Ash Wednesday and Good Friday are obligatory days of universal fast and abstinence. Fasting is obligatory for all who have completed their 18th year and have not yet reached their 60th year. Fasting allows a person to eat one full meal. Two smaller meals may be taken, not to equal one full meal. Abstinence from meat is to be observed on all Fridays of Lent. Abstinence from meat is obligatory for all who have reached their 14th year.
Those that are excused from fast and abstinence outside the age limits include the physically or mentally ill including individuals suffering from chronic illnesses such as diabetes. Also excluded are pregnant or nursing women. In all cases, common sense should prevail, and ill persons should not further jeopardize their health by fasting.
As for common sense you do not need a "Dispensation" from a priest to make these decisions. If you my choose another day or another penance based on your circumstances and needs. Always remember all church practices are to improve our relationship with God and the practices are not God Himself.