At the pointy end of the IF debate is British physician and science journalist Michael Mosley, the personable populariser of its most celebrated version: the 5/2 diet. For Mosley, the adventure started in 2012 when a BBC project nudged him into the orbit of Mark Mattson, an American neuroscientist who’d been researching fasting for more than a decade. Mattson believed you could slow ageing by slashing food consumption by roughly 75 per cent on two days per week while eating normally on the other five. Mosley, who’d recently been diagnosed with type-2 diabetes and was seeking alternatives to life-long medication dependence, put himself on the plan.

“After eight or nine weeks I’d lost nine kilograms,” he recalls. “I lost 11 inches [28cm] from around my waist, my cholesterol came down, I reversed my diabetes and I stopped snoring.”

Within a year, Mosley had co-authored his first book, The Fast Diet, which suggested that men who were overweight or had issues with their blood sugars restrict themselves to 600 calories (2510 kilojoules) on two nonconsecutive days each week. The Fast Diet became a bestseller. Active in the field ever since, Mosley spoke with Men’s Health in a Sydney hotel lobby on the eve of his “Wonders of the Human Body” tour of Australia.

“I do want to change the world,” he says. “I hear the stories of people who say [IF] has changed their life, as well as the stories of other people who say they’d rather die than eat this way. That’s how it is. But it’s unbelievably exciting feeling you’re part of this thing.”

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Six years after The Fast Diet caught fire, how have your views on fasting evolved?

Everything in that book I stand by. It has stood the test of time. But one of the things that’s changed is that I’ve moved up the calories on fasting days to 800. The original 600 for men was based mainly on rat work – there hadn’t been many human trials at that point. Since then, a lot of the science points to 800 calories as being low enough to trigger desirable metabolic changes but high enough for people to stick to.

There’s also a new 5/2 variant where you restrict yourself to 800 calories everyday. Is that shock therapy for the very overweight guy?

The main reason you would do it that way is because it’s more motivating. In studies of 800 calories done by Professor Roy Taylor at Newcastle University [in the UK], participants, on average, sustained a 10-kilogram weight loss for a year. The results were so impressive that the British Health Service is going to roll out a program testing it on 5000 people. It’s gone from crazy stuff to mainstream. As well as Roy Taylor there’s Professor Susan Jebb at the University of Oxford: she’s just done a big clinical trial looking at rapid weight loss and says [the results were] phenomenal. Yes, you have to make sure you’re getting all the necessary nutrients. It’s not suitable for everyone. My advice is to give it a go for two weeks. If you feel dreadful, stop. If you feel good, [persist]. I do recommend you don’t go on it for more than 10-12 weeks because we don’t really know what happens beyond that. You want to be conservative with this stuff.

Do you know how fasting fixed you back in 2012?

I asked Roy Taylor in 2014 why my diabetes had gone away. He basically said, “Look it’s all about rapid weight loss: you lost about 10 per cent of your body weight, you drained the fat out of your liver and pancreas, and they came back to life”.

Have you stayed on 5:2 for the past seven years?

No. I always saw it as an initial phase that would help people lose weight. I still try to do it but it’s more like 6/1. Except when I put on weight on a holiday or something like that, then I’ll go back to it. Some people stay on it because of the potential brain benefits. The data on that is not all there yet but Mattson is doing a trial and hopefully we will see the results on that sometime soon.

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But your diabetes has never come back?

Gone. If it ever did return I’d do something aggressive about it again.

What do we know about the benefits of 5/2 for the fit, normal-weight guy?

To be honest, all the studies have been done on people who are either overweight or obese, or have a metabolic disorder. But there have been two small randomised controlled studies done on time restricted eating [specifically, the 16-8 protocol, in which all eating for a 24-hour period occurs within an eight-hour window] involving fit men, and these showed participants lost fat without losing muscle. But I wouldn’t put my hand on my heart and tell you that if you’re already fit and lean you should be doing 5/2 or anything like it because we don’t have the data.

In front of everybody, though, many fasting advocates dangle this carrot of autophagy – or cell renewal. Does that excite you?

It does. It’s a lovely notion, isn’t it? The body gobbling up all the bad cells. It’s a real phenomenon. There was a Swedish study published where they put people on 5/2 and it did indeed trigger autophagy. Basically, when you’re in a negative energy balance your body stops trying to produce more and more new cells, and instead tries to get rid of the garbagy old cells. The first person who told me about this was Valter Longo [a biogerontologist at the University of Southern California and author of The Longevity Diet]. We need periods of time without food. Valter Longo has shown that, particularly for things like your immune system, an initial effect of going on a fasting diet is that your white blood cells reduce. That doesn’t sound good. But what’s actually happening is you’re getting rid of the rubbishy ones, and when you eat again you get a lot of new ones that have been generated by stem cells. This may happen in the brain as well. Early days, but I do think that is one of the more exciting things. You strip the house in effect and then start fitting it out with new furniture.

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Have you dabbled in 16/8?

Yes, though I prefer 14/10. This is based on the work of Professor Satchin Panda, who’s based at the Salk Institute and has written a brilliant book recently all about it (The Circadian Code). He’s a guy I spoke to back in 2012 when he just had rat work. Out of this spun all sorts of things, including 16/8. I saw him just before Christmas at the first ever intermittent fasting conference and he reckons 14/10 is more realistic and that’s where he is focusing his research. He says what happens is that after about 8-10 hours without eating your fat-burning starts to soar but, realistically, most people aren’t going to stick to 16/8. What he recommends is that you start by moving to 12/12. Just go 12 hours overnight without eating, then try shifting to 14/10 and see if you can stick with that. What I would love is to do a trial with Dr Panda where we combine time-restricted eating with 5/2, because he reckons they would work brilliantly together.

Would you say that any guy who doesn’t at least dabble in fasting is missing a trick?

I’m a great believer in self experimentation. I think if you’re at all curious, you might want to give it a go. You may find you keep all that lovely muscle and shed all that fat.

We’ve talked about the “when” of eating. What’s your approach to the “what”? I saw a clip of you tucking into a plate of pulled pork and fried calamari. Are you a fan of the meat-only diet?

No, that was for an Horizon episode called “The Truth About Meat”. I put myself on a heavy meat diet. I put on quite a lot of weight, my blood pressure went up and my cholesterol went crazy. I’m utterly sympathetic with the vegans, but the truth is I just don’t think I could do it. I’m more of a flexitarian. I like red meat. I’ll have it occasionally.

Why do it, Michael – commit to a lot of fuss and denial around food? Is it to squeeze out extra years at the end in a nursing home?

I think it’s to be happier. We’re all going to live a long time. Statistically, we’re likely to hit 84 and we’re probably going to hit 90. No male member of my family has hit 74 but I’m pretty confident I’m going to do it. But I don’t want to live my later years in declining health – miserable, impotent, diabetic, blind. I don’t want to deal with those things. What would be the point of being alive? I want to hit the age of 84, be super healthy and then die hang gliding or something like that. So that’s why you do it: you want to feel young and vibrant for as long as possible.