I'm cooking lunch for Michael Mosley, the medical journalist whose Horizon programme launched the 5:2 diet, which became a bestselling book – and changed my life.
Melodramatic, yes, but true. In the past five months on the 5:2 diet I have lost a stone and a half, reducing my body mass index from a lumbering 27 to a healthy, if admittedly hardly Kate Moss-like 23. Already I can fit into clothes I bought years ago, no longer feel self-loathing about occasionally ordering cake with my coffee and have a positively Tiggerish spring in my step.
I'm not the only one to have jumped on the 5:2 wagon. The diet book has been reprinted 13 times, selling more than 340,000 copies and e-books and topping the Amazon charts for weeks.
Mosley himself arrives promptly for lunch. He is youthful-looking for his 56 years and, encouragingly, slim but not scrawny.
He is very much Mosley of the Beeb: fast, clear, incisive and also curious, as he quizzes me about my experiences on the diet – an interest I find so flattering I have to remind myself I'm the one meant to be doing the interviewing.
He makes frequent references to expert advocates of various forms of "intermittent fasting" – experts such as Dr Krista Varady in the United States and Dr Michelle Harvie in Britain, who have done the serious scientific research, while he has merely collated the information and presented it in a way he feels is accessible.
And – crucially – he has tried it himself. Self-experimentation is a theme of much of his work. He has gone to some bizarre lengths in the name of research – swallowing a camera for the programme Inside the Human Body, which turns out to have been even more gruesome in reality than those riveting inner-space shots suggest. "The really unpleasant part was that the night before I had to drink four litres of laxative. I was meant to be going to dinner with the director general of the BBC and the gastroenterologist said, 'Not a good idea'."
Mosley's interest in intermittent fasting is highly personal. His father died aged 73 of complications related to type 2 diabetes, a disease inextricably linked with being overweight.
"He's been both a role model but also a sort of threat going forward, because I see myself in him," Mosley remarks ruefully.
So when he was diagnosed as pre-diabetic last June, he tried out various intermittent fasting diets, having heard about their potential beneficial effects on insulin levels. Not that these are the only positive aspects being investigated. Harvie and Varady have focused on the cancer preventive benefits of fasting, and other researchers are looking into how it may slow Alzheimer's. There is, however, no conclusive proof yet.
Lunch is ready, and as it's a fast day, I dish up shiitake mushroom, lemon grass and ginger broth, laced with low-carb Japanese shirataki noodles, one of the ingredients that have kept me on the straight and narrow over the past five months of dieting.
Mosley is delighted, or at least very polite, about being served up fewer than 200 calories. But will it hit the mark nutritionally? I worry about satiating hunger on fast days, and bulk up with loads of veg rather than worrying about vitamins. Mosley, however, disagrees.
"I think nutritional content is absolutely key. If you are going to eat less food it should be as good as possible. You need high protein because while you store carbohydrate and fat, your body doesn't store protein."
My loads-of-veg policy is fine, too, as it has lots of fibre – as long as I have some high-quality protein such as chicken or fish, too. I quickly add some poached chicken to our bowls. Mosley looks approving. "Fibre and protein, those are the things that fill you up. But it turns out that those are the things that are pretty good for you as well."
Rather than working out BMI, Mosley recommends measuring your waist around the belly button, now considered by some health professionals to be a better indicator of a healthy weight.
"Your waist measurement should be no more than half your height. Men tend to go by their trouser size, which is wrong – it's generally smaller than your actual waist."
To keep his svelte figure, favourite dishes on fast days include mushroom and spinach frittata and marinated steak and Asian cabbage salad, both recipes from his literary collaborator Mimi Spencer's recipe book.
He has, he says, learnt to love vegetables. "I like using the griddle for things like courgette. Touches of lemon and orange juice jazz up salad leaves."
The family have had to be on his side in the battle to lose weight. "We don't have cookies or crisps in the house. Otherwise I find 11 o'clock at night I'm looking around for a biscuit.
"I've told my wife that if she ever has any chocolate in the house, she has to hide it or I will eat it."
After black coffee – Mosley, once a latte drinker, has taught himself to like it without milk – he has to go back to the studio. There are tweaks to be made to his new Horizon programme, on meditation and mindfulness. "I've had a go at the body, now I'm having a go at my brain," he says.
I'll be watching.
The fast show
Choose any two days a week to stick to 500 calories for women or 600 for men.
The 2-Day Diet restricts what you eat on two consecutive days. Eat a healthy Mediterranean diet the rest of the week.
Alternate Day Fasting (ADF): details vary, but the higher number of fast days is offset by looser restrictions on non-fast days.
Skinny Weeks and Weekend Feasts by Chef Gizzi Erskine suggests a gentler five-day regime with tempting recipes.
The Fast Diet by Dr Michael Mosley