During a 72-hour fast, you can have water, black coffee, plain tea, and electrolytes. A small number of other items fall into a gray zone depending on your goals, but those four form the core of what’s widely considered acceptable. The key principle: anything with significant calories, protein, or sweetness can trigger metabolic responses that undermine the reasons you’re fasting in the first place.
Water and Electrolytes Come First
Water is the foundation of any extended fast, and during 72 hours you’ll need more of it than you might expect. Without food, your body loses sodium and water faster than usual because insulin levels drop and your kidneys excrete more fluid. Plain water alone can actually work against you if it dilutes your remaining electrolytes without replacing them. Adding a pinch of salt to your water, or using mineral water, helps prevent the headaches, dizziness, and muscle cramps that catch many first-time fasters off guard.
Sodium is the most important electrolyte to replace. A quarter teaspoon of salt in a glass of water a few times a day keeps most people stable. Potassium and magnesium matter too, especially by days two and three. You can get these through sugar-free electrolyte packets or drops, but check the label for sweeteners and calories (more on that below). Some people sip diluted sole water (water saturated with unrefined salt) throughout the day.
Black Coffee and Plain Tea
Black coffee, both regular and decaf, is one of the safest things to drink during a fast. It contains essentially zero calories and does not appear to suppress autophagy, the cellular cleanup process that many people are trying to activate with a 72-hour fast. Research published in Cell Cycle found that coffee actually triggers autophagy in animal models, and the effect comes from polyphenols in the coffee itself, not just caffeine. Decaf produced the same result.
Plain tea, whether green, black, or herbal, is similarly fine as long as you add nothing to it. No milk, no honey, no sugar. Even a splash of cream introduces enough fat and protein to trigger a metabolic response. If you find black coffee too harsh on an empty stomach, green tea tends to be gentler and still provides a mild energy boost through its combination of caffeine and the amino acid L-theanine.
One practical note: caffeine on an empty stomach can spike cortisol and cause jitters or nausea, especially if you’re not a regular coffee drinker. If this happens, switching to tea or cutting back to one cup in the morning is a simple fix.
What About Bone Broth?
Bone broth is the most debated item in fasting circles. It contains calories, a small amount of protein, and some electrolytes. Whether it “breaks” your fast depends on what you’re fasting for.
If your primary goal is autophagy, protein intake matters a great deal. Research published in Nature Metabolism identified a threshold of roughly 25 grams of protein per meal as the point where mTOR, a growth-signaling pathway that shuts down autophagy, becomes activated. A typical cup of bone broth contains about 6 to 10 grams of protein, which falls under that threshold. However, even smaller amounts of protein and amino acids (especially leucine) begin nudging your metabolism away from a fully fasted state. A cup of bone broth won’t completely cancel autophagy, but it does reduce the depth of it.
If your goal is metabolic rest, calorie restriction, or practicing discipline, a cup of bone broth on day two or three can help you finish the fast rather than abandon it. Many people use it as a tool to manage symptoms, not as a regular addition throughout. Think of bone broth as a compromise: it keeps you closer to fasted than eating would, but it’s not a zero-impact choice.
Apple Cider Vinegar
A tablespoon of apple cider vinegar diluted in water is a popular addition during fasts. It contains roughly 3 calories per tablespoon, which is negligible. Its main active component, acetic acid, actually works in your favor during a fast. It inhibits enzymes that break down carbohydrates and has been shown to reduce fasting blood glucose levels in clinical trials. In one randomized controlled study of diabetic patients, the group consuming apple cider vinegar saw a significant decrease in fasting blood glucose compared to controls.
The practical downside is that vinegar on a completely empty stomach can cause nausea or acid reflux in some people. Diluting it well and sipping slowly helps. It won’t break your fast in any meaningful metabolic sense.
Sweeteners: Proceed With Caution
This is where many people unknowingly compromise their fast. Artificial and non-nutritive sweeteners, even zero-calorie ones, can trigger insulin release. The sweet taste activates receptors on your tongue and in your gut that signal your body to prepare for incoming sugar. Research has shown that sucralose in particular raises blood insulin levels compared to plain water, even without any actual glucose entering the bloodstream. This “cephalic phase” insulin response is the opposite of what you want during a fast designed to keep insulin low.
Stevia and monk fruit are generally considered less problematic than sucralose or aspartame, but the research is not definitive enough to call them completely safe for fasting purposes. If you’re going through the effort of a 72-hour fast, avoiding all sweeteners is the most reliable approach. This includes flavored electrolyte products, diet sodas, and flavored sparkling waters with sweeteners.
Sugar-free gum and mints fall into this category too. Each piece of gum contains about 5 to 10 calories from sugar alcohols, and the sweet taste can stimulate hunger by triggering saliva production. One piece probably won’t derail your fast biochemically, but chewing gum throughout the day adds up and can make the psychological challenge of fasting harder by keeping your appetite engaged.
What to Skip During the Fast
Fat-soluble vitamins (A, D, E, and K) need dietary fat to be absorbed properly. Taking them on an empty stomach means your body can’t use them, and they’re more likely to cause nausea. If you normally take a multivitamin or fish oil, pause these during the fast and resume when you eat again. Water-soluble vitamins like B and C can technically be taken, but many people find that any supplement on a completely empty stomach causes discomfort.
Anything with calories from carbohydrates or protein, no matter how small, shifts your metabolism out of a fasted state to some degree. Juice, milk, coconut water, protein shakes, and smoothies all break the fast immediately. Bulletproof coffee (coffee with butter or MCT oil) is sometimes promoted as “fasting-friendly,” but the 200-plus calories from fat will stop autophagy and raise insulin, even if it keeps you in ketosis.
What Happens During 72 Hours
Understanding the timeline helps explain why the rules above matter. In the first 12 to 24 hours, your body depletes its stored glycogen and transitions to burning fat for fuel. Between 24 and 48 hours, animal studies suggest autophagy begins ramping up. By 48 to 72 hours, you’re in a deep fasted state where autophagy, fat oxidation, and ketone production are all elevated.
Research from USC found that a 72-hour fast can trigger the recycling of old and damaged immune cells, prompting the body to regenerate new ones from stem cells. This study was conducted over six months with subjects undergoing chemotherapy, but the underlying mechanism, the body conserving energy by breaking down unneeded immune cells, applies broadly. This regeneration effect is one reason people specifically choose the 72-hour mark rather than a shorter fast.
Breaking the Fast Safely
What you eat after 72 hours matters almost as much as what you consume during the fast. Refeeding syndrome, a dangerous shift in electrolytes caused by suddenly reintroducing food, is a clinical concern primarily for people who have been malnourished or fasting for seven days or more. A 72-hour fast in an otherwise healthy person carries low risk, but your digestive system will still be sensitive.
Start with something small and easy to digest. A cup of bone broth, a small portion of cooked vegetables, or a few bites of avocado gives your gut time to wake up. Wait 30 to 60 minutes before eating a fuller meal. Keep that first real meal moderate in size and relatively low in carbohydrates, since a large carb load after three days of fasting can cause a sharp insulin spike, bloating, and fatigue. Protein and healthy fats are easier for your body to handle as a reentry point. By the second meal, most people can return to normal eating without issues.