I’ve had a lot of requests for a Fast 800 diet review. I had seen all of the media headlines and the actual research when the first 800 calorie diet study – DiRECT – came out. I knew that a guy named Michael Mosley had parlayed that research into a diet that supposedly ‘reverses diabetes,’ but I had no idea how popular that diet was actually going to get.

And how much press it was going to receive.

800 calories a day. As a dietitian, it goes against everything I’ve learned and experienced about weight loss and nutrition. But what is the Fast 800 diet all about, and is it really that bad?

Let’s take a look.

What is the Fast 800 Diet?

The Fast 800 is promoted by Dr. Michael Mosley, who is not a licensed physician – he’s a journalist and TV presenter. He did qualify as a psychiatrist, but hasn’t been working as an MD since 1985.  

Mosley has been credited for inventing 5:2 fasting, having followed this way of eating and apparently healing his own health issues.

I find a lot of diet programs led by doctors or entrepreneurs like Dave Asprey have some sort of ‘rags to riches’ health story attached to them. This is all well and good, but someone’s story doesn’t mean sh*t when applied to your own health.

In other words, just because Michael Mosley used this diet to ‘reverse’ his diabetes doesn’t mean that this way of eating is easy or smart. Personal anecdotes are often used to sell a diet product with emotion, but I want you all to be careful about falling for this.

I put quotes around the word ‘reverse,’ since diabetes goes into remission..it doesn’t ‘reverse.’ The word ‘reverse’ implies a ‘cure,’ which is not possible. It’s a red flag when anyone uses this term to describe a chronic disease remission. 

Followers of Fast 800 can choose from three different ‘approaches’: 

The Very Fast 800, which is an 800 calorie diet done over 2-12 weeks. It’s for ‘people looking for rapid weight loss’, or to ‘reverse their diabetes.’ Followers of the Very Fast 800 are encouraged to incorporate fasting into the program as well, although I’m not sure what part of 800 calories a day is NOT fasting.

For the general population, three months of 800 calories a day is not safe or recommended by reputable healthcare providers. 800 calories a day falls into the category of VLCD, or Very Low Calorie Diet. If someone needs to undertake this sort of eating plan, they should be followed closely by a medical team that includes a doctor and dietitian. 

Fast 800 does give an exhaustive list of who should not do the program. This includes people under 18, pregnant or nursing women, anyone with an eating disorder history, type 1 diabetes or anyone on insulin, and people who have had a recent heart attack. 

The New 5:2, which is an intermittent fasting program that has 800 calorie days twice a week. It’s for ‘gradual weight loss’ and to ‘improve overall health.’ 

Way of Life, which is a lower-carb ‘maintenance’ plan. 

All of the plans are moderate to low-carb, and are styled after the Mediterranean diet. Which, FYI, is typically not low in carbs.

fast 800 5:2

Fast 800 research.

There have been several recent studies that put subjects on an 800 calorie diet to examine the impact on their diabetes, risk for diabetes, and cardiometabolic risk factors.

In 2018, a research study called the DiRECT trial put 68 out of 149 subjects with type 2 diabetes into remission using diet alone. Weight loss in 36 of the participants was at least 15kg. The trial was funded by Diabetes UK, lending it some credibility in scientific and nutrition circles.

Normally, these results would be great news. There are a lot of type 2 diabetics in the world, and achieving remission from this chronic disease would no doubt improve their quality of life and their overall health, not to mention their disease risk. 

Predictably, the DiRECT trial results were all over the media. But what the media didn’t emphasize was that the participants in the study were subjected to an 800 calorie liquid diet for the duration of 3-5 months. As in, you can put your diabetes into remission, but you’ll have to starve yourself first. 

No thanks.

Following DiRECT, there was DROPLET, a similar study done on ‘overweight people’ with BMI >30, that used an 800 calorie liquid diet for 8 weeks, followed with food re-introduction. The subjects on the 800 calorie plan lost more weight and saw more improvements than the control group in biomarkers of metabolic and cardiometabolic risk (HbA1C, cholesterol, blood pressure, etc.) But at the 3-year follow-up, intervention group weight gain was more than the control group (but some of the weight was still kept off), and differences in cardiometabolic risk factors were not significant between the two groups (except for blood pressure).

In 2020, the results from another study, the PREVIEW intervention study, were published in the journal Diabetes, Obesity, and Metabolism. 

PREVIEW had 2326 pre diabetic adults randomized to either a high protein medium glycemic load, or moderate protein higher glycemic load diet. 

Interestingly, while participants lost significant weight, at the end of 3 years, the incidence of type 2 diabetes was lower than researchers expected. In addition, there was no significance between the two groups.

Are we done putting people on starvation diets, already?

Losing weight does often improve blood glucose, blood pressure, cholesterol, and other metrics. But the stress of eating almost nothing for weeks on end, and then transitioning to a 5:2 fasting regimen, is something only a very small segment of the population can physically stomach. And even then, what’s it doing to their relationship with food and their bodies?

It’s important to note that the above trials were medically supervised. Fast 800 is not.

I decided to sign up for the Fast 800 to really see what it’s all about (and to see if I could game the system). I did not do the diet, because it would be super triggering to me. 

fast 800


The first step after signing up was to complete a questionnaire to determine which diet approach was recommended for me. As I said above, I tried to game the system by putting in a much lower weight and a low weight goal – something completely inappropriate.

To the credit of Fast 800, the app immediately told me that because of my weight, the program is not appropriate for me, and that I should request a refund. This was exactly what I wanted to happen! 

fast 800 review

Interestingly, I’ve tried the same thing with other diet programs *ahem* Noom *ahem* and ended up getting a low calorie diet anyhow. Not okay. 

Read my Noom review here. 

I then went back and put in my actual weight and a goal, and because my anthropometrics are not indicative of high diabetes risk, the app gave me the Way of Life approach.

This ‘maintenance phase’ plan offered me between 1300 and 1600 calories a day, which is far lower than what I need, but still not shockingly low (again, see Noom). The app didn’t think I needed to lose weight, so presumably it thinks that this level of calories will sustain me.

I believe that most of the people who use the Fast 800 are doing it to lose weight. I suspect that most of them will receive the Very Fast 800 plan as their first step. 

‘Starve the fat people,’ if you will. I feel a bit sick thinking about it.


There are meal plans and shopping lists and online communities and coaches (who aren’t dietitians) to support Fast 800 followers. I found the recipes to be fairly easy, with a good variety of plant-based and animal proteins. Clearly though, if you’re too busy to cook meals, or you don’t have access to a wide variety of foods for whatever reason, you’ll probably have issues with this diet.

fast 800 meal plan

There are no snacks at all in any of the plans, because Mosley tells us that snacks ‘make us hungrier’ and may cause insulin resistance, which can lead to diabetes.

He gives this explanation:

If you are not using energy effectively, and frequently topping up with snacks, your pancreas releases more insulin to try and bring down your regularly peaking blood sugars. Over time, insulin receptors will become resistant meaning less glucose is transferred to energy cells and your blood sugar levels remain high – this is often the starting point of type 2 diabetes.10

 He offers up this reference to support this theory. Except that this paper doesn’t support it at all – it’s just a clinical review of insulin as a hormone. It confirms what we already know – that insulin resistance can lead to diabetes, but nowhere in the paper does it talk about snacks. 

He goes on to say this:

As less fuel becomes available to keep your body going, tiredness will kick in and hunger levels will increase as your brain tells you that more energy is needed. Despite the intention, snacking actually makes you hungrier and breaking out of this cycle is the key to stopping.

Nope. Telling people that they can’t eat if they’re hungry, and that snacks will result in insulin resistance for EVERYONE, isn’t productive. It’s this sort of all-or-nothing thinking that’s a red flag for me…especially when there’s no evidence to support it.

You can lose weight having snacks, and they are not directly linked to insulin resistance. This is completely ridiculous, and suggests that although Mosley invented this diet, his knowledge about nutrition and physiology is very limited.

That’s another red flag. Some of what he says is accurate, other things (and this entire diet in general) are really, really not.  As a professional, I’ll say that anytime this happens, it leads me to distrust what the person says. 

Can you hack your blood sugar? Read my Glucose Goddess review here.


Mosley has faced some serious backlash from promoting this diet.

In 2021, he went on TV during Eating Disorder Awareness Week to promote the Fast 800 for ‘losing the lockdown weight.’ 

In 2020, he appeared on a UK morning show telling people that after three or four days of eating 800 calories, the stomach ‘shrinks’ (this is untrue – the stomach does not shrink) and we ‘go into fat-burning mode.’

Why do people continue to call ‘ketosis,’ ‘fat burning mode’? He also says the diet ‘burns fat by flipping a metabolic switch.’

fast 800 diet

This is language used in marketing to make potential customers believe that this diet is as easy as turning on a light. It isn’t. 

The response of many people on social media to both appearances was overwhelming outrage. The television station that aired the 2021 segment got 2000 complaints. 

People were not happy.

Mosley also had a TV show called, ‘Lose a Stone in 21 Days,’ which honestly sounds like a hot garbage mess. Because although losing that much weight in that short of a time might be doable (a stone equals 14 pounds or just over 6 kilograms), actually achieving it isn’t necessarily a good idea.

Biggest Loser, anyone? Anyone?

And although the Fast 800 site claims that there is no evidence that the diet causes eating disorders, BEAT, a UK eating disorder charity, gave an official statement after Mosley’s ‘Lose a Stone’ show aired. It included the following paragraph:

Research has shown that dietary restraint, including the restriction of calories, has been found to be a risk factor in the development of an eating disorder. The programme did not appear to look after the participants’ mental health in any way, such as consultations with a psychiatrist. However, it did have an alarming focus on the participants’ weight, BMI and measurements, as well as encouraging them to carry out activities that we know those affected by eating disorders may engage in, such as destroying food to avoid eating it or fixating on eating a restricted number of calories.

The charity also reported a 51% uptick in calls on their helpline after the show aired. 

Recently, GP and menopause specialist Louise Newsom had Mosley and his wife and fellow Fast 800 developer Clare Bailey, on her podcast for a chat. During that episode, Mosley promoted Fast 800 to menopausal women – and again, the backlash was swift.

Menopausal women (and people in general) should never starve themselves. In fact, in midlife and beyond, people have increased protein needs to prevent muscle loss aka sarcopenia – something that’s likely to happen with an 800 calorie a day diet – and should not be adding more stress to their lives. 

While it’s true that our risk for insulin resistance increases with menopause, it’s not a forgone conclusion that it’s going to happen – and VLCDs can result in rebound overeating and diet cycling, which research shows can actually increase health risks. 

They can also cause nutrient deficiencies, which is probably why Fast 800 recommends vitamins to followers. Red flag. 

A recent study published in the BMJ found a higher than expected prevalence of eating disorders in women in midlife. People in positions of authority – like self-proclaimed ‘menopause experts’ – should never be promoting diets like Fast 800. 

A dietitian’s last word: The Fast 800.

The Fast 800 developers should not be advertising this diet as an easy and safe way to lose weight. It’s unnecessarily extreme, and there’s something unsavoury about the undertone of telling fat people to starve themselves to be healthier. I don’t like it. 

Fasting isn’t better than normal caloric restriction. Fast 800 is a low-calorie diet, and any improved bloodwork results that people may see are likely due to weight loss. This can be achieved without going to such extremes. 

VLCDs should not be done without the approval and oversight of a physician. As a dietitian, I don’t recommend them, and never have. I see absolutely no point in taking calories down that low, which can lead to rebound overeating, food obsession, and nutrient deficiencies, in particular if you’re doing it for months on end. 800 calories a day is disordered eating, in my opinion. 

Menopausal women are the new targets of fad diets, supplements, and extreme weight loss programs, including this one. I’ve said in once, and I’ll say it again: nobody needs to eat only 800 calories in order to lose weight. 

Is diet soda unhealthy? Here’s what the research says.

You should never have to take supplements to ‘make up’ for what your diet plan isn’t giving you. If you’re on a diet that’s that restrictive, it’s a red flag.

Lastly, losing weight themselves does not make someone a diet expert. If they’re selling a product associated with that weight loss, be very suspicious.