In LENT Catholics between the ages of 14 and 59 fast on Ash Wednesday and Good Friday to honour the passion of Christ. Fasting means we can have only one full, meatless meal. Some food can be taken at the other regular meal times if necessary, but combined they should be less than a full meal. Liquids are allowed at any time, but no solid food should be consumed between meals. 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. We also abstain from meat every Friday during Lent. These practices are disciplinary in nature and often more effective if they are continuous, i.e., kept on Sundays as well. That being said, such practices are not regulated by the Church, but by individual conscience.
From the website of the US Catholic Bishops.
Also, a regular practice of fasting can improve digestive efficiency, increase mental clarity, increase physical and mental vigor, remove toxins, improve vision and give a general feeling of well being. A 24- hour fast, once a week, is a common religious practice in many