Doctor Robert Atkins is telling me about a moment in history when the world changed for ever. 'In November 1963,' he says, 'two things happened. I gained an awful lot of weight from Thanksgiving, and John F. Kennedy was murdered.' This was the moment that Atkins, a chubby young doctor from Dayton, Ohio, decided to lose weight. 'I was just sitting there watching television,' he tells me, 'watching all this sad stuff. And I made up my mind. I felt I had to do something. So I went on a diet.' Historians were quick to grasp the importance of the Kennedy assassination. But it has taken the world four decades to understand the huge significance of the Atkins diet.

Now, Atkins is the most influential diet guru in the world. His first book, Dr Atkins Diet Revolution, was a hit in the early Seventies. But his more recent book, the very similar Dr Atkins New Diet Revolution, is much, much bigger. This is his second coming. He has been cited as one of the 10 most influential people in the world. He believes he knows how to save Western society from its devastating twin epidemics of obesity and diabetes. He thinks he has unlocked the mystery of why we put on weight, and why we find it so difficult to lose the weight we put on. The key, he believes, is insulin, a hormone secreted by the pancreas gland. He describes insulin as 'the fattening hormone'.

In terms of diet, his idea is very simple. If you cut down radically on carbohydrates, you'll lose weight, even if you eat steaks and chops and fried eggs - and, importantly, bacon. To Atkins, carbohydrates, particularly refined carbohydrates, are the core of the obesity problem. Calorie intake is comparatively unimportant. Fats are not so worrying. 'If you believe that weight loss requires self-deprivation,' he says, 'I'm going to insist on teaching you otherwise.'

And the Atkins diet seems to work. People who follow the Atkins plan - it is known as 'doing Atkins' - do lose weight. He has sold more than 10 million books, which have been read by perhaps 30 million people. This is much bigger than Audrey Eyton's F-Plan Diet, which advocated eating large amounts of fibre or Harvey and Marilyn Diamond's Food Combining Diet, followers of which tried not to eat proteins and carbohydrates at the same time.

Atkins estimates that tens of millions of people have lost weight by following his methods. Geri Halliwell and Julia Roberts have lost weight, and kept it off, on the Atkins diet. Victoria Adams does Atkins, and look at her. Before she did Atkins, new mother Davina McCall described her body as 'cellulite city'. Now she is trim. Minnie Driver, a follower of Atkins, looks sharp and angular these days. Catherine Zeta-Jones slimmed down for her performance in the musical Chicago on Atkins. Al Gore does Atkins. Fleetwood Mac's Stevie Nicks, who had blobbed out, got her figure back on the Atkins diet. 'He is a god among men,' she said.

Atkins is sitting in his office at the Atkins Centre. This is a man who goes to work in a big, chunky office block in midtown Manhattan with his name above the door. He is a trim, nice-looking 72-year-old wearing a tweedy jacket and spiffy tasselled loafers. You would never guess that he used to weigh 16 stones. In 1963, he says, 'I looked at a picture of myself and realised I had a triple chin'. He was 33. 'I was eating junk food. Nobody had ever told me junk food was bad for me. Four years of medical school, and four years of internship and residency, and I never thought anything was wrong with eating sweet rolls and doughnuts, and potatoes, and bread, and sweets.'

As a man with a big appetite, Atkins knew he would not last on a traditional low-calorie or low-fat diet. But he'd just read an article in the journal of the American Medical Association about a low-carbohydrate diet. He says, 'It was so simple! I hadn't tried a diet before that. It was the only diet that looked like I'd enjoy being on it. I ate a lot of meat, and a lot of shrimp, and a lot of duck, and a lot of fish. And omelettes in the morning, and salad vegetables.' The diet worked a treat. The pounds fell off rapidly, and, significantly, Atkins did not feel hungry. His cravings for buns and doughnuts had gone. He felt perky and energetic. He began to realise that something was happening, something which contradicted general medical opinion. He was eating a lot of calories, and a normal amount of fat, but he was losing weight. Why was this? He decided to try his diet on patients, to see if it worked on them, too.

Meanwhile, all over the Western world, overweight people were being told to eat less fat, and to replace the fat in their diet with 'healthy' alternatives - complex carbohydrates such as pasta, potatoes, and rice. The trouble was, they were getting fatter. Even when they cut their caloric intake, they still got fatter. In the mid 1970s, fat made up 40 per cent of the typical Western diet. Twenty years later, that figure had dropped to 32 per cent. Health-conscious people were replacing the fried foods in their diet with cereals. And they were putting on more and more weight. The more people tried to avoid fat, the fatter they got.

Back in the mid-Sixties, Atkins put 65 overweight patients on an early version of the Atkins low-carbohydrate diet. All 65 reached their target weight. Atkins looks across his desk at me and says, 'I knew I had something wonderful'.

But how does it work? How can you lose weight without cutting calories? How can you eat fat and not get fat? How can you switch to a diet which is less bulky, and yet feel less hungry?

Atkins shifts in his seat. He speaks in a slow, measured way, but you can tell he is burning with zeal. As a diet guru, he was in the wilderness for decades, but he feels that now, at last, the world is listening. 'You have to understand the role that stored fat plays in our bodies,' he explains. 'It is the number two fuel system for energy. And there are only two fuel systems. The number one system is glucose, which is made from carbohydrate. When you cut out carbohydrates, you cut out glucose, and then fat plays the role of the primary system.'

According to Atkins, then, if we eat fat, but no carbohydrates, our bodies burn fat, but if we eat carbohydrates, our bodies find it more difficult to burn fat. Cutting out carbs, says Atkins, gives you what he calls a 'metabolic advantage'. 'You actually lose more weight, calorie for calorie, than you do on balanced diets, or low fat diets,' he says. I can see the excitement mounting in him. You lose fat because of a process called ketosis - the transformation of fat into fuel. 'And that's been proven by study after study,' he says. 'All of a sudden, we've reached an era where people are doing these studies, even people who felt that my diet was wrong, and finding out that it gets better results than the diet they previously recommended.'

Why does he think carbohydrates are so bad for us? This is where insulin comes in. Carbs, says Atkins, tamper with the insulin in our bodies. Refined carbs, those with a high 'glycaemic index' - those that produce a lot of glucose - are worst. When we eat carbs, the glucose released into our bloodstream raises our blood-sugar, and sometimes gives us a blood-sugar rush. That's the feeling you get in the middle of the morning when you eat a doughnut; your mid-morning torpor is suddenly gone. You feel boosted. When glucose enters the blood, the pancreas releases insulin, the hormone which turns glucose into energy, if you are physically active, and fat, if you are not.

But it's easy to eat too much carbohydrate, and, when this happens over time, the pancreas begins to overproduce insulin, a condition known as 'hyperinsulinism.' Eating a doughnut will give you a blood-sugar rush, but, if you eat doughnuts for a few weeks, this rush will be followed rapidly by a blood-sugar crash, because the large amount of insulin in your blood is metabolising the glucose very rapidly. And what do you want when your blood-sugar crashes? Another doughnut, of course. Low blood sugar produces cravings. When you see an obese person, this is likely to be the reason for their problem. 'Food compulsion,' says Atkins, 'isn't a character disorder; it's a chemical disorder.' The poisonous combination of carbs and too much insulin makes us fat, and, horribly, the food we crave makes us hungry. It's a vicious cycle.

Of course, all of this must have implications for diabetes. In the last four decades, when Western society reduced its fat intake, and increased its carb intake, diabetes, as well as obesity, has sharply increased. 'We eat our way towards it, three meals a day, a thousand meals a year, ten thousand meals a decade,' says Atkins. Over-consumption of carbs, which leads to over-production of insulin leads in turn, he believes, to a condition known as 'insulin resistance' - with so much insulin aboard, our bodies become desensitised to it.

So, in the worst-case scenario, eating carbs makes you want to eat, which makes you fat, which makes you tired, which makes you want to eat even more carbs; you're hungry all the time, and eventually you significantly upset the balance of your blood-sugar. Atkins believes firmly in the link between diabetes and obesity. 'So much so,' he says, his eyes glinting, 'that I call it "di-obesity". And that'll be the title of my next book.'

Atkins, at 72 - or, as he puts it, '72 and a quarter', shows no sign of retiring. 'Oh, I love working,' he says. He's the son of a restaurateur from Dayton; his father, he says, was an incredibly well-liked figure. 'He was just an extremely likeable person that everybody fell in love with.' Atkins tells me he wishes he could be like his father. 'I will never be as nice as him,' he says. 'I wish I were. I try. But I'm not nearly as nice as my dad was.' Atkins' parents retired to Florida, where they 'did Atkins'; his father died at 84, and his mother is still alive at 93. Does his mother still do Atkins? 'My mom cheats too much,' says Atkins. 'But she tries.'

He says he loves helping patients. 'They're so grateful,' he says. 'It's very gratifying. They come back and they say, "Thank you, doctor". It's incredible.' He could, of course, have retired years ago - when I point out that the sale of millions of books must have made him very rich, he says that the lion's share of the money comes from the retail arm of his business - he sells 'Atkins nutritionals', such as food supplements and low-carb bars. He lives with his wife in an apartment along the road from his office, and owns a house in the Hamptons. He has no children, and got married at 56; having been a chubby doctor, he became a slim, attractive doctor with lots of girlfriends.

In the beginning, he just wanted to lose weight. 'I didn't realise I was going to be fighting the whole world,' he says. He is aware that many people doubt him, and he wants to know what his critics are saying in the UK. He's very big on his critics. He finds it upsetting that people should set themselves against him so staunchly. 'What I wanted to find out,' he says, 'is what exactly they're saying. Because there's nothing to back up what they're saying. Everything that we say has back up. There aren't any exceptions. And I'd like to know - what have they said?'

What do Atkins' critics say? They say that ketosis is a bad thing. Atkins says that's because they are confusing it with ketoacidosis, a different condition altogether. They say the Atkins plan is low on roughage. Atkins says this is not true - if you replace potatoes with broccoli or salad or cabbage, you're going to gain it. Also, Atkins recommends psyllium husk, a soluble fibre, to be drunk with large amounts of water if you feel you're not getting enough fibre. They say that too much protein is bad for the kidneys. Only if you already have a kidney condition, says Atkins. They say you don't get enough calcium if you don't drink milk. He says you'll get enough calcium from cheese, which, of course, is milk and green vegetables such as broccoli.

Another thing is that Atkins comes in several phases, the most important of which are 'induction', which is supposed to last for two weeks, during which you cut out almost all carbohydrates, and 'maintenance', which is much less spartan. People who rail against Atkins, says Atkins, are often railing against induction.

Atkins, of course, has been doing Atkins for longer than anybody - he's been doing it for over 39 years. He tells me what he had for breakfast - 'Ham and eggs. And onions. Very dark fried onions.' He has long since lost his carb-induced food cravings. It's getting dark outside, and he hasn't had lunch yet. 'I sometimes just eat some macadamia nuts or something like that, when I'm working. Just something to tide me over until dinner.' He's looking forward to dinner. 'Tonight it's going to be some fish and about three or four vegetables, plus a salad. My wife gives me about five or six different vegetables in that one meal.'

I decided to do Atkins for a while. Two years ago, I'd tried a diet suggested by the French diet guru Michel Montignac, which is very similar - you cut down on carbs, particularly refined ones, and eat protein and vegetables in normal quantities. Both times the diets worked. The striking thing, at first, is not that you immediately lose weight, but that you feel less tired and hungry; after a while, the food you eat seems to fill you up more. You get used to looking at the breadbasket, or the bowl of mash, and turning away from it, knowing that the slump you will feel in an hour is not worth the rush you will feel in five minutes. The pounds begin to drop off.

Is Dr Atkins a quack? Possibly not. For instance, his diet definitely works. Lots of people stay on it because it cuts out food cravings, rather than producing them. Now he's paying for research to be done to back up his theories. 'Nowadays,' he writes in his recent book Dr Atkins New Diet Revolution, 'the tide is flowing strongly in my direction.' The New York Times recently published an article in favour of Atkins, in which Richard Veech, a prominent scientist who studied under the Nobel laureate Hans Krebs, was quoted as saying, 'ketosis is a normal physiological state. I would argue it's the normal state of man'.

Atkins and I walk down the stairs and into the lobby of the Atkins Centre. There is a cartoon on the reception desk which depicts Santa, having come down the chimney, looking at a plate of food which has been left out for him. The plate contains a steak but no chips. The caption is, '******* Atkins diet'.

He shakes my hand and walks down the street, towards his wife, towards the fish and possibly six vegetables she is preparing for him. His new book, Atkins for Life (out here next month) is just hitting the shops. 'Di-obesity' will be next. Of course, he might be unmasked as a fraud. For one thing, a new generation of diet gurus will want to find flaws in his thinking. But for now, things are looking up for Atkins. The world is listening. If I were a potato farmer, I might just start to be worried.

· To order Dr Atkins New Diet Revolution for £7.99 wtih free UK p&p, or his new book, published on 7 March, Atkins for Life for £8.99 plus p&p (rrp £10.99), call the Observer book service on 0870 066 7989

The doctor's prescription

So how does it work?

According to Atkins, Westerners are overweight, not because we eat too much fat, but because we eat too many carbohydrates. Too much starchy food like potatoes, pasta, rice and bread, which contain refined carbohydrates, and sugar, honey, milk and fruit, which contain simple ones can lead to hyperinsulinism - a condition in which too much insulin is created by the body when it struggles to convert glucose to fat. Hyperinsulinism makes us hungry, leading to a vicious circle. So if we stop our bodies producing too much insulin, we start burning fat instead. This state is known as ketosis.

So what am I allowed to eat?

Atkins starts by asking you to picture, in your mind, a table laden with food, including 'a lobster in drawn butter, well-seasoned fish, turkey and duck and certainly a juicy steak'. These are the kinds of foods you can eat on the Atkins diet.

For the first two weeks - the Induction stage - a typical daily menu would be omelette with tomato, avocado and ham for breakfast, a caesar salad with grilled chicken for lunch, and for dinner, steak au poivre served with roasted asparagus and a mixed green salad, followed by chocolate truffles. The On-going Weight Loss stage allows you add 5 grams of carbs every day, so you can have poached eggs and bacon for breakfast, turkey burgers for lunch and pork chops for dinner. Adding 10grams of carbs during the Pre-Maintenance stage means Belgian waffles with maple syrup for breakfast, ham and cheese tortillas for lunch and roasted salmon for dinner. Finally, the lifetime Maintenance stage allows you to eat chocolate cakes, grilled chicken with wild rice, and ricotta omelettes.

Who has been on the diet?

Who hasn't? Geri Halliwell, Brad and Jen, and Matthew Perry swear by it, as do Julia Roberts, Minnie Driver, Sir Bob Geldof, and Sarah Jessica Parker. Catherine Zeta-Jones lost over two stones on it, and Renée Zellweger famously used it to slim down after she piled on the pounds to star in Bridget Jones's Diary.

Why do they love it?

Because it provides rapid weight loss and you can eat the things other diets forbid, like steak, sausages, bacon, cream, butter, and peanuts.

What do the critics say?

That it's high in fat and protein, and low in health-boosting antioxidants. (Many vegetables including leeks, squash and sweetcorn are carbohydrate-rich, so their intake is limited.) Experts warn that this combination can cause health problems in the long term. High-protein diets make the body acidic, and calcium is released from bones to counteract this, so brittle bones could be a result in old age. Some scientists say that deliberately inducing ketosis can lead to muscle breakdown, dehydration, headaches, nausea, and kidney problems. Oh, and of course there's bad breath.

Catrin Rogers