Marika Sboros wrote:
No, it's not an observational study. An observational study is a study of data such as patient records and food questionnaires, then a study of correlations between two variables with the assumption of causation. Here, it's a study of three case reports. A case report is a report on an intervention, i.e. we applied this treatment and here are the results of this treatment. The study part of this study is the amalgamation of the three case reports' results.

Typically, a case report is written when the diagnosis and/or treatment and/or outcome is unconventional and extraordinary. The treatment - intermittent fasting to treat diabetes type 2 - is indeed unconventional. The fact that this treatment is a cure for two of the three patients is extraordinary, considering the disease is officially deemed incurable.

What the researchers are aware of or not in the grand scheme of things is irrelevant. Case reports are intended to be read by other practicing doctors who deal with the same diagnoses and are searching either for solutions to their failed prescribed treatment or for alternative treatments. This study of three case reports should fit those needs.

It's important to note that a case report rests squarely on the author's reputation as a practicing doctor. As such, it's deemed much more clinically relevant by other practicing doctors.