples could be 16 hour fasts (16:8), 24 hour fasts, 3*48hour fasts etc as described in the sections below.
There are a number of metrics that can make up a fasting protocol, including the number of hours each day we fast, but also for shorter fasts can include as well as how many days a week we perform the fast.
So a 5:2 fasting protocol (5 “normal” days and 2 fasting days) could be 16:8 fasts on the fasting days. You need to think about both terms.
Historically I did most of my fasting on a 2:5 schedule. 2 “normal” eating days at weekends, with 5 weekdays of fasting. And generally I did most of my fasting using the OMAD protocol.
Intermittent Fasting – Intermittent Fasting is a term generally used to refer to shorter fasts between 16 hours and 2-3 days.
In principle of course there is an argument that all fasting is intermittent, occurring at roughly irregular intervals and not constant. But we digress – let’s stick to the definition of IF as shorter fasts.
- 16:8 Fast / 2MAD- Fasting for 16 hours, then eating only in an 8 hour window. Also often referred to as 2MAD for “2 Meals A Day”. Aka “skipping breakfast!” 😀
16:8 is generally done every day continuously as long as required to get results. It is the most “tame” of the fasting protocols, effectively just requiring you to skip breakfast, and therefore may be more suited as a stepping stone to longer fasts when you are beginning, or as a maintenance schedule once you are at your target weight. It’s a common misconception that “breakfast is the most important meal of the day” – skipping breakfast and doing a 16:8 fast actually has many health benefits.
Of course, from 16:8 it’s easy enough for most people to do 17:7, and then 18:6, 19:5. These again are typically stepping stones to build towards OMAD. - 20:4 / 22:2 / 23:1 / OMAD – OMAD is the next major step up from 16:8. Now you are skipping breakfast and lunch, and fasting for a longer period, with a significantly reduced eating window. Some prefer to define OMAD more strictly as 23:1 in that it is then only effectively one meal, while longer windows such as 20:4 would involve grazing multiple distinct meals, but for me I treated 20:4 as one meal, with a starter, main course and dessert, just eaten over a leisurely period with a break between courses.
Ultimately it doesn’t matter what you choose to call it, I never saw any difference in results between 20:4 and 23:1.
Extended Fasting – a term usually applied to longer constant fasts going over a full 24 hour day.
36 hour fast – a fast effectively lasting 1 1/2 days / 36 hours approximately. Often achieved by fasting for one day (say 8pm – 8pm, but then not eating before bed and using your sleep period to take you through to 8 am the next morning. Most people find it easiest to fast when they sleep, as of course, this is when we all naturally fast!
ADF / 3 * 36 – alternate day fasting is another popular weekly protocol. Fast from for example 8pm Sunday through to 8am Tuesday, eat normally on Tuesday, then repeat from 8pm Tuesday to 8am Thursday and so on.
There are many variations to this theme – a 42 hour fast would mean extending your fast through until lunchtime and can be done as 3 * 42 hr and so on.
48 hour fast – a fast lasting 2 days / 48 hours approximately. So for example, finish eating 8pm Sunday then eat again 8pm Tuesday. Note that in reality, this may well be more like a 47 hr fast as you may have an eating window on your last day, but it’s just an easier shorthand to approximate.
60 hour fast – a fast lasting 2 1/2 days / 60 hours approximately. Like the 36 hour fast, taking a 48 hour fast that finishes in the evening, and then going to bed without eating so you continue through until breakfast.
72 hour fast – a fast lasting 3 days / 72 hours approximately.
96 hour fast – a fast lasting 4 days / 96 hours approximately.
120 hour fast – a fast lasting 5 days / 120 hours approximately.
It goes without saying that you should gradually build up your tolerance to fasting. Fasting is like a muscle – learning to ignore the fake hunger signals your body sends you. The fact is that you cannot actually be hungry as long as you have one single lb of excess body fast. That lb of fat equates to 3,500 calories which is more than enough to get you through the day. But it takes a while to get used to fasting and ignoring those fake hunger signals, which is just a hormone called “ghrelin” – sometimes called the “hunger hormone” which simply goes off through the waking hours of the day at a schedule based on your habits. I like to call ghrelin my “hunger gremlin”, as telling him to shut up is more fun that way. 🙂
So – you can start with 16:8 fasts, work up to OMAD, then if you like try longer fasts. But you DON’T HAVE TO. If 16:8 works to get the results you need – stick with it! If you hit a plateau, then maybe try OMAD. If you plateau with OMAD, you can kick start results with a 36 hr fast. If you are seeing great results from one 36hr fast, maybe try ADF / 3*36. You get the idea.
Personally I tried longer fasts just to see if I could. I was amazed to find that I could fast for 5 days, exercise up to an hour each day, and not feel light-headed or any hungrier than I did during a regular fast.
Based on my research, an extended fast up to 5 days can be done at almost any time with little or no supplements – as long as you then have a refeeding window afterwards that allows your body to recover and top up its essential nutrients. I generally try to take a one-a-day multivitamin supplement but also, if I’m exercising, I generally use electrolyte tablets in my water – this ensures that key minerals like potassium, sodium and magnesium do not get low due to sweating/excretion.
If you do consider 5 day or longer fasts, it’s definitely not advisable to then jump straight into another extended fast without a decent break of a few days for proper refeeding. Failing to do this could be asking for trouble, as you will severely limit your intake of vitamins/minerals during an extended period.
You should definitely research essential fasting supplements before embarking on fasts for more than 5 days. I don’t recommend these types of fast as I have not seen any evidence at all that they offer any benefit above the “regular” extended fasts I’ve mentioned above.
Common weekly fasting protocols
5:2 diet / aka “The Fast Diet” – often you will see 5:2 mentioned in fasting circles, but mainly in the UK. The 5:2 diet was popularised by Dr Michael Mosley’s book.
Very simply, you fast just 2 days a week and eat normally for 5 days – but during those 2 days you are allowed up to 600 calories (500 for women). So it was not even a “true” fast. There were no restrictions placed on whether you spread out your 600 calories over three small 200cal mini-meals (we call them snacks here), or just had one “big” meal of 600 calories. I was generally able to fit in an omelette for my 600 calories.
I tried it for just two weeks and lost 6lbs both weeks for a total of 12 lbs. It must be stated that I was training heavily for a 3.9km river swim at the time but it was still a phenomenal weight loss in a short period.
At the time it was considered quite radical but is definitely one of the lightest fasting approaches. Nevertheless, it should not be discounted and is a great way to break yourself into fasting gently. By spreading your fasting days out (say Monday and Thursday), and having a small meal each of those days, it doesn’t feel terribly difficult.
When I started fasting “properly” at the start of this blog back in 2018 I did exactly this. I began effectively with the Fast Diet approach – 600 calories twice a week, then extended to three days a week (effectively, 4:3), and then four days (3:4) and then 5 (2:5). So effectively I fasted during weekdays and ate “normally” at weekends. However, note that as I introduced more fasting days I did NOT keep to 600 calories on my fast days. I ate a normal meal without ANY restrictions, and I still saw great results. I calculated to get to my 2lbs per week weight loss target 2,000 calories a day would work, and unless I was eating junk food I genuinely would struggle to exceed 2,000 calories in just one meal!
2MAD / 16:8 – can be done either every day (typically for weight loss), or a specific number of days (more for maintenance) each week. Many people find it easier to fast while busy at work rather than at home at weekends so might do 2MAD 5 days a week.
Which fast should I do?
This is the 64 million dollar question – and the answer is – it depends. It depends on what results you are after (weight loss, maintenance, improved health, autophagy), and also your timeframe.
In my view – start with the results you want and work backwards from there. Initially I wanted to lose 100lbs in 1 year. I set 1 year as I felt this was a comfortably achievable time frame to completely transform my body. I know more rapid transformation is possible, but it can be very hard and stressful both physically and psychologically.
Starting with 100lbs in 1 year gave me a very easy goal. 100lbs in 52 weeks = approximately 2lbs per week. So I had a very measurable ongoing target. Again 2lbs a week also feels very achievable. Many health and nutrition experts warn against losing weight significantly faster than this – but of course, it can be done.
See my separate post on “general dieting terms and concepts” for more information about how to calculate your calorie deficit as well as things like fasting supplements and what you should be eating.