This mystical fast is also known as the gift of "inedia"
"For the seven year period prior to her death, Saint Catherine of Siena took no food into her body other than the Eucharist. Her fasting did not affect her energy, however. She maintained a very active life during those seven years. As a matter of fact, most of her great accomplishments occurred during that period. Her death had nothing to do with malnutrition, or anything connected with lack of food."
Table of Contents of St. Catherine