This was a metabolic ward study, very controlled:
To Reduce Body Fat, Eating Less Fat May Be More Effective than Eating Less Carbohydrate, Endocrine Society Annual Meeting, 5 March 2015
“Calorie for calorie, reducing dietary fat results in more body fat loss than reducing dietary carbohydrate when men and women with obesity have their food intake strictly controlled,” said lead study author Kevin D. Hall, PhD, senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health (NIH) in Bethesda, Maryland.
“Ours is the first study to investigate whether the same degree of calorie reduction, either through restricting only fat or restricting only carbohydrate, leads to differing amounts of body fat loss in men and women with obesity,” Dr. Hall said
The authors studied 10 men and 9 women with obesity.
All participants were admitted to the metabolic ward of the NIH Clinical Center and resided there 24 hours per day. All food eaten was strictly controlled and the daily activities of the participants were monitored.
It was a crossover design so each group ate each diet for 6 days. From BBC:
Half were placed on a low-carb diet of 30% carbohydrate, 49% fat and 21% protein.
The rest were placed on a low-fat diet – 72% carbohydrate, 7% fat and 21% protein.
7% fat is a true low-fat diet. It’s about the amount of fat the traditional Okinawans ate, the people who are known to live long and healthful lives.
Compared to the reduced carbohydrate diet, the reduced fat diet led to a roughly 67% greater body fat loss.