A lot of people want to test themselves during Lent by learning how to fast. That is a laudable endeavor. But not everyone can fast.
Diabetics, children, the aged, pregnant women... there is a long list of people who, for various medical reasons, simply cannot fast. Or, you might not have a medical reason, but you might still be unable to fast. For instance, you might find yourself in a social situation where it would insult the host if you did not consume food. Catholic Faith is not meant to insult people. As long as you break no Church law, then breaking your fast with your host is the charitable thing to do. The person you interact with is an image of Christ, the Bridegroom. You don't want to let your personal fast turn you into the man at the wedding feast who refuses to eat.
So, does this mean that if you have a medical condition or are in an awkward social situation, you cannot fast?
No, not at all.
Remember, apart from the prescribed fasting times, such as Ash Wednesday, Good Friday or the Eucharistic Fast, there is no "minimum length" to a fast. There are people who have tried to fast for 40 days (the Church recommends against this, by the way). Bully for them. But that is really not necessary. The Eucharistic fast used to be a lot longer than the single hour we now observe. But the fact that it is shorter is not really the problem people make it out to be.
As St. Thérèse of Lisieux observed:
I will seek out a means of getting to Heaven by a little way—very short and very straight, a little way that is wholly new. We live in an age of inventions; nowadays the rich need not trouble to climb the stairs, they have lifts instead. Well, I mean to try and find a lift by which I may be raised unto God, for I am too tiny to climb the steep stairway of perfection. [...] Thine Arms, then, O Jesus, are the lift which must raise me up even unto Heaven. To get there I need not grow; on the contrary, I must remain little, I must become still less.In honor or her brilliant, insightful way of presenting the ancient teaching, we recognize the truth: it isn't the length or difficulty of the task that makes it holy, it is the love which with the task is done. The rich man may give away his wealth, but if the beggar woman throws her penny into the collection with great love, than her contribution is greater than that of the rich man who loves less.
And so we find the meat of the matter. If the length of the fast isn't really relevant (and, apart from the Church's prescribed fasts, it isn't), then a Catholic's voluntary fast can be of any length. Even ten seconds is enough. And anyone can do a 10-second fast.
Have you just become aware that you are thirsty?
Then, while you contemplate God with love, simply refrain from drinking anything for a count of ten.
You have fasted.
Now take your drink of water, satisfy your health needs, make your host smile, and no one is the wiser but you.
Done with great love, this ten-second fast is a greater accomplishment than a 40-day fast. You can fast for the whole of Lent, in ten-second, five-second, one-second increments, here and there, scattered throughout your day. Diabetic, pregnant, child - it matters not. You still consume all you need for health of the body, while fasting as you need for the health of the soul.
And so the day becomes holy, and God is woven into your life, and Lent fulfills its purpose.