During a 3-day fast, you can have water, black coffee, plain tea, and electrolytes. That’s essentially the full list. The goal is to consume zero calories (or as close to zero as possible) while keeping your body hydrated and your electrolyte levels stable. What sounds simple on paper gets more nuanced when you’re actually 36 hours in and wondering whether a squeeze of lemon will ruin everything.
Water Is the Foundation
Plain water is the single most important thing you’ll consume over 72 hours. Still or sparkling, it doesn’t matter. What does matter is drinking enough. Without food, you lose a surprising amount of water that you’d normally get from meals (roughly 20 to 30 percent of daily water intake comes from food), so you’ll need to drink more deliberately than usual. Aim for at least 2 to 3 liters per day, and more if you’re physically active or in a warm climate.
Black Coffee and Plain Tea
Unsweetened black coffee and plain tea are generally considered acceptable because they contain essentially zero calories. Both can make fasting more tolerable, and coffee in particular helps with alertness and appetite suppression on days two and three when energy tends to dip. Green tea, black tea, and herbal teas are all fine as long as nothing is added to them.
The key word is “plain.” Adding milk, cream, sugar, or honey introduces calories and triggers metabolic responses that break the fast. Even a splash of milk can add enough protein and fat to shift your body out of the fasted state.
What About Sweeteners?
This is where opinions split, but the research leans toward caution. A study comparing beverages sweetened with stevia, monk fruit, and aspartame found they had minimal effects on blood glucose and insulin compared to sugar. So in pure metabolic terms, these sweeteners don’t cause the same spike that sugar does. However, some evidence suggests artificial sweeteners can still affect blood sugar regulation in subtler ways, and during an extended fast, even small disruptions matter more than usual. The safest approach is to skip sweeteners entirely for 72 hours.
Electrolytes Are Non-Negotiable
This is the part most people underestimate. When you stop eating for three days, your body flushes electrolytes at an accelerated rate, especially sodium. Without replacing them, you’ll likely experience headaches, dizziness, muscle cramps, fatigue, and brain fog. These symptoms aren’t hunger. They’re electrolyte depletion, and they’re what makes most people quit a fast early.
The daily targets to aim for during a 72-hour fast:
- Sodium: 1,500 to 2,300 mg per day
- Potassium: 1,000 to 2,000 mg per day
- Magnesium: 300 to 400 mg per day
You can get sodium simply by adding a pinch of sea salt or pink salt to your water throughout the day. For potassium and magnesium, sugar-free electrolyte powders or capsules are the most practical option. Some people make “sole water” (salt dissolved in water) or add a combination of salt and a potassium-based salt substitute. Bone broth is another popular choice, though it does contain a small number of calories (typically 15 to 40 per cup), which means it technically breaks a strict fast. Many people accept this tradeoff because it provides sodium, potassium, and is easier on the stomach than salt water.
What About Lemon Juice or Apple Cider Vinegar?
A small squeeze of lemon in your water adds roughly 1 to 3 calories. Apple cider vinegar (one tablespoon) adds about 3 calories. In practical terms, these amounts are unlikely to trigger a meaningful insulin response or knock you out of a fasted metabolic state. Most people who do extended fasts consider these acceptable. If you’re fasting for strictly metabolic reasons and want to be as precise as possible, skip them. If they help you drink more water and stay compliant, the tradeoff is reasonable.
Vitamins and Medications
If you take prescription medications, continue taking them. A 3-day fast is never a reason to stop prescribed medication without guidance from whoever prescribed it.
Supplements are trickier. Vitamins A, D, E, and K are fat-soluble, meaning your body needs dietary fat to absorb them properly. Taking them on an empty stomach during a fast means you’ll absorb very little. Many multivitamins also cause nausea when taken without food. The practical move is to pause fat-soluble vitamins and multivitamins for the three days and resume them when you start eating again. Water-soluble vitamins like vitamin C and B vitamins are less likely to cause stomach upset, but there’s also less reason to take them during a fast since you won’t be metabolizing food.
Electrolyte supplements (magnesium, sodium, potassium) are the exception. These you should actively take, as described above.
What Happens in Your Body Over 72 Hours
Understanding the timeline helps explain why the fast feels different on each day. In the first 12 to 24 hours, your body burns through its stored glucose (glycogen). This is when hunger is often strongest, because your body is signaling that its preferred fuel source is running low.
By 24 to 48 hours, you shift into ketosis, where your liver starts converting fat into ketone bodies for fuel. Many people report that hunger actually decreases during this phase, though energy levels can fluctuate. Between 48 and 72 hours, most of the key metabolic processes peak: your body ramps up ketone production and increases its rate of cellular cleanup, the process where cells break down and recycle damaged components. This cellular recycling is one of the primary reasons people pursue multi-day fasts in the first place.
Exercise During the Fast
Light activity like walking is fine and can even help with the mental challenge of not eating. But this is not the time for intense workouts. Research on fasted exercise consistently recommends keeping intensity low to moderate to prevent drops in blood sugar. High-intensity training, heavy lifting, and long endurance sessions should wait until you’re eating again. On days two and three especially, your body is running on ketones and has limited capacity for explosive or sustained high-output effort. A gentle walk, light yoga, or easy stretching is the ceiling for most people.
Who Should Not Do a 3-Day Fast
Extended fasting is not safe for everyone. People with type 1 diabetes face a serious risk of ketoacidosis during prolonged fasting. Anyone who is underweight (BMI under 18.5), pregnant, breastfeeding, or has a history of eating disorders should not attempt a 72-hour fast. The same applies to children and adolescents, whose bodies need consistent nutrition for growth.
If you take medications for blood sugar or blood pressure, fasting for three days can cause dangerous drops in either one. This isn’t a situation to manage on your own.
How to Break the Fast Safely
What you eat when the 72 hours end matters almost as much as what you consumed during the fast. Your digestive system has been essentially idle for three days, and hitting it with a large or heavy meal can cause bloating, cramping, nausea, or diarrhea.
Start small. Your first meal should be something gentle and easy to digest: a cup of bone broth, a small portion of cooked vegetables, some avocado, or a few bites of soft scrambled eggs. Avoid greasy foods, raw vegetables, nuts, seeds, and anything high in sugar or fiber for the first meal or two. These are harder for a dormant digestive system to process. Over the next 12 to 24 hours, gradually increase portion sizes and reintroduce more complex foods.
For a healthy person doing a 3-day fast, refeeding syndrome (a dangerous shift in electrolytes that can occur when malnourished people start eating again) is unlikely. Clinical guidelines identify the primary risk group as people with negligible food intake for more than five days, or those who are already malnourished, significantly underweight, or have a history of alcohol misuse. A 72-hour fast in an otherwise well-nourished person falls below that threshold, but easing back into food is still the smart approach.