How many calories on a Fast Day?

5:2 has changed my life – so am I about to change how I do 5:2 after this article?

Michael Mosley, 5:2 pin-up and TV science presenter, has just posted his thoughts on 5 years of intermittent fasting in the Mail on Sunday – and many people are asking if they should change what they do. Here’s my verdict… (PS: the quick answer is, probably not, if it’s already working well for you!)

5:2 and intermittent fasting is incredibly flexible. As the author of the first published book on the approach, I always suggest you start with the basics of 2 Fast Days a week, at around 500-600 calories. Once you’re used to that, and losing weight, you can experiment with other patterns – 3 fasts a week, for example.

The central benefits of the Fast Day are:

  • reducing calories to encourage weight loss
  • encouraging awareness of healthy choices and of understanding appetite/listening to your body
  • the health benefits of giving the body a rest from digestion and other functions (like dealing with high blood sugar) by leaving gaps between eating. This encourages the body to ‘clean up’ cells and processes

Like Dr Mosley, I lost weight (31lbs) and cut my very high risk of diabetes doing this. The original BBC Horizon programme and his interviews with some amazing scientists, inspired me to make changes and do research that changed my life – after decades of weight loss hell. So what are we to make of his new article?

What Michael Mosley is saying in his new article talking about his updated thoughts on the Fast Diet 每 which he wrote with journalist Mimi Spencer after the Horizon show每 isn’t radically different.

Dr Mosley says: &Going up to 800 calories isn*t going to make that much of a difference, particularly if you go low carb on the other 5 days.*

As you*ll see from my calculations later in this article, this does depend on your own weight/goals and there are pros and cons to this: it may be less daunting if you tried fasting before and got too hungry on Fast Days. The cons might include the calorie reduction slightly reducing the pace of weight loss (see below for an example), and perhaps the less strict limit making us less careful/aware of what we eat.

Kate says: I*ve always suggested that if you find 500/600 hard 每 and especially if you*re larger or do a lot of exercise 每 you might start with 750 and reduce each fast by around 50 calories until you find your comfort zone.

The low-carb/Mediterranean advice is great, of course每 the principles of Med diet are generally great and this is the advice given in Michelle Harvie’s brilliant ‘The 2-day Diet Book’ which was written after pioneering dietary research aimed at cutting the risk of breast cancer in women. Cutting out processed carbs and sugary foods is also a great idea 每 cakes, sweets, white bread/rice/pasta also makes sense. But I think to ban foods you love long-term and completely actually LEADS us to crave them 每 so following the 80/20 rule (80% good eating, 20% more relaxed) is also helpful. Before 5:2, I low-carbed for a year or so and found it impossible to keep up forever.

Kate says: The freedom to eat foods we love 每 in moderation – is one of the flexible aspects of 5:2 that makes it possible to live for life!

What about the fasting &period* 每 without food?

Cutting out snacking and aiming for long fasts 每 especially between the last meal on the day before a fast and the first meal on the next day 每 has always been part of our advice.  

We*ve been talking about delaying/skipping breakfast and cutting out snacking since the start 每 and the advice to try to lengthen the overnight fast is a good one. 16:8 almost certainly does have cellular health benefits 每 while it also aids self-control as we feel we*re controlling our appetite, rather than the other way around.

Kate says: For me, the important thing to realise is that 5:2 is pick and mix 每 if you*re in good health and have no history of eating disorders – you can play around with it. But it*s always best to start with the basics first.

500 cal fast versus 800 cal Fast Day examples: the figures

We do all burn calories at slightly different rates, depending on our size, gender, age, and even the microbes in our stomach. But we do need to use more calories than we consume to lose weight. 5:2 does this by limiting calories intermittently rather than every day. But look at these two examples.

The first is a man, 30 years old, 6 foot tall, weighing 15 stone, who plays sport 3 times a week.

His TDEE 每 the amount he*d need to eat to stay the same weight 每 is just under 3200. So if he eats normally 5 times a week and then eats MM*s new idea of 800 twice a week gives a deficit of 4800 calories. We estimate it takes a deficit of 3,000-3,500 calories to burn off 1lb in weight.  So he*ll lose at around 1.5 lbs a week

Now take a woman, 59, 5 foot tall, weighing 10 stone who does no exercise and is quite inactive. Her TDEE is just over 1402 At the 500-calorie Fast Day limit, she*ll have an 1800 deficit, so will lose around ?-1lb a week on average. With 800 calories, that deficit reduces by 600 calories which may well slow weight loss and be discouraging. Actually, many of us lose more: we naturally eat a little less on non-Fast Days, and also the lack of snacking/increased times not eating may help us burn more fat.

You can calculate your own TDEE on our calculator page.

The fact is, these are all averages. So whether you go to 500, 800, or zero calories on a Fast Day, it*s your choice. But do remember 500/600 has been working well for many of us for almost five years!

My conclusions:

Kate says: Whichever way you do it, remember you can be flexible 每 but you have responsibility for your own health 每 including making the decisions that are right for you (and following the advice about avoiding the diet if you have certain medical conditions).

And we will be here to share your success! Come and join the debate over in our free Facebook group.



Male: 7 days at 3200: 22,400

5 days at 3200, 2 at 800 = 17,600, deficit of 4800 calories

Female: 7 days at  1402: 9184

With 2 Fast Days per week at 500 calories: 8010, a deficit of 1802

With 2 Fast Days per week at 800 calories, 8610, a deficit of 1204

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