Intermittent fasting is an eating pattern that cycles between periods of eating and periods of fasting. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat. The fasting windows typically range from 14 to 36 hours depending on the method, and during those windows, your body shifts how it produces energy, processes blood sugar, and maintains cells.
The Main Types of Intermittent Fasting
There are three broad approaches, and each structures the fasting window differently.
Time-restricted eating is the most popular form. You eat all your meals within a set window each day and fast the rest. The 16:8 method (eating within an 8-hour window, such as 8 a.m. to 4 p.m., and fasting for 16 hours) is the most common version. Some people use a tighter window, like eating only between 8 a.m. and 3 p.m. Others follow a 15:9 pattern, eating within 15 hours and fasting for 9. The key variable is how many consecutive hours you spend without food.
Whole-day fasting means going one or two full days per week with very little food. The well-known 5:2 approach has you eat normally five days a week and limit intake to roughly 400 to 500 calories on the other two days. Those two low-calorie days don’t need to be consecutive.
Alternate-day fasting switches between unrestricted eating days and fasting days. On fasting days, you eat one meal providing about 25% of your normal calorie intake. This is the most intensive common protocol and tends to be harder to maintain long term.
What Happens in Your Body During a Fast
When you stop eating, your blood sugar drops and your body begins drawing on stored energy. In the first several hours, it burns through glycogen, the sugar reserves stored in your liver and muscles. Once those reserves run low, your body starts breaking down fat into molecules called ketone bodies, which your cells can use as fuel. Ketone bodies can supply up to 60% of the brain’s energy needs during prolonged fasting, effectively replacing glucose as the primary power source.
This shift from burning glucose to burning fat is sometimes called the “metabolic switch.” It doesn’t happen at a fixed hour for everyone, but it generally kicks in after your glycogen stores are depleted, which for most people occurs somewhere between 12 and 36 hours of fasting depending on activity level and what you last ate. The 16:8 method is specifically designed to push you into the early stages of this transition on a daily basis.
Effects on Blood Sugar and Insulin
Every time you eat, your pancreas releases insulin to help move sugar from your blood into your cells. When you eat frequently throughout the day, insulin stays elevated for long stretches. Over time, cells can become less responsive to insulin, a condition called insulin resistance, which is a precursor to type 2 diabetes.
Fasting gives your pancreas extended breaks from producing insulin. Research on people with type 2 diabetes shows that longer daily fasting windows improve fasting glucose values. Time-restricted eating in particular has shown results in improving insulin sensitivity, meaning cells respond more efficiently to insulin when it is released. The fasting period also appears to support the function of the pancreatic cells that produce insulin in the first place, helping normalize blood sugar regulation over time.
Autophagy: Your Body’s Cleanup Process
During extended fasts, your cells activate a recycling process called autophagy. Think of it as a deep clean: cells break down damaged or dysfunctional components and repurpose the raw materials into functioning parts. This process also destroys pathogens like viruses and bacteria that may be lurking inside cells.
Animal studies suggest autophagy ramps up significantly between 24 and 48 hours of fasting. There isn’t enough human research yet to pinpoint the exact timing in people, which means shorter daily fasts like 16:8 may trigger only modest autophagy compared to longer protocols. If autophagy is your primary goal, alternate-day or whole-day fasting likely gets you closer to the relevant threshold.
What You Can Consume During the Fast
The fasting window doesn’t require complete avoidance of all liquids. Plain water and sparkling water are fine and help you stay hydrated. Black coffee and unsweetened tea are also widely considered acceptable since they contain negligible calories. Some people add a very small amount of milk or fat to their coffee to manage hunger, though this technically introduces calories. Diluted apple cider vinegar (1 to 2 teaspoons mixed into water) is another common choice for curbing cravings without meaningfully breaking the fast.
The general principle: anything with significant calories, sugar, or protein will trigger an insulin response and pull you out of the fasted state. Some experts suggest staying under 50 grams of carbohydrates to maintain the fat-burning state, but for most people practicing time-restricted eating, the simplest rule is to stick to zero-calorie or near-zero-calorie drinks.
Side Effects and the Adjustment Period
The first few weeks are the hardest. Common side effects include headaches, low energy, irritability, and constipation. These are largely a response to your body adapting to longer stretches without food, and they tend to fade as you settle into the routine.
Rather than jumping straight into a 16-hour fast, gradually narrowing your eating window over several months makes the transition easier. If you normally eat from 7 a.m. to 10 p.m., you might start by cutting off food at 8 p.m., then 7 p.m., and slowly work toward your target window. Hunger during the fasting period is normal early on, but most people report it becomes much more manageable after two to four weeks as their body adjusts to the new schedule.
Heart Health: A Mixed Picture
Short-term studies have linked intermittent fasting to improvements in blood pressure, cholesterol, and markers of inflammation. But the long-term cardiovascular picture is less clear. An observational study analyzing data from over 20,000 U.S. adults (from the National Health and Nutrition Examination Survey, 2003 to 2018) found that people following an 8-hour time-restricted eating schedule had a 91% higher risk of dying from cardiovascular disease compared to those who spread their meals across 12 to 16 hours.
That finding generated significant attention, but it’s important context: this was an observational study, meaning it identified an association, not a cause. People who eat in very narrow windows may differ from the general population in ways the study couldn’t fully account for. The current body of evidence from human trials doesn’t yet provide a comprehensive picture of the long-term safety of intermittent fasting for heart health. Some studies point to benefits, others to risks, and many show no significant difference.
Who Can and Can’t Do It Safely
Clinical guidelines from researchers at the University of Illinois Chicago identify a broad range of people who can practice intermittent fasting: adults at a normal weight, those who are overweight or obese, people with high blood pressure or high cholesterol, and those with insulin resistance, prediabetes, or type 1 or type 2 diabetes. Adolescents with severe obesity may also be candidates under supervision.
If you take medications for blood pressure, cholesterol, or blood sugar, those prescriptions may need adjustment as you lose weight or change your eating patterns. Vitamin and mineral levels should also be monitored, since compressing all your nutrition into a shorter window can make it harder to hit your daily requirements. Most side effects and potential nutrient gaps show up in the first three months, making that early period the most important time to pay attention to how your body responds.
People who are pregnant, breastfeeding, or have a history of eating disorders are generally advised to avoid intermittent fasting. Children and older adults with frailty or low body weight are also poor candidates for extended fasting protocols.