When you fast, you voluntarily stop eating for a set period while staying hydrated with non-caloric drinks. What you actually do during that time, from what you can drink to how you handle hunger and exercise, matters just as much as the decision to fast itself. Here’s a practical walkthrough of what fasting looks like hour by hour, and how to do it well.
What Happens in Your Body as You Fast
Your body moves through distinct metabolic stages during a fast, and understanding them helps you know what to expect. Around 3 to 4 hours after your last meal, you enter the early fasting state: blood sugar and insulin levels start declining, and your body begins tapping into stored glycogen (the sugar reserve in your liver and muscles) for energy. Most popular intermittent fasting methods, like the 16:8 approach, cycle between the fed state and this early fasting phase.
If you fast beyond 18 to 24 hours, your body shifts toward burning fat for fuel, a state called ketosis. Fasts shorter than 24 hours typically don’t reach full ketosis unless you’re also eating very low carb. Around 24 to 48 hours, animal studies suggest the body ramps up autophagy, a cellular cleanup process where damaged components are broken down and recycled. Not enough human research exists to pinpoint the exact timing in people, but this process is one of the main reasons some people pursue longer fasts.
By around 48 hours, you enter what’s considered the long-term fasting state, where fat burning is fully established and your body is deeply relying on its own energy stores. Your stress hormones shift too. Epinephrine (adrenaline) levels rise significantly during food deprivation, which partly explains the surprising mental alertness many people report while fasting.
What You Can Drink
Water is the foundation. Beyond that, black coffee and plain tea are generally considered safe because they contain negligible calories and don’t trigger a meaningful insulin response. The key is drinking them without sugar, milk, or cream. Even small additions of dairy or sweetener introduce calories that can shift your body out of the fasting state.
Bone broth technically breaks a fast because it contains calories and protein, but some people use it during longer fasts to get minerals and ease hunger. Protein powder, collagen supplements, and branched-chain amino acids all trigger an insulin response and interrupt autophagy, so they’re best saved for your eating window. If your primary goal is fat loss rather than cellular cleanup, a splash of cream in your coffee is unlikely to derail your results, but it does end a “clean” fast in the strict sense.
Managing Hunger and Energy
Hunger during a fast comes in waves rather than building steadily. Most people find the first fast is the hardest, and the sensation gets easier to manage with experience. Staying busy is genuinely one of the most effective strategies. Boredom and habit drive a surprising amount of what feels like hunger.
Drinking water or sparkling water throughout the day helps. Some people find that black coffee blunts appetite effectively. Light movement like walking can also redirect your attention and stabilize energy levels. If you feel dizzy, shaky, or genuinely unwell rather than just uncomfortable, that’s a signal to eat. There’s no prize for pushing through symptoms that suggest your blood sugar has dropped too low.
Electrolytes During Longer Fasts
For fasts lasting 24 hours or more, electrolyte balance becomes a real concern. Without food, you’re not getting sodium, potassium, or magnesium from your meals, and your kidneys excrete more sodium when insulin levels are low. The result can be headaches, muscle cramps, fatigue, and brain fog that people mistakenly blame on the fast itself when it’s really just a mineral deficit.
During extended fasts beyond 24 hours, aim for roughly 2,000 to 3,000 milligrams of sodium, 200 to 400 milligrams of potassium, and 60 to 120 milligrams of magnesium per day. If you’re fasting beyond 48 hours, those targets increase: sodium needs can climb to 3,000 to 4,000 milligrams, potassium to 400 to 600 milligrams, and magnesium to 120 to 180 milligrams daily. Adding a pinch of salt to your water is the simplest way to address sodium. Sugar-free electrolyte supplements or mineral drops are another option.
Exercise While Fasting
Light to moderate exercise is generally fine during a fast. Walking, yoga, and easy jogging don’t appear to harm muscle tissue when done in a fasted state. Some people prefer fasted cardio specifically because the body is already drawing on fat stores for fuel.
High-intensity workouts and long endurance sessions are a different story. Without available energy from food, your body may start breaking down muscle protein for fuel during hard efforts. If you want to do intense training, schedule it near the end of your fast so you can eat afterward, or save it for a non-fasting day. Pay attention to how you feel: lightheadedness, unusual weakness, or a racing heart during exercise while fasting means you should stop and refuel.
Vitamins and Medications
Most vitamins are better taken with food. Fat-soluble vitamins (A, D, E, K) need dietary fat to be absorbed properly, so taking them during a fast means you’re largely wasting them. Vitamins containing calcium, vitamin C, or iron are especially likely to irritate an empty stomach, causing nausea, stomach pain, or diarrhea. If you take a daily multivitamin, move it to your eating window.
Medications that need to be taken with food to prevent nausea or stomach irritation can be a real obstacle to fasting. If you’re on blood pressure or heart disease medications, extended fasting can alter your sodium and potassium balance in ways that interact with those drugs. Anyone on medication that requires food should plan their fasting schedule around their dosing times, not the other way around.
How to Break Your Fast
What you eat first after a fast matters more the longer you’ve gone without food. For a standard 16 to 18 hour fast, your first meal doesn’t require special planning. For fasts of 24 hours or longer, your digestive system has slowed down, and hitting it with a large, heavy meal can cause bloating, cramping, and nausea.
Start with something easy to digest and low on the glycemic index. Fruits, vegetables, beans, nuts, and minimally processed grains all produce a slower, gentler rise in blood sugar and a steadier insulin release compared to refined carbs. White bread, bagels, rice cakes, and sugary cereals spike blood sugar rapidly, which can leave you feeling worse after a long fast. Practical swaps that help: choose steel-cut oats over instant oatmeal, whole-grain bread over white, brown rice over white rice, and leafy greens over starchy corn. A small meal with some protein and healthy fat, followed by a normal-sized meal an hour or two later, is a smoother reentry than one giant plate.
Who Should Be Cautious
Fasting isn’t safe for everyone. People with diabetes face real risks from extended periods without food, particularly if they take insulin or medications that lower blood sugar. People on blood pressure or heart medications may develop dangerous imbalances in sodium and potassium during longer fasts. If you’re already at a low body weight, fasting can push you into territory that weakens bones, suppresses your immune system, and drains your energy reserves.
Pregnant or breastfeeding women, children, and anyone with a history of eating disorders should avoid fasting protocols. Even for healthy adults, fasts extending beyond 24 to 48 hours carry more risk and less established benefit than shorter daily fasting windows. Starting with a 14 to 16 hour overnight fast, which mostly just means skipping a late-night snack and delaying breakfast, is the most practical entry point for most people.