Just in time for...Lent 2017, here are some comments on what the (Latin) Church requires in regard to fasting.
Part I: When, and how much, to eat on fast days
From the early centuries of the Church, there has been a practice of fasting but it has never been uniform throughout the Church and it has changed over time: the customs of time and place were essential in determining what was required. In addition to limiting the overall quantity of food, as well as the time when it could be consumed, there were also limitations on what food could be eaten. There was a combination of "fasting" and "abstinence."
By the end of the first millennium, fasting was generally observed for the 40 (more or less) days of Lent, with a possible break from the fast on Sundays, as well as on other days throughout the year. Fasting was understood as a day on which people did not eat or drink anything until around sunset. As time went on, this was changed--more by customary practice than legislation from the hierarchy--so that "breaking the fast" at about 3 pm was deemed acceptable (so said St. Thomas Aquinas). As the years continued to progress, the time for the "break-fast" meal was moved even earlier in the day so that noon was the practical time for the meal on fast days. This migration from evening to afternoon to noon was first justified in cases of necessity. Necessity, over time, was not required. Eventually, it was acceptable to eat the meal even an hour before noon for a good reason. According to the old Catholic Encyclopedia:
According to general usage, noon is the proper time for this meal. For good reasons this hour may be legitimately anticipated. Grievous sin is not committed even though this meal is taken a full hour before noon without sufficient reason, because the substance of fasting, which consists in taking but one full meal a day, is not imperiled.Here we have the fundamental nature of fasting, as it was understood at around the turn of the 20th century: fasting limits food intake to one meal a day and the timing of this meal is not a matter of grave sin. How much could a person eat at this meal? This was never stated in the law of the Church. (Moral theologians tended to state that a full meal was 32 ounces.)
As this change in the time of the "breakfast" took place, people began to also have a small meal in the evening. This "collation", as it was called, was first just some wine and a bit of bread. Drinking only wine might have some unwanted effects so, therefore, some bread was also allowed. By the time of the 16th century, it was customary to have fruit and even other food, as long as the total quantity did not exceed five...six...seven...eight ounces. According to St. Alphonsus (1770), eight ounces was seen as the maximum amount. (This would be 1/4 of a full, 32 ounce meal). Once again, however, local practice was variable. There was no legislated, universal norm.
During the next 75 years or so, people began to have some food even in the morning. This "frustulum". as it was called, was allowed by "the Vatican" (the Sacred Penitentiary, to be specific) in an 1843 response which said: "Those who in the morning on fast days take a small quantity of coffee or chocolate with a piece of bread should not be disturbed." "Chocolate", in this context ("...coffee or chocolate...") refers to a chocolate drink, not a big chunk of chocolate. Over time, it was said that this little snack was supposed to consist of no more than two ounces of food but some authors stated that people are allowed to eat as much as they truly need in order to carry out their duties. Once again, there was no legislation from the Holy See which determined how much could be eaten in these "snacks." In any case, I doubt people had a scale to measure their food intake.
In 1891, the Baltimore Catechism defined a fast days as: "days on which we are allowed but one full meal" (q. 1337). Question 1338 asked whether or not it is "permitted on fast days to take any food besides the one full meal" and said that it is permitted "to maintain strength, according to each one's needs. But together these two meatless meals should not equal another full meal." (This should sound familiar since it is how we (at least in the USA) often define what it means to fast.)
In the 1917 Code, we can see a universal law that formally allows both a morning and evening "snack": "The law of fasting ordains that only one full meal a day be taken, but does not forbid a small amount of food in the morning and in the evening. As regards the kind of food, and the amount, that may be taken, the approved customs of one's locality are to be observed" (canon 1251). This canon went on to say that the "main meal" could be taken in the evening and the "collation" at noon.
As usual, the Church did not specifically define what this "small amount" of food is. After the 1917 Code was promulgated, some authors continued to say that the two "snacks" should be limited so that, together, they do not add up to more than the full meal. The Bishops of the United States, in 1951, continued to use this sort of limitation by saying that the two meals would be "sufficient to maintain strength, may be taken according to each one's needs; but together they should not equal another full meal" (quoted in Callan's Moral Theology, n. 2588).
In 1966, Paul VI promulgated a document on fasting and abstinence. Therein, he repeated canon 1251 of the 1917 Code. The 1983 Code does not define what fasting is--it only says that 18-59 year-olds are bound to fast on Ash Wednesday and Good Friday and local conferences of Bishops can define it more precisely (cc. 1251-1253). The Bishops of the USA have not made any such definition, subsequent to the 1983 Code.
Part II: What is food/liquid?
All of the foregoing dealt with the amount and timing of food intake on days of fasting: we are allowed, at most, one regular meal and two smaller "snacks" and these meals can be taken at any time of the day.
What about drinking/liquids? For hundreds of years, we've had this axiom: "liquids (or drinks) do not break the fast." Great, that's clear...or is it? Unfortunately, it's not as clear as it seems. Many authors, for example, have said that milk breaks the fast (for example, the old Catholic Encyclopedia said this). Other authors said it doesn't. From what I've seen, more recent (let's say, from 1950) authors say milk is a drink but older ones said it is food.
We will not find an "official" list of foods/drinks. These days, "drinkable meals" are becoming more and more popular. These things aren't the same as fruit juice, which has been long considered to be a drink. Yet, we say we "drink" them, not "eat" them. We will have to depend on principles to decide what is a food and what is a drink--don't expect the Church to formally define this. How about "if it passes through a straw, it's a drink" principle? That's a good start but not enough.
It seems to me that we drink liquids when we are thirsty and/or when we need to assist the digestion of food. When we are hungry, we eat food. If I think "I'm thirsty", I won't say "I think I'll make a banana/blueberry/apple/pear/mango/grape/kiwi/date/milk smoothie in the blender." No, I'd consume such a thing so that I can eat something and not be hungry, even though it can go through a straw.
On the other hand, I would not say "I'm hungry. I'll have a glass of milk." (I think of milk as a drink...maybe because I grew up in Wisconsin.) Other people, though, might consider milk to be a snack. There is a lot of room for disagreement here, based on customary/cultural differences.
I've been looking for recent commentaries on what is considered to be a liquid, and why. I haven't been able to find anything more than a 1990s paragraph from a Philippine canon law journal In Callan's Moral Theology (which comes from the 1920s, with a 1950s revision), he has this to say:
The law speaks of eating, that is, of solid food, and hence the Lenten and other similar fasts are not broken by liquids which are beverages rather than foods, or which are used to allay thirst, or carry food or assist digestion, and not chiefly to nourish (e.g., water, teas, coffee, light cocoa, wine, beer, lemonade, fruit juice). Likewise, sirups taken as medicines are not considered foods, even though they contain nourishment, unless one drinks a large quantity for its food content. Light ices may be considered drink, but ice-cream is food. On the contrary, liquids that are chiefly nourishing are regarded as food (e.g., soup, oil, honey). Finally, some liquors vary between food and drink, according to their richness or weakness, their great or small quantity. Thus, hot chocolate as made in the United States contains only a small amount of solid and may be considered as a drink, but as made in Europe it is stronger and rather food than drink.This makes sense to me, even the part about ice cream being food.
Wondering about what is a food or liquid might be fun but it's not all that important, practically. The Latin Church's fasting requirements are extremely limited today: only two days of the whole year are fast days. If we are physically able to fast, we shouldn't need to even wonder about whether or not soy milk is a liquid or food: just don't eat/drink it until it's time for your meal/snack. Have a glass of water.