Experts in nutrition are warning that crash diets could lead to weight gain rather than weight loss.
According to the Irish Nutrition and Dietetic Institute (INDI), the average Irish person gained 5lbs during Christmas and over the next few weeks many people will opt for fad diets in efforts to shed the weight.
However, according to the INDI - which represents the majority of qualified dieticians in the Republic - most of these diets are unhealthy and any weight loss will be temporary.
"At this time of year you see all kinds of diets aimed at detoxing and weight-loss," said Margot Brennan, INDI spokeswoman. "It may be a case of eating nothing but popcorn, nothing but grapefruit or buying a bottle of liquid meal replacement and having that. The reality is these things make you fatter, because they lower your metabolism and you soon begin to gain weight.
"Nutritionally, these fad diets are unbalanced - you're not getting enough iron or calcium to meet your daily requirements and this, in time, can lead to health problems such as osteoporosis," she said.
"Just remember that behind every book or diet is someone trying to make money and, in many cases, the people behind these diets know nothing about nutrition.
"There is no quick fix to weight-loss. There's no point making a mad effort for two weeks; you need to change your lifestyle step by step and learn to eat the right foods in the right portions with regular exercise," said Ms Brennan.
The INDI said yo-yo dieting could lead to eating disorders. An estimated 85 per cent of sufferers of eating disorders started out as yo-yo dieters.
Dr John Griffin, medical director of the eating disorders unit at St Patrick's Institute in Dublin, agrees there is a link.
"Yo-yo diets are very popular, especially at this time of the year," said Dr Griffin. "They can lead to eating disorders and we are seeing an increase in the number of cases of binge eating disorder, anorexia nervosa and bulimia. Also the sufferers are getting much younger, with some children as young as nine or 10 coming in for treatment and we're seeing more men," he said.
"There are so many people who read books or watch something on TV about the latest diet. In fact, the best way to gain weight is to go on one of these yo-yo diets.
"You lose weight quickly initially, but your body has a set-point weight and once you start eating normally again you will go back to that weight and even gain weight - it's the body's way of surviving," Dr Griffin said.
According to Dr Griffin, no one should attempt to lose more than one-two pounds a week. He also said that any diets should be cleared by a GP, who can suggest an eating plan that takes into account your metabolism and any health requirements.
"I could write a book on losing weight in the morning too, but it would consist of only two sentences: reduce your calorie intake and increase your activity," said Dr Griffin.
A healthy eating makeover plan is available to download from the INDI website: www.indi.ie