Written by Amber Charles Alexis, MSPH, RDN — Fact checked by Alexandra Sanfins, Ph.D.
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 types 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 eating 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 rhythm, 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 research 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-nutrition.
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.
Here are some of the science-backed benefits of intermittent fasting.
1. Improved cholesterol levels
Intermittent fasting has the potential 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 factors for heart disease.
2. Blood sugar control
This results in lower 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
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.
A 2015 review 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 research of 26,092 adult males over a 16-year period suggested that r .
Despite the many touted benefits of intermittent fasting, there are also some downsides.
Intermittent fasting may be safe for heart and metabolic health, but according to a 2017 review, it may induce negative side effects in some people, such as:
- increased feelings of hunger
- heightened irritability
- worsened mood
- increased thoughts about food
- 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 little long-term human research available.
Furthermore, a 2021 review 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.
Intermittent fasting is not the only type of diet to result in the aforementioned benefits.
Calorie restriction 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 comparable 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.
The Mediterranean diet is a renowned dietary pattern based on the traditional eating patterns of the Mediterranean basin.
Like the potential heart-friendly 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% after approximately 5 years.
A major benefit of the Mediterranean diet compared to intermittent fasting is that similar results can be achieved without the need for fasting.
The bottom line
Intermittent fasting describes a variety of eating patterns that alternate periods of fasting and eating with a consistent, recurring pattern over the course of a week.
Time-restricted eating is the most popular form of intermittent fasting and uses the principles of chrono-nutrition to lengthen night-time fasting and potentially reduce chronic disease risk.
Intermittent fasting may improve cholesterol levels, blood sugar control, weight and/ or fat loss, lower inflammation, promote longevity, and support neurodegenerative conditions like Parkinson’s disease.
However, most of the research on intermittent fasting is based on animal studies and human research is sparse and often of low quality.
Alternative non-fasting diets that produce similar results to intermittent fasting include calorie restriction and the Mediterranean diet.