Obesity in the UK has trebled over the past 20 years, and the government is in a quandary over it. A couple of months ago, the prime minister's strategic unit developed a proposal which was known as the "fat tax". The idea is to put a tax on fatty foods such as butter, full-fat milk, and cheese.

But people such as Dr Arthur Agatston, the cardiologist who developed the South Beach diet, believe that the obesity problems we see in the west are not due to consuming excess fats. "Putting a tax on those foods isn't going to decrease obesity," he says. "There has to be more to it than that."

Agatston, a Miami cardiologist and director of the Mount Sinai cardiac prevention centre, is in the UK to promote his new book, The South Beach Diet, Good Fats Good Carbs Guide (Rodale, £4.99). The 56-year-old New Yorker, is the perfect advert for his diet. He looks 10 years younger than his age.

The original South Beach diet book has sold more than 7m copies worldwide. Its message is that bad carbohydrates are the enemy. They cause surges in blood sugar levels, which lead to increased production of insulin, which in turn drives blood-sugar levels to subnormal levels, leading to hunger again soon afterwards. If Agatston were to put a tax on any food it would be refined carbohydrates, such as those found in white bread, bagels and pastries. I show Agatston a packet of crisps and a health bar that I had picked up at Boots on my way in. "This is British health food. What do you make of it?" I ask the good doctor. After a brief examination of the labels, Agatston starts to laugh. "That's about as bad as you can get," he says.

The South Beach diet is based largely on the glycaemic index (GI). In the early 1980s, Dr David Jenkins of the University of Toronto, introduced a concept called the glycaemic index, or GI. This measures how quickly food is digested and converted to energy in the form of glucose. The quicker food is digested, the sooner the person will be hungry again. The index runs from 0 to 100, and, generally speaking, the South Beach diet recommends adherents stick to foods with an index of 55 and below (All-Bran, sponge cake, baked beans and peanut M&Ms, all fall within this range). Foods in the 56-69 range are also tolerated, but only in moderation. Any food with a GI of 70 or above is best avoided. The "health foods" I showed Agatston were high GI - ie, mostly refined carbohydrates with a soupçon of artificial flavouring.

The Commons health committee on obesity recently lambasted the food industry for not doing enough to encourage people to eat healthily. Shortly afterwards, Tesco, Britain's biggest supermarket, announced it would be putting GI labels on food so that people could shop more healthily. Tesco plans to label 900 own-brand products as either medium or low GI. "You guys are ahead of us," says Agatston, clearly impressed.

Agatston believes that the US has developed into a lardy nation because the wrong message was sent out about carbohydrates. The American Heart Association championed a high-carbohydrate, low-fat diet. But Agatston believes there is an obesity epidemic now not because people ignored the message, but because they stuck to it religiously. Americans aren't greedy - they are just constantly hungry. All those high-GI pizzas, burger buns and chips are leaving them famished and fatigued. If you over-stimulate insulin production, you stifle the body's ability to use fat as fuel. Low GI foods are the answer. They release sugar into the bloodstream more slowly and stave off hunger for longer.

The British Dietetic Association (BDA) is not sure that basing a diet on the GI is the right answer. It says that it is a laboratory-based measure which doesn't translate easily into eating habits in reality. "They look at how your blood sugar rises and dips in comparison to a test food," says Ursula Arens, a BDA spokeswoman. "But as soon as you add protein or carbohydrate, those differences are much smaller than if you look at just pure carbohydrate. So, for example, if you have plain white bread, your blood sugar goes up and then it goes down. But as soon as you have it with scrambled eggs, or baked beans or anything else, that rise is slower and that dip is slower."

I could live on Snickers bars and sponge cake (both low GI foods) and lose weight, but that isn't a balanced diet by anyone's standards. Thankfully, the South Beach diet has two other important principles. One is good carbs vs bad, and the other is good fats vs bad. Where Atkins pooh-poohs all carbohydrates, the South Beach diet advocates healthy carbohydrates, such as wholegrains and rye bread (which have a lower GI than white bread). Where Atkins says to eat all the butter, you like, the South Beach says to avoid saturated fats as much as possible, and use monosaturated fats or oils rich in omega 3 fatty acids instead.

The diet is divided into three phases. Phase one involves going cold-turkey on sugar and the source of blood sugar surges (ie, bad carbs). Phase two allows the dieter to add back in some healthy carbohydrates and some fruit, and phase three is the maintenance phase, which goes on indefinitely. By phase three, you know which foods to eat and which to avoid. "It's the best diet for anybody to be on, whether you have to lose weight or not," says Agatston.

The maintenance phase is lifelong because the South Beach diet was developed as a healthy heart diet, not a diet to make you look good in a bikini (as the name might suggest). Agatston originally developed the diet to improve the blood chemistry of his patients. The fact that people lost weight (especially around the stomach) and kept it off, was a nice side-effect that led to a lucrative side-line in diet books.

Phase one bans fruit as many, especially tropical fruits, have a high glycaemic index. Diabetes UK says cutting out fruit is inadvisable as it contains vital vitamins and minerals which help to reduce the risk of heart disease and certain cancers. So isn't Agatston throwing the baby out with the bath-water? "The purpose of the first two weeks is to get rid of cravings," he says. "It's to stop the big swings in blood sugar and insulin levels. We encourage plenty of vegetables, and don't go into ketosis like Atkins. Fruit is very important; that's why we add it back in phase two."

He says that the obesity epidemic in the US was caused by the myth that all carbohydrates are good for you. But as we now know, refined carbs stimulate further hunger by producing excess insulin. However, Dr Toni Steer, of MRC Human Nutrition Research, Cambridge, says that Agatston has got it the wrong way round. "People become hyperinsulinemic [excess insulin in the blood] because they are overweight and they are overweight because they have eaten too many calories. Whether those calories come from fat, carbohydrate or protein is almost irrelevant."

She also says that to claim overeating has only one cause - hunger - is an over-simplification. There are a myriad causes for overeating, some of them complex and psychological. But Agatston says that much of the perceived psychological stresses that cause people to overeat are simply dips in blood-sugar levels. Even those dips out, and your temperament evens out too. "Also, when you're under stress," says Agatston, "the adrenalin exacerbates the swing in blood sugar as well." It's one nasty merry-go-round.

Poverty and stress often go hand in hand. A lot of obese people in Britain are on low incomes. It is cheaper to buy processed food than it is to buy organic produce. The people who would benefit from the South Beach diet might be put off by the ritzy sounding name. Before I read the book, I imagined lots of emphasis on balsamic vinegar, grilled swordfish, and extra-virgin olive oil. And there is an element of that, but the diet isn't as expensive as the name suggests, and it is easy to follow. There is no calorie counting and once you know the principles, it is easy to follow phase three.

One of the strengths of the South Beach diet is that it is constantly open to change. When new evidence comes to light, Agatston isn't afraid to say, "we got it wrong" and adjust the guidelines accordingly. Where he was once hostile to carrots - because the evidence suggested they have a high GI - he has changed his stance in the light of new evidence. Where coffee was once frowned upon because it caused a rush of insulin, the evidence is now being re-examined. A study, conducted in Finland, found that people who drink more coffee are less susceptible to diabetes. "But," says Agatston, "most of what has come out has reinforced the basic principles of the diet."

With research-based evidence to back it up and a raft of charismatic supporters, such as Sex and the City actor Kim Cattrall and Bill and Hillary Clinton, the South Beach diet may soon supplant the Atkins diet in the UK. As I pack away my tape recorder, I notice that there are a couple of delicious-looking biscuits next to the dregs of my coffee. My "health" crisps and bar are still on the table. Not having had a snack for a while, I ponder which to take - the antiseptic-looking health food or the buttery biscuits. "I'd go with those," says Agatston, pointing to the biscuits. No wonder the man's diet is a success.

· The South Beach Diet Cookbook will be published by Rodale this October, priced £20. The South Beach Diet is published by Headline, £10.99