You wake up on day 1 of your fast, pour yourself a steaming cup of coffee, bring it to your lips, and then, just as you’re about to take a sip… oh wait: “Am I allowed to have this?” It smells soooo good, but will it derail your fast?

Probably not. But it depends on your goals.

Most people fast for one of three reasons: weight loss/metabolic health, gut rest, or longevity. Whether coffee breaks your fast depends on why you’re fasting. It also depends on how you drink your coffee. Do you take it black? With cream? With butter? So let’s pour our coffee talk through that filter.

Black Coffee

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.

One literature 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 resistant 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.

Another 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.

The Verdict:

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. 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. 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.

The Verdict:

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

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. 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.

The Verdict: