For the Christian, all foods are clean. When no fast is prescribed, there are no forbidden foods.
For fasting days, the following information may be helpful.
The Orthodox church does not make the distinction between "fasting" and "abstinence" that is sometimes made in the West. The strict rules simply indicate that on any fasting day the following foods should be avoided: Meat, including poultry, and any meat products. Fish (meaning fish with backbones, since shellfish are permitted). Eggs and dairy products (milk, butter, cheese, etc.). Olive oil. Wine and other alcoholic drink. (In the Slavic tradition, however, beer is often permitted on fast days because - until recently - beer was considered food.)
When the feasts of some major saints fall on what would otherwise be fasting days, the fast is still kept but wine and olive oil are permitted. These days are indicated in the calendar. Similarly, Saturdays and Sundays in the main fasting seasons (see below) are also "wine and oil" days.
Although these rules may appear quite strict to those who have not seen them before, they were developed with all of the faithful in mind, and not only monks. (Monks do not eat meat, so rules regarding the eating of meat cannot have been written with them in mind). However, we should bear in mind that the circumstances and dietary habits in which they were developed were different to those in which many Orthodox now live, and it is the spirit rather than the letter of the rules that is important. Nevertheless, although few laypeople are now able to keep these rules completely, it is desirable for those wishing to follow the Orthodox way to find as close a compromise to the strict rules of fasting as fits in in with their own lives. (Those eating in works canteens or living in households containing non-Orthodox members will find this issue more difficult than others and in these cases, in particular, compromise is quite proper but should ideally be discussed with a priest or spiritual guide).
It should be noted that it is neither permitted nor required that you endanger your health by fasting. This means, for example, that abstinence from food that leads to dangerously low levels of blood sugar - which in turn endangers yourself or others - is not good practice. However, avoiding prohibited foods usually poses no health risk as long as adequate amounts of other foods are taken. When fasting, we should eat simply and modestly. Monastics eat only one full meal a day on fast days, two meals on "wine and oil" days. However, laypeople are not usually encouraged to limit meals in this way.
The Church has always exempted small children, the sick, the very old, and pregnant and nursing mothers from strict fasting. However, while people in these groups should not seriously restrict the amount that they eat, no harm will come from doing without some foods on two days out of the week — simply eat enough of the permitted foods. Exceptions to the fast based on medical necessity (as with diabetes) are always allowed. It also must be acknowledged that where households consist of both Orthodox and non-religious members, the unity of the family transcends the rules of fasting, where to fast would lead to conflict.
The fasting seasons - of which the main one is the Great Lent - are indicated in the calendar. Note that Saturdays and Sundays in these seasons are usually "wine and oil" days.
In addition to the main fasting seasons, Orthodox Christians are encouraged to keep a fast every Wednesday and Friday unless a fast-free period is indicated in the calendar.
So that the Eucharist may be the first thing to pass our lips on the day of communion, we abstain from all food and drink from the time that we retire (or midnight, whichever comes first) the night before. When communion is in the evening, as with Presanctified Liturgies during Lent, this fast should, if possible, be extended throughout the day until after communion. For those who cannot keep this discipline, a total fast beginning at noon is sometimes prescribed.