Somewhere in the course of childhood, I got the impression that “fasting” meant “not eating anything.” This isn’t wrong, but it’s incomplete: as shown by the well-known custom of Lent in the Roman Catholic Church, there are degrees of fasting, from the token to the incredibly strict.

On the lighter end, one might fast by abstaining temporarily from a particular food. For this to be meaningful, the change in diet should be more than trivial; if I abstain from mushrooms, which I don’t particularly like and almost never eat, how is that different from my ordinary life? Abstaining from meat or dairy is much more significant (and common) — though for a vegetarian or a vegan, again, that’s just business as usual. The stricter the fast, the less nourishment is permitted, until you reach the point at which no food may be eaten at all.

What about liquids? Again, it’s a spectrum. In religious contexts, temporary abstention from alcohol is (unsurprisingly) a common prohibition. People fasting before a medical procedure like a colonoscopy are allowed to have clear liquids, so they can get some calories in them for the day or so preceding. Other kinds of fasting might permit only water, to avoid dehydration; the absolute strictest forbid anything at all.

As you can imagine, that latter is not very sustainable, which is why timing also matters. During Ramadan, observant Muslims take nothing during daylight hours — which, depending on where you live, makes the fast much harder to maintain in the summer! (There are various religious opinions on how to handle this at extreme latitudes, or in space.) Once the sun sets, however, they’re free to eat and drink as needed. If your fast means continual abstention from food, or especially from drink, then you can only keep it up for a little while before you’re in medically dangerous territory. Which people have voluntarily entered, for all kinds of reasons!

Medical fasting usually tries to avoid negative side effects; it’s done for safety purposes before going under anaesthesia, or to test baseline blood sugar levels. Some people use intermittent fasting (which, like Ramadan, limits consumption to certain times of day) as a weight loss method: done right, fasting can indeed bring some health benefit.

Religious fasting sometimes goes much further. It often has overtones of purification, with the participants exerting conscious control over the base needs of their bodies and ridding them of the filth of excretion; because of this, a short fast is a common element before rites of passage or other such rituals. It may also be a form of penitence, showing your contrition for your sins by denying yourself the comfort of food. Extremely pious individuals, seeking to focus on the spiritual rather than the material, have taken it to great extremes — including all the way to the end. The Jain practice of sallekhana (fasting to death), meant to destroy karma that would influence your next rebirth, is officially not considered suicide; the Cathar endura was somewhat similar, with people on their deathbeds refusing everything other than water after they received the funerary rite of consolamentum. In historical Japan, some Buddhist monks underwent a process of lethal fasting designed to ensure their bodies would become natural mummies after their deaths, known as sokushinbutsu.

Or perhaps your fast goes beyond orthodox religion and into the realm of magic. Despite its dramatic-sounding name, a “black fast” is merely a strict form of Catholic fasting, akin to Ramadan, with nothing during the day and only a vegan meal and water at night. According to folklore, undertaking a black fast on certain holy days could bring special benefits, like protection from harm or sudden death. But some people took it further, using it as a form of spiritual vengeance: they would “fast upon” someone who had wronged them, with the intent of harming or even killing them outright.

Take away that magical angle, and you have the notion of fasting as a protest against injustice. In this form we often call it a hunger strike, and in Ireland, this appears to have roots stretching back to before Christianization. The practice there was usually to fast on the doorstep of the person from whom you wanted justice, and scholars theorize that this interfaced with the deep-seated customs around hospitality. (Letting somebody starve on your literal doorstep: the opposite of being a good host.) It’s not just an Irish practice, though, being found in India as well — and in modern times, Gandhi might well be one of the most famous examples of a hunger striker. His fame played a key role in the success of his fasts, as the Brits knew very well how bad it would look if somebody that well-known died in one of their prisons. Meanwhile, a no-name agitator might be allowed to starve to death without comment.

Because for a protest fast to work, it generally needs publicity. In pre-modern times it’s enough for a fast to be known within the community — which is another reason doing it on someone’s doorstep is useful; even if you get forcibly evicted, people will notice that happening. As society gets bigger, hunger strikers depend on news media to report their actions . . . and, if necessary, their deaths. In 1981 Ireland, Irish Republican prisoners made a point of fasting one at a time for more sustained attention, ultimately resulting in ten deaths.

Not all such fasts end fatally, of course. Some hunger strikers flinch; others succeed in achieving their aims and return to normal eating. But sometimes, their fasts end involuntarily, as authorities either decide they have a moral duty to prevent death or refuse to let the guilt of the striker’s death land on their heads. In these cases prisoners get force-fed, usually by means of a tube placed down their throats, though some slavers at the height of the trans-Atlantic trade used a speculum to pry open the mouths of slaves who refused to eat. Suffragettes in particular received this treatment, as the chauvinism of the era meant deaths of women would have cast their captors in a particularly bad light.

Before wrapping this up, I should make mention of the concept of inedia, i.e. the ability to live without eating. Some claimants of this ability insist they need only air, and are called breatharians; others say that sunlight is all they need. Mythologically, the concept seems to have its roots in South and East Asia — and of course, in speculative fiction it might be literally possible! Lots of cultivation novels assume their characters can build up the spiritual power necessary to survive without food, though they might still enjoy it as a sensory pleasure afterward.

In real life, however, there are no confirmed cases, because this simply doesn’t work. By all means, abstain from food or drink for short periods of time if it benefits you medically, spiritually, or politically. But be careful not to take it too far!