Intermittent fasting: Is it all it’s cracked up to be?

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Intermittent fasting is a term used to describe a variety of eating patterns that have alternating periods of fasting — abstinence from foods — and eating.
The fasting period may last from 12 hours per day to several consecutive days, with a consistent, recurring pattern over the course of a week.
The main typesTrusted Source of intermittent fasting are:
• modified fasting or the 5:2 diet — this protocol involves fasting for 2 non-consecutive days of the week, and eating normally for 5 days
• alternate-day fasting — fasting days are alternated with days where foods and beverages are consumed normally, without restrictions
• time-restricted eating — a type of intermittent fasting that limits the “eating window” to 4–12 hours, inducing a daily fasting period of 12–20 hours. Persons eat to satiety during their eating windows without caloric restrictions.
Of these, time-restricted eatingTrusted Source is the most popular, and may be what most people refer to when they mention intermittent fasting.
The 16:8 pattern — eating during an 8-hour window and fasting for 16 hours each day — may be the most recommended time-restricted eating pattern.
The circadian rhythm
Much of the research on intermittent fasting and time-restricted eating considers the impact of fasting on the body’s natural circadian rhythm.
The circadian rhythmTrusted Source, also called the circadian clock, represents the 24-hour cycle of metabolism in the body, including control of the sleep-wake cycle, blood pressure, mood regulation, and hormonal balance, to name a few.
It is influenced by light and darkness over the course of the day, eating behaviors, and the timing of meals.
A growing body of researchTrusted Source suggests that eating for lengthy periods in the day, ranging from 12–15 hours, may disrupt the circadian rhythm and increase the risk of chronic diseases including heart disease, cancer, and type 2 diabetes.
Thus, a major goal of fasting, specifically time-restricted eating, is to reduce the time spent eating in the day by extending the overnight fasting period.
The study of the relationship between circadian rhythms and food timing is called chrono-nutritionTrusted Source.
Benefits
Many of the benefits of intermittent fasting are attributed to daily fasting periods of no less than 12 hours, although some research suggests that a minimum of 16 hours of fasting may be required.
Generally, during 12–36Trusted Source hours of uninterrupted fasting, the liver glycogen stores become depleted, overall metabolic processes are altered, and positive health effects are observed.
Here are some of the science-backed benefits of intermittent fasting.
1. Improved cholesterol levels
Findings across animal and human researchTrusted Source show favorable changes in cholesterol levels.
Intermittent fasting has the potentialTrusted Source to reduce total cholesterol, triglycerides, LDL cholesterol or “bad” cholesterol, and increase HDL cholesterol or “good” cholesterol.
Elevated total cholesterol, LDL cholesterol, and triglyceride levels are risk factorsTrusted Source for heart disease.
2. Blood sugar control
Intermittent fasting can improve blood sugar controlTrusted Source by reducing insulin resistance, and increasing insulin sensitivity.
This results in lowerTrusted Source fasting blood sugar and glycated hemoglobin — HbA1c — levels.
In fact, experimental research in adult males with type 2 diabetes showed the potential for intermittent fasting as a therapeutic approach that may reduce the need for insulin therapy.
3. Changes in body composition
Changes in body weight and compositionTrusted Source are among the most studied effects of intermittent fasting.
Several studies have shown that weight loss of between 3–7% body weight in an average of 8 weeks was achievable through intermittent fasting. Research also noted that this method could result in fat loss.
Fasting in a 14:10 patternTrusted Source — an eating window of 10 hours and a daily fast of 14 hours — can act on the risk factors of metabolic syndrome, including by reducing waist circumference, body fat percentage, and visceral fat.
Intermittent fasting can thus ease metabolic syndrome, a set of risk factors that increase the risk of heart disease and type 2 diabetes.
4. Other healthbenefits
A 2015 reviewTrusted Source of 2,650 adult females indicated that reducing calorie intake in the evenings, and fasting for longer periods at night, may lower inflammation and the risk of breast cancer and other inflammatory conditions.
Observational researchTrusted Source of 26,092 adult males over a 16-year period suggested that reducing late-night eating through time-restricted eating may significantly reduce the risk of heart disease.
Other areas of health that intermittent fasting is being explored in include longevityTrusted Source and neurodegenerative conditions like Parkinson’s diseaseTrusted Source.
Potential downsides
Despite the many touted benefits of intermittent fasting, there are also some downsides.
Side effects
Intermittent fasting may be safe for heart and metabolic health, but according to a 2017 reviewTrusted Source, it may induce negative side effects in some people, such as:
• increased feelings of hunger
• heightened irritability
• worsened mood
• increased thoughts about food
• fatigue
• fears of feeling out of control around food
• overeating during eating windows
• difficulty concentrating.
Quality of evidence
Additionally, most of the research on intermittent fasting is based on animal research, with littleTrusted Source long-term human research available.
Furthermore, a 2021 reviewTrusted Source found that only six out of 104 alleged health benefits of intermittent fasting were supported by moderate- to high-quality evidence, and most findings were based on low-quality research.
This means that more rigorous human research on the long-term health benefits of intermittent fasting is warranted.
Alternatives
Intermittent fasting is not the only type of diet to result in the aforementioned benefits.
Calorie restriction
Calorie restrictionTrusted Source involving a reduction of about 25% of daily energy needs without a change in mealtimes had a positive effect on promoting overall health.
Some research suggests that the health outcomes of intermittent fasting are no greater than those observed in calorie restriction diets.
In fact, outcomes for weight and/ or fat loss, body fat percentage, and metabolic risk factors are comparableTrusted Source between the two.
However, research on intermittent fasting shows a greater adherence over longer periods compared to calorie restriction, and suggests that it may be a more sustainable approach.
Mediterranean diet
The Mediterranean dietTrusted Source is a renowned dietary pattern based on the traditional eating patterns of the Mediterranean basin.
Like the potential heart-friendlyTrusted Source benefits of intermittent fasting, research shows that long-term adherence to the Mediterranean diet reduces the occurrence of heart attack and stroke by up to 30%Trusted Source after approximately 5 years.
Additional research on the Mediterranean diet demonstrates its protective nature against the development of colorectal cancerTrusted Source and the loss of nerve cells in Parkinson’s diseaseTrusted Source.
A major benefit of the Mediterranean diet compared to intermittent fasting is that similar results can be achieved without the need for fasting.