Intermittent fasting

Intermittent energy restriction that refers to an umbrella term for various meal timing schedules that cycle between between voluntary fasting and non-fasting over a given period.

The three methods of intermittent fasting are alternate-day fasting, periodic fasting, and time-restricted feeding.


IF may have similar effects to a calorie-restriction diet.


IF may help with obesity, insulin resistance, dyslipidemia, hypertension, and inflammation.


IF  may produce weight loss, reduce insulin resistance, and lower the risk of cardiometabolic diseases.


IFs long-term sustainability is unknown.

Data suggest limiting food intake to an eight-to-12-hour window can improve insulin sensitivity, blood pressure, fat metabolism, gut function, and immune function. 

It can also lower inflammation and help regulate uric acid.

Alternate day fasting schedule, that alternates between usual eating, described as feast days, and fast days with reduced or no calories.


Alternate-day fasting involves alternating between a 24-hour fast day when the person eats less than 25% of usual energy needs, followed by a 24-hour non-fasting feast day period. 


Alternate-day fasting  is the strictest form of intermittent fasting because there are more days of fasting per week.


There are two subtypes of alternate-day fasting: 


Complete alternate-day fasting, or total intermittent energy restriction, where no calories are consumed on fast days.


Modified alternate-day fasting, or partial intermittent energy restriction, which allows the consumption of up to 25% of daily calorie needs on fasting days instead of complete fasting. 


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.


The more extreme version with several days or weeks of fasting.


During the fasting days, consumption of approximately 500 to 700 calories, or about 25% of regular daily caloric intake, may be allowed instead of complete fasting.


Time-restricted feeding refers to eating only during a certain number of hours each day.

Time-restricted feeding is examples are: 


Skipping a meal and the 16:8 diet (16 fasting hours cycled by 8 non-fasting hours.


Time-restricted feedings are thought to leverage the circadian rhythm.


Intermittent fasting may be effective for weight loss, may decrease insulin resistance and fasting insulin, and may improve cardiovascular and metabolic health, although the long term sustainability of these effects has not been studied.


There is limited evidence that intermittent fasting produces weight loss comparable to a calorie restricted diet.


Most studies on intermittent fasting in humans have observed weight loss, ranging from 2.5% to 9.9%.

Alternate-day fasting does not affect lean body mass.


Alternate-day fasting improves cardiovascular and metabolic biomarkers.


The findings are similar to a calorie restriction diet in people who are overweight, obese or have a metabolic syndrome.


It ie uncertain whether intermittent fasting could prevent cardiovascular disease.


Obese women over 10 weeks showed that those who used both calorie-restriction and intermittent fasting were more successful at losing weight than the group who used only calorie restriction, or only intermittent fasting.


A review found that intermittent fasting may help people lose more weight than regular eating patterns, but was not different than energy restriction diets.


Intermittent fasting has not yet been studied in children, elderly, or underweight people, and may be harmful in these populations.


Intermittent fasting is not recommended for people who are not overweight, and the long-term sustainability of intermittent fasting is unknown.

Intermittent fasting can improve the immune system and the body’s resistance to bacterial infection.

Afew small-scale clinical studies suggest that it may reduce chemotherapy side effects.


Intermittent fasting has shown signs of reducing risk factors for certain disorders, including insulin resistance and cardiovascular disease.


Intermittent fasting does not affect bone health.


Athletic performance does not benefit from intermittent fasting.


Overnight fasting reduces performance in prolonged exercise of more than 60 min.


Short-term intermittent fasting may produce minor adverse effects, such as continuous feeling of hunger,


irritability, and impaired thinking, although these effects disappear within a month from the fasting practice.


Intermittent fasting is not recommended for pregnant or breastfeeding women, or growing children and adolescents, or individuals vulnerable to eating disorders.


The  drop-out rates varied widely from 2% to 38% for intermittent fasting, and from 0% to 50% for calorie restriction diet.


In the fed state (or absorptive state) during which satiety occurs, is when the primary fuel source is glucose and body fat storage is active, and lasts for about 4 hours.


The postabsorptive state, lasts for up to 18 hours, and is when glucagon is secreted and the body uses liver glucose reserves as a fuel source.


In the fasting state, there is progressively transferring to other reserves, such as fat, lactic acid, and alanine, as fuel sources, when the liver glucose reserves are depleted. 


This occurs after 12 to 36 hours of continued fast.


The shift from preferential lipid synthesis and fat storage, to the mobilization of fat as free fatty acids, that are metabolized into fatty acid-derived ketones to provide energy.

After 8-12 hours of fasting, the liver starts to break down fatty acids to produce ketone bodies. 

The body uses ketone bodies as an alternative for fuel to sustain vital organs and tissues when the typical source of glucose is not available. 

Intermitt ent fasting decreases inflammatory markers and improves glucose regulation.

While studies lasting between 12 weeks and 12 months have shown no difference in weight loss when intermittent fasting is compared with a daily calorie restricted diet. 

However, 27-40% of these study participants dropped out of intermittent fasting demonstrating the challenge of adherence to intermittent fasting.

Intermittent fasting can result in decreases in blood pressure, resting heart rate, cholesterol and triglycerides, glucose and insulin.

Intermittent fasting may produce weakness, hunger, dehydration, headaches, difficulty concentrating, hypotension, or fainting.

Intermittent fasting is not recommended for pregnant or lactating women, frail older adults, individuals with immunodeficiencies, and individuals with risk of eating disorders.

The above transition is referred to as a metabolic switch.


Adult studies have not found intermittent feeding to increase glucose variability or gastrointestinal intolerance.


Meta-analyses found intermittent feeding had no influence on gastric residual volumes aspiration, pneumonia, mortality nor morbidity in people with a trauma.


There is an increased risk of diarrhea with IF.


While many diets focus on what to eat, intermittent fasting is all about when one eats.


The 5:2 approach, involves eating regularly five days a week. 


For the other two days, the limit is  500-600 calorie meal. 


Longer periods without food, such as 24, 36, 48 and 72-hour fasting periods, are not necessarily better and may be dangerous. 


It can take two to four weeks before the body becomes accustomed to intermittent fasting. 


A range of health benefits associated with intermittent fasting: longer life, a leaner body and a sharper mind.

Intermittent fasting boosts verbal memory in adult humans, improves blood pressure and resting heart rates as well as other heart-related measurements.


Intermittent fasting not for:


Children and teens under age 18.


Women who are pregnant or breastfeeding.


People with diabetes


History of eating disorders.


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