Millions of people consume cup upon cup of coffee every day. To the office worker or the early riser, it's a godsend; a nectar sent from the heavens to give you the energy and focus to go about your day.
That's why it can be so difficult to start a program of fasting, only to find in the instructions some rule against coffee. How can you deal with fasting without your coffee? Or do you even have to? If the rules of your fast don't say anything, maybe you can still drink a cup or two with no adverse effects. Let's look into it, shall we?
The Effects of Coffee While Fasting
The first thing you want to know about coffee while fasting is what effect it may have on your body.
Normally, of course, coffee is an energizing beverage. It's full of caffeine, and if you add stuff to it, it might have a bunch of sugar and cream as well. These give you energy and fuel to go about your day.
The thing about normal coffee drinking, though, is that you do it when you're eating breakfast or afterwards. You have some food in your stomach, which helps to even out the beverage. The sugar and caffeine doesn't hit you all at once; it's partially absorbed by your food and lasts a little longer before it works its way through your system.
When you're fasting, you don't have anything in your stomach. This means a few things.
First, it means the coffee will hit you much faster. Caffeine is readily absorbed through the stomach lining and intestinal walls, so it hits your bloodstream almost immediately. You'll feel the effects within minutes. Additionally, without the delay of food, caffeine hits harder. The effects – both in terms of energy and in focus – will be more pronounced. You may feel like it's more like when you first started drinking coffee, or that a regular coffee hits you more like a shot of espresso.
Second, those pronounced effects don't have anything to temper them. If you're prone to jitteriness when you drink too much coffee, that "too much" becomes a lot lower. You may find yourself fidgeting or jittering more easily when drinking coffee while fasting.
Third, there's also the potential digestive issues. Coffee stimulates the production of stomach acid, since it's something you're consuming to be digested. But, since coffee isn't a solid that needs much digestion, that excess acid doesn't have much to neutralize it. This can lead to issues, ranging from heartburn to irritable bowel syndrome, or even to stomach ulcers.
Normally, we wouldn't recommend drinking coffee on an empty stomach, whether it's first thing in the morning or because you're in the midst of a fast. However, there may be some exceptions.
Types of Fasting
There are several kinds of fasts you might be trying. Since we can't divine which kind of fast you're on, we're going to cover them all.
In addition to the major categories below, there are also types of fasts within those categories. You have food fasts, which are fasts where you're allowed to drink liquids but not eat foods. You also have water fasts, which limit you to nothing at all except water. There are even total, strict, dry fasts, which limit you to nothing at all, not even water.
Liquid fasts tend to come in diet programs, like a juice cleanse. These programs limit the solid foods you can eat because they intent to purge your system of any lingering half-digested food, any nebulous "toxins", and any other potential issues in your gut. They allow you to drink liquids, including smoothies and shakes, so you can still get some nutrition while you're cleansing. They aren't really true fasts, though, so you can feel free to drink coffee with them. In fact, the diuretic effects of coffee may even help with the cleanse! Just don't dehydrate yourself.
Water fasts, which allow you to drink water but not consume any other beverages or foods, tend to be more religious in nature, but they can also stem from intermittent fasting programs.
If you're doing an intermittent water fast for weight loss purposes, you should probably not drink coffee. You can, if you really want to, but you absolutely should not drink any of those fancy Starbucks coffees with the whipped cream and the chocolate and the creamer and sugar. The point of intermittent fasting is to minimize caloric intake, so drinking a fancy coffee full of sugars is just going to cancel out any benefit from the fast.
Plain black coffee is probably fine for intermittent fasting, but again, be aware of the side effects. You may find it's not as pleasant to drink while fasting as you expect it to be.
Dry fasts, of course, prohibit you from consuming anything and are most often either religious or medical in nature. If you're doing this kind of fast, you absolutely cannot drink coffee without breaking the fast. There's just no way around it.
So, what about those other reasons to fast? Let's look at it from that perspective too.
Medical Fasting
First up is medical fasting. Medical fasting is usually something doctors instruct you to do before something like bloodwork or surgery. In the case of bloodwork, you should avoid drinking coffee, because the caffeine in the coffee can still mess with your hormones and other bodily processes. When they're taking blood, they want to test it for various proteins, chemicals, hormones, and other identifiers.
These markers are used to look to see if something is wrong with you, anything from high blood sugar or blood pressure to high cholesterol to signs of cancer. Drinking coffee can throw off some of these metrics, and can make it look like you have a problem when you don't. Or, worse, it could hide a problem you have.
In the case of surgery, eating or drinking anything except maybe water is hugely risky. Major surgeries are a large trauma on the body, and as such, they carry numerous risks. One of those risks, if you have food in your stomach, is that your body may reflexively vomit up something. This in turn can lead your unconscious body to breathing in some of that mess, or aspirating it, which can be anything from a harmless cough to choking you enough to call off the surgery, or to even extreme cases where you can die.
That's not to say that coffee is likely to kill you – it's more of an issue with solid foods – but it's still a risk you shouldn't take when your doctors expressly forbid it. Additionally, caffeine can cause a spike in blood pressure, which can cause issues for the people monitoring your vitals during surgery.
In general, if a doctor tells you to avoid eating or drinking anything, no, coffee is not okay.
Religious Fasting
Religious fasting has a long history throughout multiple different religious traditions. Islam has fasting as part of Ramadan. Jews fast for Yom Kippur and a few other observances. Some Christians observe the Daniel Fast. Lent is a well-known fast as well, though it's often giving up one specific thing rather than everything except water, and it doesn't have to be about food at all.
As such, there's no hard and fast rule as to whether or not you can have coffee during this kind of fast. It's a question you need to ask yourself and a conversation you need to have with your religious leader. If you come from a very orthodox tradition and don't want to compromise your religious beliefs, coffee may be forbidden during your fast. If you're more of a casual observer of your religion, you may be willing to technically break your fast with coffee to avoid the caffeine withdrawal symptoms. It's really up to you.
Intermittent Fasting
Intermittent fasting is usually a dietary style of fasting, meant to help you lose weight by cutting calories. There are a few types of intermittent fasting:
The 16/8 method. This method has to fast for 16 hours a day, including when you're sleeping. During the remaining 8 hours – typically during the workday – you are free to consume whatever you want. During this style of intermittent fasting, you are free to consume coffee as long as it's during your 8 hours of active time. Some proponents also allow coffee during your fasting hours, so long as it's black coffee, as a calorie-free way to satiate some hunger cravings.
The 5/2 method. This divides your week up into days, rather than your day up into hours. Five days a week, you can eat whatever you want. Two days of the week, you fast. 
This method would be very harsh if it were a total fast or dry fast, so it's often simply a very restricted calorie day instead. You generally limit yourself to 400-500 calories each fast day. During these fasts, you are free to drink coffee, though if you drink coffee with sugar and cream and other additives, be aware that it definitely eats up a significant portion of your caloric intake for the day.
The Eat Stop Eat method. This is very similar to the 5/2 method, except instead of a restricted calorie diet, it is a total fast on the days where you don't eat. It can help you lose weight much more quickly than the 5/2 method, because it's cutting an additional 500-1,000 calories per week.
Proponents of this method allow coffee, again, so long as it's black coffee. Coffee has no calories on its own, so as long as you're not adding a bunch of stuff that does have calories to it, you aren't going to be breaking your fast with a cup of coffee in the morning or what have you.
The Alternate Day method. This is a very harsh method of fasting, where you alternate every other day with fasting. So on Monday you would eat normally, on Tuesday you would fast, on Wednesday you would eat normally, on Thursday you would fast, and so on.
Coffee is typically allowed during these kinds of fasts, again because of its zero calorie nature. However, given that this is a very harsh fast, the diuretic effects of coffee might be causing more trouble than they're worth and you could end up with some form of malnutrition if you keep it up for a prolonged period.
The Warrior Diet. This is a sort of 16/8 method but expanded and focused around a lifestyle. The idea is to fast during the day – when a "warrior" would be out fighting or training – and then having one large feast in the evening. Typically, even when you've been fasting all day, you won't eat a full day's worth of calories in your nightly feast, so you're still operating at a caloric deficit and thus losing weight.
The warrior diet typically allows any zero-calorie beverage during your fasting time, and recommends black coffee as an option.
The Bottom Line
At the end of the day, depending on why you're fasting, coffee may or may not break the fast. If you're fasting for weight loss purposes, black coffee is fine, but adding cream, sugar, or other additives is not. If you're fasting for a religious or medical reason, you're more likely to have to avoid coffee. In general, though, you should avoid calorie-filled coffee drinks simply because they're not all that good for you anyway.
