Fasting properly means choosing a method that fits your life, staying hydrated, and knowing what happens in your body at each stage so you can get the benefits without the risks. Whether you’re trying a simple overnight fast or a longer protocol, the details matter: what you drink, when you eat again, and which supplements to take can all determine whether fasting helps or harms you.
Common Fasting Methods
Fasting protocols fall into three broad categories, each with different time commitments and levels of difficulty.
Time-restricted eating is the most popular starting point. You compress all your meals into a set window each day:
- 16:8 — 16 hours fasting, 8 hours eating (skip breakfast or dinner)
- 18:6 — 18 hours fasting, 6 hours eating
- 20:4 — 20 hours fasting, 4 hours eating
- OMAD (one meal a day) — 23 hours fasting, 1 hour eating
Intermittent fasting works on a weekly schedule rather than a daily one. The 5:2 method means eating normally five days per week and limiting yourself to 500 to 600 calories on the other two days. Alternate-day fasting takes this further: every other day you eat nothing or cap intake at 500 calories.
Extended fasting means going beyond 24 hours on water only, typically 48 to 72 hours. If you already practice intermittent fasting, this type of fast doesn’t need to happen more than about four times a year (once per season). Fasting beyond 72 hours carries real risks, including dangerous drops in blood pressure and severe low blood sugar, and should not be done without medical supervision.
If you’re new to fasting, start with 16:8 for a few weeks before considering anything longer. Your body adapts to fasting gradually, and jumping straight to extended protocols often leads to miserable side effects that make you quit.
What Happens in Your Body While Fasting
The first 12 hours of a fast are mostly about burning through your stored sugar. Your liver holds a reserve of glycogen (the storage form of glucose), and your body taps into it as blood sugar drops between meals. Between 12 and 24 hours, that reserve depletes significantly, with blood glucose falling 20% or more.
Once glycogen runs low, your body shifts to burning fat. The liver converts fatty acids into ketone bodies, which your brain and muscles can use as fuel. This transition, sometimes called the “metabolic switch,” is why many people report mental clarity kicking in around the 18- to 24-hour mark. It’s also why the first few times you fast past 12 hours, you may feel foggy or irritable: your body isn’t yet efficient at making that switch.
Autophagy, the cellular cleanup process that recycles damaged proteins and organelles, activates around 12 hours of fasting and appears to peak around 24 hours. This is one of the most-cited benefits of fasting, and it’s worth noting that you don’t need multi-day fasts to trigger it. A consistent 16:8 or 18:6 routine gets you into that window daily.
What You Can Drink Without Breaking Your Fast
Water is the obvious choice, and you should drink more of it than usual. When you’re not eating, you lose a significant source of daily water intake (food accounts for roughly 20 to 30% of your hydration), so aim for at least an extra glass or two beyond your normal amount.
Black coffee and plain tea are fine. They contain essentially zero calories and don’t trigger an insulin response. If you want a sweetener, artificial options like stevia, monk fruit, sucralose, and aspartame don’t affect blood sugar. What you should avoid are sugar alcohols like sorbitol, xylitol, and mannitol, which do raise blood sugar and contain about half the calories of regular sugar. And skip the cream or milk: even small amounts of fat or protein can signal your body to stop fasting-related processes.
Electrolytes During a Fast
The headaches, fatigue, and muscle cramps people blame on fasting itself are usually caused by electrolyte depletion. When insulin levels drop during a fast, your kidneys excrete more sodium, and potassium and magnesium follow it out. This is especially common during fasts longer than 16 hours or when you’re new to fasting.
The minimum daily requirements for key electrolytes are roughly 500 mg of sodium (about a quarter teaspoon of table salt), 1,600 to 2,000 mg of potassium, and adequate chloride (which generally tracks with sodium intake). During fasting, your losses increase, so these minimums become harder to hit. Adding a pinch of salt to your water or using a calorie-free electrolyte supplement can prevent the cluster of symptoms people call “keto flu.” If you’re doing an extended fast of 24 hours or more, electrolyte supplementation moves from optional to important.
When and How to Take Supplements
Vitamins A, D, E, and K are fat-soluble, meaning they need dietary fat present in your gut to be absorbed properly. Taking them during a fasting window is essentially wasting them. Save these for your eating window and take them alongside a meal that contains some fat.
Water-soluble vitamins (B vitamins, vitamin C) don’t require fat for absorption and can technically be taken during a fast, though some people find they cause nausea on an empty stomach. If that’s you, move them to your eating window as well. There’s no fasting benefit to taking vitamins on an empty stomach.
Exercise While Fasting
Light to moderate exercise during a fasting window is generally safe and can even accelerate the metabolic switch to fat burning. Walking, yoga, and easy cycling all work well. The issue arises with resistance training and high-intensity work.
Your muscles build new protein most effectively when exercise and food work together. Resistance exercise stimulates muscle protein synthesis on its own, and eating protein afterward amplifies that effect in a way that’s greater than either stimulus alone. If you lift weights in a fasted state and then don’t eat for several more hours, you’re missing that synergistic window. The practical takeaway: schedule intense workouts near the end of your fasting window so you can eat relatively soon afterward, or move them into your eating window entirely. A fasted morning walk is great; a fasted heavy squat session is counterproductive for muscle growth.
How to Break a Fast Safely
For fasts under 24 hours, this is simple. Eat a normal meal. Your body hasn’t been deprived long enough for refeeding to pose any risk.
For fasts lasting 24 to 72 hours, ease back in. Start with something small and easy to digest: a handful of nuts, some bone broth, eggs, avocado, or cooked vegetables. Give your digestive system 30 to 60 minutes before eating a full meal. Avoid immediately loading up on high-sugar or heavily processed foods, which can cause sharp blood sugar spikes and digestive discomfort after your body has been running on ketones.
True refeeding syndrome, a dangerous shift in electrolytes that can affect the heart, is a concern only after very prolonged fasting (five or more days of little to no intake) or in people who are severely malnourished. Clinical guidelines recommend restarting calories at no more than 50% of normal energy needs for people who have eaten little or nothing for more than five days, then building back up over four to seven days. This level of caution isn’t necessary for standard intermittent fasting, but it’s worth knowing about if you’re ever tempted to attempt a week-long fast.
Who Should Not Fast
Fasting is not safe for everyone. People with diabetes face the most obvious risk: going without food while on blood sugar-lowering medications can cause dangerous hypoglycemia. If you take insulin or similar drugs, fasting requires direct coordination with your prescribing doctor.
Others who should be cautious include people taking blood pressure or heart medications, since longer fasts can throw off sodium and potassium balance in ways that interact with those drugs. If any of your medications need to be taken with food to prevent nausea or stomach irritation, fasting may force you into an uncomfortable choice between compliance and side effects.
People who are already at a low body weight should avoid fasting. Losing additional weight when you’re already lean can weaken your bones, suppress your immune system, and drain your energy. Pregnant or breastfeeding women, children, and anyone with a history of eating disorders should also steer clear. Fasting can easily reinforce disordered patterns around food restriction, and the psychological risks outweigh any metabolic benefit.
Building a Sustainable Routine
The most common mistake with fasting is treating it as something you white-knuckle through rather than something you build into your routine. Start with a version that feels only slightly challenging. For most people, that’s 16:8, which often just means skipping breakfast and not snacking after dinner. Once that feels normal (usually within one to two weeks), you can extend your fasting window if you want to.
Consistency matters more than duration. A daily 16-hour fast will produce more cumulative benefit than an occasional 48-hour fast followed by weeks of unstructured eating. Track how you feel, not just the clock. Persistent irritability, inability to concentrate at work, or disrupted sleep are signs you’ve pushed too far too fast. Hunger in the first week is normal and fades as your body becomes more efficient at switching fuel sources. Feeling genuinely unwell is not normal and means you should shorten your window or add electrolytes.