FATS, CALORIES, carbohydrates are yesterday’s bad guys — the new enemies are toxins, with toxic “excess” blamed for a host of ills from weight gain and poor digestion to acne and excessive tiredness. Such is our obsession for the internal clean-out that the early months of the new year are now well established as the detox months.

Tesco has reported that the “Detox Season” has become “an essential fixture on the retail calendar”, and that “anything that has a well-known connection with detoxing has flown off the shelves”; its sales of brown rice in January were up by 67 per cent on the previous month, and those of the Nutri Centre @Tesco’s Detox Plan Drink up by more than 200 per cent. And detox plans regularly top the diet bestsellers lists at Waterstone’s — with around 200 detox titles in the UK.

It’s hard to believe that just five years ago detoxing was considered the preserve of celebrities and alternative types. Now we’re all at it.

The popular perception is that a detox will probably do some good, and it can’t do us any harm. Or can it? Some experts believe that detoxing is at best a waste of time, effort and money, and at worst it can be dangerous.

Last year Catherine Collins, chief dietitian at St George’s Hospital, London, saw a 23-year-old man who had been rushed to accident and emergency after slipping into a coma. Doctors reported that his serum sodium levels had plummeted to a dangerous level, causing his brain to swell. Constricted by the skull, the brain has little leeway for expansion and once it has enlarged by 6 per cent it hits bone and starts to compress against the hard surface. This can be serious; the patient remained in a coma for four days. It was not alcohol or drugs, nor a mystery illness or reaction to medication that was to blame, but almost certainly a detox diet.

The patient had attended a family party where several close relations had commented on his pasty appearance and expanding waistline. As a result, he started a three-week detox diet, which consisted almost exclusively of fruit, vegetables, fruit juice and water.

While his was an extreme case, it is by no means isolated, says Collins. She sees dozens of people with detox side effects ranging from bowel problems to chronic dehydration and potassium depletion as a result of their attempts to detox. More seriously, dizziness, respiratory problems, sometimes even potentially fatal water toxicity, are symptoms of hyponatraemia, when sodium levels and other body salts, or electrolytes, in the blood are too dilute, often as a result of consuming more water and less salty foods sometimes in conjunction with increased activity.

“There is this fixation with the notion that we can detoxify the body through what we eat and drink, but the whole idea has no scientific basis and anything that promises to help you to detox is a rip-off,” Collins says. “Sticking to a detox regimen for a day or two won’t be harmful for most people, although neither will it have any effect on their long-term health. But when detox plans promote longer periods of severe dietary restriction, which many do, they can cause problems.”

The detox theory is that too much of the wrong sorts of food, a polluted environment, and unhealthy habits such as drinking and smoking contribute to a build-up of poisonous substances in the body, which eventually buckles under the strain. The signs of this strain include headaches, body aches, chronic fatigue, allergies, chronic digestive problems, muscle aches, autism, schizophrenia, even drug reactions and Gulf War syndrome fatigue. The substances commonly considered poisonous include caffeine, alcohol, drugs, cigarette smoke, exhaust fumes, high protein diets, organophosphate fertilisers, paint fumes, saturated fat, steroid hormones, and the list goes on.

According to the detox theory, by sticking only to foods that are considered pure and unadulterated, and by avoiding anything that might strain the internal organs, you will purge yourself of poisons and undo the damage wreaked on your health.

The avoidance of food, or fasting, is not new, and has been practised for religious reasons for centuries with the idea that, at certain times, it is beneficial to concentrate more on keeping faith and less on the concerns of everyday life such as eating and drinking. But detoxing usually differs from religious fasting in duration, or in the stipulations about what cannot be consumed. Prolonged spiritual fasts, such as the month-long Ramadan fast practised by Muslims, do allow food to be consumed at certain times; others, such as the Jewish Yom Kippur, restrict the practice to 24 hours. Most detox programmes advocate 48 hours to seven days of a strict dietary regimen. Some suggest sticking it out for longer; the latest hit with the Hollywood set is a 21-day programme devised by the nutritional therapists John Woods and Richard DeAndrea .

If detox plans vary in length, their content is pretty similar. Typically, they prescribe drinking two litres or more of water a day along with dandelion coffee and herb teas. Most also recommend half a pint of fruit or vegetable juice — carrot or apple are favourites because of their “digestive-boosting” properties — and some allow unlimited consumption of raw fruit and vegetables considered beneficial for detoxification purposes, such as fresh apricots, citrus fruits and mango or peppers, watercress and bean sprouts. Meat, fish, dairy, processed foods are definite detox no-nos.

So, does detoxing fulfil its promise? “No, it’s rubbish,” says Professor Tom Sanders, of the department of nutrition at Kings College London.