When considering whether a food, beverage, or supplement might break a fast, it’s important to first consider what your goals are around fasting. The three most common reasons people fast are for weight loss/metabolic health, gut rest, or longevity. Let’s take a look at coffee—the beverage we see the most questions about—through that lens. Read More
Coffee itself has almost no calories, so it already has that going for it. Several studies have examined whether fasting still demonstrates health and disease-prevention benefits if the people fasting drink coffee. The answer: yep! The benefits are intact. These studies also considered whether people got those same benefits from a restricted calorie fast (RCF) accounting for less than 25% of energy needs, and saw the same result.
review demonstrated that coffee was associated with a decrease in insulin sensitivity, but scientists have observed those same short-term effects in fasting in general, with or without coffee. During nutrient deprivation, cells become slightly more insulin resistance likely due to the body prioritizing fuel to go to the brain instead of other cells in the body. This was also a short-term study, so further research would be necessary to show whether coffee has any detrimental effects on insulin sensitivity in the long term. Conversely, there have been many long-term studies linking regular coffee consumption to positive health benefits including reducing the risk of type 2 diabetes.
study showed that consumption of both caffeinated and decaffeinated coffee trigger autophagy in mice, which is good news for longevity. The authors of this study related the increase in mTOR inhibition and other cellular processes to the polyphenols in coffee. This same study hasn’t been replicated in humans yet, so we have to take these results with a hefty grain of Himalayan sea salt.
What if you’re fasting for gut rest? Even though black coffee has negligible calories, it does trigger some digestive functions. Coffee stimulates gastrin (a hormone that triggers the secretion of gastric acid) and gallbladder contraction, both of which have an impact on our gastrointestinal tract. Coffee intake may also elicit a reflux sensation, which isn’t ideal for those with heartburn issues.
Fasting for metabolic health/weight loss: likely does not break a fast
Fasting for gut rest: does break a fast
Fasting for longevity: likely does not break a fast
Butter (Bulletproof) Coffee
Butter coffee, popularized by the bulletproof coffee brand, typically has added butter and/or medium chain triglyceride (MCT) oil. Even though MCT oil is calorically dense, it’s been shown to improve insulin-mediated glucose metabolism . Plus, the body easily converts MCTs into ketones to use for energy. Doctors have also used MCT oil to induce ketosis in the management of epilepsy , demonstrating that the consumption of MCT oil can still produce a ketogenic environment.
Butter and MCT oils have a different chemical composition though. MCTs are produced from coconut oil and, as the name suggests, are 100% made-up of medium-chain triglycerides. Butter, on the other hand, is mostly composed of long chain triglycerides (LCTs). MCTs and LCTs have a different effect on our gastrointestinal system . MCTs are absorbed directly through the portal vein and taken immediately to the liver, whereas LCTs stimulate pancreatic enzymes and require the release of bile into the GI tract. So, butter does trigger some digestive processes while MCTs are less likely to do so.
Butter also has a small amount of protein in it. Typically, protein inhibits autophagy, but butter contains such a small amount it’s unlikely to matter or affect your blood glucose. MCT oil contains no protein at all, but it is highly caloric. Energy restriction is also important for autophagy, so overconsumption of butter or MCTs may not provide the low nutrient environment necessary for autophagy’s longevity benefits. For example, a typical bulletproof coffee calls for 2 Tbsp. grass-fed butter and 1 Tbsp. MCT oil, which provides ~320 kcal. It’s possible that this amount of energy intake either slows or stops the longevity benefits of fasting.
Fasting for metabolic health/weight loss: likely does not break a fast
Fasting for gut rest: though MCT oil has minimal impact on the digestion, coffee and butter break a fast focused on gut rest
Fasting for longevity: likely breaks a fast
Coffee + Cream
Plain, high-quality dairy by itself likely does not contribute to weight gain or increased risk of metabolic disease such as insulin resistance and high blood pressure. One study was even able to show an association between consumption of trans-palmitoleate (a fat found in milk) and lower fasting insulin levels. However, quantity is key since a couple Tbsp. of cream/milk in coffee is very different than a few cups.
Cream, milk, and other dairy products contain carbohydrates, protein, and fat that do require digestion, so the gut is activated after consumption. Research is limited on dairy and its role in autophagy, but a few studies have shown that high-quality dairy consumption does not increase risk of chronic disease.
Fasting for metabolic health/weight loss: likely does not break a fast in small quantities
Fasting for gut rest: breaks a fast
Fasting for longevity: likely does not break a fast, but research is limited in this area
How many calories break a fast?
Hate to break it to you, but “technically, any caloric intake breaks a fast,” says Benjamin Horne, Ph.D.
, a genetic epidemiologist who has published research on the effects of intermittent fasting. Even a few calories’ worth of food can inactivate some of fasting’s perks. According to Horne, some mechanisms behind fasting’s benefits, like ketosis (which increases fat burn), remain active with the consumption of certain macronutrients; but others, like autophagy , may or may not remain active. “In humans, it appears that autophagy does not remain as active when any food is consumed,” he says.
But what if we’re talking about a measly 2 to 5 calories in a cup of tea or coffee? This ultra-low-calorie territory is where things get a little tricky. According to Horne, it likely needs to be a water-only fast to maintain the maximum benefit from autophagy.
Others aren’t so sure we need to be quite this nitpicky, though. “I’ve heard good debates about whether coffee can break a fast. We don’t have any good evidence to show either way,” says Vincent Pedre, M.D.
, an integrative physician and gut health expert who frequently recommends intermittent fasting diets to his patients. “I would say if you’re drinking organic black coffee no cream, no sweeteners then you should be fine. That said, I would say stick to as close to zero calories as you can during your fasting period
with plenty of clean filtered water; herbal teas are also good.”
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. Keatley says you can add the following to your coffee and still not break your fast:
- A sprinkle of cinnamon
- A touch of nutmeg
- A teensy bit of cocoa
- Low-calorie sweeteners like stevia
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, she adds 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 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 not only that it can raise your blood sugar levels a lot.
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.