You Can Drink Coffee During Your Fasting Window, But There's A Catch

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Intermittent fasting (IF) is one of the most popular weight loss methods out there. And while holding off on food for a couple of hours throughout the day is part of the plan, staying away from caffeine adds a whole other challenge to this eating plan. Can you drink coffee while fasting? After all, it's only fair to consider it an essential.

Here's a refresher on why so many people opt for IF in the first place: It may help improve blood pressure, reduce liver fat, and lower cholesterol on top of helping you lose weight, according to the University of Michigan Health Lab.

When you deprive your body of calories, it goes into a temporary state of starvation and slowed metabolism that forces your fat cells to give up the glucose they’re storing in order to fuel your body, says nutritionist Barbie Boules, RDN, of Barbie Boules Longevity Nutrition. If you repeat that process a few times, you may start to experience weight loss.

But an important side note: Fasting all the time would, in theory, permanently slow your metabolism and counteract the benefits, so undereating all the time isn't ideal.

“We don't yet know conclusively what happens in humans [during fasting], and if it's any more beneficial than simply reducing overall calories,” says Boules. So far the studies that have shown those positive results have mostly been done in rats, but not humans.

If you're still concerned about having to kick your coffee habits, read on to learn all about how an intermittent fasting diet will affect your coffee-drinking habits.

Meet the experts: Barbie Boules, RD, is a registered-dietician nutritionist and certified health coach at Barbie Boules Longevity Nutrition. Scott Keatley, RD, is a nutritionist and member of the Academy for Nutrition and Dietetics. Keri Gans, RD, is a New York-based registered dietician and author of The Small Change Diet.

Can I combine coffee and intermittent fasting?

Because a cup of brewed coffee is fat-free and almost no-cal, it won’t screw up your long as it's black coffee, says Boules. All your regular coffee variations and add-ins will cost you fat and calories—just so you know, consuming fat and calories means you are no longer fasting.

Plain black coffee has around two to five calories (per cup), according to the USDA. But once you start pouring in sugar, milk, or cream, you’re adding anywhere from 16 to nearly 100 more calories. So while your cup of Joe can be a decent way to tide you over your fasting window, make sure you’re drinking it straight. The more calories you feed your body, the more outside fuel it has to use as energy, which means it won't utilize the glucose stored up in your fat cells, in theory.

But you don’t necessarily want to start a coffee-drinking habit just to soothe your hunger pains if you're doing intermittent fasting. Caffeine can temporarily speed up your metabolism and potentially take you out of that starvation mode. "But if you're [already] a regular caffeine consumer, the thermogenic effect is minimal,” explains Boules.

In other words, if you’re already addicted to an a.m. coffee, you should be okay: The calories are minimal and your body is adjusted to the caffeine. But if you were a decaf tea drinker before, don’t start chugging espresso, mmkay?

Can coffee enhance the effects of intermittent fasting?

Since coffee makes you feel revved up, it’s only natural to wonder if it can give you an added boost when you’re fasting. Here’s the thing: There’s no data to either support this idea or shut it down, so it’s really hard for experts to say one way or another.

But it’s unlikely that coffee will do much, if anything, to give you a weight-loss boost of any sort when you’re fasting, says Scott Keatley, RD, of Keatley Medical Nutrition Therapy.

Black, unsweetened coffee “increases metabolic rate by about three to four percent,” Keatley says, noting that this can be seen in a very slight increase in your body temperature. “The effect peaks at about 90 minutes after consumption, and the effect is seen in subsequent cups of coffee as long as they are spaced out by about two to three hours,” he says.

Got all that? Cool, cool. Here’s the slight issue with this, Keatley adds: Fasting actually slows down your metabolism as your body tries to decrease your metabolic rate to conserve energy. So, while coffee might increase your metabolism slightly during a fasting state, your body is also operating at a slower speed. Essentially, you’re really not gaining anything special by downing coffee.

Still, your caffeine habit may serve up other benefits unrelated to the scale. Coffee may boost brain health by activating certain neurons and impacting the local release of dopamine, a 1992 study published in Brain Research Reviews found.

Additionally, there's a clear association between increased coffee consumption and a lower risk of developing certain conditions such as cardiovascular disease, type 2 diabetes, and liver cancer, according to a 2021 study published in Advances In Nutrition.

What can I put in my coffee that won't break my fast?

“The general rule for what to put in your coffee is that it basically has zero calories,” says Keri Gans, RD, author of The Small Change Diet. “In other words try and enjoy it black.”

But having your coffee black just might not be your ~thing~ and that’s okay. You can add the following to your coffee and still not break your fast, Keatley says:

  • A sprinkle of cinnamon

  • A touch of nutmeg

  • A teensy bit of cocoa

  • Low-calorie sweeteners like Splenda

Unfortunately, buzzy supplements such as collagen powder or MCT oil *can't* be added to your cup of coffee, "as one serving does provide too many calories," Gans says. Ditto for a splash of milk or unsweetened almond milk in your coffee—it contains too many calories.

What else can I drink while I'm in a fasted state?

Obviously, you can drink water—you can even punch it up with some fresh fruit or a squeeze of lemon for flavor as long as you aren’t actually eating the fruit. But Boules says you can also reach for unsweetened tea, which is zero calories without anything extra added to it.

What you can’t drink, however, are things like fruit juices, green juices, or smoothies. This trips people up sometimes, because we tend to think stuff in liquid form is calorie-free, just like water. But juices and smoothies are made with food, which means they have enough calories to break your fast.

“The whole idea behind the alleged health-promoting benefits is actually fasting (i.e., zero calories or energy intake),” says Boules. “[If you’re] consuming something caloric during ‘fasting hours’ in the form of sweetened beverages, juices, or smoothies, you're not fasting.”

Remember, the whole concept of intermittent fasting relies on a prolonged, uninterrupted period of not consuming any calories (otherwise, your body won’t go into the state of starvation necessary for your insulin levels to drop and your body to burn fat).

Most people opt for a 16:8 diet—fasting for 16 hours per day and eating during an 8-hour window—though it's not totally clear as of now if 16 hours of intermittent fasting is the magic number of fasting hours to see weight-loss results. But, that's a reasonable rule to follow until science reveals more about IF. (A 2016 study actually proposed an 18-hour fast for maximum weight loss—but the sample size was tiny, only 11 people.)

Can I eat *anything* during my fasting window?

Unfortunately, Boules says that consuming any calories your body could convert to energy for fuel means you are not fasting anymore. Period.

So what can you chew on? Well, it’s not terribly exciting, but feel free to reach for some sugar-free gum or hard candy with sugar alcohols or non-nutritive sweeteners like xylitol, which Boules says don’t affect your calorie intake or blood glucose in the way that regular sugar does, so you won't be breaking your fast. (But the artificial kinds could cause some bloating or mild GI upset, FYI).

But if you’re finding that you’re a raging hunger monster while doing intermittent fasting or just otherwise totally miserable, it’s okay to quit and try something else—fasting certainly is not for everyone. “If you are genuinely physically and emotionally nourished and satisfied [while] confining food to a set number of hours per day, do you,” says Boules. “If you're not, just understand this is not required to achieve your goal of good health.”

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