January, and intermittent fasting is all the rage. But it's about to get even more radical

Eating is now a numbers game. Depending on who you listen to, you should fast for two days a week, on alternative days, or even over five consecutive days every two months, if you want to keep the weight off. But as the diets get more radical, the perfect ratio of healthy eating is proving more elusive to pin down.

It all began with the 5:2 diet, the intermittent fasting eating plan championed by everyone from Beyoncé to Alex Salmond. It struck a chord in its simplicity. For two days a week you eat only 500 calories (600 for men), and the rest of the week you eat normally.

The Fast Diet, the book which advocated the fasting plan – written by Dr Michael Mosley and Mimi Spencer – reportedly ended last year as Amazon's best-selling book. An estimated four in 10 British slimmers tried the diet.

But now, nutritionists are upping the ante. Dr Krista Varady, one of the women whose clinical trials helped to start the 5:2 fasting plan, has published a new book lauding the Every Other Day diet, or the 4:3 diet – where you fast on alternate days.

Scientists at the University of Southern California have started a clinical trial to see how eating very little for five days in cycles can affect weight and even how it could be used to manage blood pressure.

The British Nutrition Foundation is hosting a symposium on the evidence behind popular diets this week, with a whole section dedicated to "intermittent fasting". Men seem keener on the fasting plan than, say, the low-carbohydrateAtkins diet. David Ryan, 54, a journalist who has never tried a structured diet before, said he plans to start the 5:2. "It's the carrot-and-stick approach of being able to resume normal eating after each day of fasting that's appealing," he said. "It's more attractive than constantly being on a diet."

Champions of the diet claim that it can bring health benefits, including longer life expectancy, increased cognitive function and protection against conditions such as dementia. Clinical trials have suggested a reduction in the risk of developing certain obesity-related diseases, such as breast cancer. However, the NHS describes the evidence for 5:2 as "limited" and the diet as "a fairly radical approach to weight loss".

But is it just another way to encourage structured dieters – usually women – to obsess about their weight? Nutritional therapist Deborah Colson, director of the British Association for Applied Nutrition and Nutritional Therapy, is not so sure. She said the basic principle of the restriction of calories was "clearly sound". She added: "When we were hunter gatherers, we didn't have a constant supply of food. When there is a lack of food, we become sharper, with more energy to find food.

"It's not all about calories; we have to make sure what we do eat counts in nutrients. Could it be that we could [just] eat less every single day?"