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Dr Michael Mosley tests out The Fast 800 rapid weight loss diet on himself

Book extract from The Fast 800 by Dr Michael Mosley

To test out his new diet, The Fast 800, Dr Michael Mosley experimented on himself. 

As I mentioned in the introduction, when I was researching this book – and following my own personal rationale that I should try the programmes I recommend – I tested out the Fast 800, by putting on weight and then seeing how quickly I could lose it again.

Before starting, I did a range of tests, including measuring my fasting glucose, my blood pressure, my weight and my waist. The tests showed I was basically healthy. I came in at 172lb (78kg), with a waist of 32 inches (81cm), blood sugars and blood pressure both excellent. Time to undo all that. To put on weight, I stuck to a relatively healthy diet but increased my consumption of starches. I ate more bread, potatoes, rice and pasta, plus the occasional biscuit.

Here’s an entry from my diary: ‘It’s now a couple of weeks since I started doing my experiment and the biggest surprise has been that, so far, I have put on remarkably little weight. I think my body has just got used to my current weight and is resisting my attempt to pile the fat on. In some ways that is immensely reassuring. I could get used to this new lifestyle.’

It didn’t last.

‘It’s now a month into my experiment and the scales are beginning to move. My blood sugar levels are also starting to rise. The strangest and most unsettling thing is that I am now really beginning to crave sweet things again. I find it almost impossible to pass a shop and not buy a small bar of chocolate. Clare says that I am beginning to snore again and she is anxious that I stop soon.’

In the end, it took me nearly four months to put on 14lb (just over 6kg) and by then the rot had really set in. My blood sugars were almost back in the diabetic range. My waist had ballooned to 37 inches (94cm) and my blood pressure was in the red zone. I felt hungry much of the time.

Read more: Crash diet or smart choice? 5:2 creator explains the theory behind The Fast 800The benefits of a Mediterranean diet

I was doing a lot of filming during that period and I worried that people would notice that I’d put on lots of weight and ask why I had allowed myself to go to seed, but no one did. It helps that when I put on fat, it is mainly internal. Wearing baggy shirts also helps.

My wife told me I was beginning to look older. I was sleeping badly and feeling increasingly moody.

So, after a final, indulgent summer holiday in Greece, I knuckled down to losing weight. I started by doing the Very Fast 800 programme [the Rapid Weight Loss stage], sticking to 800 calories a day, and using menus from this book. I included TRE [time-restricted eating, which means eating within a relatively narrow time window each day, usually 8 to 12 hours], doing a 12-12 programme. My plan was to finish eating by 8pm and then not eat anything till at least 8am the next morning. So, how did I get on?

Well, it was easier than I feared. Perhaps because I am used to occasional fasting, sticking to 800 calories a day was not as challenging as I thought it would be. I knew what to expect, which helps, and I imagine that my body is more used to “flipping the metabolic switch”. I was certainly hungry and a bit grumpy to begin with, but after a few days the cravings and the bursts of hungriness passed. Mostly.

Since I was trying to fit this rapid weight loss diet around a busy filming schedule, I had to combine using meal replacement shakes when I was on the road, with recipes from this book when I was at home. On a couple of occasions, I had to go for business-related meals, but I managed to stick to fish and vegetables.

The weight loss was fast and the metabolic changes impressive. In the first four days I lost 6lb (2.7kg), some of it water. My blood sugars and my blood pressure also fell. Optimistically, I tried to tighten my belt by a notch. Not there yet.

Lots of water and black tea helped prevent headaches and constipation when Mosley tried the Fast 800 himself. Photo/Ken Downie.

I kept up my exercise regimen, but I did notice that going for long walks or runs was tougher than it had been. Although I was in mild ketosis (I used my ketone sticks to check), my energy levels were definitely lower.

I was glugging back lots of water and black tea, so neither headaches nor constipation were a problem.

I had some bad moments, including one occasion when I was stranded on a railway platform at 10pm, not having eaten that day, with nothing for company but a chocolate machine. Fortunately I didn’t have any change, or I would probably have cracked.

There were lapses. There was an evening when I gave myself a night off and drank several glasses of wine, followed by way too much cheese. And another occasion, when I gave into one slice of hot buttered toast, then another. But on the whole, I stayed on track.

After two weeks, I had managed to lose 11lb (5kg) and get my blood sugars and blood pressure back to normal. I could have continued on the rapid weight loss programme but I thought this would be a good moment to switch to the 5:2.

As an experiment, I did my fast days back-to-back (Mondays and Tuesdays) and noticed, thanks to my keto measuring sticks, that I was in mild ketosis for some of the first and most of the second day. Doing exercise became easier. I could push myself harder without feeling drained.

I continued to eat the Mediterranean-style menus from the book on my fast days and eat more freely on the non-fast days. I also returned to drinking wine on my non-fast days. It was, dare I say it, easy.

Three weeks and five days after I started, I was back to my previous healthy weight and everything else had returned to normal.

What had I learned?

  • That this diet is very doable.
  • That if I let myself go, then the diabetes and other health issues will return.
  • That what I eat really does affect my mood.
  • That TRE helps, but it is tricky to stick to rigidly if you have a social life. That said, I will persist with trying to do it on as many occasions as possible, as I think the science is convincing.
Fish and vegetables – a good bet when dining out.

Where do we go from here?

It is six years since I first suggested that intermittent fasting might offer a new and exciting alternative to the standard ‘slow, steady, eat low-fat’ message. Today, I am more confident than ever that it does.

I am stopped on the streets almost every day by people who want to tell me about their weight-loss success. Do I mind? Not at all. I love feedback and even if we never cross paths, you can always get in contact with me or the Fast 800 team via the website.

Just as importantly, I feel the science is coming along in leaps and bounds. Although there are still some very important questions that remain unresolved, answers are on their way.

So will Professor Valter Longo’s Fast Mimicking Diet turn out to be as revolutionary as his early studies suggest it might be? Will Professor Mark Mattson’s 5:2 study on the brain open the door to a new way of combating dementia? Will doctors and other health professionals respond positively to the latest research showing just how effective a rapid weight loss diet can be?

I’m obviously hoping the answers to all these questions will be “yes”.

Intermittent fasting has changed my life. I hope it changes yours.  

This book extract from The Fast 800 (Simon & Schuster, $35) was published in the February 2019 issue of North & South.

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