We consulted with health and nutrition experts on this comprehensive guide to drinking coffee while fasting: the pros, the cons, and how to do it most effectively for metabolic health and longevity benefits. Time-restricted eating (TRE) is a type of intermittent fasting that involves limiting food intake to a short hourly window each day and fasting for the remaining time. Example ratios include 16:8 and 18:6, with the first number being the hours you spend not eating during a 24-hour period. “Food or absence of food can help regulate our internal clocks1, turning our genes off and on and regulating metabolism,” adds Jaime Seeman, M.D., board-certified OB-GYN, and the host of the Fit and Fabulous podcast. “Independent of weight loss, people who use time-restricted eating have improved insulin2 and glucose levels3 as well as improvements in blood pressure2.” Fasting also has benefits that go beyond metabolic health to potentially aid longevity4. “Eating in a narrower window each day, particularly if most of the calories are taken in the morning and early afternoon, has been shown to increase the amount of time the body spends making vital repairs that can save us from diseases5 we don’t yet have and might halt or even reverse some diseases we do have,” says journalist Steve Hendricks, author of The Oldest Cure in the World: Adventures in the Art and Science of Fasting. You can think of the fasting period as a time for your body to go into “cellular housekeeping6” mode. “Among the repairs the body accelerates during time-restricted eating,” Hendricks adds, “are patching up damaged or miscopied DNA, producing more antioxidants to fight off the daily assault from free radicals, and increasing autophagy7, which is the recycling of worn-out cellular parts.” The answers aren’t so clear-cut. A cup of black coffee contains very few calories, only about two, so the caloric impact on the fast is likely minimal. The caffeine in coffee, however, will technically affect your fast, though it’s not entirely clear how much it gets in the way of fasting benefits. “Caffeinated drinks, even noncaloric ones,” Hendricks explains, “almost certainly disturb a fast, but scientists don’t yet know precisely in what ways or to what extent. What we know with certainty is that even a modest amount of caffeine, particularly in the morning, resets our circadian clocks8. But to what extent morning caffeine also fires up our metabolism and pulls us out of our body’s fasting-and-repair mode is, so far as I know, still uncertain.” One thing we do know, notes Pedre, is that many popular coffee additives will further break your fast. “What breaks any fast,” he says, “is when you cause an insulin spike, which would be caused by eating carbohydrates or taking sugar in your coffee, for example.” The caveat is that coffee can trigger insulin release9 even without sugar or carbohydrates, he adds. “Anything that contains a calorie can impact fasting,” Seeman echoes. “This goes for carbs, fats, proteins, and alcohol. These nutrients activate our internal clocks and essentially ‘break’ our time-restricted eating window.” This means that ultimately, the decision about whether to drink coffee (and how to take it) while fasting is largely a personal one. It depends on how much of a purist you are about fasting and how much you value that morning cup of coffee. It’s worth noting that coffee consumption in general may also have some metabolic health benefits (noted below), so those are also worth considering when weighing your decision to caffeinate or not. Here’s the scoop on several common coffee adds and how they impact a fast: MCT oil may have some metabolic health benefits, including supporting weight management16. The fat consumption may provide energy, help you feel fuller, and prevent a subsequent blood glucose (sugar) drop brought on by the caffeine-driven insulin release. “Some people do quite well with coffee, butter, and MCT oil without tanking their blood glucose,” explains Pedre, “and these individuals may be able to have coffee early in the morning while still fasting and maintain the fasted state until lunchtime in the afternoon.” Seeman adds that someone may consume MCT oil during a fasting window as a way to boost ketone levels and enhance cognition17. “In this case,” she says, “it all depends on what someone’s goals are.” Here’s the science behind why this can be a beneficial pairing: Low-grade inflammation is a hallmark of many chronic conditions, including Type 2 diabetes and metabolic syndrome. Coffee consumption specifically (not just caffeine) is associated with a reduction in pro-inflammatory markers22. “The benefits of coffee are mostly related to the polyphenols it contains,” Seeman says. Polyphenols23 are plant compounds that act as antioxidants. “Polyphenols help with glucose regulation in the liver,” Seeman says. This could explain some of the associations we see with coffee consumption and reduced risk of Type 2 diabetes and metabolic syndrome. But there’s likely more to the link. “Coffee boosts both thermogenesis24 immediately and metabolic rate25, meaning using fats as fuel, for up to 24 hours,” Pedre adds. Like coffee, intermittent fasting also helps control inflammation by boosting a protein called galectin-326, according to preliminary research. Fasting also drives thermogenesis, promoting the browning of white adipose tissue27, important for weight management and insulin sensitivity. Finally, “[Polyphenols] also stimulate autophagy within the liver,” Seeman adds, “clearing out dead and damaged cells.” And intermittent fasting boosts autophagy7, as well. The short-term benefits of coffee on the body and brain include increased mental focus and alertness31 and increased exercise performance32. The caffeine is at work here, and it may provide a welcome jolt if you’re feeling low energy from fasting. And some coffee drinkers are going to feel this more than others. “Some people are going to be very sensitive to coffee triggering an insulin response, even if the coffee is black or only has fat but no sugar,” Pedre says. “When the pancreas releases insulin, blood sugar will drop subsequently. And within 30 to 60 minutes of drinking coffee, a blood sugar drop will cause uncomfortable weakness, shaking, and cravings for carbs or sugar.” These regular crashes could have long-term impacts. “It may even lead to the overconsumption of calories over the rest of the day,” Pedre says, “which will cause weight gain in most individuals.” If you experience these “crash” symptoms, you may wish to wait until you break your fast to drink your coffee. Alternatively, you could wait until just 30 minutes before you break your fast, Pedre adds. That way you can use food to mitigate any blood sugar plummet. Another potential way to limit a crash is to add MCT oil and ghee to your cuppa. Huberman also says adding 100 mg of L-theanine can offset some of the jitteriness. Another potential side effect of drinking coffee on an empty stomach is gastrointestinal upset. “Coffee on an empty stomach can trigger the release of hydrochloric acid33,” Pedre explains, “leading to an acid stomach, heartburn, and acid reflux.” Excessive coffee consumption in a fasted state, over time, can also lead to abdominal pain and nausea, he adds. If you have a history of gallbladder attacks, you may wish to avoid coffee, especially on an empty stomach. “Coffee induces the release of a hormone called cholecystokinin34 [CCK],” Seeman adds, “which causes gallbladder contractions. People with underlying gallstones can get symptomatic with ingestion of coffee.” Finally, be aware of how much coffee you consume and if you have any restrictions on caffeine intake regarding underlying conditions. “Excessive caffeine intake has risks35, including poor sleep, high blood pressure, headaches, anxiety, and more,” Seeman notes. “The recommended upper limit of caffeine is no more than 400 milligrams per day for adults.”
