NEW DELHI: Intermittent fasting (IF) refers to a range of eating patterns that alternate between periods of fasting when no food is consumed and designated eating windows. Rather than focusing on what to eat, intermittent fasting emphasises when to eat. Fasting periods can range from as little as 12 hours a day to multiple consecutive days, following a fixed and recurring weekly schedule.
Common types of intermittent fasting include:
- Modified fasting or the 5:2 diet: Eating normally for five days a week while significantly restricting calories on two non-consecutive days.
- Alternate-day fasting: Days of fasting alternate with days of unrestricted eating.
- Time-restricted eating: Daily eating is confined to a specific window of 4-12 hours, resulting in a fasting period of 12-20 hours.
People typically eat to satiety during the eating window without strict calorie counting. Among these, time-restricted eating is the most popular and is often what people mean when they talk about intermittent fasting. The widely followed 16:8 method—fasting for 16 hours and eating within an 8-hour window—is one of the most commonly recommended approaches.
The role of the circadian rhythm
Much of the research around intermittent fasting focuses on its interaction with the body’s circadian rhythm, also known as the internal biological clock. The circadian rhythm governs the body’s 24-hour metabolic cycle, influencing sleep-wake patterns, hormone release, blood pressure, mood, and metabolism. It is shaped by light exposure, meal timing, and eating habits.
The scientific study of how meal timing affects health is known as chrono-nutrition. Emerging research suggests that eating over long periods—often 12 to 15 hours a day—may disrupt this rhythm and increase the risk of chronic conditions such as heart disease, cancer, and type 2 diabetes. Time-restricted eating aims to counter this by shortening the daily eating window and extending overnight fasting. The scientific study of how meal timing affects health is known as chrono-nutrition.
Potential benefits
Many of the proposed benefits of intermittent fasting are linked to fasting periods lasting at least 12 hours, though some studies suggest that 16 hours or more may be necessary. During 12 to 36 hours of uninterrupted fasting, liver glycogen stores are depleted, metabolic pathways shift, and several physiological changes associated with improved health may occur.
Here are some evidence-backed benefits:
- Improved cholesterol levels: Animal and human studies show that intermittent fasting may lower total cholesterol, triglycerides, and LDL (“bad”) cholesterol, while increasing HDL (“good”) cholesterol. Since elevated cholesterol and triglycerides are major risk factors for heart disease, these changes are considered beneficial.
- Better blood sugar regulation: Intermittent fasting may improve blood sugar control by reducing insulin resistance and enhancing insulin sensitivity. This can lead to lower fasting blood glucose and HbA1c levels. Some experimental studies in adults with type 2 diabetes even suggest that intermittent fasting could reduce dependence on insulin therapy.
- Changes in body weight and composition: Weight loss is one of the most studied outcomes of intermittent fasting. Research indicates that a 3-7% reduction in body weight over roughly eight weeks is achievable, along with a reduction in body fat. A 14:10 fasting pattern—fasting for 14 hours and eating within a 10-hour window—has also been shown to reduce waist circumference, visceral fat, and other risk factors linked to metabolic syndrome.
- Additional health effects: A review involving over 2,600 adult women found that reducing evening calorie intake and extending nighttime fasting may lower inflammation and reduce the risk of breast cancer and other inflammatory conditions. Long-term observational studies in adult men suggest that limiting late-night eating through time-restricted eating may significantly lower the risk of heart disease. Intermittent fasting is also being explored for its potential role in longevity and neurodegenerative conditions such as Parkinson’s disease.
Possible downsides
Despite its popularity, intermittent fasting is not without drawbacks.
- Side effects: A 2017 review reported that some individuals experience negative effects, including increased hunger, irritability, low mood, fatigue, difficulty concentrating, and heightened preoccupation with food. Others may struggle with overeating during eating windows or feel a loss of control around food.
- Quality of evidence: Another concern is the strength of the evidence. Much of the research on intermittent fasting is based on animal studies, with limited long-term human data. A 2021 review found that only six out of 104 claimed health benefits were supported by moderate to high-quality evidence, highlighting the need for more robust human trials.
Alternatives to intermittent fasting
Intermittent fasting is not the only dietary approach shown to improve metabolic health.
- Calorie restriction: Reducing daily calorie intake by around 25%—without altering meal timing—has been shown to produce many of the same benefits, including weight loss and improved metabolic markers. Research suggests that outcomes for weight and fat loss are comparable between calorie restriction and intermittent fasting. However, intermittent fasting may be easier to sustain for some people due to higher long-term adherence.
- Mediterranean diet: Based on traditional eating patterns of Mediterranean regions, it is widely recognised for its heart-protective benefits. Long-term adherence has been shown to reduce the risk of heart attack and stroke by up to 30% over five years. Research also links the Mediterranean diet to a lower risk of colorectal cancer and protection against neurodegenerative conditions like Parkinson’s disease without the need for fasting.
The bottom line
Intermittent fasting includes a range of eating patterns that cycle between fasting and eating on a regular schedule. Time-restricted eating is the most common form and aligns with chrono-nutrition principles by extending nighttime fasting. Evidence suggests intermittent fasting may improve cholesterol levels, blood sugar control, body weight, inflammation, and potentially support longevity and brain health.
However, much of the existing research is limited in quality, and long-term human studies are still lacking. Importantly, similar health benefits can be achieved through non-fasting approaches such as calorie restriction or the Mediterranean diet. Whether intermittent fasting is “worth it” ultimately depends on individual preferences, lifestyle, and how well the approach can be sustained over time.
