He's been teaching us how to diet and ditch the fat for over three decades.

Dr Michael Mosley is a pioneer of the weight loss industry and regarded as one of the world's most trusted health presenters.

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First, it was his famous 5:2 diet - feasting for five days and fasting for two - that had the whole world talking, and now it's the Fast 800 that has people counting calories in a whole new way.

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Mosley's new book suggests people should restrict themselves to only 800 calories a day, seven days a week.

"You'd lose an average of one to two kilos a week," Mosley said.

"And in the clinical studies, they found that people not only lose weight, but they keep it off."

Is 800 enough?

"If you eat the right stuff, it's amazing how much you can get with 800 calories," Mosley said.

"What you want is stuff that is really nutrient-rich - food designed to give you all the nutrients you need as well as keep you full.

"You can do it as two meals or three meals - so for example, breakfast can be scrambled eggs and smoked salmon, which comes to roughly 250 calories.

"For lunch, I'll have vegetables with a bit of meat - maybe some soup - which is normally a couple hundred calories, and then 300 calories in the evening, which is a big pile of vegetables with fish or meat or something like that."

Recognising legitimate diet plans

"The reality is losing weight is really hard, and if it was easy people would do it," Mosley said.

"People do say, just cut your calories and do your exercise, that's it. But that advice has been out there for a hundred years, and we're getting fatter and fatter. It clearly doesn't work.

"You need precise advice - it's all about how you do it."

Dr Michael Mosley is in Australia for a new national tour in September. Find out more here.