Intermittent fasting (often misspelled as “intermediate fasting”) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat. The most popular version involves fasting for 16 hours and eating within an 8-hour window each day, though several other approaches exist. It works by giving your body enough time without food to exhaust its sugar reserves and start burning stored fat for energy.
How It Works in Your Body
When you eat, your body breaks food down into glucose and stores the excess in your liver as glycogen. As long as those stores are full, your body has no reason to tap into fat. When you stop eating for long enough, those glycogen reserves run out, and your body shifts to burning fatty acids and molecules called ketones for fuel instead. This shift, sometimes called the “metabolic switch,” typically happens between 12 and 36 hours after your last meal, depending on how much glycogen you started with and how active you are during the fast.
This is why the length of the fasting window matters. A 16-hour fast puts most people right at the threshold where their body begins relying on stored fat. Shorter fasts may not get you there. Longer fasts push you deeper into fat-burning territory but are harder to sustain.
The Most Common Approaches
There are several ways to practice intermittent fasting, and they differ mainly in how long you fast and how often.
- 16:8 (time-restricted eating): Fast for 16 hours, eat within an 8-hour window. Common schedules include eating from 10 a.m. to 6 p.m. or noon to 8 p.m. This is the most popular starting point.
- 18:6: A tighter version with an 18-hour fast and a 6-hour eating window, such as noon to 6 p.m.
- 20:4: Twenty hours of fasting with a 4-hour eating window, often from 2 p.m. to 6 p.m.
- OMAD (one meal a day): A 23-hour fast with a single meal, usually dinner.
- 5:2: You eat normally five days a week and restrict calories to 500 to 600 on two non-consecutive days.
- Alternate-day fasting: You alternate between normal eating days and fasting days where you consume 0 to 500 calories.
Most beginners start with 16:8 because it essentially means skipping breakfast and stopping eating after dinner. From there, some people experiment with longer fasts as their body adapts.
What You Can Have During a Fast
Water, black coffee, and plain tea are all fine during a fasting window because they contain no calories. Carbonated water works too. Some people add a teaspoon or two of apple cider vinegar to water to help with cravings. Adding sugar, milk, or cream to coffee or tea introduces enough calories to interrupt the fasted state.
Things like bone broth, butter coffee, or anything with coconut oil technically break the fast because they contain calories. However, because they’re low in carbohydrates, they won’t knock your body out of fat-burning mode. Whether that matters to you depends on your goals. If you’re fasting purely for weight loss and fat burning, small amounts of fat may not be a problem. If you’re fasting for cellular repair benefits, stricter is better.
Weight Loss Results
Across 27 clinical trials involving nearly 950 participants, intermittent fasting consistently produced weight loss ranging from 0.8% to 13% of baseline body weight. Most of that loss comes from fat specifically. One study calculated that 79% of the weight lost through intermittent fasting was fat rather than muscle or water.
Here’s the catch: when researchers compared intermittent fasting head-to-head with traditional calorie-restricted diets across 12 studies and more than 1,200 participants, both approaches produced equivalent weight loss, ranging from 4.6% to 13%. Intermittent fasting doesn’t appear to be metabolically superior to calorie counting. Its advantage is practical. Many people find it easier to follow a time rule than to track every meal.
Effects on Blood Sugar and Inflammation
A meta-analysis looking at metabolic syndrome found that intermittent fasting significantly improved insulin resistance, the condition where your cells stop responding efficiently to insulin. This is one of the strongest and most consistent findings in the research, rated as high-quality evidence. Improved insulin sensitivity means your body handles blood sugar more effectively, which lowers the risk of type 2 diabetes over time.
The picture on inflammation is more mixed. Intermittent fasting reduced one key inflammatory marker (IL-6) significantly, but had no statistically meaningful effect on two others (CRP and TNF-alpha). So while there’s some anti-inflammatory benefit, it’s not as clear-cut as the insulin improvements.
Cellular Cleanup
One of the most talked-about benefits of fasting is autophagy, a process where your cells break down and recycle damaged components. Think of it as cellular housekeeping. Animal studies suggest autophagy ramps up after 24 to 48 hours of fasting. However, there isn’t enough research yet to pinpoint when this kicks in reliably in humans, or whether shorter fasts like 16:8 trigger meaningful levels of it. Most of the autophagy buzz around intermittent fasting is extrapolated from animal data and longer fasting protocols.
Considerations for Women
Intermittent fasting affects hormones differently in women, and this is worth understanding before diving in. In animal studies, alternate-day fasting disrupted reproductive cycles and altered estrogen and other hormone levels. Time-restricted eating (like 16:8) had a much milder effect and in some cases actually improved reproductive function. The type of fasting protocol matters significantly.
Thyroid hormones are also sensitive to fasting. In humans, levels of the active thyroid hormone T3 start dropping rapidly after fasting begins, with noticeable reductions within 48 hours. Both short-term (4-week) and long-term (6-month-plus) alternate-day fasting reduced circulating T3. Even 8-week time-restricted eating produced the same result. For women who already have low thyroid function, this could be a concern worth monitoring.
Cortisol, the stress hormone, rises during fasting. It begins increasing immediately after food intake stops, and longer fasts of several days can dramatically elevate cortisol levels and shift its natural peak from morning to afternoon. Even a short 4-day early time-restricted eating protocol slightly but significantly raised morning cortisol. Women who are already dealing with high stress may want to start with a gentler fasting schedule and pay attention to how they feel.
Common Side Effects
Most side effects are mild and tend to improve as your body adapts over the first week or two. In a survey of people practicing intermittent fasting, the most common complaints were headache (reported by about 61% at some level of severity), lethargy (68%), mood swings (58%), dizziness (56%), and increased urination (46%). Headaches during fasting are typically caused by low blood sugar and feel like a dull, diffuse pressure rather than the pulsing pain of a migraine. They’re most common when fasting stretches past 8 hours.
Staying hydrated helps with most of these symptoms. The headaches and dizziness in particular tend to be worse when water intake drops. Easing in gradually, starting with a 12-hour fast before jumping to 16, gives your body time to adjust and reduces the severity of early side effects.
Exercise During Fasting
You can exercise while fasting, but intensity matters. Low to moderate intensity workouts, like walking, yoga, or easy cycling, are well-suited to the fasted state. High-intensity training while fasting increases the risk of low blood sugar and can impair recovery. If you prefer intense workouts, scheduling them inside your eating window or shortly before your first meal gives you better fuel and faster recovery.
Interestingly, research suggests that training in the evening while fasting may be more effective at improving aerobic performance than morning sessions. If your schedule allows flexibility, experimenting with timing can help you find what feels best.
Who Should Avoid It
Intermittent fasting is not appropriate for everyone. People with a current or past eating disorder should avoid it, as the rigid rules around food timing can trigger or worsen disordered patterns. It’s also not recommended during pregnancy or breastfeeding, when consistent calorie and nutrient intake is critical. People at high risk for bone loss and falls, including some older adults, should also steer clear, since calorie restriction combined with fasting may accelerate bone density loss.
Anyone taking medications that require food, particularly diabetes medications that lower blood sugar, needs to coordinate with their prescriber before starting a fasting protocol. The blood sugar drops that naturally occur during fasting can interact with these medications in dangerous ways.