Does coffee break my fast? This is one of the questions we get most frequently from our readers and LIFE Fasting Tracker users. The simplest answer is, probably not. In fact, coffee may boost many of the physiological impacts of fasting.

But because all of us here are into exploring, tracking and measuring the impacts of our lifestyles and interventions like intermittent fasting on our bodies and our health, let’s dive deeper into this story of coffee and fasting. Because the real answer to “Does coffee break my fast?” is, as always, more complicated than yes or no!

Have questions about coffee and fasting that we haven’t covered here? We’ll be responding to your questions LIVE on Facebook Messenger from 10AM – 1PM Central US time on Weds, May 22nd. Click here to pre-register for our Coffee AMA and be notified when it starts!

Will coffee break your fast? Probably not! You can even enjoy a tad of cream or butter in your coffee while fasting, but avoid sugar. Image Credit: Elle Hughes.
Will coffee break your fast? Probably not! You can even enjoy a tad of cream or butter in your coffee while fasting, but avoid sugar. Image Credit: Elle Hughes.

Coffee and Fasting – The Big Picture

When consumed in moderation, black coffee should not break your fast. In other words, having some coffee won’t kick you out of ketosis or a fat burning state. This is usually true as long as you are a healthy individual who hasn’t been diagnosed with diabetes and who metabolizes coffee quickly (there are gene variants that cause slow clearance of coffee from the body).  However, there is some controversy over how coffee impacts the fasted state in other ways.

Coffee, for starters, has many different compounds in it, not just caffeine. Plant compounds in coffee have actually been found to increase autophagy, a cellular recycling stage of intermittent fasting! Learn more here with LIFEApps blogger Zach about coffee and autophagy. Antioxidants in coffee may also help reduce inflammation and insulin resistance when consumed over time.

Caffeine in coffee, however, can temporarily raise levels of stress hormones like cortisol and epinephrine. These can in turn raise your blood glucose levels by signaling your liver to create new glucose molecules from proteins and fatty acids. Stress hormones can also temporarily block glucose uptake into your muscles (although exercise can help). Dose matters for these effects. Consuming two cups of black coffee in 10 minutes has been found to double levels of plasma epinephrine. However, epinephrine infusions in humans that cause plasma levels to reach 3-5 times normal levels have only been observed to cause small changes in fasting blood glucose levels (~5 mg/dl).

Overall, moderate coffee intake is unlikely to raise your fasting blood sugar levels enough to dampen or delay ketone body production or ketosis. You won’t have to worry about black coffee breaking your fast, especially if you isolate your consumption to the morning and don’t drink coffee continuously throughout your fast. The bigger concern is for individuals with diabetes or prediabetes consuming large amounts of coffee (or other caffeinated beverages) during high carbohydrate meals. In this case, the blocking effect of caffeine on glucose uptake can cause heightened hyperglycemia (high blood sugar).

If you are concerned, you can try drinking decaffeinated coffee or coffee that doesn’t have much caffeine (try darker roasts) while fasting. For individuals participating in clinical studies of intermittent fasting, Dr. Courtney Peterson recommends a single cup of coffee or less during a fast, to be safe.

“The degree to which moderate or typical caffeine consumption impacts circulating glucose or insulin response is minimal in healthy people,” says Dr. Imogen R. Coe, founding dean of the Faculty of Science at Ryerson University and a researcher who studies nucleoside transporters. “In fact, caffeine consumption may be correlated with better glucose responses in healthy people to some extent.”

Coffee and Fasting – The Gritty Details

If you welcome nuance, there is a vast, complicated system of physiological, cellular signaling and metabolic responses to coffee intake to explore. We’ve highlighted some of them below.

Just want the takeaways? Click here!

Some metabolic processes and signaling pathways are activated or increase in the presence of coffee or caffeine, while others are blocked or decrease. To make it more complicated, these effects vary from individual to individual and also from tissue to tissue, cell type to cell type.

Caffeine and other compounds in coffee have one effect on brain cells and another effect entirely on immune cells, for example, even when acting on the same receptors or cellular signaling targets. This is partly because caffeine is a non-selective antagonist or blocker of adenosine receptor activity. Adenosine receptors are extremely common – they are found on cells throughout the human body. They have many functions that vary by cell type, playing a role in biological processes as diverse cardiac rhythm and circulation, lipolysis (fat breakdown), renal blood flow, immune function, sleep regulation, angiogenesis, inflammation and neurodegenerative disorders.

“It can be calculated that the daily consumption of three to four regular cups of coffee results in approximately 50% A1 and A2A receptor occupancy for several hours.” – Adenosine Receptors as Drug Targets

Caffeine non-selectively blocks its cousin adenosine from activating all four different types of adenosine receptors known as A1, A2A, A2B and A3. By blocking these receptors, caffeine can at least temporarily have widespread effects throughout the body. Many of these effects are beneficial, such as protection of brain cells from death and injury, synaptic plasticity (growth of new brain cell connections), reduced inflammation, reduced insulin resistance, airway dilation and lowered activation of immune mast cells, which helps to reduce allergy and asthma-related symptoms.

However, some of the effects of adenosine receptor blocking by caffeine are less ideal in the context of fasting. By blocking adenosine receptors, caffeine can increase insulin secretion and reduce bone and immune stem cell growth and differentiation. Prolonged fasting and fasting mimicking diets have been shown to rejuvenate the immune system in animal models by activating immune stem cell renewal. Whether caffeine could interfere with these benefits of fasting is up for debate. What we do know is that it would likely take large and sustained doses of caffeine or strong coffee throughout the fasting period to have such an effect.

An A1 adenosine receptor in a cell membrane. Caffeine can bind to the receptor but doesn't activate its normal internal signaling cascade.
An A1 adenosine receptor in a cell membrane. Caffeine can bind to the receptor but doesn’t activate its normal internal signaling cascade.

Explore more of the nitty gritty details of the impacts of coffee on your fasting body below!

+/- Insulin Signaling

Coffee and caffeine have a somewhat complicated relationship with insulin in our bodies. Habitual moderate coffee drinking has actually been associated with increased insulin sensitivity and lowered risk for type 2 diabetes. But in acute and high doses, caffeine can decrease insulin sensitivity and raise plasma insulin levels. This effect runs counter to the ideal goal of a regular intermittent fasting practice, which is to lower insulin levels and related mTOR activity (the “gas pedal” of cellular growth and the “brake pedal” of autophagy).

The insulin-raising effect of caffeine is more likely to happen for individuals already at risk of developing diabetes, as well as individuals consuming higher doses of caffeine/coffee than they are accustomed to. For example, a series of randomized controlled trials in healthy individuals found that heavy coffee consumption (the equivalent of 13 conventional cups of coffee per day after several weeks of complete coffee abstinence!) significantly raised insulin levels. However, a more moderate intake of 5-8 cups of coffee or 870 mg of caffeine per day only slightly raised plasma insulin levels.

So if you are worried about coffee raising your insulin levels and disrupting the benefits of your fasting practice in terms of lowering insulin and mTOR activity – just drink in moderation! A few cups of black coffee in the morning should be fine.

A few cups of black coffee won't break your fast or raise your blood sugar much, but you might want to avoid breaking a fast with coffee and high-sugar foods due to risk of hyperglycemia. Image credit: Sarah Swinton.
A few cups of black coffee won’t break your fast or raise your blood sugar much, but you might want to avoid breaking a fast with coffee and high-sugar foods due to risk of hyperglycemia. Image credit: Sarah Swinton.

+/- Glucose

Coffee can also impact glucose levels, although it gets complicated because caffeine may have different impacts on glucose levels than other compounds present in coffee, including coffee’s polyphenols.

Acute caffeine intake at high levels can slightly raise fasting blood sugar levels but more importantly raise post-meal blood sugar levels. This is more of a risk for people who already have blood sugar control issues, including people with type 2 diabetes. For example, 1,5-AG or GlyoMark is a marker of glycemic control that decreases with acute coffee intake in people with type 2 diabetes.

“[The] acute effect of caffeine is an exaggeration of glucose and insulin responses to a carbohydrate challenge; large amounts of caffeine with meals may exaggerate hyperglyemia in type 2 diabetes patients.” – Lane et al., 2012

However, other studies have shown that polyphenols or plant-based antioxidant compounds in coffee can lower peak post-meal glucose levels in healthy individuals. So if you do need a hit of “pure energy” caffeine while fasting, coffee may be one of the best ways to get it thanks to the health benefits of chlorogenic acids and other antioxidants in coffee.

“[C]offee polyphenol consumption improves postprandial hyperglycemia and vascular endothelial function, which is associated with increased GLP-1 [Glucagon-like peptide-1, which stimulates insulin and reduces blood sugar] secretion and decreased oxidative stress in healthy humans.” – Jokura et al., 2015

Overall, you probably don’t have to worry about coffee significantly impacting your blood sugar levels as long as you are an otherwise healthy individual drinking moderate amounts of coffee (up to 5 cups per day). Polyphenols in coffee in particular may actually improve your glycemic responses to food over time, an effect that mimics intermittent fasting.

It's ok to drink coffee during a fast! Just enjoy in moderation.
It’s ok to drink coffee during a fast! Just enjoy in moderation.

+ cAMP

Caffeine and other compounds in coffee including theophylline, a caffeine-like compound, can affect levels of cyclic AMP (cAMP) within cells. cAMP is an important messenger that can activate many other proteins, including protein kinase A (PKA). Coffee can raise levels of cAMP through the adenosine receptors, which activate cAMP, as well as by inhibiting phosphodiesterases (PDEs) that normally degrade cAMP.

By raising levels of cAMP, theophylline has been found to dilate airways and reduce inflammation in asthma patients. Caffeine has also been found to lower levels of proinflammatory cytokines including tumor necrosis factor TNF-α in human blood, through the cAMP/PKA pathway.

+ PKA Signaling

By raising levels of cAMP and blocking adenosine receptors, caffeine and other compounds in coffee can also activate PKA or protein kinase A. PKA is a family of enzymes that have many downstream impacts on the body and its metabolic functions. PKA helps to regulate biological processes like glucose production and metabolism, lipid metabolism, contraction and relaxation of heart muscle, and even nerve cell activity and activation of the brain’s reward system.

As always, PKA can have both beneficial effects on the body as well as less beneficial ones in the context of intermittent fasting.

On the beneficial side, coffee-activated PKA signaling slows cognitive decline, increases working memory and reduces both the risk and symptoms of Alzheimer’s disease, among other neurodegenerative diseases. Just 1-3 cups of coffee per day can produce plasma concentrations of caffeine shown to prevent the formation of amyloid plaques in the brain of an animal model of Alzheimer’s disease. A dose of 4-6 cups of coffee per day (400-600 mg) has therapeutic potential for Alzheimer’s disease and reduces the risk of type 2 diabetes, Parkinson’s disease and liver cirrhosis. This level of coffee intake has thankfully not been shown have any adverse impacts on the heart. This may be due to the other antioxidant and anti-inflammatory plant compounds found in coffee; caffeine consumed not as coffee (e.g. energy drinks, etc.) can raise blood pressure.

Caffeine raises levels of cAMP and activates PKA, which in turn lowers the activity of RAF-1, an enzyme involved in cancer, and the activity of the pro-inflammatory NFkB pathway. The NFkB pathway increases inflammation levels but also elevates levels of an enzyme that helps produce the amyloid beta protein that gums up brains affected by Alzheimer’s disease. Caffeine also suppresses the activity of GSK-3, an enzyme that helps produce the protein tangles of Alzheimer’s disease and has also been implicated in type 2 diabetes, inflammation, cancer and mood disorders. Finally, other plant compounds in coffee have antioxidant properties and can reduce the levels of inflammatory cytokines in the brain.

Brain Inflammation from Alzheimer's Disease
Research suggests that chronic inflammation may be caused by the buildup of glial cells normally meant to help keep the brain free of debris. One type of glial cell, microglia, engulfs and destroys waste and toxins in a healthy brain. In Alzheimer’s, microglia fail to clear away waste, debris, and protein collections, including beta-amyloid plaques. Credit: National Institute on Aging, NIH.

But elevated PKA activity can also have unwanted side effects in tissues other than the brain, including in bone and in the immune system. Likely through cAMP and PKA activity, coffee can suppress the immune system. This is a good thing for some people with overactive immune systems, but potentially a negative thing in contexts where you’d want your body to be producing plenty of new antibodies and immune cells.

The good news is that a single cup is unlikely to increase PKA activity generally across the entire body – this would likely require systemic levels of caffeine sustained over a period of time. However, different people metabolize coffee differently, meaning that the amount of sustained coffee intake required for PKA activation differs by individual. Pregnant women, for example, metabolize coffee more slowly; it can take twice as long for caffeine to leave their systems as compared to before they were pregnant. Some people also have a genetic predisposition to metabolize coffee slowly – a 23andMe report can actually give you an indication of your ability to metabolize coffee quickly or slowly! (If you feel wired after a small amount of coffee, you may want to look into whether you have a CP1A2 coffee metabolism gene variant).

To get the positive impacts of PKA signaling without negative immunosuppressive impacts, you may want to simply moderate your caffeine intake, especially during a prolonged fast.

– mTOR

While caffeine can activate mTOR via PKA signaling in some cells, coffee tends overall to inhibit mTOR and enhance autophagy via cAMP and AMPK pathways. The nutrient sensing enzyme AMPK inhibits mTOR and activates autophagy, a cellular component recycling process, in response to both intermittent fasting and various plant compounds found in coffee!

Coffee has been shown in cell culture and animal model studies to enhance autophagy in liver, muscle and even brain cells! Learn more about coffee and autophagy here.

+ Circadian Clock, – Melatonin

Something all individuals practicing intermittent fasting should be aware of is that caffeine in coffee can reset our daily or circadian biological rhythms. Intermittent fasting also impacts of our circadian rhythms, usually in a positive way if we eat in tune with our daylight and active hours and fast longer overnight. But drinking coffee, especially later in the day, can delay our circadian melatonin rhythms by 40 minutes or more depending on the dose. Cyclic AMP actually plays a role here too; the rising and lowering of cAMP levels helps our cells keep time, so to speak. By preventing the degradation of cAMP, caffeine lengthens the period of cellular circadian rhythms.

In other words, when exposed to caffeine our cells go through a kind of jetlag where their days get longer.

In summary, caffeine intake in the form of coffee can impact our circadian rhythms and lower our production of melatonin, the sleep hormone. It’s best for this reason to confine your coffee intake to early in the day, especially if improved sleep is one of the benefits you’d like to glean from your daily intermittent fasting practice.

Both fasting and coffee can have positive impacts on the diversity of probiotic bacteria in our guts!
Both fasting and coffee can have positive impacts on the diversity of probiotic bacteria in our guts!

+ Gut microbe metabolism!

Researchers recently found that coffee consumption increases levels of health-promoting compounds in the body that are produced as gut microbes break down antioxidant plant compounds in coffee! Coffee contains polyphenols, ring-shaped compounds that absorb ultraviolet and other light and protect the body against free radicals. There are many polyphenols in nature including catechins in wine, tea, apples, grapes, blueberries etc.; isoflavones found in soybeans; and chlorogenic acid found in coffee.

Coffee and its polyphenols can promote the growth and metabolic activity of healthy gut microbes including Bifidobacterium (may help prevent colorectal cancer and inflammatory bowel disease) and Akkermansia (associated with improved blood sugar control and weight) species.

“[W]hen overweight or obese people undergo calorie-restricted diet therapy, the effect of improving insulin resistance has been reported to be more pronounced in humans with a higher abundance of Akkermansia in the intestine. […] Polyphenols derived from cranberries have also been reported to increase the abundance of Akkermansia, as well as help suppress obesity, insulin resistance, and intestinal inflammation.” – A next-generation beneficial microbe: Akkermansia muciniphila

Coffee’s impact on the activity of healthy gut microbes might even be tied to its positive impacts on risk and symptoms of neurodegenerative diseases including Parkinson’s disease and Alzheimer’s disease.

“After consumption, polyphenols need to be degraded by gut microbiota into small molecules that are easily absorbed by human body. Therefore, in order to sufficiently obtain the health benefits and increase the bioavailability and activity of polyphenols, you need a healthy gut microbiota (Moco et al., 2012). The protective role of other antioxidants and nutrients may also depend on the balance of gut microbiota to some extent. Healthy gut microbiota may increase their biological activity and utilization and thus maximum exert their brain protective roles and reduce the risk of AD.” – Hu et al., 2016


Congratulations, you’ve just learned a LOT in a small amount of time about the nitty gritty details of how coffee impacts the human body and metabolism!

We leave you with some takeaways below, as well as some questions to help you decide whether coffee is a healthy addition to your fasting practice!

  • Moderate! Moderate coffee consumption (up to 3-5 conventional cups per day) is safe and shouldn’t break your fast. Drink coffee with less of a caffeine content (darker roasts have less caffeine) if you are concerned about its impacts on your glucose and insulin levels.
  • Cut sugar. Have you been diagnosed with diabetes or prediabetes? Ask your physician how caffeine may impact your blood sugar levels. Generally try to avoid large doses of coffee (several cups) in conjunction with high-carb meals.
  • Coffee for allergy relief! Do you suffer from asthma or chronic inflammation? Coffee may help improve your symptoms! Enjoy moderate coffee with your prolonged fasts.
  • Immune check. Do you have an immune disorder or concerns about the function of your immune system? You may want to speak to your physician or avoiding caffeine due to its immunosuppressive impacts.
  • Don’t get wired at night. Do you have trouble sleeping or are you practicing time-restricted eating to improve your sleep? Go easy on the coffee and only drink it in the mornings.
  • Exercise and avoid overdoing coffee for bone health. Have you been diagnosed with osteoporosis or are you concerned about your risk? Especially if you are a postmenopausal woman with low estrogen levels, you may want to cut back on your caffeine intake to preserve bone density. Speak to your physician about this if you have concerns.
  • Say it with me – Autophagy! Are you fasting for autophagy, mental clarity or to preserve cognitive function as you age? Coffee may actually boost the impacts of fasting in this area! Enjoy a few cups of ideally black coffee per day! It’s ok to put a little cream or butter in your coffee while fasting, but avoid sugar, sweetened creamer or artificial sweeteners, which can activate insulin signaling and mTOR.

Paige Jarreau

I am the Director of Social Media and Science Communication for LifeOmic and an avid blogger. I'm interested in how scientists use social media to promote public engagement and health behaviors.

LifeOmic is the software company that leverages the cloud, machine learning and mobile devices to offer disruptive solutions to healthcare providers, researchers, health IT companies and patients.


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