Crash diet or smart choice? 5:2 creator explains new rapid weight loss theory

by Sarah Lang / 13 February, 2019

Can intermittent fasting help you lose weight quickly and safely, prevent or reverse diabetes, or even lower the risks of dementia, cancer and heart problems? And can you actually stick to it? Dr Michael Mosley of the “5:2 diet” fame has written a new book about all this. He talks to Sarah Lang.

When we speak just before Christmas, Dr Michael Mosley badly needs his holiday break. After all, he’s had a hectic two years, even by his standards.

In 2017, the UK-based author, science journalist and medically qualified doctor wrote The Clever Guts Diet, which shows how an eating regime based on the Mediterranean diet can improve not just gut symptoms, like bloating, cramping and diarrhoea, but also boost the gut microbiome and overall health. Then, Mosley collaborated on the accompanying recipe book – and got some serious jetlag while flying in and out of various countries to promote the books.

Mosley – who also presents and produces BBC documentaries on science, health and human biology – gets stuff done. Late last year he wrote another book, The Fast 800, published in January by Simon & Schuster. It’s the successor to his bestseller The Fast Diet (2013), which Mosley co-wrote shortly after reversing his newly diagnosed type 2 diabetes with a new-fangled approach: intermittent fasting.

Read more: 5:2 diet: Three Kiwis share whether intermittent fasting works for them

Dr Michael Mosley’s new book, The Fast 800, counters some old dieting myths with new science: “Frankly, people who say the brain runs out of energy without carbs don’t understand the basic science.” Photo/Ken Downie

Dr Michael Mosley’s new book, The Fast 800, counters some old dieting myths with new science: “Frankly, people who say the brain runs out of energy without carbs don’t understand the basic science.” Photo/Ken Downie

Mosley’s “fast diet” is now commonly known as the catchier “5:2 diet”, because you fast on two days a week, not the other five. The Fast 800 is, in part, an update on both The Fast Diet (revised in 2015) and Mosley’s following book The 8-Week Blood Sugar Diet (2015). The former has helped numerous people lose weight and improve their health through eating between 500 and 600 calories on those two “fasting” days of the week. The latter has helped numerous people prevent or reverse type 2 diabetes (a blood-sugar disorder usually associated with being overweight) – and reverse pre-diabetes (raised blood sugars) – by eating 800 calories a day for eight weeks, without using medication. Of course, there are overlaps between the two diets and their aims and results.

Read more: Dr Michael Mosley tests out The Fast 800 rapid weight loss diet on himselfWhat happened when I tried Michael Mosley's 'Clever Guts' diet 

People often stop Mosley on the street or tweet to say that one of his books changed their lives. His latest, The Fast 800, certainly isn’t a case of cutting and pasting the other books together with a new title and a few tweaks to score more sales. Importantly, it describes strong evidence from recent years that shows the 5:2 can lead to health benefits from preventing diabetes through to potentially preventing dementia. Mosley also outlines a new, step-by-step 5:2 programme which ups the fasting-day limit to 800 calories, and incorporates new methods and techniques. Bear with the numbers for a sec. In the first stage, Rapid Weight Loss, you eat 800 calories every day for two weeks – or longer – to activate the process of ketosis, which burns fat fast. In the second stage, the New 5:2, you eat 800 calories on two days of the week, and a Mediterranean-style diet on the other five days. In the third stage, Maintenance, you eat Mediterranean-style with no calorie counting but perhaps one fasting day. You can also skip the Rapid Weight Loss stage and go straight to the New 5:2.

As is his norm, Mosley, 61, became his own human guinea pig for the new regime. Now he’s about to hazard jetlag again to spread the New 5:2 word.

North & South: Have you timed this book to come out right after people make New Year’s resolutions?

Michael Mosley: Of course! That’s when people are keenest to try something new. 

N&S: What would you say to someone who complains you’re flogging all these different diets and books: The 8-Week Blood Sugar Diet, The Clever Guts Diet, The Fast Diet, now The Fast 800?

MM: I’d say each book is a different way of viewing health – and different approaches suit different people with different priorities. But the books are also evolutions. With each, I’ve tried to build on and extend what I’d previously learned. For example, intermittent fasting is good for gut health. Something that’s just come out – unfortunately, after this book went to press – is a big study showing intermittent fasting encourages the growth of anti-inflammatory gut microbes. The books are complementary – and this one draws together important elements of previous books, plus incorporates new science and new methods.

N&S: Why increase your earlier recommendation of 500-600 calories to 800?

MM: It’s the magic number. The original numbers were, frankly, based largely on rat studies. Since then, several 5:2 studies have shown that, in practice, many people are actually eating 800 calories. It’s more comfortable. More doable. And we now know that, at 800 calories, you can still achieve significant weight loss and other health benefits.

N&S: Compared with your first 5:2 book, this programme is based much more on the Mediterranean diet, isn’t it?

MM: Yes. There’s now overwhelming evidence for the benefits of the Mediterranean-style diet, rich in natural fats, fish, veges, nuts and legumes. Studies show it cuts your risk of heart disease, cancer, type 2 diabetes, depression and dementia. And there’s now lots of strong science behind intermittent fasting, so The Fast 800 combines these two approaches.

N&S: Explain the process of ketosis.

MM: The Fast 800 is a mildly ketogenic diet. You eat a much-higher ratio of fat and protein compared to carbohydrates than normal, especially during the rapid weight loss stage. Ketones produced by your liver see your body go from burning sugar to burning fat stores. Recent research shows ketosis is key to intermittent fasting – and surprisingly good for the body and the brain. It triggers autophagy: a self-repair mechanism to get rid of old, junky cells.

N&S: This process sees some people experience side effects, including intense hunger, headaches, light-headedness, and irritability.

MM: Some do, but all that tends to drop off after two weeks, sometimes earlier. Don’t give up on Day 7. I predict after two weeks, you’ll feel a hell of a lot better. The body adapts rapidly.

N&S: Don’t many health professionals say ketogenic and low-carb diets are unhealthy and unsustainable?

MM: Many health authorities and nutritionists still see low-carbohydrate diets as the devil. Frankly, people who say the brain runs out of energy without carbs don’t understand the basic science. Carbs are not obligatory. Certainly, very-low-carbohydrate diets are really difficult to sustain long-term. And your microbiome needs some good quality complex carbs, for fibre, which is why you move onto the maintenance stage.

N&S: During the rapid weight loss stage, you may lose five kilos after two weeks, nine kilos after four weeks, and 14 kilos after eight weeks. This challenges the common advice against “crash dieting”, right?

MM: Unfortunately, despite strong evidence to the contrary, we’re told repeatedly that rapid weight loss is futile. Some well-informed weight-loss specialists have told me this drives them crazy, and it frustrates me too, because it’s profoundly unhelpful yet it persists. We sort of vaguely believe these things because we were told them once many years ago. If you manage to lose weight slowly, terrific, but many people struggle that way. You can lose weight quickly and safely through the 5:2 – it’s a matter of doing it right. And studies show rapid-weight-loss dieters don’t only lose but keep off more weight.

Two examples, from The Fast 800, of an 800-calorie day: (left) boiled eggs with spiced asparagus soldiers, root vegetable turmeric soup and Spanish eggplant stew with chorizo; (right) smashed avocado on dark rye bread, sausage and mushrooms with spring greens.

Two examples, from The Fast 800, of an 800-calorie day: (left) boiled eggs with spiced asparagus soldiers, root vegetable turmeric soup and Spanish eggplant stew with chorizo; (right) smashed avocado on dark rye bread, sausage and mushrooms with spring greens.

N&S: When so many diets fail, why does 5:2 work for many people? Because you’re not restricting calories every day so are less likely to “blow it”? Because there are fast results?

MM: Yes, and 5:2 is flexible – people choose the days that suit them – and it isn’t expensive. Plus, while some people struggle, others feel good physically. There’s evidence that ketones stimulate production of the protein BDNF, a natural anti-depressant that stimulates the creation of new brain cells and connections.

N&S: My barista says he has better memory, focus and “cosmic clarity” when he’s fasting.

MM: Taxi drivers have told me they drive better.

N&S: A recent story about intermittent fasting featured a photo of a slim woman staring at a plate which contained only a tiny carrot. Is it a misconception that you eat next to nothing on fasting days?

MM: Yes. No one’s advocating starvation. For starters, you must eat enough protein; 800 calories doesn’t sound much, but the recipes in this book are filling and nutrient-rich. Spread the calories over two meals or three – many people skip breakfast. If necessary, try meal-replacement shakes.

N&S: You say that, on non-fasting days, it’s important to eat Mediterranean-style and use portion control. If you eat lots of white carbs for five days, does that undo the good work?

MM: Two days of intermittent fasting are better than none. But I hope habits like eating more veges and protein would seep into non-fasting days. People are smart enough not to wolf down ice cream then. There are bastardised versions of 5:2 that say eat whatever you want on non-fasting days – I’ve never, ever said that.

N&S: You suggest people simultaneously practise time-restricted eating [TRE], where you don’t eat during a 12-16 hour window, including overnight?

MM: Some people find TRE easier than 5:2 – anyone can use a watch. You get modest weight loss doing TRE on its own, but combine it with 5:2 and you’ll see impressive changes.

N&S: Is there now good evidence that 5:2 can reverse type 2 diabetes?

MM: Yes. The strongest evidence comes from the DIabetes REmission Clinical Trial (DIRECT), a randomised, controlled study, with results published in 2018. Nearly half of the type 2 diabetics on an 800-calorie-a-day diet brought their blood sugars back to normal, despite having come off their diabetes drugs. If you’re already taking insulin [often prescribed for type 2 diabetics after first-line medications like Metformin], be careful and consult your doctor. However, my GP wife, Clare Bailey, is currently involved with an Oxford University trial of type 2 diabetics both taking insulin and doing 5:2, and they’re doing well. Similar trials are running.

N&S: The UK’s National Health Service [NHS] originally called the 5:2 a “fad diet”, but its recent “Top Diets Review” said 5:2 can be more achievable than dieting every day – and that 5:2 can lead to greater reductions in body fat, insulin resistance and other chronic diseases. Were you expecting that move?

MM: Actually, the speed of that move astounded me. Just after The Fast 800 went to press, the NHS announced it’s about to roll out a programme trying this 800-calorie approach with up to 5000 people who have type 2 diabetes, in the hope of reversing their condition.

N&S: Do you think New Zealand will follow the lead of the “Mother Country”? Currently, the NZ Ministry of Health’s “Popular Diets Review” recommends the Mediterranean diet, but advises you consult your doctor before doing 5:2 – and doesn’t recommend very low-calorie diets (800 or fewer a day) over a short time period.

MM: I suspect things will change in New Zealand. In Australia, the CSIRO [Commonwealth Scientific and Industrial Research Organisation], an independent federal government agency, is looking at low-carbohydrate diets.

N&S: Given the new evidence, is it wrong for doctors to immediately suggest medication for type 2 diabetics or pre-diabetics without suggesting intermittent fasting?

MM: I’m surprised by how many doctors haven’t heard of this approach. I think it’s wrong if they don’t look into it, or don’t at least suggest patients investigate it. There’s a fear patients will feel offended if their weight is mentioned, but actually they generally don’t.

N&S: A couple of women have told me that, when they were obese, their GPs never brought up their weight. Both wonder whether that’s because their doctors were overweight themselves. Another woman told her doctor she’d reversed her pre-diabetes [raised blood sugars] through 5:2, and the doctor didn’t show any interest.

MM: I’m surprised [the latter GP] wasn’t curious, because if a patient transformed themselves, I’d want to know how. And do you really want advice about weight loss from someone with weight issues? But please, do tell your doctors about 5:2 – that’s how these things often spread.

N&S: Dementia is expected to affect 150 million people by 2050. It’s as yet unproven that 5:2 could help prevent it?

MM: Frankly, we’re waiting on US neuroscience professor Mark Mattson’s current study to see if 5:2 can protect and boost the brains of people who are at increased risk of developing dementia. He says interim results are encouraging. His findings are expected to be out later this year. He’d previously done animal studies that show intermittent fasting can help combat memory loss and delay dementia.

N&S: You write that 5:2 can help prevent cancer, because carrying too much fat or having high insulin levels are risk factors for cancer. What if you already have cancer?

MM: It’s early days with research on that. But Dr Valter Longo, a human-ageing expert, found that time-restricted eating seemed to have a significant impact on the risk of breast cancer recurrence. He believes fasting has the potential to delay ageing and prevent the onset of diseases such as cancer and heart disease. On his “Fasting Mimicking Diet” [FMD], people eat 800 calories a day for a fixed number of consecutive days, mainly vegetables, olive oil and nuts. A dozen clinical trials worldwide are looking at FMD’s effect on various conditions, including breast cancer in women who had already had chemo.

N&S: How might 5:2 improve your heart health?

MM: First, by helping you lose weight. Second, by improving your insulin sensitivity. And a small study showed 5:2 had a significantly bigger impact on blood pressure than losing weight by another method. Watch this space.

N&S: When asked about intermittent fasting, Dr Fredrik Karpe from the Oxford Centre for Diabetes, Endocrinology and Metabolism said it’s “very important to critically investigate health plans for interventions giving great promises”. So many experts are still sceptical about intermittent fasting.

MM: I’m very sceptical by nature. But scientists have critically investigated all this – and still are. I sometimes collaborate on a few small studies. Mainly, though, I weigh up various claims and studies. People get understandably confused about conflicting evidence so, in the book, I explain different types of studies and the most reliable ones – particularly randomised, controlled trials. Government advice actually ranks amongst the weakest evidence. And I always look at, for instance, how big the trial is, who conducted it, who funded it, etcetera. Plus, I acknowledge gaps in scientific knowledge, and very much hope others will fill those gaps.

N&S: Back to the practical stuff. You’ve included lots of tips for doing 5:2, such as talking to others about the approach, writing things down, etcetera. How important are psychological techniques to success?

MM: Do your psychological techniques really matter that much if you feel starving? The practical steps are most important. For instance, actually doing the rapid weight loss stage will help change hunger hormones and suppress your appetite. However, psychology is clearly important regarding things like doing 5:2 with others, having clear goals, practising mindfulness, managing stress, and trying to sleep well.

N&S: If it’s too hard, you say take a break?

MM: Yes. Perhaps try intermittent dieting. It seems counter-intuitive but the Australian MATADOR study’s small trial of obese men showed “two weeks on, two weeks off” was beneficial. The intermittent dieters lost – and kept off – more weight than men who dieted throughout. Why? Perhaps it counters dieting fatigue.

N&S: Plus everyone on a diet needs breaks, especially over the Christmas season! You also say that, if you find 5:2 difficult, try 2:5 where you eat 800 calories a day during the week and don’t fast on weekends. That sounds much harder than 5:2.

MM: I’m being pragmatic. I’m providing a toolkit you can dip into to suit your needs. I think being overly prescriptive is unhelpful. For instance, it doesn’t matter if you’re 50 or 100 calories over or under.

N&S: On weekdays, my friend eats 1100 calories and exercises, then on the weekend eats junk food and drinks a fair bit of alcohol. Is that bad for her?

MM: Our ancestors had long periods when they wouldn’t eat much, and occasionally gorge, but there are no particular studies on this. Is she a decent weight, sleeping okay, and are her cholesterol and blood sugars under control?

N&S: Yes, and she’s got a body like Charlize Theron’s.  

MM: I wouldn’t knock her approach then!  

N&S: Should people of a healthy weight do 5:2 for health benefits?

MM: The honest truth is I don’t know. There’s some evidence it can benefit slim people, but right now I’m reluctant to suggest it. I’ve just been talking to researchers about doing a study looking at healthy people on the 5:2 diet. We just need to find funding, because slim people are not any health authority’s priority.

N&S: In the book, you note that people with certain health conditions, etcetera, shouldn’t do 5:2, or should consult their doctor. And people who once had eating disorders shouldn’t do it?

MM: I say that based on caution, not on science. One small study showed that people with current or past eating disorders actually do better on intermittent fasting, but I don’t want to put anybody at risk, because as a doctor I’ve sworn to “first, do no harm”.

N&S: You do a lot. Do you overwork yourself?

MM: I write books relatively fast but, before that, I collect data, studies, people’s stories, and talk to experts. I haven’t totted up my hours, but, blimey, I probably should slow down a bit.

N&S: Has anyone asked you if you’re just in it for book sales?

MM: Regularly. Mainly online. When one person asked that online, I replied “It’s all free online”, in my articles, and there’s a website for each book. But some people prefer something they can hold and flick through. Money has never been my main motivation. I just really want people to know this stuff.

This article was first published in the February 2019 issue of North & South.

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