Intermittent fasting

• Lose weight and body fat without counting calories every day.
• Improve blood sugar control and insulin sensitivity.
• Lower cardiovascular risk factors such as blood pressure, triglycerides and LDL cholesterol.
• Trigger the metabolic switch from burning glucose to burning fat and ketones.
• Slow aging and protect the brain, which is the most ambitious and least tested aim.

Intermittent fasting tells you when to eat, not what to eat. All versions share one feature: regular stretches long enough that the body runs low on stored glucose and starts relying more on fat [15].

The versions differ in how long each fast lasts and how often it comes round:

• Daily windows. You eat every day but only within a set window, such as 8 hours in 16:8. This is time-restricted eating, which also covers when in the day the window should sit.
• Weekly patterns. The 5:2 diet keeps five normal days and two days of about 500 to 600 kcal [6]. Whole-day fasting uses one or two 24-hour fasts instead.
• Alternate-day fasting. A fast day of about 25% of energy needs alternates with a normal day [3]. A stricter version allows only water on fast days [9].
• One meal a day. The most compressed daily version, with a fast of about 23 hours. It has its own evidence and risks, covered on the OMAD page.

Researchers group these together because they share a mechanism, but they are not interchangeable. A 12-hour overnight fast and a 36-hour water fast put very different demands on the body.

After a meal, your body runs mainly on glucose, and insulin helps store the surplus. As hours pass without food, the liver releases its stored glycogen to keep blood glucose steady. When that store runs low, usually beyond 12 hours, fat tissue releases fatty acids and the liver turns some of them into ketones such as beta-hydroxybutyrate [15]. Researchers call this the metabolic switch.

What the switch does in people is easier to measure than what it means:

• Ketones rise modestly. In healthy young men who fasted 20 hours every other day, beta-hydroxybutyrate rose from 0.06 to 0.10 mmol/L after each fast [7]. A strict alternate-day fasting trial found raised ketones even on eating days [9]. Some people track this at home with finger-prick ketone and glucose meters, as one N-of-1 study did [16].
• Insulin action improves. In the same 15-day study, with body weight held steady, insulin-driven glucose uptake rose from 6.3 to 7.3 mg/kg/min [7].
• Calories usually fall. Most of the benefit in free-living people comes from eating less. Strict alternate-day fasting cut intake by 37% on average [9].

Proponents argue that repeated switching also turns on cellular stress-resistance and repair pathways, including autophagy, which might slow aging [13]. That part of the theory rests largely on animal and cell studies, discussed below.

Intermittent fasting reliably causes weight loss compared with no diet. The bigger question is whether it beats ordinary calorie restriction, and the best evidence says it mostly does not.

• Network meta-analysis, 2025. Pooling 99 randomized trials with 6,582 adults, all fasting schedules and daily calorie restriction lowered weight versus usual eating. Alternate-day fasting was the only schedule to beat daily restriction, by 1.29 kg (95% CI 0.59 to 1.99 kg), and the edge disappeared in trials of 24 weeks or more [1].
• Cochrane review, 2026. In 22 trials with at least six months of follow-up, intermittent fasting gave little to no difference in weight loss compared with regular dietary advice [2].
• Umbrella review, 2021. Of 104 associations from meta-analyses of 130 trials, only one, modified alternate-day fasting for 1 to 2 months, was backed by high-quality evidence [5].

Head-to-head trials tell the same story. Over 12 months, alternate-day fasting and daily restriction gave 6.0% and 5.3% weight loss, and 38% of the fasting group dropped out [3]. In a 6-month trial in 107 women, 5:2 and daily restriction gave 6.4 kg and 5.6 kg [4]. A one-year Norwegian trial found 8.0 kg versus 9.0 kg, with similar regain afterwards [8].

The practical conclusion: pick the pattern you can keep. Intermittent fasting is a way to create a calorie deficit, and for some people it is easier than counting calories every day.

Blood sugar. In 137 adults with type 2 diabetes, two days a week of 500 to 600 kcal lowered HbA1c by 0.3 percentage points over 12 months, equivalent to daily calorie restriction (0.5 points) [6]. In women without diabetes, 5:2 lowered fasting insulin slightly more than daily restriction, by 1.2 microunits/mL [4]. Across 99 trials, no fasting schedule changed HbA1c more than daily restriction [1].

Fatty liver. In adults with fatty liver disease, alternate-day fasting with 600 kcal fast days plus exercise cut liver fat by 5.48 percentage points in 3 months, more than exercise alone [17].

Cholesterol. The picture is mixed. Pooled trials found alternate-day fasting lowered total cholesterol, triglycerides and non-HDL cholesterol more than time-restricted eating [1]. But in the longest alternate-day trial, LDL cholesterol was 11.5 mg/dL (0.30 mmol/L)0.30 mmol/L (11.5 mg/dL) higher than with daily restriction at 12 months [3]. If you fast, recheck your lipids after a few months.

Blood pressure and inflammation. Blood pressure and triglycerides improve with weight loss on 5:2, about as much as with daily restriction [8]. hs-CRP fell equally on 5:2 and daily restriction in one trial [4] and did not differ between alternate-day fasting and daily restriction in another [3].

The most exciting claims for intermittent fasting come from animals:

• Mice on alternate-day fasting ate about as much as fed mice yet had lower glucose and insulin and brain cells more resistant to injury [14].
• In mice, a long daily fast was needed for the anti-aging effects of calorie restriction, and fasting alone reproduced many of them [10].
• Reviews by leading fasting researchers argue these stress-response pathways could slow aging and diseases such as Alzheimer's in people [13].

Human data are thinner. Strict alternate-day fasting for 4 weeks lowered some markers linked to aging, such as the inflammatory protein sICAM-1 [9], but markers are not outcomes. In Utah, heart patients who reported routine fasting, a practice common among Latter-day Saints, had lower odds of coronary artery disease, which cannot be separated from the rest of their lifestyle [11]. Pointing the other way, in 2,981 Swedish adults aged 60 and older, a habitual daily fast of 14 to 24 hours was linked to faster build-up of chronic disease than a fast of 6 to 11.5 hours, mainly after age 78 [12].

Autophagy, the cell's recycling process, is often named as the reason to fast. It is hard to measure in living people, and one study found fasting raised autophagy markers in mouse liver but not in human muscle [18]. Some leading trialists now say the field was oversold on the strength of mouse results; the people section below covers that debate.

Direct comparisons are few and short:

• 5:2 versus 16:8. In a 4-week trial in 68 women, both lowered weight (2.7 kg on average) and glucose, with faster fat loss on 5:2. Both groups lost about 0.6 kg of muscle [19].
• Alternate-day fasting versus the rest. In pooled trials, alternate-day fasting lowered weight 1.69 kg more than time-restricted eating and 1.05 kg more than whole-day fasting, with moderate certainty [1].
• Hunger and dropout. Participants on 5:2 reported higher hunger than those on daily restriction [8]. Alternate-day fasting had a 38% dropout rate over a year, and participants ate more than prescribed on fast days [3].

For most people the comparison that matters is intermittent fasting versus a calorie deficit. Trials show the two give about the same result [1], so choose on preference. If you like structure and dislike counting, fasting may suit you. If fast days leave you ravenous, daily restriction is the better tool.

Women's hormones. Only a few human trials have measured reproductive hormones. They suggest fasting lowers androgens and raises SHBG in premenopausal women with obesity, especially when eating ends before 4 p.m., and does not change estrogen, gonadotropins or prolactin [20]. That could help women with high androgens, as in PCOS, but the data are too sparse for firm conclusions. Pregnancy and breastfeeding are standard exclusions from fasting trials.

Eating disorders. Intermittent fasting is very common in young people: in a Canadian national survey, 47.7% of young women and 38.4% of young men had tried it in the past year, and it was linked to eating disorder symptoms [21]. The survey cannot show which came first, but anyone with an eating disorder history should avoid fasting schedules.

Older adults. Most trials enroll people under 65. In Swedish adults aged 78 and older, longer habitual fasts were linked to faster accumulation of chronic disease [12]. Older adults also need enough protein to keep muscle, which is harder in short eating windows.

Medication. People on insulin or sulfonylureas can fast safely only with dose changes; trials reduced these drugs at the start [6], and published cases of stopping insulin were done under close medical supervision [22].

Rare reactions. Combining fasting with a ketogenic diet has triggered prurigo pigmentosa, an itchy rash linked to ketosis [23].

• Eating more than usual on non-fast days or in the eating window, which cancels the deficit.
• Treating fasting as permission to eat ultra-processed food when the window opens.
• Under-eating protein, which speeds up muscle loss.
• Jumping straight to long fasts and quitting after a week of hunger and headaches.
• Fasting on glucose-lowering drugs without a plan, which risks low blood sugar.

• You are pregnant or breastfeeding.
• You have had an eating disorder, or you notice fasting feeding binge eating or anxiety about food.
• You are under 18 or underweight.
• You take insulin, sulfonylureas or SGLT2 inhibitors: check with your clinician first, since doses may need changing.
• You are an older adult, especially over 75, with frailty or several chronic conditions: check with your clinician first.
• You have type 1 diabetes, kidney disease, gout or a history of gallstones: check with your clinician first.

• Hunger, irritability and headaches while you adjust.
• Higher hunger on fast days than with daily calorie restriction.
• Loss of lean mass along with fat, especially if protein is low.
• Low blood sugar in people taking glucose-lowering medication.
• A rise in LDL cholesterol in some people, seen in an alternate-day fasting trial.
• Rarely, a ketosis-related rash (prurigo pigmentosa) when fasting is combined with a ketogenic diet.