I'm not a serial dieter, or any kind of dieter, but I am interested in what we eat, how we eat, and how it affects our health. Sometimes I think, with mounting impatience, that it's all so bloody obvious. Michael Pollen has it pretty much right in his book In Defence of Food: "Eat food, not too much, mostly plants." Five a day is the government's rather patronising version of that – and woefully fails to emphasise how important it is that most of those plants are freshly cooked in their whole, natural form (or better still, eaten raw).

I can't claim too much high ground here. At the turn of the year, like so many, I consumed way too much meat, cheese, cream, sugar and alcohol. And despite a garden bursting with brussels sprouts, kale and winter salads, and a weekly delivery of organic apples, oranges, clementines and bananas, I know I didn't eat nearly enough fruit and veg to offset the gluttony. And so I've been patting a tummy I didn't have a few weeks ago, and wondering what to do about it.

But now I find myself beguiled, for the first time ever, really, by a new diet. The Fast Diet, by Michael Mosely and Mimi Spencer, makes a compelling promise that with regular fasting (they propose two days out of every seven) you will quickly lose weight, while on non-fast days you can continue to eat (and, importantly, drink) whatever you like.

This isn't the first time I've tried to shed a few pounds. Previous, successful attempts have not been guided by books, or any form of calorie-counting, but by a self-imposed period of abstinence from the two things I know put pounds on me – alcohol and dairy products. They are also two things I am not planning to abstain from in the longer term – so it isn't really a sustainable approach to keeping trim.

But The Fast Diet says I can continue to butter my bread, cheese my butter, and raise my glass – at least for five days a week. It also promises much more than mere weight loss. It will reduce my bad cholestrol, protect me against cancer and even sharpen my mind. It pretty much promises that I will live longer, and healthier. As my half century approaches, that's quite a punchy proposition. So let's take it seriously for a moment.

The reason we're in such a mess with food is evolution, progress. We no longer fight for our meals. We don't even need to burn a few calories acquiring the next one. The high-energy foods that were once such a prize that we'd risk life and limb for them are now constantly within reach. We can mainline sugar and fat effortlessly. So any diet that claims to offer a solution to our crisis needs to make evolutionary sense. If I was going to write a diet book (I'm not), I would call it The Bonobo Diet, and it would recommend that we all sit in extended family groups, for a couple of hours every day, eating a large pile of something good: apples one day, nuts the next, perhaps roast chicken the day after that. It wouldn't catch on, would it? But I think it would be sound.

Many of the problems we give ourselves through the modern diet stem from the fundamentally flawed habit of piling a lot of different, unrelated ingredients on to a plate and scoffing them at high speed. Meat, wheat and cheese (the burger) – clearly foolhardy. Chocolate, sugar, butter, flour (the brownie) – delicious, but insane. These compound meals, too often glued together with synthesised products our bodies don't even recognise as food, curdle and rot in our stomachs, giving us varying degrees of nausea, acid reflux, gaseousness and cramp, and pushing our stressed digestive systems to the absolute limit. No wonder we're bloating like dead whales and dropping like flies.

Of course, I realise that all my books and television shows are largely complicit with this disastrous approach to eating. I like to think I am at the healthier, more natural end of the spectrum – in fact, my professional self-respect is predicated on that. But I think it would be unwise not to acknowledge that even the "River Cottage diet" – rich as it ought to be in fresh vegetables and fruit – is open to abuse. And I should know; I abuse it often enough. The fact is that even those of us who know exactly what a sensible, restrained and healthy diet looks like still struggle to keep to one much of the time. Hardly anyone on the planet eats for optimal health. The industrialisation of food is now universal, and even the supposedly healthier culinary cultures are losing their way. Did you know that Italy now has the biggest obesity problem in Europe, or that China farms more pork intensively than the rest of the world put together? The bad eating habits we've acquired are species-wide, and they're not about to go away.

This is what's fascinating about fasting. It seems to offer a possible way out of this tragic culinary cul de sac – for human health, at least, if not for global food production. Starve yourself once in a while, as our antecedents did for millions of years, by force of circumstance, and your body and digestive system go into recover and repair mode, giving rise to a whole host of physical benefits.

Could this diet, and the knowledge that underpins it, be harnessed to make a genuine impact on global health and the obesity epidemic? In principle, the answer would seem to be yes. (Though it wouldn't be popular with the supermarkets, would it? Imagine if we all started shopping for a five-day eating week. That'd be more than 25% of Tesco's turnover down the pan.)

But it's just one option. We know there are others. So what's easier, ultimately? Persuading, educating or forcing our citizens to cut down on fatty, sugary, processed foods and to eat loads more fresh fruit and veg? Or getting them to eat nothing at all for a couple of days a week? The authors of The Fast Diet are betting on the latter, of course. And my initial hunch would be with them.

But one aspect of the book rather undermines its conviction. The clearest benefits are said to come from fasting for considerable, and quite challenging, lengths of time – two, three or four days even, with no calories at all (but plenty of water, vitally). They are quick to acknowledge (and Mosely knows from his own self-guinea-pigging) that this just isn't practical.

So they've formulated the approach of two non-consecutive days out of seven, where a reduced calorific intake of 500 calories (women) or 600 (men), taken as two meals, gets you through your fast period. So a fast day would allow me a 250-calorie breakfast and a 350-calorie supper, giving me, effectively, 36 hours divided into three periods of 12 hours without food, twice a week. The authors say these shorter periods are still effective periods of fasting, and I've no reason to disbelieve them. So far so good.

But then they have a wobble. Having made a really convincing case for regular food-free periods of at least 12 hours or more, Spencer tells us it's OK to cheat a bit. When the hunger pangs strike, as they will intermittently through your fast day, it's apparently allowed to nibble a few carrot sticks, or munch an apple. Sorry, Mimi, I'm not convinced. Surely any snacking at all is going to set off gastric processes that will undermine to some degree the proven benefits of fasting? And suddenly this is in danger of becoming a book that advises you not to fast two days a week, but to diet two days a week. And that's a much less radical idea, with far less potential to reach out and change the world.

My worry is that the authors' wobble, unknowingly, gets to the heart of the problem, of this or any radical idea about how we improve our health, through diet, exercise or any other known means. They all involve self-discipline, and fasting is no exception. So is fasting a more realistic, more manageable discipline? Are we up to it? Can we see it all the way through to reap the long-term benefits – as individuals, as a society, as humanity?

As I write, I am on my sixth day of fasting since I started on New Year's Day. I feel lean and sharp, and although I've had some pangs of hunger through the day, a few cups of black tea and rooibos have kept them at bay. I'm just an hour away from my light supper of bream fillet baked with chilli, ginger and soy, a spoonful of rice and a large leafy salad, and I'm certainly looking forward to it.

So I believe in this fasting thing, I really do. With my strictly non-snacking version, I've lost eight pounds already, and I find the whole thing rather exhilarating. I feel I might just be part of a health revolution. But is it really sustainable, for me or for significant numbers of others?

Can I honestly say I'm backing myself to be fasting regularly a year from now? I very much want the answer to be yes. Then I look at the rowing machine propped up in the corner of the kids' playroom. This time last year I was pounding it for 8,000 metres twice a week. It didn't last past the first chocolate egg.

Dr Luisa Dillner on The Fast Diet

"I'm on a diet," says a friend tucking into treacle tart and cream. It can only be the 5:2 – or Fast Diet. Made popular by a BBC Horizon programme, Eat, Fast and Live Longer, it promises not only weight loss but a reduced risk of cancer and heart disease. While a normal calorie intake is 2,000 for women and 2,500 for men, the 5:2 diet advocates eating normally for five days, but restricting calories to almost fasting levels of 500 for women and 600 for men on the other two days. It is not recommended for people with diabetes, pregnant women or those with a history of eating disorders (there's no evidence it triggers anorexia, but fasting can be addictive). Supporters say it's easier to stick to than daily calorie-counting.

Evidence supporting the health benefits of fasting is only strong in studies on rats, looking at fasting every other day. There are limited studies on humans suggesting that regular fasting or calorie restriction, even when overall calories per week are not reduced, improves health. A Spanish study in 1957 of 120 people over 65 found those eating a normal (or slightly more calories than normal) diet on one day, alternating with 900 the next, had almost half the number of days in hospital than those on the regular diet. The number of deaths in the fasting group was halved, but numbers were small (13 versus six in the fasting group).

A review in the American Journal of Clinical Nutrition found evidence for animals reducing their risk of diabetes and lowering cholesterol and blood pressure by fasting every other day, but merely an increase in good kinds of cholesterol in humans. It reduces weight in humans – 2.5kg in three weeks.

The most relevant evidence is a randomised study of regular calorie restriction versus intermittent fasting (5:2 with 75% fewer calories on fasting days) in 107 overweight women. It found that both groups lost the same amount of weight after six months (on average 6kg) – just under a third lost between 5% and 10% of their body weight. Both groups saw reductions in risk markers for diabetes, cardiovascular disease and cancer. The 5:2 wasn't any easier to stick to than usual calorie-restriction diets. At the end of the study, only 58% of the 5:2 group said they'd continue, compared with 85% on the regular restricted diet. The women on the 5:2 diet were also more likely to feel tired, cold and have headaches.

Trying a 5:2 diet may work – there's not enough research to tell us. If you don't overfast, then it's likely to be safe, if you're sensible. The two fasting days shouldn't be back-to-back, and you shouldn't run a marathon on one of them. You should also be aware that your stomach is likely to rumble loudly.