Fast 800


So after a few nasty injuries and then recovering from cancer treatment I’ve found myself quite drastically overweight. Not good for climbing and I’m finding it very frustrating not being able to perform anywhere near my peak. I reckon I can lose about 3st and still be a healthy weight. [35yr old, female]

So - I’m a big fan of Michael Mosley’s work and his latest book the Fast 800 basically proposes a crash diet of 800kcals a day for at least a couple of weeks to get your weight loss off to a good start. Yeah this goes against everything we’re led to believe about metabolism and weight loss etc. etc. but it seems to be backed up by a lot of studies so I’m willing to give it a shot.

He proposes a Mediterranean style diet, however the vegan options for this are pretty limited so I thought I would try getting my 800kcals a day from Huel. This probably has a slightly higher carb content than desirable but I’m not pre-diabetic and am pretty active so I’m not sure it’ll be a big deal. I presume the low calories alone will trigger ketosis. I also plan to add in a multivitamin as I won’t be getting the full RDA as not taking in enough calories.

No idea if this is even doable - I tracked my calories this week when I was eating sensibly and was averaging 2100kcals a day. Bit apprehensive that I’ll start gnawing on the desk at work or something, but nothing ventured nothing gained!

Has anyone tried the Fast 800 (or the Blood Sugar Diet which I believe is fairly similar)?


Congrats on recovering from cancer!

Now, 1) this crash diet won’t put you into ketosis 2) you don’t need to be in ketosis 3) I don’t see how having low energy for weeks is considered “a good start to weight loss” when you could very likely eat 1200-1500 calories a day, have more energy because of it to move your ass and end up losing at a great rate without feeling like shit 4) figure out how many calories you burn a day (approximately) and set a sensible calorie intake goal from there 5) crappy books are great to use in arm workouts.


Hi, thanks for your reply.

I should perhaps also mention that the guidelines are to reduce your eating window as well to ensure longer periods of fasting during the day (not that I could make 800kcals last particularly long!) I’ve experimented with both ketogenic diets and intermittent fasting in the past and actually found I had a lot of energy after the initial couple of days. Very low calorie diets are known for inducing ketosis - once there are no more carbs to be burned your insulin levels drop and your body can switch to burning fat.

There are also proposed benefits of fasting for cancer prevention through autophagy which I’m pretty keen on!

Anyway - only one way to know if this works, so I’ll crack on!

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Hi and welcome to the forum :smile: Wow, you really have been through the mill! How long since you recovered from your injuries and finished the cancer treatment?

Hi! Thanks for the welcome!

I had two surgeries and radiotherapy in 2017 and then my last surgery in August 2018 and got the all clear from the physio for my last injury (hamstring tear) in October.

Don’t you think the 800 calorie diet might be too much for your body to cope with after all the assaults has recently taken? Perhaps better to lower your calorie intake by a smaller amount, eat healthily and spread weight loss over a longer period of time, not go jumping into starvation rations right now.


Hi - I am actually starting this today though I am going to be consuming 900 calories of Huel. I am currently 12st 4 lbs and am aiming for 11st. Having tried intermittent fasting before I find it easier to not eat until as late as possible in the day - if I have any breakfast I am just starving all day…

Good luck with this and I will keep you posted on my progess !

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I haven’t read the book, I’d normally be quite sceptical, but having off my own back done something similar to what you are suggesting and found it works, would suggest giving it a go for a SHORT term.

I didn’t go as low as 800kc a day, but I did aim for a 1000kc deficit, as well as using the one meal a day OMAD theory…so I would only eat between 3pm and 7pm, so not technically one meal but a short eating window.

I did drink plenty of fluids throughout the day.

So I did this for a month, and then followed it with 5 weeks of 100% Huel (at regular mealtimes, but with breakfast later than normal). Huel was also part of the original month but not exclusively.

I agree with Juke, aim to start as late as possible in the day.

Warning. you will be VERT hungry for the first few days, but you have to push through that. Alas hunger is what stops many people reaching their goal. It will get better; drink plenty of water.

Intermittent fasting is a more natural way of eating than 3 meals a day; many creatures have that way of living when food is not always readily available.

I lost 2.5 stone, and although I fluctuated a bit over xmas period have managed to keep at my new weight.

I did intend to document my journey after I had completed it, rather than a regular journal, but things got in the way.


Congratulations on kicking cancer’s ass! I think my only concern with this plan would be that you would gain back a big amount of what you lost once you started consuming a higher number of calories. I’m the same as @Juke in that if I eat breakfast I’ll be hungry all day, but I also like the idea of limiting your eating hours (and try to do this myself when I can) because that kind of naturally limits how much you’re consuming unless you eat constantly for that entire window. I also think it’s better to give your body more hours to consume the energy you put in, but I have zero science to back that up. Would you consider starting on a higher calorie total? Maybe 1200 if you’re rather sedentary.

Hi, thanks for your concern but I’m happy with my approach

Hi Juke, thanks for your reply!

I would be very interested to hear how you get on!

I have about 3st I can lose and still be a healthy weight but I don’t think I’ll continue with the 800kcals for the whole lot! Have you read the Fast 800 book which suggests switching to a 5:2 style diet after the initial crash diet period? That’s what I’m planning on but will just have to see how it all goes!

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Hi! thanks for your reply - very encouraging that you’ve managed to maintain your new weight! I’m definitely not expecting this to be easy, but I’ve done a fair amount of fasting in the past so am hoping I can manage!


Yes, i do believe that you need to change your mindset around eating though; having done 5 weeks of 100% Huel my desire for snacking has decreased, and that was my downfall. I do still have occasional snacks.
Over Christmas I was drinking (alcohol) every day, so that more than food may have accounted for many of the unwanted/empty extra calories. I have not had a drink since NYD.

I have also been a vegan for decades.

Rather thought you might say that. Good luck anyway :grinning:

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That’s a different (and the correct) way to look at it though. It seems foolish to just aim for a calorie intake of 800 because a book tells you so, without knowing your personal calorie burn.

On what I consider moderately active days I easily burn 2500 calories (female, 27yo) so an intake of 800 calories would be a ridiculous aim, while a 1000 calorie deficit is actually very doable.


I am losing a steady 1lb ish per week on a daily intake of 1200 calories of nutritious food. I don’t feel hungry or deprived in any way so it suits me fine. If I cut my intake down a further 400 calories I would definitely feel bad and no doubt put the extra lbs lost right back on once I upped my calories again.


Hi Climbergirl, I too am looking to lose about 3 stone, have started the Fast 800 today in order to kick start and was thinking of using Huel for some of my food options. My understanding is that the ‘rules’ of the diet mean it is carb limiting anyway however obviously this is where real food is being eaten. I am aiming to keep my carbs under 50.

As regards Huel, I only ever tend ot have one scoop in a large glass with lots of ice and often coffee mixed in. According to Myfitness pal, one scoop is 14g carbs and 152 cals.

I understand the other pov on this thread re going down to 800 however for some of us this way of gaining control of our weight can work. For example they say to eat nuts on this plan however there is no way I am going near nuts as I can’t just eat 5! Having read the information this approach is now being actively recommended to people who are very overweight to get their weight under control relatively quickly. I am looking for this woe to be a long term approach, I will be switiching to 5:2 when I get some control.


Hi! Thanks for your reply Foxykiwi!

Yeah the recommended Mediterranean diet is pretty limited for vegans and hard to limit carbs - I think I end up with 73g carbs per day using Huel for my 800kcals which should be okay.

Yeah I can understand the resistance towards this method as it goes against everything we’ve been told about weight loss in the past but there’s a decent amount of research out there in it’s favour.

I’d be interested to hear how you get on!


It goes against nothing. It’s a severe caloric deficit, nothing more, nothing less. Weight is lost through caloric deficit, that’s the fundamental physics of energy in/energy out.

Everyone here is pretty much spot on, aiming for 800 a day is unsustainable and as has been said, teaches you nothing of how to maintain the weight once you stop the Fast 800 program.

I’d still advise calculating TDEE and creating a 1000 deficit on this value, but I see you’re happy to follow the approach you’ve chosen which is fine. I’d warn that there will be consequences, but we’ll see.


Yeah it’s going to be difficult, no question about that, but I’m still convinced that it’s something I want to try.

Once you stop the initial phase you move to the 5:2 with 2 800kcal days per week until you’re ready to enter maintenance where that drops to 1 800kcal day per week.

Interestingly, the NHS are starting to prescribe 800kcal a day diets for patients to reverse type 2 diabetes, intending the patients to follow the diet for about 3 months.

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