Fasting is the voluntary decision to go without some or all food, or food and beverages, for a set period of time. It can last anywhere from 12 hours to several days, and people do it for weight management, metabolic health, religious practice, or medical preparation (like before blood work or surgery). While skipping a meal might seem simple, your body goes through a predictable series of metabolic shifts once food stops coming in.
What Happens in Your Body When You Fast
Your body doesn’t flip a single switch when you stop eating. Instead, it moves through distinct stages as it searches for alternative fuel sources.
Around 3 to 4 hours after your last meal, you enter what’s called the early fasting state. Blood sugar and insulin levels start dropping, and your body begins converting its stored form of sugar (glycogen, mainly kept in the liver) into usable glucose. This phase carries you through roughly the first 18 hours without food, and it’s the stage most people experience overnight between dinner and breakfast.
Between about 18 hours and two days, your liver’s glycogen stores run out. Your body starts breaking down fat and, to a lesser degree, protein for energy. This fat breakdown produces molecules called ketone bodies, which your cells can burn for fuel. Over the course of this window, your metabolism gradually shifts into ketosis, a state where fat becomes the primary energy source. The transition isn’t instant; it ramps up as the fast continues.
Beyond roughly 48 hours, the body enters what researchers call the long-term fasting state. Your liver ramps up a process of manufacturing new glucose from non-sugar sources to keep the brain supplied, since the brain can’t run entirely on fat. At this point, the body is working hard to preserve muscle mass while relying heavily on fat stores. Extended fasts at this level carry real risks and aren’t something to attempt casually.
Common Types of Fasting
Most people who fast aren’t going days without food. The popular approaches build fasting windows into a normal weekly routine.
Time-restricted eating (16:8 or 14:10): You eat all your meals within a set window each day. In the 16:8 version, you fast for 16 hours and eat during the remaining 8, for example between 11 a.m. and 7 p.m. The 14:10 version is more lenient, with a 10-hour eating window. This is the most common starting point because it essentially means skipping one meal and avoiding late-night snacking.
The 5:2 method: You eat normally five days a week and cap your intake at about 500 calories on the other two days. Those two low-calorie days shouldn’t be back to back. On fasting days, people typically split the 500 calories between two small meals.
Alternate-day fasting: Every other day, you either eat nothing or limit yourself to about 500 calories (roughly 25% of a normal day’s intake). Non-fasting days are unrestricted. This is more aggressive and harder to sustain socially.
24-hour fasts: You go a full day without eating, usually once or twice a week. Most people time it from breakfast to breakfast or lunch to lunch so they still eat something every calendar day.
Effects on Weight and Metabolism
A 2022 systematic review of 13 controlled trials compared intermittent fasting to standard daily calorie restriction, matching the total calories consumed in both groups. The result: when calorie intake was equal, weight loss was generally comparable between the two approaches. Fasting doesn’t appear to have a magic metabolic advantage over simply eating less every day.
That said, two studies within that review found that certain fasting protocols (the 5:2 diet and time-restricted eating) produced greater reductions in body fat specifically, even at the same calorie level. Two other studies found fasting was better at improving insulin sensitivity, which is how efficiently your cells respond to blood sugar. The evidence is limited, but it hints that fasting may offer some metabolic benefits beyond the number on the scale.
For many people, the real advantage of fasting is practical: it’s simpler to follow a rule like “don’t eat before noon” than to count every calorie at every meal. The structure removes decisions, which can make it easier to stick with over time.
Cellular Cleanup and Brain Health
One of the most talked-about effects of fasting is autophagy, your body’s process of recycling damaged or worn-out cell components. Think of it as cellular housekeeping. Animal studies suggest autophagy ramps up somewhere between 24 and 48 hours into a fast, though researchers haven’t pinpointed exact timing in humans yet.
Fasting also appears to boost levels of a protein that supports brain cell growth and connection. In animal studies, this protein (BDNF) increases during fasting and is linked to improvements in learning and memory. It also appears to help protect against age-related cognitive decline and reduce brain inflammation. The animal research is consistent and promising, but human studies on fasting and cognition are still limited, so it’s too early to make strong claims about brain benefits in people.
What Breaks a Fast
Anything with calories technically ends a fast. That includes bone broth, cream in your coffee, protein powder, and oils. Even small amounts of protein can trigger an insulin response that signals to your body that food has arrived.
Black coffee and plain tea are generally considered fine during a fasting window, as long as you skip the sugar, milk, and cream. Water, sparkling water, and plain electrolyte supplements without added sugar also won’t break a fast.
If you take supplements, check the label. Gummy vitamins and anything containing maltodextrin, cane sugar, or fruit juice concentrate will add calories and trigger a metabolic response. Standard capsule-form vitamins without fillers are usually safe during a fast.
Who Should Be Cautious
Fasting isn’t appropriate for everyone. People with diabetes face real risks from extended periods without food, since blood sugar can drop dangerously low, especially on certain medications. If you take blood pressure or heart medications, longer fasts can throw off your electrolyte balance, particularly sodium and potassium levels.
Anyone who is already underweight or at the lower end of a healthy weight range risks losing too much, which can weaken bones, suppress the immune system, and drain energy. People who need to take medications with food to avoid nausea or stomach irritation will also find fasting impractical or uncomfortable.
Most of the research on intermittent fasting has been conducted in young and middle-aged adults over relatively short time periods. The evidence for older adults is thin, so the benefits seen in studies may not apply equally across all age groups.