Intermittent fasting is a strategy for timing meal schedules. Fasting is an ancient tradition practiced by many cultures and religions over centuries. Religious fasting practices may only require abstinence from certain foods or last for a short period and cause negligible effects.
Intermittent fasting, also known as intermittent energy restriction, is an umbrella term for various meal timing schedules that cycle between voluntary fasting (or reduced calorie intake) and non-fasting over a given period.
The studies say intermittent fasting may produce weight loss, reduce insulin resistance, and lower the risk of cardiometabolic diseases, although its long-term sustainability is unknown. Intermittent fasting helps with obesity, insulin resistance, dyslipidemia, hypertension, and inflammation. The science concerning intermittent fasting is preliminary and uncertain due to the absence of studies on its long-term effects.
Types
An alternate day fasting schedule that alternates between usual eating (feast days) and fast days with reduced or no calories
Three methods of intermittent fasting are alternate-day fasting, periodic fasting, and daily time-restricted feeding. It is similar to a calorie-restriction diet.
- Alternate-day fasting involves alternating between a 24-hour “fast day” when the person eats less than 25% of everyday energy needs, followed by a 24-hour non-fasting “feast day” period. It is the strictest form of intermittent fasting because there are more days of fasting per week. There are two subtypes:
- Complete alternate-day fasting where no calories are consumed on fast days.
- Modified alternate-day fasting allows the consumption of up to 25% of daily calorie needs on fasting days instead of complete fasting. This is almost similar to alternating days with normal eating and days with a very low-calorie diet.
- Periodic fasting or whole-day fasting involves any period of consecutive fasting of more than 24 hours, such as the 5:2 diet where there are one or two fast days per week, to the more extreme version with several days or weeks of fasting. During the fasting days, approximately 500 to 700 calories, or about 25% of regular daily caloric intake, may be allowed instead of complete fasting.
- Time-restricted feeding involves eating only for a certain number of hours each day. Skipping a meal and the 16:8 diet (16 fasting hours cycled by eight non-fasting hours) are examples.
Intermittent fasting has not yet been studied in children, the elderly, or underweight people and can be harmful in these populations. Intermittent fasting is not recommended for people who are not overweight. The long-term sustainability of intermittent fasting is unknown yet.
Other effects
Night eating is linked to impaired sleep quality, whereas periodic fasting may negatively affect chronic pain and mood disorders. Intermittent fasting does not affect bone health.
Athletic performance does not benefit from intermittent fasting. Overnight fasting before exercise increases lipolysis but reduces performance in prolonged exercise (more than 60 min).
Adverse effects
Short-term intermittent fasting may produce minor adverse effects, such as the continuous feeling of hunger, irritability, and impaired thinking. However, these effects disappear within a month from the fasting practice. Intermittent fasting is not recommended for pregnant or breastfeeding women, growing children and adolescents, or individuals vulnerable to eating disorders.
Tolerance
Tolerance of a diet is a determinant of the potential effectiveness and maintenance of benefits obtained, such as weight loss or biomarker improvement.
Putative mechanisms
Preliminary research indicates that fasting may induce a transition through four stages:
- The fed state or absorptive state during satiety, when the primary fuel source is glucose and body fat storage, lasts for about 4 hours.
- The postabsorptive state, lasting for up to 18 hours when glucagon is secreted, and the body uses liver glucose reserves as a fuel source.
- The fasted state, transitioning progressively to other reserves, such as fat, lactic acid, and alanine, as fuel sources, when the liver glucose reserves are depleted, occurring after 12 to 36 hours of continued fast.
- The shift from preferential lipid synthesis and fat storage to the mobilization of fat (in the form of free fatty acids), metabolized into fatty acid-derived ketones to provide energy. Some authors call this transition the “metabolic switch.
