The ketogenic diet, best known as a fat-busting fad, holds promise for treating anorexia nervosa. Following the diet – which contains high amounts of fat, moderate amounts of protein and very few carbohydrates – caused 3 in 4 people with the eating disorder to drop below the threshold for diagnosis in a small study. This is thought to be due to the diet restoring malfunctioning energy release in brain cells, which has been linked to anorexia, thereby lowering anxiety and reducing the compulsion to restrict food.

Mimicking starvation by restricting carbohydrates in a condition characterised by extreme dieting, and with one of the highest mortality rates of all mental health conditions, sounds risky. But Guido Frank at the University of California, San Diego, argues that when properly supervised, it could remove the compulsive drive to self-starve. “People tell me clinically, it’s like an addiction, [saying] ‘I crave this’,” he says. “Perhaps if you create that state that they crave while giving them enough food, it can be beneficial.”

Frank and his team asked 22 women with anorexia, whose body mass index (BMI) had risen enough to sit in the healthy to slightly underweight range, to follow a ketogenic diet for 14 weeks, supervised by a dietician, psychiatrist and a peer support counsellor who had experienced anorexia. Their weight, mood and anorexia symptoms were monitored weekly, using questionnaires to track any changes in body image, depression, food-related anxiety and fear of weight gain.

The 18 women who stuck to the diet for the full 14 weeks showed a significant improvement in anorexia symptoms and scores of depression, which commonly occurs alongside anorexia. Thirteen of them (72 per cent) even improved enough to drop below the threshold for clinical diagnosis for both anorexia and depression. “The level of recovery was far better than what we see in other anorexia treatments,” says Frank.

The aim of the study was not to see if the keto diet made the participants gain weight, however, they all stayed in a healthy to slightly underweight BMI range, and didn’t relapse.

Ketogenic diets are named for the way they prompt a metabolic shift that evolved to help us survive times of famine. As plant-eaters, our metabolism runs mostly on carbohydrates, which are broken down into glucose to be burned in the energy-releasing mitochondria in cells.

When carbs are unavailable, the body adapts to burn fat, releasing it from storage and converting it in the liver to molecules called ketone bodies. These can be burned in the mitochondria in place of glucose.

The diets were invented in the 1920s, not for weight loss, but as a treatment for epilepsy. It was known that fasting for several days could reduce or stop seizures, but as a treatment, it was unsustainable. The ketogenic diet provided a solution: restricting carbs enough to mimic starvation, while providing enough dietary fat so those on it didn’t lose weight.

Research since suggests that epilepsy and many mental health conditions, including anorexia, are associated with problems related to releasing energy from glucose in the brain, and ketone bodies can relieve these problems by providing an alternative fuel.

Sahib Khalsa at the University of California, Los Angeles, who researches and treats eating disorders, sounds a note of caution for anyone considering trying a keto diet for anorexia. “It is important to distinguish between close monitoring from an eating disorder psychiatrist, dietitian and treatment team, and attempting to do this independently.” Until we have more data from large, randomised controlled trials, it is too early to change the way we treat anorexia, he says, which typically involves therapy and nutritional support.