Intermittent fasting – what you need to know

Health and Lifestyle Editor at Bupa UK
04 October 2017

If you’re looking for information on nutrition, a quick search of the world wide web could leave you feeling more confused than informed. While instant access to a wealth of information can be empowering, it can also be overwhelming and sometimes misleading. So it’s more important than ever to make sure you’re armed with the correct facts before embarking upon a new diet. Here I’ll explain what you need to know about intermittent fasting and what you should take into consideration if you’re thinking of trying it.

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What is intermittent fasting?

Fasting means to limit your food and drink intake for a period of time, and has traditionally been practised around the world for religious reasons. But more recently, intermittent fasting has been used as a method of losing weight. Traditional weight loss diets might involve cutting out certain foods altogether or reducing how many calories you eat by a small amount each day. But with intermittent fasting, the focus is more on when you eat, as opposed to what you eat. Intermittent fasting requires you to switch between times when you eat and times when you fast. There’s still an element of calorie restriction involved, but instead you choose set times or days of the week to do this. So it’s often described as a pattern of eating, rather than a diet.

How does intermittent fasting work?

To understand how intermittent fasting can help with weight loss, you first need to understand how your body loses weight. In order to lose weight, your body has to experience a shortage of energy (calories), also known as a caloric deficit. You do this either by eating less, exercising more or a combination of the two. Intermittent fasting works by allocating times when you eat, and times when you fast. During these fasting periods, you either eat fewer calories or none at all. Either way – by the end of the week, your overall energy intake should be less than your body needs, which should lead to weight loss.

Types of intermittent fasting

There are a number of different methods of intermittent fasting and within each method there are various ways of doing them. Ultimately, they all involve dividing up your days or weeks into periods of eating and fasting. Three of the most common methods are outlined below.

Whole Day Fasting

Whole Day Fasting (WDF) is the method of intermittent fasting you might be most familiar with as it’s the approach behind the celebrity-endorsed 5:2 Diet. The 5:2 Diet involves eating a normal diet on five days of the week and fasting on two days of the week. During fasting days, you significantly reduce how many calories you consume, normally to around a quarter of your required daily amount. That’s roughly 500 calories for women and 600 calories for men per day. Some approaches to WDF will ask you to eat nothing at all during fasting days.

The days of the week you choose to fast is up to you and you can change your two selected fasting days to suit you week on week. This style of intermittent fasting can also be adapted to a 6:1 or 4:3 pattern. In other words, you fast for just one day a week, or on as many as three days a week.

Alternate Day Fasting

Similar to the WDF approach outlined above, Alternate Day Fasting (ADF) involves eating normally on some days of the week, while fasting on others. The difference here is that rather than fasting on just one or two days per week, you fast every other day. For example Monday would be a normal eating day, Tuesday a fasting day of 500 to 600 calories, Wednesday a normal eating day and so on. Similarly to WDF, there are a variety of approaches to this. During fasting days you may be asked to refrain from eating completely. Or you may choose to reduce your calories significantly and eat only a small amount on normal eating days. How you fast will depend on the method you choose.

Time Restricted Fasting

The Time Restricted Fasting (TRF) method of intermittent fasting involves setting an eating ‘window’ – a set time frame each day where you allow yourself to eat. For example, you could choose to fast for 16 hours a day, and eat normally during the other eight hours. Your eating window might start at 1pm each day and finish at 9pm. So perhaps you eat your evening meal and then wait until lunchtime the next day to begin your eating window again. You may choose to fast for 20 hours each day, and allow a four hour eating period. The theory behind this method of intermittent fasting is that you won’t be able to consume as many calories during this allocated eating period as you would if you ate throughout the entire day.

Things to consider about intermittent fasting

Before you make any changes to your diet, it’s important to choose a plan that works for you. One that fits around your lifestyle, is realistic, sustainable and helps you to lose weight safely and healthily. Here are a few things you may wish to consider about intermittent fasting.

Fasting versus continuous calorie restriction

There’s no evidence that intermittent fasting is better for losing weight than eating slightly fewer calories every day. In order to lose weight, the main thing that counts is that your body experiences a shortage of energy (calories) by the end of the week. Both approaches should help you to achieve this. But for some people, reducing calorie intake significantly on just one or two days a week, rather than by a little every day, might seem more manageable.

A balanced diet

When it comes to intermittent fasting, there’s no focus on completely cutting out particular food groups. There’s no need to increase certain food groups either, which may appeal to you. However, if you’re eating less, you’ll need to make sure the meals you do eat have all the nutrients you need to stay healthy and avoid missing out on essential vitamins and minerals.


On the days when you’re not fasting, it’s important not to overcompensate and overeat. If you overeat during your allocated eating periods, chances are you’ll tip the balance of calories in versus calories out back in the other direction. So normal eating days or times should be nutritionally balanced to ensure there’s still a caloric deficit in your body by the end of the week.

Side effects

Reducing how much you eat by a large amount can sometimes lead to feelings of hunger, irritability, poor concentration, tiredness, headaches or dizziness. So it might not be easy to stick to intermittent fasting in the long term.

Many studies report that being able to stick to a weight loss plan in the long run has a greater effect on overall weight loss. But alot of the research on intermittent fasting is based on short-term studies. So it’s unclear if intermittent fasting can help you to lose weight and keep it off long term. It’s also hard to determine which method is most effective and if there’s an optimum number of calories to eat when you’re fasting. So more long-term studies, including those looking at the effect on overall health and not just weight loss, are needed.

Is intermittent fasting suitable for you?

Fasting isn’t appropriate for everyone. For example if you’re pregnant, have a history of eating disorders or have certain medical conditions such as diabetes. Both the National Institute for Health and Care Excellence and the British Dietetic Association recommend that very low calorie diets such as these should only be done under the supervision of a medical professional. Speaking to your GP or a qualified Dietitian is a good place to start. They can help you decide on the right plan for you and your health.

Here’s what Bupa Dietitian Rachael Eden had to say about intermittent fasting:

“Intermittent fasting definitely has its place for some, particularly those who find applying moderation to their diet, such as to portion size and frequency, more difficult. Many find that it’s less restrictive than trying to eat less calories on all days of the week. What’s really important to bear in mind however, is that non-fasting days are still meant to be healthy eating days. Planning ahead is important for success on fasting days. Try looking up some low calorie recipes to keep fasting days interesting and keep you feeling as full as possible.

"Although many methods of intermittent fasting may now exist, it’s worth noting that the 5:2 Diet was in fact adapted from an evidence-based diet known as the 2-Day Diet. The 2-Day Diet involves reducing your carbohydrate intake on two days of the week, followed by nutritionally balanced meals on the other five days of the week. So for anyone thinking about trying intermittent fasting, I’d recommend looking at the information around the original diet first.”

Do you know how healthy you truly are? Bupa health assessments give you a clear overview of your health. You’ll receive a personalised lifestyle action plan with health goals to reach for a healthier, happier you.

Michelle Harrison
Health and Lifestyle Editor at Bupa UK

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