Low carbohydrate, high protein diets may help to shed weight quickly but researchers warned on Friday that they can be unhealthy.

The diets can cause constipation, diarrhea, headache, bad breath and ketosis, which causes raised levels of ketones, or acids, in the body.

“Low-carbohydrate diets for weight management are far from healthy,” said Lyn Steffen and Jennifer Nettleton, of the University of Minnesota School of Public Health in Minneapolis, writing in the British Medical Journal.

They cited the unpleasant effects and a lack of trials to test the long-term effects of low-carbohydrate diets.

In a case report in the journal, doctors at New York University School of Medicine said they had treated a 40-year-old obese woman, who had followed the Atkins diet, for a life-threatening illness known as ketoacidosis.

The diet, based on research by Dr Robert Atkins who died in 2003, involves eating proteins such as meat and cheese and limiting carbohydrates such as bread and pasta.

“Our patient had an underlying ketosis caused by the Atkins diet and developed severe ketoacidosis,” said Tsuh-Yin Chen, adding that mild pancreatitis or stomach infection may have contributed to the problem.

Ketoacidosis is caused by dangerously high levels of ketones in the blood. It can lead to coma and death if untreated.

Steffen and Nettleton said special care should be taken with diet plans because patients wanting to shed weight may have a wide range of risk factors for diseases.

“As researchers and clinicians, our most important criterion should be indisputable safety, and low-carbohydrate diets currently fall short of this benchmark,” they added.