The diet—which asks adherents to limit their caloric intake to specific time windows or days of the week, depending on the specific plan—has strict rules about what one can and cannot consume during a fasting period in order to enjoy the purported benefits of more energy, less brain fog, and healthy weight management. And surely, drinking something as delicious as coffee goes against those rules, right?
New to IF? Here's the 411 straight from a top RD:
Does drinking coffee interfere with intermittent fasting?
The good news: Neither expert gave coffee a hard no for anyone doing intermittent fasting—but it did come with some caveats.
If you're using IF primarily to help with weight management, Minno says straight coffee shouldn't interfere with your goals because it has nearly zero cals (a plain cup of brewed coffee has less than 5). It also won't take you out of ketosis (the state where your body is burning fat instead of carbs for fuel), which is great news if you're on the keto diet and want to incorporate IF.
But once you start adding in anything more caloric to your coffee, like milk or MCT oil, that's more of a no-go. "Technically, when you're fasting, you're not supposed to eat anything," Dr. Axe says. And some more elaborate coffee drinks can come packed with calories—even Bulletproof coffee, which adds MCT oil and/or butter to the brew, has around 440 calories per serving, according to Minno. There's nothing wrong with calories, but consuming them means you're no longer in a fasted state.
Additionally, "part of the benefits of fasting is letting systems of your body rest," says Dr. Axe. "With coffee, you're extending your adrenals; they start working." Basically, your adrenal glands release cortisol (which helps your bod respond to stress), and excessive caffeine intake can overwork those glands. Letting them rest during a fasting period only works so well if you're not consuming something like coffee that will trigger them into action again.
But that's not to say you'll completely cancel out all benefits of a fast by taking a sip of coffee, he adds. "Even with coffee, your liver, pancreas, and other organs are still resting and you're still getting big benefits through IF," he says. They just might not be as pronounced as they would if you skipped the coffee altogether during a fasting period.
Could coffee help with intermittent fasting?
If you're sticking with black coffee, there could be some benefits to having it between meals. "If you're mentally or physically tired because you're not getting your energy from calories from food, the caffeine can definitely help you be more alert, focused, and help you physically power through," she says. For example, if you're working out in the morning before eating your first meal, it can especially be helpful—Minno says that caffeine has been shown to help with athletic performance.
She issues this with a major caveat though: No one should ever use coffee as a replacement for food when they need nutrients. "Anyone doing intermittent fasting should really work with an expert to make sure they're getting everything their body needs," she says. Don't just tough it out if you really feel poorly. There are also some iterations of IF that are straight-up unsafe, such as the OMAD diet (which stands for "One Meal a Day"), which is why it's so important to work with an expert. (Intermittent fasting in general is also not recommended for people with a history of disordered eating, or for people who are pregnant or trying to become pregnant.)
The bottom line: While some IF diets might call for zero coffee during a fasting period, sneaking in a cup or two won't totally undo the benefits (as long as it's black coffee). And let's be real—it might be what will keep you sane.
This story was originally published on November 18, 2018. It was updated on February 18, 2021.
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