I think that determining cause of death might be difficult owing that she died in 1380.
Also, there were a number of saints, very very few who survived wholly on the Eucharist. This was purely a supernatural Grace from God. Her Confessor, blessed Raymond of Cappua, has this to say:
“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.”
I do agree with you, however, on the imprudence of overly-long fasts. I would say it’s very ill advised without the consent of your confessor.