"The obvious answer 'because we eat too much and don't do enough exercise' is too simplistic," he says.

"It's like going to a tennis coach to improve your game and being told all you need to do is 'win more points than your opponent'."

It's true, he writes, but not useful. If it was that easy why, he asks, has there been such an explosion in obesity worldwide, over the past 40 years?

He puts it down to many things, increased anxiety, stress, poor sleep and becoming less active, but at the top of his list is more snacking and lots more junk food, cola, cake, candy and refined carbohydrates - up by 20 per cent since 1980.

He first became interested in intermittent fasting when he was diagnosed with type 2 diabetes. He tested a number of different forms and settled on the 5:2, which restricts calories two days a week. A few years later he came across some startling research being carried out by Professor Roy Taylor, a diabetes specialist from the University of Newcastle. Professor Taylor had started a major trial hoping to prove that an 800 calorie a day rapid weight loss diet would not only lead to massive weight loss but also help most patients with type 2 diabetes come off all medication and restore their blood sugars to normal.

In 2018 Professor Taylor published the results of his four-year trial and the results were better than expected. The 800 calorie diet worked.

Again, Mosley put the science to the test, doing a "super size me" type experiment on himself. He deliberately put on weight before embarking on the Fast 800 diet. The results were so successful he knew he had his next project.

This diet is designed to be as flexible as possible, while incorporating the best science-based advice. All the meals are based on a lowish-carb Mediterranean-style way of eating, but how you manage the 800cals days is down to you.

Most people choose to kick start their diet with a fast-track plan of 800cals a day, every day, for two or more weeks. They then move on to the New 5:2, cutting their calories for two days a week, and on the other days continuing to eat a healthy Med-style diet, keeping carbohydrate intake low and controlling portion size. But you can tailor your regime to suit your needs. If you find the idea of the fast-track plan daunting, you could go straight to the New 5:2 and stick with that, in which case you will lose weight but not quite so quickly.

As Mosley describes in his latest book, you can also add in a relatively new form of intermittent fasting called Time Restricted Eating, whereby you eat all your calories within a narrower time window each day - usually within eight to 12 hours. This extends the length of your normal overnight fast (when you are sleeping and not eating) and gives your body an opportunity to burn fat and do essential repairs.

As with any diet, the Fast 800 may not suit everyone, so check with your health professional first.

Now there is a companion cookbook to The Fast 800, filled with delicious, easy, low carb recipes and essential weekly meal planners, all carefully formulated by Dr Clare Bailey and Justine Pattison to help you lose weight, improve mood and reduce blood pressure, inflammation and blood sugars.

Here are three recipes to get you started.

The Fast 800 Recipe Book, by Dr Clare Bailey and Justine Pattison. Simon and Schuster, $35.

Challenge anyone to name the main ingredient.

Ingredients

100g coconut oil, plus extra for greasing

275g cooked beetroot, drained and cut into small chunks

3 large eggs

60g cocoa powder

100g soft pitted dates

100g wholemeal self-raising flour

1 tsp ground cinnamon

1 tsp bicarbonate of soda

75g plain dark chocolate (around 85% cocoa solids), roughly chopped

Method

Step 1. Preheat the oven to 200C. Grease and line the base and sides of a 20cm loose-based square cake tin with non-stick baking paper.

Step 2. Place the beetroot, eggs, cocoa powder, dates and coconut oil in a food processor and blend until thoroughly combined. You can also blend the ingredients together in a bowl using a stick blender.

Step 3. Add the flour, cinnamon, a pinch of sea salt and the bicarbonate of soda and blend until well combined. Add an extra tablespoon water to loosen the mixture, if needed.

Stir in the chocolate, then spoon into the prepared tin, spreading to the sides. Bake for about 20 minutes, or until risen and just firm to the touch.

Step 4. Cool in the tin for 10 minutes, then turn out and cut into squares to serve.

Cook's tips: If you use a pack of ready-cooked beetroot from the supermarket, make sure it doesn't contain vinegar or spices. Otherwise, you can cook them yourself - wash the beetroot, place in a pan of water, bring to the boil and cook for 40-45 minutes, or until tender, then peel. Freeze leftover brownies in a lidded container, wrapped in foil.

5:2 non fast days: To add a bit of crunch and extra protein, throw in 150g roughly chopped pecans with the chocolate. Serve after a meal with a handful of berries, a dollop of full-fat yoghurt or crème frache. (You can reheat in the microwave for a few seconds first, if you like.)

Serves 20. 128cal per serving.

A filling Moroccan-inspired casserole with lovely fibre-rich chickpeas. Don't be put off by the number of ingredients - once the chicken is browned it's an easy throw-it-in-the-oven number. Serve with a large portion of green beans or a generous leafy salad.

Ingredients

2 tbsp olive oil

1 medium onion, peeled and thinly sliced

3 boneless, skinless chicken thighs (around 300g), quartered

11/2 tsp ground cumin

11/2 tsp ground coriander

1/4 tsp ground cinnamon

1 red capsicum, deseeded and cut into roughly 3cm chunks

1 400g can chopped tomatoes

1 210g can chickpeas, drained (around 130g drained weight)

4 dried apricots (around 25g), roughly chopped

1 chicken stock cube

handful fresh coriander or parsley, leaves roughly hopped, to serve

Method

Step 1. Preheat the oven to 200C.

Step 2. Heat the oil in a medium flame-proof casserole over a medium heat. Add the onion and chicken and gently fry for 6-8 minutes, or until the onion is lightly browned, stirring regularly.

Step 3. Sprinkle with the spices and cook for a few seconds more, stirring.

Step 4. Add the capsicum, tomatoes, chickpeas, apricots and crumbled stock cube. Pour in 250ml water, season with sea salt and plenty of ground black pepper and bring to a simmer. Cover with a lid and cook in the oven for 45 minutes, or until the chicken is tender and the sauce has thickened.

Step 5. Sprinkle with coriander or parsley to serve.

Cook's tip: For a more fiery taste, add 1 tablespoon harissa paste with the tomatoes. For a meat-free version, omit the chicken, which is 163cals per serving, use a veggie stock cube and add 200g cubed butternut pumpkin instead.

5:2 non fast days: Increase the portion size and serve with 3 tablespoons of quinoa or bulgur wheat.

Serves 2. 447cal per serving.

This ratatouille is wonderfully versatile, delicious served warm or cold. It can be easily reheated in the microwave, so also makes a handy packed lunch. Serve with a large leafy salad.

Ingredients

2 capsicums (1 red and 1 yellow), deseeded and cut into roughly 2cm chunks

1 medium eggplant (around 250g), cut into roughly 2cm chunks

1 medium onion, peeled and cut into 12 wedges

3 tbsp olive oil

2 garlic cloves, peeled and crushed

handful fresh basil leaves (around 10g), thinly sliced, plus extra to serve

1 400g can chopped tomatoes with herbs

225g block halloumi cheese, cut into 8 slices

Method

Step 1. Preheat the oven to 220C.

Step 2. Place the capsicums, eggplant and onion in a bowl, add 2 tablespoons of the olive oil and season with sea salt and ground black pepper. Toss everything together well then scatter into a shallow baking dish and bake for 25 minutes.

Step 3. Remove the dish from the oven, turn the vegetables, then cook for a further 5-10 minutes, or until well softened and lightly browned.

Step 4. Remove the dish from the oven, stir in the garlic, basil and tomatoes, arrange the halloumi on top, drizzle with the remaining oil, season with more ground black pepper and return to the oven for a further 15 minutes, or until the halloumi is hot and lightly browned.

Step 5. Scatter with more fresh basil to serve.

Cook's tip: If you don't have canned tomatoes with added herbs, simply stir 1/2 teaspoon dried oregano through the tomatoes before adding to the vegetables.

5:2 non fast days: Add a 400g can of drained and rinsed cannellini beans to the ratatouille at the same time as the tomatoes. Drizzle the halloumi and vegetables generously with olive oil when you serve.

Serves 4. 321cal per serving.

Thanks to publishers Simon & Schuster we have two copies of The Fast 800 Cookbook to giveaway. Email food.wine@canberratimes.com.au by COB Wednesday June 26 and tell us why you deserve a copy.