Hello, Waggy123 – yes, the 500/600cal figures are based on Ms/Mr Average, as is the 2000cal women’s recommended daily intake figure, and, yes, many other factors (such as height, age, activity levels, etc, etc) need to be taken into account for each individual.
The loss of ‘1 lb per fast day’ is described in ‘The Fast Diet’ by Mimi S. as a ‘basic guide’ to what anyone might anticipate when first starting out, but she says this would include things like ‘water and the digested food in your system’. She then says ‘You should, however, lose around ten pounds of fat over a ten week period’ – so, one pound of FAT per week.
It’s a minefield of conflicting information out there, if you do what I just did, and search for ‘calorie deficit to lose 1 pound’ and also look at the various calorie calculators.
So, regretably, I don’t know what ‘the answer’ is to your query on the 400 or 500cals issue – or even whether you can trust the basis on which your1600cal figure is calculated – other than try what sounds right to you and see what happens.
A major foundation of the 5:2 system is that it should be ‘do-able’ on a long-term basis, so that you can reap health benefits and, hopefully, a steady, gradual fat loss, without losing muscle mass.
For many people, being allowed 500cals on fast days and then ‘normal’ eating – better to read that as ‘reasonable’ eating, to my mind – on non-fast days, means it IS much more sustainable than ‘everyday’ calorie restriction diets. Maybe, if the fast day allowance was quoted as even less than 500cals, some people would not stick to it – or might not even try the 5:2 system in the first place?
Other people’s stories will not necessarily be your story – experiment with what works for you and I hope you will be delighted with some good results. Best wishes to you.

Jeanius, you seem to be the resident expert! I too have not had success with this diet…YET. I have about 10 lbs to lose. I’d estimate on non-fast days I eat approx. 1500-1800 calories. And stick to about 550 on fast days. I just feel bloated. Maybe I should increase my water intake, to flush away the sodium? I assume I just need to be more vigilant in tracking calories, and maybe then I will see the difference. I’ve been doing this for 3 weeks. Your insights are quite helpful!

Sad to say, I am finding the set-up and layout of this website increasingly frustrating and awkward to use and I would ask the managers of this site to review and redesign the whole thing.
To view an example of an attractive, user-friendly and technically far more comprehensive website, check out that of Kate Harrison, author of ‘The 5-2 Diet Book’. (I suspect that if I quote the full website address here this post will be vetoed so, just search the www by her name and/or the book title.)
Kate H’s forum is open and anyone can read all the lively contributions without having to register. (To contribute or seek members’ advice, registering is easy and, usually, quickly confirmed.)

A brief summary of some of main problems I experience here, on this site:
– The range of given threads is narrow and so the list of comments in most threads quickly gets longer and longer – the scrolling down seems endless.
– There is no indication given of the whereabouts of any new postings since the last time I signed-in. (I, for one, waste a lot of time searching all the threads, only to find there are no new comments, because I can’t remember what the number totals were from my previous visit.)
– There is no means to check back on each individual’s full collection of comments – to get an overall picture of their experiences and views – before replying to their most recent posting.
– Valuable forum subjects are hidden away in obliquely titled links like ‘in depth’ whilst two-thirds of the home-page space is given over to stale and unchanging items under ‘twitter’ and ‘in the press’.
– The input/response from either Michael M or Mimi S seems minimal and disappointing nowadays. Many members clearly are expecting MM and MS to be participating in the forum.

(As there is no obvious place for this subject, I will put these comments in more than one topic heading, to ensure they can be read.)

Hello, minki – Once you reach your preferred weight, try a 6:1 maintenance programme, i.e. just one fasting day per week rather than two. It’s great that you are doing so well using this system. All good health and well-being to you.

Thank you Jeanius, no problem with the name. Thanks for that advice, I thought I saw it somewhere in the book, but couldn’t find it again. I also Just have to say that this way of eating works so well for me, I am usually quite a healthy eater but I have always eaten more than I need! Doing this, my body has responded by giving me very clear signals that I’ve had enough to eat, so now I leave things on the plate…unheard of for me. I’ve told so many people about this and now have two friends also doing it with great ease and success. Thank you so much.

minki and Enaj, I blush but thank you for your kind words. The variety of available sites about intermittent fasting in general and the 5:2 approach in particular can be a bit overwhelming. So far, I have been either monitoring or contributing to just three and, speaking personally, I compare their technical layout and ease of use to villages, towns and cities:
Sad to say, this current site is still a bit of a village, with few facilities and rather confusingly spread out. Hopefully, a team of planners will get in here and come up with a great, new design. (Ahh, JUST seen Michael M’s recent posts on this subject in another thread – so that’s all good news!)
I find ‘the 52diet.co.uk’ brilliant for breadth of information and for its efforts to collect and collate statistics about members’ results but I often get confused and lost there – it is definitely a big city, with almost too much available.
I’m a townie and, for me, the clearest and most easy to navigate is Kate Harrison’s site based on her ‘The 5-2 Diet Book’. The forum design is neat, colourful and uncluttered, with many excellent features (e.g. being able to edit your contributions if you realise you’ve mis-typed/mis-spelled something or thought of a better way to express yourself, to name just one.)
I use all three and get different things from each. What I hadn’t realised originally is: one can dip in to all the postings on each of the different forums without having to register. (You need to register only if you actually want to make a comment.) If you have enough time, more than one forum can be helpful. Go explore, if you feel like it. (No actual web addresses given, as they tend to get vetoed.)

I did it and within budget by 7 cals! Phew!
The interesting thing is that I don’t feel compelled to race off and eat lots today, as I imagined, which is good. I had 55g couscous and a small piece of poached haddock 57g for evening meal and that sustained me through the night.

I bought lots of sweets for Easter but I will eat careful.

I will continue 2 fasting days maybe more with or without support. Some of my family say I should stop now BMI 24.5, 76kg

My doctor say to continue what I do to improve health. I haven’t told him I’m fasting.