I sometimes get questions from inquirers and others who are not Orthodox asking what our fasting rules entail. Here in brief are the rules for the Lenten Fast.
Starting on the first day of Lent (we begin on Monday not Ash Wednesday) Orthodox Christians are to abstain from all meat, fish, wine, oil and dairy/animal products during weekdays. Also normally only one meal is taken each day. On weekends this is relaxed somewhat with two meals permitted and wine and oil are allowed. The first week of the Fast (called Clean Week) as also Holy Week are observed very strictly. Those able to do so keep a strict fast on Clean Monday (no food) and eat as little as possible thereafter until the weekend. The monastic discipline (which is the standard towards which we should strive) calls for only two meals between Monday and Friday. Holy Week also gets rough starting on Great and Holy Wednesday until the Fast is broken by taking Communion on Pascha (Easter). Great and Holy Friday is a strict fast day (no food) and Holy Saturday is very nearly a strict fast with only a little fruit and nuts normally allowed.
Now that I have probably frightened off any inquirers let me throw out some mitigating words. First the Church calendar contains a number of days during lent where wine and oil are permitted. Also there are two days (the Feast of the Annunciation and Palm Sunday) where fish wine and oil are allowed. Secondly (and very importantly) we are not insane and we do understand human limits. In practical terms and for obvious reasons few laymen can keep the fast this strictly. But for everyone the Fast during Clean Week should be kept as strictly as individual circumstances permit as also the last several days of Holy Week.
Food and drink taken for reasons of health are always permitted. Most priests routinely relax some aspects of the fast for their spiritual children especially for reasons of age or health. The rules outlined above were written in a day and age when people generally ate much less and had far more limited diets all year. Also the influence of diet on health was not understood as it is today.
The object in the fast is not to satisfy some legalistic requirements dating to late antiquity or to see who can inflict the most damage on their health. Rather it is to stretch ourselves spiritually by denying the flesh one of its favorite comforts. The Fast is first and foremost a form of spiritual medicine intended to help with the process of decluttering our lives for the 40+ days of Lent and get us refocused on God.
Now a few closing thoughts on fasting. The above rules are the guidelines in the book. The real rules are the ones your spiritual father/confessor prescribes for you. One of the most frequent mistakes committed by those new to the Faith is to dive into the Fast and try to do it all the first time around. That is in almost every case a recipe for failure and frustration. I have been Orthodox for four years now. I have yet to keep Lent strictly and probably never will for various reasons. Few people outside of monasteries do. But we need to try as best we can. If you fall down (we all do) then pick yourself up and try again.
Finally, avoid discussing the fast with others. Your fast is your business. The only one you should be talking with about it in detail is your spiritual father. Remember you may be keeping a stricter fast than others and discussing it could cause them to feel bad or make them attempt something they are not ready for. Or you may be doing less than someone and that too can cause issues. The Fast should be kept discretely. Efforts should be made to avoid those circumstances where you may be tempted to break the fast or show off to others who are not fasting. This last part is quite challenging in our culturally Western country in which fasting as a religious discipline has been more or less abandoned. My own spiritual father has gone so far as to say that it is better to break the fast than cause scandal or show off. If you are a guest and someone puts food in front of you, you eat it. With that view in mind keep your social engagements to a minimum.