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The truth about fasting


You will know that what you eat is very important for your health and wellbeing. But not eating is just as important. Having extended periods of not eating is called ‘intermittent fasting’, and when the body fasts, all kinds of magic happens, from accelerated weight loss (and specifically fat loss) to the normalising of your blood sugar levels, cholesterol, and blood pressure. Your body gets the chance to rest and repair in a way that it can never do when you are constantly grazing on food. Plus, when you choose to fast, you’ll save time and money.

How fasting works

When you eat, often you’re taking in more food (and energy) than you need at the time so the extra is stored away for use at a later time. The reason for it is this: you eat a meal, the body makes insulin in response to the amount of carbohydrates (which turn to sugar) in your food. The hormone insulin stores energy in the liver as long chains of sugars called glycogen, and then, when that storage facility is full, the liver turns the remaining sugar into fat and stores it in fat deposits around the body. The most basic difference between these two storage spaces is that one is limited but easily available (the energy stored in the liver) and the other is harder to get to, but there is an almost unlimited reserve (the energy stored in fat cells). So, if eating involves the body making insulin and storing excess sugar as body fat, fasting has the process work in reverse. Not eating means insulin drops and the body has to start using the stored energy – simply, your body switches to burning your body fat instead. In a nutshell, you are either in a fed state (eating or not long having eaten) or a fasting state. When your eating and fasting are balanced, you maintain a stable weight. But when you are either eating meals or grazing on snacks all through the day, you are constantly supplying your body with energy and will start to put on weight, because you are not giving your body any opportunity to use up the stored food energy. To get your body back into balance and to lose weight, you need to burn the energy you have in storage by fasting.

The benefits of fasting

The most obvious benefit of fasting is weight loss, but there are many other positive benefits reported by those who fast regularly. These include:

  • More energy

  • Improved concentration and feeling more alert

  • Lower insulin and blood sugar levels (including reversing type 2 diabetes)

  • Lowered cholesterol

  • Reduction in inflammation

  • Better stress management

  • Better digestion

  • Faster recovery after exercise

  • Better immunity

  • Lower risk of cancer

  • Slower ageing

  • Improved brain function

  • Lower risk of dementia

Different types of fasting

Shorter Fasts

Probably the fast most recommended by nutrition practitioners is Time Restricted Eating (TRE) and it involves quite literally restricting the hours in which you eat. 12-hour fast: eat breakfast at 7am and dinner at 7pm balanced by 12 hours fasting. This is how we all used to eat. 3 meals a day with no snacks in between. You would do this on a daily basis. Extending the fast to 14 (with an eating window of 10 hours) or – better still – 16 hours (with an eating window of 8 hours) brings increased benefits. That would mean having breakfast at 9am (14-hour fast) or 11am (16-hour fast), then having your last meal at 7pm. A 16-hour fast is sometimes referred to as the ‘lean gains method’. You would do this on a daily (or almost daily) basis, adjusting the time you eat your breakfast (or even skipping breakfast entirely) depending on the timing of the evening meal the night before. The 5:2 diet, made popular by Dr Michael Mosley, involves eating normally for 5 days during the week and ‘fasting’ for two non-consecutive days. Strictly, these are not actually fast days, because you are allowed to eat, but you restrict your calories to 600 (for men) and 500 (for women) a day. In his latest book, The Fast 800, he opts for 800 calories a day, which is high enough to be manageable, while low enough to bring in some metabolic changes. In The Fast 800 he also suggests readers combine the low-calorie days with time restricted eating.

Longer fasts

20-hour fast: this involves eating one or two small meals from, say 3pm-7pm. This is sometimes called ‘The Warrior Diet’. You would do this one or two days a week. A 24-hour fast extends the benefits further, with the body spending longer burning stored fat. You might eat dinner at 7pm one evening, fast through the night and right through the next day until dinner at 7pm. You would do this one or two days a week. An ‘alternate day’ fast involves fasting for 36 hours. That would mean eating on day 1 at 7pm, fasting through day 2, then eating breakfast at 7am on day 3. This might be a once in a while fast and, given its length, is harder to commit to regularly. Some people choose to go on a medical fast for 5 or even 7 days. This is called ‘periodic fasting’ and is something you might try every few months but under the guidance of a nutrition practitioner. The longest reported fast lasted nearly 400 days, during which time a man from Dundee in Scotland consumed only water, tea, coffee, and soda water, along with vitamins given to him by doctors. He lost 21 stone in weight during this period, and kept virtually all of the weight off afterwards with no reported damage to his health.

Who shouldn't fast?​

Fasting may not work for everyone. If you are diabetic or very stressed, it is important to get your blood sugar levels onto an even keel first and TRE is not recommended. Start with a 12-hour fast on just two non-consecutive days per week to see how you get on. If you have thyroid issues, speak to your nutrition practitioner before trying fasting. Also, some people with digestive symptoms may fare better on smaller, more frequent meals. They should seek professional guidance before attempting a fasting programme. It’s not appropriate for children, pregnant women or breastfeeding women, anyone very underweight (BMI less than 18) or recovering from eating disorders.

Getting started with fasting

The first job is to choose the kind of fast you are going to do and stick to it. If you know you’ve been eating a diet that’s not great, you’re a big snacker or you eat late at night, starting with a 12-hour fast is what you need at first. Do this every day for a week, eating three meals a day and no snacks. As this starts to feel manageable (or if you are already pretty much eating to this kind of pattern), stretch your fast to 14 hours or 16 hours, again eating meals but no snacks in between. Whichever of these two options you choose, the ideal scenario is that you work alongside a nutrition professional who can design a food plan that will feed and nurture your body with the right foods during your ‘eating window’, so you can get the maximum health benefits. They will also be able to tailor a programme so that it tackles any other health goals you have. Longer fasts are trickier to follow and should be carried out only when working with a nutrition professional.

What can you consume while fasting?


You must stay hydrated and drink plenty of water. Drink a minimum of 2 litres. Don’t forget that the food you would normally be eating contains varying degrees of water or other fluids (like milk), and these will need to be replaced. You can flavour the water with lemon, ginger or cucumber, but avoid artificial flavours or sugar free squash.


Any kind of tea is fine: black tea (regular tea), green tea or herbal teas.


Drinking coffee is acceptable when you are fasting. Black coffee is ideal, although fasting expert Jason Fung also allows his clients to take a full-fat cream like double cream in their coffee. While not strictly a fast, he reports that the effect on insulin levels is so small, it is not worth worrying about.


Clear broths made from bones, meat or vegetables and flavourings like garlic, ginger or herbs. These aren’t soups – they should be entirely clear with no lumps.

What to eat when you break your fast

When you have been fasting, your insulin levels are very low and the body has been using the stored fast as energy. To continue to burn fat, the ideal scenario is that you eat a diet low in starchy carbohydrates, so that there is less sugar in the blood for the body to use and store. It will, therefore, naturally continue using the fat stores. A diet low in starchy carbohydrates is called a low-GL (or low glycaemic load diet) and is based on protein (like meat, poultry and, fish), unprocessed foods full of soluble fibre (like oats, beans and lentils), foods high in healthy fats (like oily fish, avocados, nuts and seeds) and plenty of vegetables (limiting starchy veg like potatoes and parsnips). There is an emphasis on choosing smaller amounts of rice, pasta, bread and pastries.

A ketogenic diet – high in fat, moderate in protein and very low in carbohydrates – also works very well alongside intermittent fasting. The diet encourages the body to switch on a more permanent basis to burning fat for fuel rather than sugar thanks to the very low carbohydrate levels. It is sometimes referred to as a ‘fasting mimicking’ diet, because you get the benefit of fasting (low insulin) without actually having to fast. Once used to a very low-carb or ketogenic diet, many people feel much less hungry and don’t even want to eat more than once a day anymore. For them, a 24 - or 36 - hour fast is no big deal at all.

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