The quote below is taken from a much larger essay on fasting. I would recommend the entire article to you for your information and edification.
So, is it a sin to break the fast? The answer to this question depends on what is meant by breaking the fast. As we have discussed, it turns out that most people—monastic and lay alike—deviate from the rule in some way. If this deviation is meaningful and its purpose is to accommodate a real physiological need, then, it seems to me it is well within the spirit of fasting, even if it is not exactly according to monastic rules. If, however, the deviation is due to our gluttony, laziness, lack of discipline, or some other weakness, then we have something that should be corrected. Perhaps, the best way to think about sin in relation to fasting is not in legal terms—law, crime, and punishment, but in terms of preparation or exercise. Fasting is an ascetic discipline. The word “ascetic” comes from the Greekἄσκησις which means “exercise” or “training.” In other words, imagine that you are a soldier preparing for a difficult and dangerous mission. It is not so much a crime to be lazy in your training or to cut corners as much as it means that you may not be well-prepared for your task and thus will not be able to complete it or even perish in the process. So, if you choose not to exercise the discipline of fasting, you are cheating yourself out of the training necessary to fight against the enemy—sins and passions—and will be unprepared to face the snares of the devil.
