The idea of the fast-mimicking diet was to have a low caloric period of eating, but not zero calories to avoid malnutrition and complete fasting. This is how they describe it in the paper "The fasting-mimicking diet (FMD) is a low-calorie/low-protein but high-unsaturated-fat diet that has an effect on stress resistance/longevity-related markers similar to those caused by water-only fasting, but that minimizes the burden of prolonged fasting as it provides both macro- and micronutrients."