Fasting

Intermittent fasting encompasses eating patterns in which individuals go extended time periods (for example, 16 to 48 hours) with little or no energy intake, with intervening periods of normal food intake, on a recurring basis. ^[1] Time-restricted feeding is a form of intermittent fasting that limits the daily period of food intake to 8 hours or less, and periodic fasting refers to fasting or fasting-mimicking diets lasting from 2 to as many as 21 or more days. ^[1] The most studied forms of intermittent fasting include alternate-day fasting, the 5:2 diet, and time-restricted eating. ^[2]

During dietary restriction the body experiences a metabolic switch, driven by depletion of liver glycogen, that promotes a shift toward utilising fatty acids and ketones. ^[3] Fasting drives adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular protection. ^[4] The cellular and molecular mechanisms by which intermittent fasting improves health involve activation of adaptive cellular stress response signaling pathways that enhance mitochondrial health, DNA repair and autophagy. ^[1]

In rodents, intermittent or periodic fasting protects against diabetes, cancers, heart disease and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma and rheumatoid arthritis. ^[4] In a systematic review of 27 intermittent-fasting trials, all found weight loss of 0.8% to 13.0% of baseline weight with no serious adverse events, and twelve studies comparing intermittent fasting to calorie restriction found equivalent results. ^[5] A review of cardiometabolic effects found that alternate-day fasting, the 5:2 diet and time-restricted eating each produced mild to moderate weight loss of 1% to 8% from baseline with reductions in energy intake of 10% to 30%, and that intermittent fasting is generally safe. ^[2] In a network meta-analysis of 99 randomized clinical trials involving 6582 adults, all intermittent-fasting and continuous energy restriction strategies reduced body weight compared with an ad-libitum diet, and only alternate day fasting showed a benefit in body weight reduction over continuous energy restriction (mean difference -1.29 kg). ^[6] In adults with prediabetes or type 2 diabetes

Fasting has the potential to delay aging and help prevent and treat diseases while minimizing the side effects caused by chronic dietary interventions. ^{[4] [8]} Intermittent fasting is generally safe, does not result in energy-level disturbances or increased disordered-eating behaviors, and can produce clinically significant weight loss of more than 5% while improving several markers of metabolic health in individuals with obesity. ^[2] Longer-duration trials are still needed to further substantiate the effects of intermittent fasting, and possible side effects should be considered before commencing a new regimen. ^{[6] [3]}