During a 36-hour fast, you can have water, plain black coffee, plain tea, and electrolytes without meaningfully disrupting the metabolic processes that make the fast worthwhile. The goal is to keep calorie intake at zero or as close to zero as possible, because your body undergoes a significant metabolic shift partway through, switching from burning stored sugar to burning fat and producing ketones. Anything that delivers calories, even a splash of milk or a spoonful of honey, can blunt that shift.
What Your Body Is Doing During Those 36 Hours
Understanding why the “allowed” list is so short requires knowing what happens inside your body over a day and a half without food. After roughly 12 hours of not eating, your liver runs through its stored glycogen, and your metabolism flips to mobilizing fatty acids for fuel instead.1PubMed Central. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting That transition is sometimes called the “metabolic switch,” and it is the physiological gateway to most of the benefits people pursue with extended fasts: deeper fat burning, ketone production, and cellular cleanup processes.
By the time you reach 36 hours, the switch is well underway. A case report tracking a person doing twice-weekly 36-hour fasts found that the circulating concentration of the ketone body beta-hydroxybutyrate roughly quadrupled by the end of each fasting window.2PubMed Central. Twice-Weekly 36-Hour Intermittent Fasting Practice Attenuates Hunger, Quadruples ß-Hydroxybutyrate, and Maintains Weight Loss: A Case Report That level of ketone elevation indicates your body is deeply reliant on fat-derived fuel. Consuming anything with meaningful calories, especially carbohydrates, would tell the liver to stop producing ketones and start processing incoming glucose instead, effectively resetting the clock.
This is why the question of what you can “have” really boils down to what carries zero or negligible calories and does not provoke a significant insulin or metabolic response.
Water Is the Foundation
Water is the single most important thing you will consume during a 36-hour fast, and you will likely need more of it than you expect. When you stop eating, your kidneys begin flushing sodium at an accelerated rate. Sodium pulls water along with it, which is why people often lose several pounds of water weight in the first day or two of a fast. One study of 40 fasting patients found two distinct patterns: some had a steady, low-level sodium loss, while others experienced fluctuating losses that led to marked sodium depletion.3PubMed Central. Urinary sodium and potassium excretion in fasting obese subjects That fluid loss can cause headaches, dizziness, and fatigue if you are not actively replacing water.
There is no single right volume. A common guideline is to aim for at least the same amount you would drink on a normal day and then add extra if you feel thirsty, lightheaded, or notice dark urine. Sparkling water and mineral water are both fine. The carbonation in sparkling water contains no calories, and some mineral waters contribute small amounts of sodium, magnesium, and calcium, which is a mild bonus given the electrolyte losses fasting produces.
Black Coffee
A standard cup of black coffee has roughly two to five calories, which is close enough to zero that it does not interrupt the fasted state in any practical sense. More interesting is what coffee appears to do at a cellular level during a fast. A study in mice found that coffee triggered autophagy, the body’s process for breaking down and recycling damaged cellular components, in the liver, muscle, and heart within one to four hours of consumption.4PubMed Central. Coffee induces autophagy in vivo The researchers noted that coffee also suppressed a growth-signaling pathway called mTOR, mimicking the same signals that nutrient deprivation itself produces. In other words, black coffee appears to work with the fast rather than against it.
The caveats are straightforward. Adding cream, milk, sugar, butter, or flavored syrups adds calories and, in most cases, enough of an insulin response to break the fast. Even small amounts of cream (say, a tablespoon at 50 calories) can matter over 36 hours if you are having multiple cups. If you find black coffee undrinkable, try a lighter roast or cold brew, which tends to taste smoother and less bitter without any additions.
One practical note: coffee on a completely empty stomach can cause acid reflux or nausea in some people. If that happens to you, spacing your coffee later into the fast or simply switching to tea is a reasonable adjustment.
Plain Tea
Tea of any variety, whether green, black, white, herbal, or oolong, is safe during a 36-hour fast as long as you skip the sweeteners and milk. Like coffee, plain tea sits at essentially zero calories. Green tea in particular has drawn attention because of a compound called EGCG, which a meta-analysis found can modestly increase energy expenditure and shift the body toward burning more fat.5PubMed. Physiological effects of epigallocatechin-3-gallate (EGCG) on energy expenditure for prospective fat oxidation in humans: A systematic review and meta-analysis That effect is small, roughly equivalent to burning a few extra calories per day, but it runs in the same direction as what the fast is already doing. An earlier pilot study in men suggested that EGCG alone could increase fat oxidation, though the researchers cautioned that larger studies were needed to pin down the ideal dose.6PubMed. The effects of epigallocatechin-3-gallate on thermogenesis and fat oxidation in obese men: a pilot study
Herbal teas like peppermint, chamomile, and ginger are essentially calorie-free as well. Peppermint and ginger can also help with nausea, which some people experience during extended fasts. Just avoid any herbal blend that includes dried fruit, honey crystals, or added flavoring, since those can sneak in a few grams of sugar.
Electrolytes and Salt
Electrolyte depletion is one of the most common reasons people feel awful during a 36-hour fast, and it is also one of the easiest problems to fix. As mentioned earlier, your kidneys ramp up sodium excretion when you stop eating. Research from the early 1970s confirmed that this sodium loss exceeds what you would see from simply eating a low-salt diet, and that it persists throughout caloric deprivation.7The American Journal of Medicine. Fasting as a Method in Weight Loss Potassium follows a similar, though usually less dramatic, pattern.
You can replace these losses in a few ways:
- A pinch of salt in water: ordinary table salt or sea salt dissolved in a glass of water is the simplest option. A quarter to half a teaspoon per glass is a reasonable starting point.
- Electrolyte powders or tablets: many brands sell calorie-free electrolyte mixes containing sodium, potassium, and magnesium. Check the label; some include sugar or maltodextrin, which adds calories.
- Bone broth (with a caveat): a small cup of bone broth contains some protein and calories, usually around 30 to 50 per cup. Whether this “breaks” a fast depends on how strict your protocol is. For a pure water fast, bone broth is off the list. For a more relaxed approach where you just want to stay under about 50 calories total for the day, a single cup can help you feel substantially better without derailing the metabolic picture.
Magnesium deserves a mention too. Fasting can lower magnesium levels, and low magnesium often shows up as muscle cramps, especially at night. A calorie-free magnesium supplement dissolved in water is a reasonable addition if cramps are an issue.
The Artificial Sweetener Debate
This is where things get genuinely murky. Diet sodas, sugar-free gum, and zero-calorie flavored waters typically contain artificial or non-nutritive sweeteners like sucralose, aspartame, stevia, or monk fruit extract. They carry zero or near-zero calories, so in a strictly caloric sense, they should not break a fast. But the question is whether they trigger metabolic responses that undermine the fast anyway.
A recent mouse study found that non-nutritive sweeteners impaired an early-phase insulin response called the cephalic phase insulin release, the small burst of insulin your body produces in anticipation of incoming food when it tastes something sweet. After just two days of consuming non-nutritive sweeteners, the mice showed impaired glucose tolerance.8PubMed Central. Non-nutritive sweeteners attenuate reinforced cephalic phase insulin release and impair glucose tolerance in mice The concern is that tasting sweetness without actually delivering calories confuses the body’s insulin machinery.
On the other hand, a randomized controlled trial comparing stevia and sucralose in people with type 2 diabetes found no significant differences in fasting blood sugar, insulin levels, or lipid levels between the two sweetener groups, even after repeated measurement.9PubMed Central. Effects of stevia on glycemic and lipid profile of type 2 diabetic patients: A randomized controlled trial So the human evidence is not as alarming as the mouse data suggests.
The practical takeaway: if your primary goal for the 36-hour fast is weight loss and you find that a diet soda keeps you from quitting early, the trade-off is probably worth it. If you are fasting specifically for metabolic benefits like insulin sensitivity improvement, you may want to skip the sweeteners to be safe. The science has not settled this one definitively.
What About Apple Cider Vinegar?
Apple cider vinegar diluted in water is a popular fasting companion, and at roughly three calories per tablespoon, it sits comfortably in the “negligible calorie” category. Some people find it helps with hunger pangs or nausea. There is no strong evidence that it disrupts the fasted state, and a small amount of acetic acid is unlikely to provoke a meaningful insulin response. If you use it, dilute it well. Drinking it straight can damage tooth enamel and irritate your esophagus.
Exercise and Physical Activity During the Fast
You can exercise during a 36-hour fast, but your performance will take a hit, particularly for longer or harder efforts. A study that tested endurance performance after a 36-hour fast found that the body shifted heavily toward burning fat during both rest and exercise, which makes sense given the depleted glycogen stores. However, the subjects’ endurance was significantly decreased compared to when they had eaten normally beforehand.10PubMed. Effects of a 36-hour fast on human endurance and substrate utilization The researchers noted something interesting: blood glucose levels alone did not appear to be the sole factor determining when someone hit exhaustion, suggesting that other metabolic changes during fasting also play a role.
Light to moderate activity, walking, gentle yoga, easy cycling, tends to be manageable and can even help suppress hunger for some people. Intense strength training or long cardio sessions are riskier. You are running on depleted glycogen and elevated stress hormones, which means recovery will be slower and the risk of lightheadedness or muscle breakdown is higher. If you do choose to exercise, keep your electrolyte intake up and listen to your body. A fast is not the time to chase personal records.
Medications and Supplements
If you take prescription medications, the question of whether to fast is one to settle with your doctor before you start, not during. Many common medications, including blood pressure drugs, thyroid hormones, and antidepressants, are meant to be taken daily without interruption. Skipping a dose because you are fasting can be more harmful than any benefit the fast provides.
Fasting itself alters how your body processes drugs. A review of the available evidence found that fasting modulates the activity of drug-metabolizing enzymes, and while the effects are generally modest, in the range of about 10 to 20 percent change, they can become relevant for drugs with a narrow therapeutic window, where a small shift in metabolism could push levels too high or too low.11PubMed. The effects of fasting on drug metabolism Blood thinners, anti-seizure medications, and certain heart drugs are classic examples of narrow-window medications where this matters.
For supplements, most are fine to take during a fast. A basic multivitamin, vitamin D, or fish oil capsule contains trivial calories. Fat-soluble vitamins (A, D, E, K) absorb better with food, so you might get less from them during a fast, but taking them will not break the fast. Some people find that swallowing pills on a completely empty stomach causes nausea, in which case taking them with a sip of bone broth or waiting until you break the fast is a sensible workaround.
What Changes in Your Blood Over 36 Hours
A study that tracked the full lipid profile of healthy participants during a single 36-hour water-only fast found dramatic shifts. Free fatty acids rose significantly, as expected when the body is mobilizing stored fat for fuel. Triglycerides, meanwhile, dropped compared to the overnight-fasted, fed, and refed states.12PubMed Central. A single 36-h water-only fast vastly remodels the plasma lipidome The researchers identified 22 lipid species that went up and 223 that went down compared to a normal overnight fast, painting a picture of a wholesale metabolic reorganization. These changes reversed after refeeding, but they illustrate just how actively the body rearranges its fuel economy during an extended fast.
This is worth keeping in mind if you happen to have blood work scheduled. A 36-hour fast will temporarily skew your lipid panel and potentially your blood glucose and electrolyte readings. If your doctor ordered labs, either break the fast beforehand or let them know you were fasting so the results are interpreted correctly.
How You Break the Fast Matters
What you eat when the 36 hours end deserves as much thought as what you consumed during the fast. After a prolonged period without food, refeeding triggers a burst of activity in the gut. Research in mice found that 24 hours of refeeding after a fast boosted the proliferation of intestinal stem and progenitor cells compared to both normally fed and still-fasting groups, driven by a spike in mTOR signaling.13Cancer Research. Abstract 6007: Post-fasting refeeding enhances intestinal stem cell-mediated regeneration and tumorigenesis On one hand, this regenerative response can help repair the intestinal lining. On the other, the researchers noted that in certain contexts it could also promote abnormal cell growth, a finding that is preliminary but worth noting for people who fast frequently.
From a practical standpoint, breaking a 36-hour fast with a massive, high-carbohydrate meal is a recipe for gastrointestinal distress. Your digestive system has been essentially idle, and flooding it with a large volume of food, especially refined carbs and sugar, can cause bloating, cramping, and sharp blood sugar swings. A better approach is to start with something small and easy to digest: a handful of nuts, some scrambled eggs, avocado, a small portion of cooked vegetables. Give your gut 30 to 60 minutes to wake up before eating a full meal.
The old research on sodium excretion also has a practical refeeding implication. Consuming carbohydrates after a fast abruptly interrupts the kidneys’ sodium excretion, causing the body to retain fluid and sodium rapidly.7The American Journal of Medicine. Fasting as a Method in Weight Loss This is why people often see the scale jump two or three pounds the day after breaking a fast, even if they ate moderately. That weight is water, not fat, and it normalizes within a day or two. Knowing this ahead of time can save you from the discouraging feeling that the fast “didn’t work.”
When a 36-Hour Fast Might Not Be for You
Not everyone should attempt an extended fast. People with a history of eating disorders are at particular risk, since the rigid restriction and the mental preoccupation with what is and is not “allowed” can reinforce disordered patterns. Pregnant and breastfeeding women have caloric and nutrient demands that a 36-hour zero-calorie window cannot meet. People with type 1 diabetes or insulin-dependent type 2 diabetes face real risks of dangerous blood sugar drops, especially if their medication doses are not adjusted.
Children, teenagers, and older adults with low muscle mass are also poor candidates. The elderly, in particular, tend to lose muscle more easily during caloric restriction, and 36 hours is long enough for some degree of protein breakdown to begin, particularly if the person is already undernourished. Anyone on medications with narrow therapeutic windows, as discussed above, should get medical guidance before committing to an extended fasting protocol. The benefits of a 36-hour fast, while real, are not so unique that they cannot be achieved through shorter fasting windows or other dietary approaches for people in these groups.