Fasting has been around (whether forced or voluntary) for thousands of years, but exploded in popularity in the past decade thanks in part to body builders looking to improve their physique. Lately, Silicon Valley execs are turning to fasting  as a way of improving mental performance and a broader health trend of eating less frequently is becoming more popular.
For the health and biohacking geeks, the question “ does black coffee break a fast “ is a compelling conversation. Some well-respected scientists say one thing, others say something different and most readers are caught wondering what the truth really is.
Drinking coffee during a fast does alter some elements or mechanisms associated with fasting while not impacting others. The short answer to this question is both “yes” and “no”. Each user must determine what his or her goals are to take best advantage.
The following article will breakdown the various arguments and help you to determine what might be right for you.
Does Black Coffee Break a Fast?
According to an interview between Dr. Rhonda Patrick and Dr. Satchin Panda, black coffee does start the biological clock as would anything that is not water (skip to 42:08 in the video).
Obviously both Dr. Satchin Panda (a researcher at the Salk Institute for Biological Studies) and Dr. Rhonda Patrick are trustworthy sources. In a follow up, Patrick tweeted:
“Anything that affects the brain, gut, liver etc. sets the clock. Caffeine does this.” 
In a later conversation with Joe Rogan, she explains that caffeine is xenobiotic and the liver enzymes must break it down causing the biological clock to start (as far as fasting is concerned) .
As any good scientist, Patrick did note that “…we don’t have empirical data on that…”. Hopeful biohackers may feel more comfortable knowing there isn’t empirical data, but it seems there are some compelling arguments that black coffee does break a fast.
According to a later interview, drinking black coffee (anything other than water) is the equivalent of being in 40 – 50% of a fast . Benefits exist for glucose metabolism and insulin, but many elements of the fast are broken.
The question is… which elements?
Autophagy and Longevity with Coffee
One of the greatest benefits of fasting is a cellular waste removal process known as autophagy . During autophagy, cells “self-digest”, which helps to fight disease .
A study published in Cell Cycle in 2014 found that polyphenols in coffee (either caffeinated or not) can improve elements of the autophagic process . In this video with Dr. Guido Kroemer, Dr. Patrick learns that not only is it okay to drink coffee and fast, it may actually beneficial:
The answer to the question “does black coffee break a fast?” seems to be a resounding “yes”, but yet the major benefits of fasting are unaffected (or enhanced). As with all biology, there is lots of nuance involved.
Fat Utilization and Metabolism
Another major benefit of doing an intermittent style fast is to utilize fatty acids (breaking them down in the absence of glucose) and using them as energy. As exogenous ketones and the ketogenic diet become more popular, techniques that are synergistic become compelling.
Fasting is one of those synergistic activities and according to one study, caffeine upregulates ketone production .
Aside from metabolizing fats more effectively (and often), a term Dr. Mike T Nelson has called “metabolic flexibility”, fasting also improves glucose metabolism and insulin sensitivity. Both are key indicators of health and coffee reduces insulin sensitivity and glucose tolerance (good) .
What is the Downside of Coffee on a Fast?
At the moment, there isn’t much empirical data to suggest that coffee has significant negative effects when combined with fasting. From a theoretical perspective, Patrick and Panda might be correct about black coffee breaking elements of the fast.
Based on what we know now, the most important elements of fasting are preserved (or even enhanced) when combined with coffee.
How to Use Black Coffee While Fasting
Given what we know (and realizing what we don’t), it’s possible to create a framework to make the best use of fasting. Here are some basic guidelines, rules, or tips to use when customizing your personal plan.
Try no coffee – Many people are very attached to their coffee for understandable reasons, but spending a couple of days each week without coffee (or any substance besides water) can be effective.
We don’t know what the benefits of maintaining a true water fast might be versus caffeine / coffee. If you need to do this on the weekend so your brain isn’t foggy while working, this can be a great time to do so.
Coffee is not caffeine – A common nootropic stack is to combine caffeine and L-theanine in capsule form. This may not be as beneficial because many of the benefits of coffee (for autophagy) are from the polyphenols not the caffeine itself.
Don’t add fats – While there are some benefits for mental energy, glucose metabolism, etc., the effects of autophagy will be gone adding MCT oil, coconut oil, and butter into your coffee. That doesn’t mean there isn’t a time and place for that if it is a valuable tool for you, but it is going to lose many of the longevity and anti-aging benefits this way.
Mix and match these options to get the benefits that most align with your values and commitments. Certain people cannot trade subtle benefits for anti-aging in exchange for not having caffeine to do work. These are value judgments we each have to make.
Whatever you choose, it’s best to do so informed of the latest information and research. Black coffee can break a fast, but many of the benefits remain. Happy fasting.