Fasting is generally safe for healthy adults when done with proper hydration, a gradual return to eating, and awareness of how your body responds at each stage. The biggest risks aren’t from the fast itself but from dehydration, pushing too hard physically, taking certain medications without adjustment, and eating too much too quickly when the fast ends. Here’s how to manage each of those.
What Happens in Your Body During a Fast
Understanding the basic timeline helps you know what to expect and when to pay attention. In the first 12 hours, your body burns through its stored sugar (glycogen) in the liver. After that, it shifts to burning fat and producing ketone bodies for energy. Between 12 and 24 hours, blood sugar drops by roughly 20% or more as glycogen stores empty out. This is the window where you’ll likely feel the most hungry, irritable, or lightheaded.
Cellular cleanup processes (autophagy) begin activating around 12 hours and ramp up significantly by 24 hours. This is one of the mechanisms behind the proposed health benefits of fasting. After 24 hours, your body is running primarily on fatty acids and ketones. Most people adjust to this state and find that intense hunger actually fades, though energy levels and mental clarity can fluctuate.
Staying Hydrated Is Non-Negotiable
Dehydration is one of the most common and preventable problems during a fast. Your kidneys need a minimum of about 500 ml (roughly 2 cups) of fluid per day just to handle waste, but that’s a bare minimum. General recommendations for daily fluid intake are around 3,000 ml (about 12.5 cups) for men and 2,200 ml (about 9 cups) for women. During a fast, you’re not getting the water that normally comes from food, which can account for 20 to 30 percent of your daily intake, so you’ll need to be more intentional about drinking.
Water is the best choice. If you’re fasting for longer than 24 hours, adding a pinch of salt to your water or sipping on broth can help maintain electrolyte balance. Signs of dehydration to watch for include dark urine, dizziness when standing, dry mouth, and headaches that don’t resolve with rest.
How to Handle Exercise
You don’t need to stop moving during a fast, but you do need to dial back the intensity. Research on fasted exercise consistently recommends sticking to low or moderate intensity activities like walking, light cycling, yoga, or easy jogging. High-intensity training while fasting raises the risk of hypoglycemia, and your recovery will be significantly impaired without incoming nutrients.
If you’re someone who exercises regularly and wants to maintain a routine, evening sessions tend to be better tolerated than morning ones during a fast. Save any hard training, interval work, or heavy lifting for days when you’re eating normally. Listen to your body: if you feel shaky, dizzy, or unusually fatigued during a workout, stop.
Breaking the Fast Safely
How you end a fast matters as much as the fast itself, especially if you’ve gone longer than two or three days. Refeeding syndrome is a rare but serious condition where eating too much too quickly after prolonged fasting causes dangerous shifts in electrolytes, particularly phosphorus and magnesium. It can affect heart rhythm and organ function.
For fasts under 24 hours, this isn’t a concern. A normal meal is fine, though many people find their stomach handles smaller portions better after not eating all day. For fasts of one to three days, start with something easy to digest: broth, fruit, eggs, or a small portion of rice. Avoid large, heavy, or very high-fat meals for the first meal or two.
For fasts longer than five days, the refeeding period becomes genuinely important. Clinical guidelines recommend restarting food at no more than 50% of your normal calorie intake, then gradually increasing over four to seven days as long as you feel well. Vitamin supplementation, especially B vitamins and thiamine, should ideally begin before you start eating again and continue for the first 10 days. If you’re planning a fast of this length, medical supervision is worth the effort.
Medications That Need Adjustment
Several common medications become risky during a fast because they rely on regular food intake to work properly. This is especially true for anyone managing diabetes or high blood pressure.
- Insulin: Mealtime (fast-acting) insulin should not be taken if you’re skipping a meal entirely. Basal (long-acting) insulin typically needs to be reduced by one-half to one-third, depending on how well-controlled your blood sugar is. Insulin pump users should reduce their basal rate by at least 10% and check blood sugar every two hours until a stable pattern emerges.
- Blood sugar-lowering pills: Medications like sulfonylureas (glipizide, glyburide, glimepiride) carry a significant hypoglycemia risk and should be reduced or skipped on fasting days. If you take an evening dose, the night before a fasting day also needs attention.
- Blood pressure medications and diuretics: Without normal food and fluid intake, these can cause your blood pressure to drop too low, leading to dizziness or fainting. You may need to reduce or skip them on fasting days.
- SGLT-2 inhibitors: These diabetes drugs increase urine output. If your fluid intake is lower than usual during a fast, skipping them helps prevent dehydration and low blood pressure.
The American Diabetes Association updated its 2026 guidelines to include a formal prefasting risk assessment for people with diabetes who plan to fast, including for religious reasons like Ramadan. If you take any prescription medication, talk to your prescriber before starting a fast so you have a specific plan for what to adjust and when.
Who Should Avoid Fasting
Fasting is not appropriate for everyone. Pregnant or breastfeeding women, children and teenagers who are still growing, and anyone with a history of eating disorders should not fast. People who are significantly underweight (a BMI of 18.5 or below) are at higher risk for refeeding complications and nutrient deficiency. Anyone with type 1 diabetes or poorly controlled type 2 diabetes faces serious risks from blood sugar swings without careful medical oversight.
Practical Tips for Your First Fast
If you’re new to fasting, start short. A 16-hour overnight fast (eating dinner, skipping breakfast, eating lunch the next day) lets you experience the early metabolic shift with minimal discomfort. Most of the fasting hours happen while you sleep. Once you’re comfortable with that, you can try a full 24-hour fast and see how your body responds before attempting anything longer.
Ease into fasting days by eating a balanced meal beforehand, not a large or heavily processed one. High-fiber foods and protein help you feel satisfied longer. Stock up on water, herbal tea, or black coffee (if your fasting protocol allows it) so you have something to reach for when hunger hits. Keep your schedule light on fasting days, at least until you know how your energy levels respond. Most people find the first one or two fasts harder than the ones that follow, as both the body and the habit adjust over time.
Track how you feel. Mild hunger, slight irritability, and occasional headaches are normal in the first 12 to 18 hours. Persistent dizziness, heart palpitations, confusion, or extreme weakness are not normal and are signals to end the fast and eat something.