Intermittent fasting refers to different eating patterns that switch between periods of fasting (not eating) and eating.
The fasting window can range from 12 hours a day to several days in a row, following a regular, repeating schedule each week.
The main types of intermittent fasting are:
- Modified fasting (5:2 diet) – Fasting for 2 non-consecutive days per week while eating normally the other 5 days.
- Alternate-day fasting – Alternating between fasting days and normal eating days without restrictions.
- Time-restricted eating – Limiting eating to a 4–12 hour window each day, leading to a daily fast of 12–20 hours. During eating periods, people can eat until satisfied without counting calories.
Out of these, time-restricted eating is the most popular and what many people mean when they talk about intermittent fasting.
The 16:8 method—eating within an 8-hour window and fasting for 16 hours daily—is one of the most commonly recommended approaches.
The Circadian Rhythm
A lot of research on intermittent fasting and time-restricted eating looks at how fasting affects the body’s natural circadian rhythm.
The circadian rhythm (or internal body clock) regulates the body’s 24-hour metabolic cycle, including sleep-wake patterns, blood pressure, mood, and hormone balance, among other things.
It’s influenced by light and darkness, eating habits, and meal timing.
Growing evidence suggests that eating over long stretches (12–15 hours a day) may disrupt the circadian rhythm, raising the risk of chronic conditions like heart disease, cancer, and type 2 diabetes.
Because of this, a key goal of fasting—especially time-restricted eating—is to shorten daily eating periods by extending overnight fasting.
The study of how meal timing affects circadian rhythms is called chrono-nutrition.
Benefits
Many of intermittent fasting’s benefits come from daily fasts of at least 12 hours, though some studies suggest 16 hours may be needed for optimal effects.
Generally, after 12–36 hours of continuous fasting, the body uses up stored liver glycogen, shifts metabolic processes, and starts showing positive health changes.
Here are some science-backed benefits:
- Better Cholesterol Levels
Research in both animals and humans shows improvements in cholesterol, including:- Lower total cholesterol
- Lower triglycerides
- Reduced LDL (“bad” cholesterol)
- Increased HDL (“good” cholesterol)
High LDL, total cholesterol, and triglycerides are risk factors for heart disease.
- Blood Sugar Control
Intermittent fasting can improve insulin sensitivity and reduce insulin resistance, leading to:- Lower fasting blood sugar
- Reduced HbA1c (long-term blood sugar marker)
Some studies even suggest it could help reduce insulin dependence in people with type 2 diabetes.
- Changes in Body Composition
Weight and fat loss are among the most researched effects. Studies show:- 3–7% weight loss in about 8 weeks
- Reduced body fat percentage
- Smaller waist circumference
- Less visceral fat (dangerous belly fat)
A 14:10 fasting pattern (10-hour eating window, 14-hour fast) may help lower metabolic syndrome risks.
- Other Health Benefits
- May reduce inflammation and breast cancer risk (especially with nighttime fasting).
- Late-night eating restriction could lower heart disease risk.
- Being studied for longevity and neurodegenerative diseases like Parkinson’s.
Potential Downsides
Despite the benefits, intermittent fasting has some drawbacks.
Side Effects
For some people, it may cause:
- Increased hunger
- Irritability
- Mood swings
- Constant food thoughts
- Fatigue
- Loss of control around food
- Overeating during eating windows
- Trouble focusing
Research Limitations
- Most studies are on animals, with limited long-term human data.
- A 2021 review found only 6 out of 104 claimed benefits had solid scientific backing.
- More high-quality human research is needed.
Alternatives
Intermittent fasting isn’t the only way to get these benefits.
Calorie Restriction
- Cutting daily calories by about 25% (without changing meal timing) also improves health.
- Some studies find similar results between calorie restriction and intermittent fasting for weight loss, fat loss, and metabolic health.
- However, intermittent fasting may be easier to stick to long-term.
Mediterranean Diet
- This well-known eating pattern is based on traditional Mediterranean foods.
- Like intermittent fasting, it’s heart-healthy—reducing heart attack and stroke risk by up to 30% over 5 years.
- It may also protect against colorectal cancer and Parkinson’s disease.
- Unlike fasting, it doesn’t require skipping meals.
The Bottom Line
Intermittent fasting involves structured eating and fasting cycles over a week.
Time-restricted eating is the most popular form, aligning with circadian rhythms to potentially lower disease risk.
Benefits may include better cholesterol, blood sugar control, fat loss, reduced inflammation, and brain health support.
However, most evidence comes from animal studies, and human research is still limited.
Other diets, like calorie restriction and the Mediterranean diet, can offer similar benefits without fasting.