800 calorie a day diet? Possible? Desirable

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Level 1 Valued Member
Hello friends, long-time (on and off!) exerciser according to Pavel's methods, from the original RKC book onwards (still my favourite strength book).

I am a male, 40, very overweight. In the UK an 800Cal a day diet by Michael Mosley called the Fast 800 or the Newcastle Diet (the latter using meal replacement) is across the press for reversing type II diabetes and other conditions. It espouses a low carb lifestyle on this diet for 3 months.

Added into the mix is that I don't consume any animal products at all so I would be doing this plan with vegetables and pulses.

I am hoping to do a jog/walk for 5 days a week and strength training on 2 because I want to try to minimise muscle loss (I think I will be on about 60 grams of protein a day - I know that muscle loss is almost inevitable with this cal restriction, but I saw a study on PubMed where a group of female trainees kept the vast majority of muscle with strength training).

I thought I'd ask here as the forum for these diets are all, "Yes, you can do it, you will feel wonderful," etc.

I don't have a physical job so do you think it will be possible to do this for 3 months with the training plan or should I not even attempt it?

Interestingly, Mosley himself prescribes HIIT - 3 x 20 all-out bursts, 3x per week on a bike, sprints, etc. I imagine that must be hellish.


Level 8 Valued Member

800 cal a day seems very drastic to me. In my humble opnion, this is not really sustainable on the long term.

I would calculate my daily needs, then, I would use "any kind" of diet, even if I am not a "fad" or whatever. Comparing to the daily needs, I'd cut off let say about 250 / 300 cals and would perform easy LSD for instance. Basically, I'd prefer being simply active (easy ride, hike, run, swim, stairs, etc...) then training until almost exhaustion 2 to 3 times a week.

Kind regards,



Level 7 Valued Member
@Kana, congrats on taking your health by the horns.

I think your plan is doable, but subject to a few things. Much of what you plan to do is in conflict with each other (calorie restriction - muscle loss, calorie restriction - recovery from exercice, etc). So I would say you might need to see how it goes and adjust on the way.

I would keep 1 thing rigid, that you dont change at all. That would be strength training. All other things can adapt so that this one doesnt change. So I would do like this:

- Strength training - Priority one. This one doesnt change and all others adapt.
- Diet - Priority 2 - This can change slightly only to allow for strength training. Maybe eat a bit more 1 week out of 3.
- Cardio - Use as adjust variable for the above 2. If you have to cut because you dont have energy, reduce this.

Thats how I would do it.

What strength training will you be doing?

All the best


Level 1 Valued Member
Thanks for your reply, Oscar!

Most of the strength training I'll be doing will be press-ups and lateral rows. I stupidly started to hurt my back with double C & P with 2 x 24s and bad form a few months ago (my fault!) so I am giving iron a rest for a while. I know that these seem like low-intensity moves but when you are fat they are fairly challenging!

Might throw in some pistols and/or squat jumps as finishers.

As I hopefully improve I'll use feet-up push-ups and start to do negatives pull-ups to hopefully work up to being able to do proper pull-ups.

I am just going to use ladders, to an easy stop, with both moves for around 1 hour aiming for 'fresh strength'.


Level 6 Valued Member
I lost a lot of weight by significant calorie restriction but I doubt I was as low as 800 every day. Looking back I probably averaged 1000. I used protein shakes and plain uncooked vegetables during the day with the goal of staving off hunger, then a relatively normal lean protein/vegetable meal at night. I did not drink alcohol (in those days). It certainly worked and was sustainable with the occasional blowout (maybe once weekly). The weight drops off quickly at that level of calorie restriction. From memory I lost muscle too although I diligently continued strength training. While dieting my strength dropped off significantly.

I first read about the HIIT style of exercise recommended by Mosley in Art Devany's essay on evolutionary fitness. This was two decades ago. It is a very efficient but painful way of getting fit if you do it properly. The proper way to do it is to go FLAT OUT. That is, as fast as you can! Then recover until you feel you can do it again. Then go FLAT OUT again. Repeat three times or whatever until you feel completely stuffed. This won't take long. Sometimes you read the instructions as something like "go as fast as you can for thirty seconds then rest thirty seconds ..." That's complete nonsense. If you go as fast as you can you won't last thirty seconds nor will you probably be in a fit state to continue thirty seconds later. In my experience it's more like go as fast you can for 10 seconds or so then rest for one minute (while your heart flops like a toad in your chest). Done properly HIIT like this is one of those exercises where you probably should speak to your doctor first. It's brutal ... but effective.

Good luck! Please post your progress.


Level 7 Valued Member
My guess, never been on a diet myself, is that you'll feel pretty bad and out of energy on such low calorie intake.
I would not go that low, rather, as mentioned, a deficit of maybe 500cals per day.
Also, to prevent muscle loss, aim for minimum 2g protein per kg body mass per day. 60g protein per day is way too low combined with only 800cals.

william bad butt

Level 6 Valued Member
For me, and I think it's TRUE of most people, I find that slow/sustainable/consistant lifestyle improvements in nutrition yield better body composition results vs fad diets.

Be careful of drastic changes. I only make changes that I would be willing to do, indefinitely. There can be consequences of yo-yoing, especially when done without serious strength training to maintain lean body mass.

I really like @Oscar advice above.

My opinion. Good luck!



Oso Rojo

Level 5 Valued Member
If you have the budget you should consider taking an RMR (resting metabolism rate) test. It will tell you how many calories your body is burning just existing. So from there you can get a sense of how much to cut back your calories. For example my RMR 6 months a go was 2,700. That means I need 2700 calories a day to stay the same. I'm pushing hard at strength training so I need about 3,500 a day. I tried to cut back to under 1,500 and it worked for about a week or two. Then it had serious impact on my strength training.

I personally look to build more strength/muscle, which then burns more calories and drives down body fat. I use the RMR and a Dexa Scan to measure that.


Level 5 Valued Member
Dan John has been talking about a 800 calorie diet for 5 days that’s called the fast mimicking diet. I don’t think that’s sustainable long term.

He talks about it on a few podcast with Pat Flynn.

He might have some articles about it.


Level 1 Valued Member
Thanks for all your thoughts folks. I have also come across this article. A group on 500cals a day lost the same as those on 1250 cals a day but the latter group lost less muscle.

As being as the alternative to the 800cal diet that I am considering is a 1500 limit it's food for thought. It suggests that on very low calorie diets, the preservation mechanisms slow the metabolism down so there is no point in ultra-low calories. Fast Weight Loss May Mean Muscle Loss


Level 3 Valued Member
I've done a 1000kcal/day crash-diet when I was 24 years old and obese. I was miserable for 10 months and constantly hungry and sad. I lost 40kg in those 10 months. Then I moved, got a girlfriend and gained it all back in a few years and even put on more weight.
Now I'm 38 years old and started working out last October. I haven't dropped a kilo but I'm twice as strong, my waist is down 12cm, some of my hoodies don't fit around the shoulders anymore, my blood values are "perfect" according to the doctor on my last check-up, I sleep better and have energy to work out.
I'm doing this by eating healthy. Mostly whole food plant based. Meaning... whole grains, oat meal, piles of vegetables, beans, lentils, potatoes, brown rice, fruit and protein powder. And walking (almost every day) and training with kettlebells and stones 4 times a week.

It's going to take longer do drop the weight this way, no doubt, and the scale doesn't move. But I feel great, healthy, don't go hungry and save a lot of money not eating processed food. And I'm going to keep doing this for the rest of my life, my goal is to be healthy, not loose weight. No diet, life style changes.

Those 10 months on a crash diet (with cheat days once a week) was the worst in my life.

Good luck!(y)


Level 6 Valued Member
If you have the budget you should consider taking an RMR (resting metabolism rate) test.

That sounds interesting. However, a more practical method is a...

Three Day Recall

1) Count all the calories you consume in three days.

2) Divide by three.

3) One of the days you count needs to be a weekend day. That because most individual diet changes on the weekend.

This provide you with you daily caloric average.


Weigh yourself every few days.

1) Over Consumption: If you are gaining weight with on what you are eating, you are consuming too many calories.

2) Maintenance: If you weigh is constant, you food intake and energy expenditure are balanced.

3) Under Consumption: If you are losing weight, you are in a calorie deficit.

Losing or Gaining Weight

Research by Drs Layne Norton and John Ivy (independent of each) other found that decreasing your caloric intake approximately 20% was more effective at preserving muscle mass and maximizing burning fat.

The reverse applies to gaining weight. Increasing you calorie intake by 20% minimized fat gain, ensuring more muscle gain; that provide you are following a well written training program and are following it.

Yo-Yo Dieting

This term is usually associated with individual who go on a diet and lose weight . Then for some reason go off the diet and gain back the weight they lost plus gaining even more weight.

Research shows that a Yo-Yo Diet also works for continuing to lose weigh, rather than plateauing.

Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. - PubMed - NCBI

This research found that your body will adapt to a decrease in calorie after being an the diet for two weeks. That means your metabolism decreases to accommodate your new daily calorie intake.

When adaptation occurs, weight loss stops.

The Solution

The research demonstrated that an effective method to maintain weight loss is to Yo-Yo your calorie intake every two weeks, as follows...

1) Decrease you calorie intake 20% for two week below maintenance.

2) After two weeks, increase you calorie intake back up to your previous maintenance level.

3) Alternate a deficit calorie diet intake and a maintenance calorie intake every two weeks to ensure your weight loss progress continues.


Bodybuilder have used this method of calorie rotation for decades to increase or lose weight; bulking and cutting cycles.

Kenny Croxdale
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Level 6 Valued Member
Dan John has been talking about a 800 calorie diet for 5 days that’s called the fast mimicking diet.

Fasting Mimicking Program and Longevity - Valter Longo Foundation

This is Dr Valter Longo's diet.

It is based on Dr Valter Longo's research on fasting as a means of improving heath and life span. Longo's research determined that there are substantial health benefits from fasting for three days.

However, few will ever do that. Thus, Longo came up with The Fast Mimicking Diet.

On a side note, my longest fast was 47 hours. Al Caimpa posted that he fasted for 72 hours and Snowman has done about the same

However, I have yet to understand how consuming 800 calories for 5 days replicates fasting.

An 800 calorie diet for 5 days falls into a calorie deficit diet; burning more calories than you are consuming.

The diet that closely replicates Fasting is the Ketogenic Diet.

Intermittent Fasting and the Ketogenic Diet share many of the same benefits.

Kenny Croxdale
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william bad butt

Level 6 Valued Member
In addition to tracking bodyweight, I like to track my body composition over time (%body fat for example). I do this via waist circumference at the navel. Therefore, even if your weight is not changing but your waist is decreasing, you know you are dropping body fat.


Level 5 Valued Member
Something similar is what Dr Mercola writes about in his book Keto Fast. He has moved away from long water fasts and gone to a 18/6 time restricted eating window followed by a calorie and carb restricted meal then right back to a 18/6 fast (this might not be a correct summary). On the 3rd day a full re-feeding. I found the book interesting. I'm a routine kind of guy, so mercola's keto fast is not for me.

I am a big fan of time restricted eating as promoted by Dr Ronda Patrick on her podcast Found My Fitness. I do throw in longer water fasts as they naturally fit in 24-72. Seems like 24 just happen more frequently now on their own.


Level 7 Valued Member
On a side note, my longest fast was 47 hours. Al Caimpa posted that he fasted for 72 hours and Snowman has done about the same
I have just completed my first 72 h fast. It was very doable. I will start doing it a couple of times a year. Possibly before the winter to improve the immune system.


Level 1 Valued Member
Thank you all for your input, especially Kenny with the vauilt of information - thanks for taking the time to post that, I am indebted.

Steve Freides

Staff member
Senior Certified Instructor
Elite Certified Instructor
Something similar is what Dr Mercola writes about in his book Keto Fast. He has moved away from long water fasts and gone to a 18/6 time restricted eating window followed by a calorie and carb restricted meal then right back to a 18/6 fast (this might not be a correct summary). On the 3rd day a full re-feeding. I found the book interesting. I'm a routine kind of guy, so mercola's keto fast is not for me.
The refeed every three days is similar to the book Natural Hormonal Enhancement. Look up Targeted and Cyclic Keto diets.



Level 6 Valued Member
Cyclic Keto diets

Cyclic Ketogenic Diet

Bodyopus/Dan Duchane and The Anabolic Diet/Maurio DiPasquale put the Cyclical Ketogenic Diet on the map in around 1995.

Water weight loss occurs initially when carbohydrates are dramatically cut from the diets. I and a friend did the diet for a while.

My water weight loss was around 6 - 7 pound when I dramatically cut the carbohydrates out in the 5 day period. My friend was larger. His water weight loss was around 10-11 lbs.

In two days of Carb Loading, I'd gain up to 7 pound, he gain up to 11 lbs.

The training on the 5 days of low carbs was difficult due to the water weight loss and the body being "Glucose Dependent" for energy and not having enough glucose/carbohydrates to drive it.

The workout following the two day Carb Load made what every you put on the bar easy to move. Training on the Cyclic Ketogenic Diet was a roller coaster.

With that in mind, let's look at...

The Ying-Yang Effect of Hydration

The composition of muscle is 70% water. For muscle to effectively function, you need to be fairly well hydrated.

As we know, a decrease in water (dehydration) decrease performance.

Ironically, the muscles can produce more force when they are Superhydrated; which occurs with the Carb Loading process.

That is one of the reason that creatine (Cell Voluminizing) and some anabolics work, they Superhydrate muscle.

Car Tire Analogy

Think of your muscles like car tires.

When they are under inflated, your gas mileage per gallon drops. In the case of muscle dehydration (under inflated muscle), your performance drops.

When your car tire are over inflated, your gas mileage per gallon increases. In the case superhydration (over inflated muscle), your performance increases.

For someone interested in more in depth information on the Cyclic Ketogenic Diet, below is the link for the book...

The Anabolic Diet

As you know and for those who don't, the diet is about 5 days of high fat/moderate protein/low carbohydrates. It is followed by 2 day of Carb Loading.

Research by Dr Jake Wilson, determined with a Cyclic Ketogenic Diet, Carb Loading took you out of ketosis for about about four days.

Thus, on a Cyclical Ketogenic Diet, you are only in ketosis about one days.

Wilson's research also found that the Cyclic Ketogenic Diet tended to increase body fat.

Wilson's determined that individuals on a Ketogenic Diet need to slowly add carbohydrate back into their diet rather than jumping right back to a high carbohydrate diet. Doing so minimized fat gain.

Wilson found the most effective method in coming off the Ketogenic Diet was to add 100 gram of carbohydrates to you daily intake and keep it there for three days.

On day 4, increase you intake to 200 gram of carbohydrates. Then on day 7, bump it up to 300 gram of carbohydrate per day.

Kenny Croxdale
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