Christmas can have an indelible effect on our soul... and another, which is difficult to get rid of, on our body. “At this time of year people put on about three kilos, on average. Some,if they have a tendency to put on weight anyway, can end up six kilos heavier,” says Dr Montse Folch, a Catalan nutritionist who became famous for changing Rocíito's shape with the 'artichoke diet', which will go down in history.

A long time has passed since then and now, after publishing a book called 'La dieta de los colores' (The colours diet), this famous dietician has launched another bloodless but exhausting battle against those extra kilos: a liquid-only fast.

Drinking juices isn't really anything new. Angelina Jolie has revealed that she does that two days a week and Beyoncé made the 'lemonade diet' world famous. Isabel Preysler, queen of hearts and the empress of slimness, used to purge her gastronomic excesses by ingesting nothing but pure juices and infusions for one day a week, until she had to stop because it was giving her headaches.

Dr Folch says a headache is not uncommon when people start strict diets. “They are not getting any carbohydrates or sugars, so the body can enter into ketosis and the liver releases ketones, which cause the headache” she explains.

That's why this specialist never tires of insisting that anyone who plans to fast should have a blood test first and be monitored by a specialist throughout.

Folch has a project to improve people's bodies and minds in 2018. In collaboration with fellow nutritionist Anna R. Alós, a yoga teacher, a reiki teacher and an expert on mindfulness, she plans to teach people how to fast healthily while staying in rural hotels in Girona and Ibiza. That will be the starting point. The overall objective is to learn to eat healthily. Her slogan is 'Goodbye diets, hello habits', and her favourite mantra: “We aren't what we eat. We are what we assimilate.”

These fasts are not based on lemonade and turmeric, like Beyonce's. They include fruit and vegetable juices, and vegetable protein drinks. They are also accompanied by meditation and gentle controlled exercise. “We always have to compensate with something sweet at midday to prevent a drop in sugar mid-afternoon. It could be a little honey, for example. And of course, during the process, if someone needs to eat something solid, they can chew a carrot,” she explains.

The carrot comment may seem like a post-Christmas joke but it is a fact that, after so much over-indulging on food, the body resents it. The fat from lamb or stuffed turkey, turrón, cakes and cava (not to mention prawns with mayonnaise) are an explosive cocktail for our bodies. “At this time of year we are overwhelmed with saturated fats, sugars and salt,” says Montse Folch, “and our bodies need to be cleansed in some way. Not eating solids means you don't chew and it helps the stomach function. It's a way of freeing the body from saturated fats which come from animal protein.”

Many celebrities have experimented with restrictive diets for limited periods. As well as Angelina Jolie, actor Hugh Jackman has sometimes practised the '5:2 diet' which consists of eating what you like for five days a week and only ingesting a maximum of 600 calories on the other two days.

“I don't see anything wrong with it for a healthy person, as long as it is controlled,” says Dr Folch. “Eating 500 calories a day for two days isn't disproportionate.”

In their perpetual search for the perfect shape, celebrities easily succumb to fashionable diets. Gwyneth Paltrow did some time ago with macrobiotics, like Julia Roberts. There are orthorexics like Arnold Schwarzenegger, vegans like Drew Barrimore, ayurvedics like Miguel Bosé or raw food fans like Demi Moore...

“All the fashionable diets can be good if they are controlled by a specialist and as long as they provide essential nutrients,” says Dr Folch.

Someone on a liquid-only diet can lose three kilos a week, but to prevent the feared rebound effect, the programme includes four weeks of food readaptation, a way of learning so that when the Christmas celebrations are over, it is easy to say goodbye to gluttony.