Scientific Research on Intermittent Fasting
This page summarises what scientific research has and hasn’t shown about intermittent fasting: effects on blood sugar and metabolism, body weight and composition, the brain, ageing-related processes, and meal timing, along with how the main fasting protocols compare and the important limitations of the evidence.
The overall picture is more modest than popular claims suggest. Human trials show that intermittent fasting can help with weight loss and some metabolic markers, generally to a similar degree as ordinary daily calorie reduction. Many of the more striking findings, particularly on the brain, ageing, and lifespan, come from animal or laboratory studies and have not been confirmed in people.
Important Disclaimer: This page summarises research for educational purposes only. It is not medical advice. Talk to your healthcare provider before starting any fasting regimen, especially if you have a health condition or take medication.
Table of Contents
Current State of Intermittent Fasting Research
Research on intermittent fasting has grown substantially over the past decade or so. Early work was mostly in animals; there are now many human randomised controlled trials, although most are small and last weeks to months rather than years.
How the Research Developed
Animal Studies
Early research focused on calorie restriction and fasting in rodents and other animals, where it often improved metabolic markers and, in some studies, lifespan. Results in animals don’t necessarily apply to humans.
Cellular Mechanisms
Laboratory research identified processes that change during fasting, including autophagy, ketone production, and stress-response pathways.
Early Human Studies
Small human studies examined whether fasting protocols were safe and practical, and measured short-term changes in weight and metabolic markers.
Current Research
More recent studies include randomised trials comparing fasting with standard calorie restriction, trials that match calorie intake to separate the effect of meal timing, and studies in specific groups such as people with type 2 diabetes.
Judging the Quality of Evidence
A Rough Hierarchy of Evidence
- Strongest: Systematic reviews and meta-analyses of well-conducted randomised controlled trials (RCTs), and large, long RCTs
- Moderate: Smaller or shorter RCTs, and well-designed observational studies
- Weaker: Pilot studies, case reports, and uncontrolled studies
- Not directly applicable to people: Animal and cell studies, which are useful for generating hypotheses
- Weakest: Anecdotes and opinion
Much of the human evidence on intermittent fasting sits in the “moderate” category: randomised trials that are often small, short, or both.
When reading about fasting research, check whether a finding comes from humans or animals, how long the study lasted, how many people took part, and whether fasting was compared with an equal-calorie diet.
Metabolic Research Findings
Much of the human research on intermittent fasting looks at markers such as blood glucose, insulin, blood pressure, and blood fats.
Insulin Sensitivity and Blood Sugar
Early Time-Restricted Eating
In a small, tightly controlled trial, men with prediabetes ate within a 6-hour window ending mid-afternoon, with calories set to prevent weight loss. Compared with a 12-hour window, insulin sensitivity and blood pressure improved over five weeks. The study was small and short, and it is not clear whether the same effects occur in everyday life.
Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism (2018).
Blood Sugar in Broader Trials
Across trials, intermittent fasting often produces modest improvements in fasting glucose and insulin, largely in step with weight loss. When compared with daily calorie restriction that leads to similar weight loss, differences are usually small. People with diabetes should only try fasting with medical supervision, because of the risk of low blood sugar with some medications.
Inflammation
Some studies report reductions in certain inflammatory markers with fasting, but results are inconsistent. One laboratory study, mostly in mice with a small human component, found that short-term fasting reduced the number of certain inflammatory immune cells in the blood. What this means for health in people is not yet known.
Dietary Intake Regulates the Circulating Inflammatory Monocyte Pool. Cell (2019).
Autophagy
Autophagy is the process cells use to break down and recycle their own components. Fasting increases autophagy in animal studies, and research into its role in ageing and disease is active. However, autophagy is difficult to measure in living humans, so the timing and extent of changes during typical intermittent fasting in people are not well established.
What Is and Isn’t Known
- Timing: There is no established timeline in humans for when autophagy “starts” or “peaks” during a fast; figures quoted online are mostly extrapolated from animal studies
- Tissue differences: Autophagy responds differently in different tissues
- Disease effects: Links to protection against disease come mainly from laboratory and animal research
See autophagy and fasting for more.
The “Metabolic Switch”
Shifting Fuel Sources
During a longer fast, once liver glycogen runs low, the body relies more on fat and produces more ketones. Some researchers describe this as a “metabolic switch” and have proposed that switching back and forth regularly may have health effects beyond weight loss. This idea is based largely on animal research and remains a hypothesis in humans.
Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (2018).
Ketones
Ketones produced during fasting are an alternative fuel for many tissues, including the brain, and laboratory research suggests they may also act as signalling molecules. Their long-term significance in people is unclear.
Oxidative Stress
Some studies report changes in markers of oxidative stress with fasting, but findings in humans are limited and inconsistent.
Brain and Cognition Research
Researchers are interested in whether fasting affects the brain, but most of the evidence so far comes from animals.
BDNF and Neuroplasticity
In animal studies, fasting and exercise increase brain-derived neurotrophic factor (BDNF), a protein involved in learning and memory, and improve some measures of brain resilience. A review of this research proposed that alternating between fasting and eating may support brain health. These ideas have not been confirmed in human trials.
Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience (2018).
Neuroinflammation in Animals
A study in rats found that intermittent fasting reduced brain inflammation and memory impairment caused by an inflammatory trigger. Findings like this help generate hypotheses but cannot be assumed to apply to people.
Intermittent fasting attenuates lipopolysaccharide-induced neuroinflammation and memory impairment. Journal of Neuroinflammation (2014).
What We Know in Humans
- Human studies of intermittent fasting and memory, attention, or mood are few, small, and short, with mixed results
- Some people report feeling more focused while fasting; others report poorer concentration, irritability, or low mood, particularly at first
- There is no good evidence that fasting prevents or treats Alzheimer’s disease, Parkinson’s disease, depression, or other neurological or mental health conditions in people
If you have a mental health condition, talk to your healthcare provider before fasting. See fasting and mood.
Body Composition Research
Weight and body composition are the most studied outcomes of intermittent fasting in humans.
Weight Loss
Systematic Reviews
Systematic reviews and meta-analyses have found that intermittent fasting leads to weight loss in adults with overweight or obesity over periods of weeks to months. The amount varies between studies and individuals.
Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports (2018).
Compared With Daily Calorie Restriction
Trials comparing intermittent fasting with ordinary daily calorie restriction generally find similar weight loss. In a one-year trial of alternate-day fasting, weight loss and cardiometabolic changes were similar to daily calorie restriction, and more people dropped out of the fasting group. So intermittent fasting appears to be an alternative way to reduce calories, not a more effective one. Whether it is easier to stick with depends on the person.
Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine (2017).
Waist and Abdominal Fat
Many trials report reductions in waist circumference and, where measured, in abdominal fat. These changes generally reflect overall weight loss; there is no consistent evidence that fasting targets abdominal fat more than other ways of losing a similar amount of weight.
Muscle
Lean Mass With Resistance Training
In a trial of active women doing resistance training, those following time-restricted eating gained lean mass and strength similarly to those eating normally, when protein intake was adequate. This suggests time-restricted eating need not undermine training, but it doesn’t show a muscle-building advantage.
Time-restricted feeding plus resistance training in active females: a randomized trial. American Journal of Clinical Nutrition (2019).
As with any weight loss, some of the weight lost during intermittent fasting is lean mass. Getting enough protein and doing resistance exercise are the best-supported ways to limit this. Growth hormone rises during fasting, but there is no clear evidence that this preserves muscle in practice. See fasting and muscle.
Body Composition Considerations
- Adequate protein and resistance training matter more than meal timing for muscle
- Age, sex, and starting body composition influence results
- Few studies last longer than a year, so long-term effects are less clear
Ageing and Cellular Health Research
Claims that fasting slows ageing or extends lifespan are popular, but they rest almost entirely on animal and laboratory research.
What Animal and Lab Studies Suggest
- Nutrient-sensing pathways: Fasting reduces activity of mTOR, a pathway that promotes growth, and increases activity of AMPK, an energy sensor. In animals, changes in these pathways are linked with longer lifespan.
- Sirtuins and NAD+: These are involved in cellular stress responses and are studied in relation to ageing, mostly in cells and animals.
- Lifespan: Calorie restriction and some fasting regimens extend lifespan in several animal species, but results vary with species, strain, and diet.
None of these findings shows that intermittent fasting extends human lifespan or slows ageing in people.
Fasting-Mimicking Diets
A fasting-mimicking diet (a few days of low-calorie, low-protein eating repeated periodically) was studied in mice and in a small human pilot study, which reported changes in some risk factors. Larger and longer human trials are needed. See fasting-mimicking diet.
A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell Metabolism (2015).
Chronic Disease Risk Factors
In human trials, intermittent fasting can improve risk factors such as body weight, blood pressure, blood fats, and blood sugar, generally to a similar degree as other weight-loss approaches. There is not yet evidence from long-term trials showing that it reduces heart attacks, strokes, cancer, dementia, or deaths. Fasting is not a treatment for any disease and should never replace medical care.
Circadian Rhythm Research
The body’s internal 24-hour clock influences how it handles food. Researchers are studying whether the timing of meals, not just their content, affects health.
Earlier vs Later Eating
Early Time-Restricted Eating
Some small studies suggest that eating earlier in the day may be better for blood sugar control than eating later. In one small crossover study in adults with overweight, eating between 8 AM and 2 PM for four days lowered average 24-hour glucose levels compared with a 12-hour window.
Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients (2019).
Late-Night Eating
An observational study found that eating closer to the body’s internal night (when melatonin is high) was associated with higher body fat. Because it was observational, it cannot show that late eating caused the difference.
Later circadian timing of food intake is associated with increased body fat. American Journal of Clinical Nutrition (2017).
Other Findings
- Peripheral clocks: In animals, meal timing helps set the internal clocks of organs such as the liver
- Glucose tolerance: In laboratory studies, glucose tolerance tends to be better in the morning than in the evening
- Shift work: Eating at night is associated with poorer metabolic health, and meal timing for shift workers is an active research area
What This Means in Practice
- An earlier eating window may have some advantages, but the evidence comes from small, short studies
- A schedule you can keep consistently matters more than finding the theoretically ideal window
- Avoiding large meals late at night is reasonable general advice
- Shift workers should consider their own schedules and health; see fasting and sleep
Comparison of Fasting Protocols in Studies
Researchers have studied several intermittent fasting approaches. Here’s what the evidence shows for each.
Time-Restricted Eating (TRE)
Research Support
Time-restricted eating limits daily eating to a window, usually 6-10 hours. In a small, uncontrolled study of adults with metabolic syndrome who were mostly taking medication, a 10-hour eating window for 12 weeks was associated with modest reductions in weight, blood pressure, and some blood fats. Trials that have kept calorie intake the same in both groups have generally found little additional benefit from the eating window itself, which suggests that much of TRE’s effect comes from eating less.
Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metabolism (2020).
16:8
Probably the most popular schedule in practice. Studies generally show modest weight loss and small improvements in some markers.
Early vs Late Windows
Small studies suggest earlier windows may benefit blood sugar more than later ones; this needs confirmation in larger, longer trials.
Shorter Windows
Windows of 4-6 hours may lead to larger calorie reductions, but they are harder to keep up and make it harder to meet nutrient needs.
Alternate-Day Fasting (ADF)
Research Support
Alternate-day fasting alternates normal eating days with fasting or very low-calorie days. Trials show weight loss similar to daily calorie restriction. In a small pilot trial in adults with obesity, zero-calorie ADF was tolerated and led to weight loss similar to daily calorie restriction over eight weeks.
A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (2016).
Modified Alternate-Day Fasting
Modified ADF, with about 500 calories on fasting days, is the version most often studied. Results for weight and metabolic markers are broadly similar to daily calorie restriction. See alternate day fasting.
5:2 Fasting
Research Support
The 5:2 approach limits intake to roughly 500-600 calories on two days a week. In a six-month trial in young women with overweight, intermittent energy restriction produced weight loss similar to continuous energy restriction. See the 5:2 approach.
The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity (2011).
Prolonged Fasting
Research Support
Fasts of several days have much less human research, most of it short-term and focused on mechanisms. Longer fasts also carry more risk, including electrolyte problems and, when eating resumes after very long fasts, refeeding complications. They should only be done with medical guidance. See extended fasting.
Comparing Protocols
- Weight loss: When overall calorie reduction is similar, different protocols tend to produce similar weight loss
- Sticking with it: Drop-out rates vary; more demanding protocols such as strict ADF can be harder to maintain
- Individual variation: Responses and preferences vary widely between people
Choosing a Protocol
- Personal preference and lifestyle often matter more than protocol details
- Existing health conditions affect which approaches are appropriate
- Starting with a moderate approach (a 12-14 hour overnight fast) is sensible
- Adjust or stop if a protocol doesn’t suit you
Research Limitations and Considerations
The evidence on intermittent fasting has important gaps that are worth keeping in mind.
Current Research Gaps
Long-Term Studies
Most human trials last weeks to months, and few last longer than a year. There are no long-term trials of intermittent fasting measuring outcomes such as heart attacks or lifespan.
Who Has Been Studied
Many studies involve middle-aged adults with overweight or obesity who are otherwise healthy. Results may not apply to other groups.
Sex Differences
There are open questions about whether women and men respond differently, including effects on reproductive hormones. More sex-specific research is needed.
Groups Needing Particular Caution
Pregnancy and Breastfeeding
Intermittent fasting is generally not recommended during pregnancy or breastfeeding, when energy and nutrient needs are higher.
Children and Adolescents
There is very little research in under-18s, who have higher needs for growth. Adult fasting protocols should not be applied to them without medical supervision.
Older Adults
Research in older adults is limited. Getting enough protein and maintaining muscle and bone need particular attention.
Medical Conditions
People with diabetes (especially those using insulin or sulfonylureas), a history of eating disorders, those who are underweight, and people with many chronic conditions are usually excluded from studies. They should not fast without medical supervision.
Methodological Limitations
- Adherence: Many studies rely on self-reported eating times and intake
- Timing vs calories: It is often hard to separate the effect of meal timing from the effect of simply eating less
- Blinding: Participants know which diet they are following, which can influence results
- Publication bias: Positive results may be more likely to be published than null findings
Areas Needing More Research
- Long-term effects on health outcomes, not just risk markers
- Effects on hormones, including reproductive hormones
- How fasting interacts with different types of exercise
- Effects on the gut microbiome; see fasting and gut health
- Which people are most and least likely to benefit
Practical Takeaways
Given the current evidence, some reasonable conclusions:
It’s One Option, Not a Special Solution
For weight loss, intermittent fasting performs similarly to other ways of reducing calories. Choose it if the structure suits you.
Start Gradually
Beginning with a 12-hour overnight fast and extending slowly is a sensible, low-risk approach.
Food Quality Still Matters
What you eat during your eating window affects your health regardless of timing. Emphasise vegetables, fruit, whole grains, legumes, and adequate protein.
Consistency Over Intensity
A moderate approach you can keep up is more useful than an extreme one you abandon.
Exercise
Endurance Training
Low-intensity exercise can generally be done fasted by healthy people. High-intensity performance may suffer in a fasted state. See endurance training and fasting.
Resistance Training
Resistance training can be effective whether done fed or fasted. Getting enough total protein matters more than precise timing.
Recovery
Eating after hard training supports recovery, so it helps to plan workouts around your eating window.
Tracking Your Own Response
- Track objective measures (weight trend, waist) and how you feel (energy, hunger, mood, sleep)
- If you have health conditions, ask your doctor about relevant checks such as blood glucose, HbA1c, blood pressure, and cholesterol
- Note any negative effects such as dizziness, poor sleep, menstrual changes, or preoccupation with food, and adjust or stop if they occur
Research consistently suggests that adherence is a major factor in long-term results: the most useful fasting approach is usually the one you can maintain safely over time.
Selected References
The following reviews and studies are among those discussed on this page. This is not an exhaustive list, and many of the findings are from small, short-term, or animal studies, as noted above.
Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine (2019).
Impact of intermittent fasting on health and disease processes. Ageing Research Reviews (2017).
Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (2018).
Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports (2018).
Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine (2017).
A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (2016).
The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity (2011).
Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism (2018).
Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients (2019).
Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metabolism (2020).
Time-restricted feeding plus resistance training in active females: a randomized trial. American Journal of Clinical Nutrition (2019).
Later circadian timing of food intake is associated with increased body fat. American Journal of Clinical Nutrition (2017).
Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience (2018).
Intermittent fasting attenuates lipopolysaccharide-induced neuroinflammation and memory impairment. Journal of Neuroinflammation (2014).
Dietary Intake Regulates the Circulating Inflammatory Monocyte Pool. Cell (2019).
A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell Metabolism (2015).
The Bottom Line
Research shows that intermittent fasting can help some people lose weight and improve certain metabolic markers, generally to a similar extent as other ways of eating less. Evidence on meal timing, especially earlier eating, is interesting but comes from small, short studies. Claims about brain health, ageing, and longevity are based mainly on animal and laboratory research and have not been shown in people.
Fasting research is still developing, and long-term human data are limited. If you’re considering intermittent fasting, a moderate, sustainable approach combined with a nutritious diet is the most evidence-consistent choice, and it’s important to talk to your healthcare provider first if you have any health conditions or take medication.
Explore the Basics
Learn how the main fasting approaches work and which might suit you.