Short-term, very low-calorie diets for obese teenagers are safe as long as they are closely monitored by an experienced dietitian, according to researchers in Australia whose work will be presented at the European Congress on Obesity in Venice this spring.

The study, by scientists based at Sydney University, also revealed that many adolescents involved in the investigation thought the diets were an acceptable way to lose weight – despite experiencing side-effects that included fatigue, headache, irritability, constipation and nausea.

Very low-energy diets (VLED) involve taking less than 800 calories a day and are prescribed for obese people who want to shed weight but who do not respond to conventional diets and exercise programmes.

However, concerns have been raised about the risks involved in making such rapid reductions in body mass, while ensuring young people are given all essential nutrients. There is limited data on the impact of VLEDs on the growth, heart health and psychological wellbeing of the subjects.

However, the Sydney study indicated that such fears are unjustified. “Given the associated rapid weight loss, their use should be emphasised in clinical practice guidelines for the treatment of severe obesity and obesity-related complications in adolescents, especially before pharmacological or surgical intervention,” said Dr Megan Gow, who led the study.

A total of 71 men and 70 women with obesity and at least one associated condition, such as high blood pressure or insulin resistance, took part. They were placed on four-week diets involving formulated meal replacements and low-carb vegetables, such as broccoli and tomatoes.

Of the 141 who started the study, 134 completed it, losing 5.5kg in weight on average. Nearly all (95%) had at least one side-effect, with 70% experiencing three. Hunger, fatigue and headaches were the most common.

Losing weight was the most-liked aspect, the participants reported, while restrictive diet and taste of meal replacements were liked least.

The researchers conclude that a health professional-monitored VLED can be implemented safely in the short term and, despite side-effects, is acceptable for many adolescents with moderate to severe obesity.