Oh, Mary's doing that," says my husband. I've just told him that I'm writing a piece about intermittent fasting (IF). My husband couldn't be less interested in diets. And I'm surprised to discover that Mary is "doing" IF. She's built like a pencil. "She told me all about it on the train," continues husband. "I think I might give it a go."
"Giving it a go" involves an act of extreme, almost poetic simplicity: a dramatic calorie slash two days each week. That's it. There's no bible to follow, nothing to buy, no bars or shakes. For the moment, this is the Diet With No Name. Some call it intermittent fasting, others alternate day fasting (ADF) or the 5:2 – but they are all riffs on the same premise: twice a week, you eat little more than an egg, two satsumas, three oatcakes and a carrot.
The rush of interest in IF began after a programme called Eat, Fast and Live Longer was broadcast. Dr Michael Mosley followed the method for several months, losing 14lb and 25pc of his body fat in the process.
Mosley's programme stressed the regime's health-giving benefits – how, given base-level good health, a decent set of genes and careful supervision, it could substantially lower a catalogue of risk factors for cardiovascular disease, diabetes, stroke, several types of cancer, Alzheimer's. . .
In short, intermittent fasting could inhibit the ageing process. Of course, what many viewers heard loudest was weight loss. And all you have to do is fast. A bit. Not for ever, not daily, not even completely.
Scientists have known since the 1930s that there is a link between restricted calorie intake and longevity. But recent research by Professor Valter Longo at the University of Southern California has revealed that occasional calorie restriction has a similar effect: you get the benefits without the purgatory.
A study led by research dietician Dr Michelle Harvie found that women on a restricted diet (650 calories) for two days a week can lower their risk of breast cancer by up to 40pc. The key to weight loss, says Harvie, is compliance: "The two-day diet we devised could be a life-saver for women who find it difficult to restrict what they eat every day.
"What we don't know yet," says Harvie, "are the longterm effects."
What is increasingly widely accepted, though, is that short-term fasting can benefit the brain.
Mark Mattson of John Hopkins University in Baltimore says: "When the brain goes under energy restriction we see neural activity that's associated with protection against degeneration from stroke and ageing."
If there's a poster boy for intermittent fasting, it's Canadian Brad Pilon – At 5ft 10in, 12st 8lb and a mere 9pc body fat, he has been an IF evangelist for years. Armed with a masters in nutritional science, he published Eat Stop Eat, outlining the method, in 2006.
Pilon boycotts all calories during his biweekly, 24-hour fasts; these typically run from 2pm to 2pm.
"The idea is to learn to take a total break from eating."
But how difficult is it to do? Rather than Pilon's total calorie annihilation, I go for the cheat's 5:2 version, which allows women 500 calories on two non-consecutive fast days each week (it's 600 for men).
On my first fast day, I weigh 9st 6lb (60kg) and my BMI is an OK 21.4; my body fat, though in the "normal" range, seems enormous: 30pc.
It's clear that 500 calories looks pitiful if you gather them together in one place: a mug of lentil soup, a plum, half a chicken breast, seven blueberrries and a breadstick.
Usual portions are way out; to nudge under 500 calories, you're looking at a quarter of a small avocado, a 3oz steak (around a third of an average serving), eight almonds, a bowl of carrot soup. No cheeky glass of vino with your salmon salad, no crusts from the kids' tea, no pavlova.
Instead, my husband and I share an apple for lunch. I've already had 30g of Bran Flakes for breakfast. (166 calories. When a cereal box says "a 30g serving", measure it. Go on. Be amazed. It's not enough to fill a child's cupped hand.) And there's chicken salad for supper – a no-skin, white-meat, one-slice chicken salad to book-end my day. There will be some sprouts, a radish and a cherry tomato, perhaps a handful of leaves, a shaving of raw cabbage. And no pavlova. Until tomorrow.
After a week of 5:2, it's clear that this is really not fasting, at least not as we know it. It's do-able. And the effects are immediate. I lose 3lb in a week.
By week three, the novelty has worn off. We're a bit tetchy when it comes to the applesharing. Pilon's advice is to stay busy. But over the course of a day, I don't feel particularly hungry. There are occasional spikes. But mostly the hunger is a mere background hum.
Besides there's no need to panic. The next day, just hours away, you can feed. And you really can eat whatever you please on the "off-duty" days. Three weeks in, and I've lost 5lb 8oz. My BMI is a sparky 20.4, and my body fat 23 per cent. I've had to buy new jeans. And new bras.
I'm not sure how my brain is faring, but I knocked off the crossword this morning while the kettle boiled.
But is this anything more than the elimination of excess fat? All of this is food for thought, and for further research. Until then, occasional fasters should proceed with care. There may be headaches, dizziness, fatigue, dehydration.
I haven't fainted yet, but I've scaled back my fasting to one day a week. Michael Mosley has too. "I've plateaued a bit," he tells me.
So does he still believe that intermittent fasting is a radical game-changer, a revolution for the world?
"Only time will tell if this is a fad or something more meaningful," he says. Right now, the jury's out. But smaller jeans? Who's going to argue with that?