You’re supposed to fast before certain blood tests, before surgery, during specific religious observances, and optionally as part of an intermittent fasting routine for metabolic health. Each situation has its own timing rules, and getting them wrong can mean inaccurate lab results, a canceled procedure, or unnecessary discomfort. Here’s what you need to know for each one.
Fasting Before Blood Tests
The most common reason people are told to fast is for a blood draw. You typically need to go 8 to 12 hours without eating or drinking anything other than water before the test. The two tests that most often require fasting are blood glucose (blood sugar) tests and lipid panels, which measure your cholesterol levels.
Food and drinks affect the levels of sugar and fat circulating in your blood. If you eat a meal a few hours before a fasting glucose test, the results will reflect that meal rather than your baseline metabolic health. The same goes for cholesterol. Most people schedule fasting blood work first thing in the morning so the overnight hours do most of the work. You stop eating after dinner, skip breakfast, get your blood drawn, and then eat normally afterward.
Water is fine and actually encouraged, since being well-hydrated makes it easier to find a vein. Black coffee is more of a gray area. Some labs allow it, others don’t. If your provider says “fasting,” the safest approach is water only unless told otherwise.
Fasting Before Surgery
If you’re having a procedure that requires anesthesia, you’ll be told to stop eating and drinking beforehand. This isn’t about test accuracy. It’s about safety. When you’re under anesthesia, your normal reflexes shut down, including the one that keeps stomach contents from entering your lungs. An empty stomach dramatically lowers that risk.
The American Society of Anesthesiologists sets the standard guidelines. For clear liquids (water, black coffee, tea without milk, pulp-free juice, or carbonated beverages), the minimum fast is 2 hours before the procedure. Alcohol doesn’t count as a clear liquid. For a light meal, like toast and clear liquids, the minimum is 6 hours. If you’ve eaten anything heavy, fried, fatty, or containing meat, you may need 8 hours or more.
Your surgical team will give you a specific cutoff time. If you eat or drink past that window, your procedure will likely be postponed. This applies to elective surgeries, not emergencies, where the surgical team manages the risk differently.
Fasting for Religious Observances
Religious fasting has its own set of rules that vary by tradition.
During Ramadan, Muslims fast every day for the entire month, from dawn to sunset. That means no eating, drinking, or sexual activity during daylight hours. The fast is broken each evening at sunset, and a pre-dawn meal is eaten before the fast resumes. People with medical conditions, pregnant or nursing women, the elderly who are physically unable, and those with mental disabilities that affect judgment are generally exempt. Diabetics in particular are usually not expected to fast.
Yom Kippur, the Jewish Day of Atonement, involves a complete fast from sundown one evening through nightfall the following day, roughly 25 hours. In preparation, many people reduce their caffeine intake during the week before to avoid withdrawal headaches during the fast, and cut back on salty and spicy foods that increase thirst.
Other traditions have their own fasting practices. Eastern Orthodox Christians observe several fasting periods throughout the year. Some Hindu and Buddhist practices involve fasting on specific days. The timing and restrictions vary, but the core principle of abstaining from food (and sometimes water) for a defined period is consistent.
Intermittent Fasting for Health
Outside of medical and religious contexts, many people fast voluntarily as a health practice. The most popular approach is time-restricted eating, often called the 16:8 method: you eat during an 8-hour window each day and fast for the remaining 16. For most people, this means skipping breakfast or skipping a late dinner.
The 5:2 approach is less frequent but more intense. You eat normally five days a week and restrict yourself to one 500 to 600 calorie meal on each of the other two days. Some people take it further with OMAD (one meal a day), though this is typically done just two days per week rather than daily.
The timing of your eating window matters more than many people realize. Your body’s internal clock influences how efficiently you process food. Insulin sensitivity is higher in the morning, and cortisol, which helps mobilize energy and stimulate appetite, rises naturally at dawn. Eating late at night, when melatonin levels are elevated, has been linked to impaired glucose tolerance. Research consistently shows that eating earlier in the day and finishing dinner earlier improves blood sugar levels and fat metabolism compared to the same calories consumed later. Eating during hours normally reserved for sleep is associated with weight gain and worse metabolic outcomes.
If you’re choosing when to place your eating window, earlier is generally better from a metabolic standpoint. An eating window of, say, 8 a.m. to 4 p.m. aligns more closely with your body’s natural rhythms than noon to 8 p.m., even though both are technically 16:8.
What Happens in Your Body During a Fast
In the first several hours of a fast, your body burns through its stored glucose (glycogen) in the liver. Once that’s depleted, typically around 12 to 16 hours in, your body shifts to burning fat for fuel and producing ketones as an alternative energy source for the brain. This metabolic switch is a large part of why fasting has attracted so much interest for weight management.
A cellular cleanup process called autophagy, where your cells break down and recycle damaged components, is another frequently cited benefit. Animal studies suggest this process ramps up significantly between 24 and 48 hours of fasting. There isn’t enough human research yet to pin down an exact onset time, so claims about triggering autophagy with a 16-hour fast are speculative.
Insulin sensitivity improves during fasting, but the duration matters. A randomized controlled trial comparing 2-day and 6-day fasts in healthy young men found that the shorter fast actually impaired glucose tolerance afterward, possibly due to a stress response. The 6-day fast, by contrast, improved insulin sensitivity and insulin release through an adaptive mechanism that persisted even after eating resumed. This doesn’t mean you need to fast for six days. It does suggest that the metabolic benefits of fasting build over time and that very short fasts may not produce the improvements people expect.
Who Should Be Careful With Fasting
Fasting isn’t appropriate for everyone. Pregnancy is a particular concern. The first trimester is a phase of heightened vulnerability because of placental development and organ formation, both of which are sensitive to metabolic disruptions. Women with gestational diabetes face additional risks of blood sugar swings, low blood sugar, or a dangerous buildup of ketones during fasting. Fasting in hot climates with long daylight hours adds further strain. While many religious traditions exempt pregnant and nursing women from fasting obligations, those who choose to fast anyway should do so with close medical monitoring.
People with type 1 diabetes, eating disorders, or a history of disordered eating should approach any form of fasting with caution. Children, adolescents who are still growing, and people who are underweight also fall outside the safe range for voluntary fasting.
How to Break a Fast Safely
For fasts lasting a day or longer, what you eat when you resume matters. Eating a large meal, or jumping straight to high-fat or spicy foods, commonly causes bloating, heartburn, and digestive distress. Your digestive system needs time to ramp back up.
Start with liquids: water, milk, fruit juice, or a smoothie. These deliver nutrients without overwhelming your gut. Dried fruits like dates, apricots, or raisins provide quick energy through natural sugars along with fiber and minerals. Soups, especially those with lentils, beans, or rice, are another gentle option that combines protein and carbohydrates. From there, ease into solid food with smaller portions eaten slowly. Stick with lean proteins like fish or plant-based options rather than red meat, and keep fried and fatty foods to a minimum for that first meal or two.