The Fast 800 diet was one of Michael Mosley’s most famous diets – but does it work?
Dr Michael Mosley, the late TV presenter, podcaster and columnist, dedicated his life to revolutionising the health and fitness industry, amassing a legion of devoted followers in the process. Countless Brits have come to swear by Mosley’s dieting techniques — most famously, the health guru was a pioneer of intermittent fasting, which he first introduced to the masses in 2012 in his BBC documentary ‘Eat, Fast, and Live Longer’.
Later down the line, his iconic ‘Fast 800’ diet was born — a calorie-restricting diet which involves periods of intermittent fasting that claims to promote weight loss, lower uncontrolled blood sugar, and even reduce your risk of certain conditions such as diabetes and heart disease.
Since its inception over a decade ago, intermittent fasting has experienced a renaissance of sorts – receiving glowing endorsements from celebrities and chief executives (and even our very own Prime Minister). The dieting technique also seems to have whipped up quite the frenzy on social media — nearly 200,000 videos on the subject have been posted to TikTok, many of which have garnered millions of views.
A plethora of studies have touted the benefits of intermittent fasting, including one study from 2019 which found that eating within a six-hour period of the day and fasting for 18 hours could trigger a metabolic switch, as well as reduce stress, increase longevity, and decreasing the risk of certain diseases, such as cancer and obesity.
That being said, intermittent fasting isn’t without its critics. Earlier this year, research presented at an American Heart Association conference (which hasn't yet been peer-reviewed, FYI) found that people who limited their eating window to eight hours or less per day had a nearly doubled risk of dying from heart or circulatory diseases.
‘The long-term health effects of time-restricted eating, including risk of death from any cause or cardiovascular disease, are unknown,’ said senior study author Victor Wenze Zhong, a professor and chair of the department of epidemiology and biostatistics at the Shanghai Jiao Tong University School of Medicine in Shanghai, China.
Opinions on the subject are clearly mixed – some claim this free, simple-to-follow regime is a great way to ‘bio-hack’ your health, while others believe that those who lose weight by fasting is simply because they eat fewer calories, since their time window to eat is shorter.
So, back to the Fast 800 diet – we wanted to find out for ourselves whether or not Mosley’s signature diet is actually a viable one, and whether or not nutritionists recommend following it. Here’s what we found out...
Meet the experts: Dr Linia Patel (PhD) is a registered dietician and author of Food for Menopause. Dr Megan Rossi (PhD) is a registered dietician, research Fellow at King’s College London and founder of The Gut Health Doctor and The Gut Health Clinic.
What is the Fast 800 diet?
Mosley’s infamous ‘Fast 800’ diet follows a simple set of rules. Aimed at people with obesity or type 2 diabetes, the intense, 12-week version of the plan involves eating 800 calories a day, and sticking to a moderately low-carb, Mediterranean diet.
After that, a more sustained approach is encouraged — intermittent fasting for two days of the week, consuming 800 calories on both days, while eating normally for the rest of the week.
‘The Fast 800 diet is a combination of intermittent fasting and a Mediterranean style eating plan,’ explains registered dietician Dr Linia Patel. ‘The diet is aimed at those who want to lose weight, especially around the middle (visceral abdominal fat) as well as those with blood sugar issues such as pre-diabetes and type 2 diabetes.’
How to follow the Fast 800 diet
The ‘Fast 800’ diet is unique in that it doesn’t provide a prescriptive list of what to eat, but rather when and how much to eat. It’s not a fad, it’s a lifestyle.
Dr Patel explains that the plan broadly recommends eating Mediterranean-style, whole foods.‘'The plan starts with a rapid weight loss phase where you can eat only 800 calories per day. This can be through a moderately low-carb, Med-style diet with lean protein sources and vegetables,’ she says. ‘When cooking is not possible, you can have two shakes a day although the emphasis is always on whole foods.’
This intense stage (which is only recommended for people looking to kick-start significant, quick weight loss) is followed by a more gradual approach.
‘The second stage includes intermittent fasting and restricting calories to 800 kcal a day for two days a week then eating a healthy low carb, Med diet for the remaining five,’ Dr Patel says. ‘The maintenance phase will depend on your goals and lifestyle but the same Med-style eating is recommended.’
Does it work and is it safe?
According to Dr Patel, the Fast 800 diet is not a one-size-fits-all eating plan. ‘800 calories a day is described as a very low-calorie diet. This means it can lead to rapid weight loss but it is routinely not recommended and may not be safe for everyone,’ she says.
The general recommendation from health professionals is to adopt a more slow, steady and sustainable approach. ‘However, very low-calorie diets can be used in support with a clinician for individuals who have an obesity-related complication that would benefit from weight loss,’ she adds.
Dr Megan Rossi, registered dietician and the founder of The Gut Health Doctor agrees that the Fast 800 diet is not for everyone. ‘Very low-calorie diets do lead to rapid weight loss but are only typically used in clinical settings when rapid weight loss is needed ahead of surgery,’ she explains.
‘This is because such rapid weight loss not only comes with risks, such as malnutrition, but most studies suggest that the weight loss is rapidly regained and therefore is not generally recommended as an effective approach to achieving long term sustainable weight loss.’
Despite this, Dr Rossi believes that the diet’s recommendation to follow a Mediterranean-style eating plan is a definite plus, as it leads to the inevitable reduction of ultra-processed food intake. ‘The issue, however, is that doing so with 800 calories makes it very difficult to meet your micronutrient needs,’ Dr Rossi says.
One major con of this diet is the dangerous impact it may have on long-term eating habits, particularly for people with a history of disordered eating, says Dr Patel. ‘This type of diet also may have a negative impact on one's relationship with food, masking or exacerbating disordered eating.’
Who should avoid this diet?
The question of whether intermittent fasting is for you or not, is an entirely personal one.
If you are considering intermittent fasting, make sure to discuss it with your doctor, says Dr Rossi – who adds she would be cautious to recommend such a low-calorie diet to anyone without the close supervision of a clinical dietician. ‘Indeed, I generally find calorie fixation can not only predispose clients to eating disorders but tends to lead to disappointing results in the long term.’
Why is that, you may wonder? Well, according to Dr Rossi, calories on packs are frequently inaccurate, with a ‘20% error margin’. Furthermore, she explains, calorie counting is not an exact science: ‘Calories in doesn’t equate to net calories gained as some foods burn more calories through the digestive process.’
Dr Patel agrees that while some people would like to lose weight or optimise insulin sensitivity could benefit from trying out this diet, speaking to a health professional is vital. And if you have an unstable or disordered eating pattern, it’s best to steer clear of this diet altogether.
‘Time and time again, within my clinical practice, it is common to see people following a feast and famine intermittent fasting pattern of eating which does more harm than good. There is no point fasting one day and then feasting the next day,’ explains Patel.
‘In fact, people with a history of eating disorders, or those using medications that require food intake (type 1 diabetics), those still actively growing (teenagers), pregnant or breast-feeding women should not be following this diet. Any diet like this needs to account for your age, lifestyle, commitments, sports, overall goals and stage of life, and adapt it when things change.’
The bottom line: The Fast 800 diet is a combination of intermittent fasting and a Mediterranean-style eating plan. The primary goal of the Fast 800 is to target abdominal fat – the type of fat that puts people at increased risk of lifestyle diseases, such as type 2 diabetes. If this is you, speak to your GP or a dietician about how this diet could help you. But, for the general population, extended periods of eating 800 calories per day is not recommended.
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