Intermittent Fasting: What Does the Research Show?
Intermittent fasting has become a popular approach to improving metabolic health. Supporters claim it can help with weight loss, insulin resistance, blood glucose, and perhaps even healthy aging.
But does fasting offer benefits beyond simply eating fewer calories?
Clinical trials suggest that intermittent fasting can improve weight and metabolic health. However, its advantages over conventional calorie restriction are generally modest.
What Is Intermittent Fasting?
Intermittent fasting focuses on when we eat, rather than what we eat.
Common approaches include:
12:12: Eating within 12 hours, followed by a 12-hour overnight fast.
14:10: Eating within 10 hours and fasting for 14.
16:8: Eating within 8 hours and fasting for 16.
5:2: Eating normally five days a week and substantially restricting calories on two days.
Alternate-Day Fasting: Alternating normal eating with fasting or substantial calorie restriction.
Water, plain tea, and black coffee are generally permitted during fasting periods.
What Does the Research Show?
Weight Loss
A 2025 review published in The BMJ examined 99 randomized clinical trials involving 6,582 adults.
Researchers found that intermittent fasting and conventional calorie restriction produced similar weight loss.
Alternate-day fasting resulted in approximately 2.9 pounds more weight loss than conventional calorie restriction.
The conclusion: Intermittent fasting works, but it is not clearly superior to simply reducing calories.
Insulin Resistance and Blood Glucose
Fasting allows insulin levels to decline between meals and encourages the body to use stored energy.
Some clinical trials have found improvements in fasting insulin, blood glucose, and insulin sensitivity.
A 2024 randomized trial involving adults with metabolic syndrome found modest improvements in HbA1c after three months of time-restricted eating.
However, many benefits may result from weight loss rather than fasting itself.
Does Eating Earlier Help?
Our bodies follow daily biological rhythms that influence insulin sensitivity and glucose metabolism.
Some research suggests that eating earlier in the day may provide metabolic advantages.
However, results are mixed, and the ideal eating window remains uncertain.
Cardiovascular Health
Some studies report improvements in blood pressure, triglycerides, and waist circumference.
However, there is not yet convincing evidence that intermittent fasting reduces heart attacks, strokes, or cardiovascular deaths.
Fasting and Low-Carbohydrate Diets
Low-carbohydrate diets and intermittent fasting work differently.
A low-carbohydrate diet reduces the amount of carbohydrate consumed, while fasting extends the time between meals.
Both approaches can reduce insulin exposure and may improve insulin sensitivity. For more information, see Insulin Resistance.
Some people find that combining the approaches makes fasting easier because they experience less hunger.
However, research has not established that combining them produces superior long-term health outcomes.
What About People Over 70?
Most fasting studies have involved younger adults, often those who are overweight.
For people in their 70s and 80s, maintaining muscle strength and adequate nutrition may be more important than extending fasting periods.
A 12-hour overnight fast may be a reasonable option for some older adults, but individual needs vary. People taking insulin or medications that can cause low blood glucose should obtain medical guidance before fasting.
For more information, see Fasting After 70: Benefits, Risks & Practical Advice.
What Remains Unproven?
Fasting is sometimes promoted as a way to prevent Alzheimer's disease, increase autophagy, slow aging, or extend lifespan.
These possibilities are interesting, but human clinical trials have not established these benefits.
We explore the biology of fasting and its possible effects on aging in the following articles.
Jason Fung: Fasting and Insulin Resistance
Dr. Jason Fung, a Canadian nephrologist and author of The Obesity Code and The Diabetes Code, has helped popularize intermittent fasting for obesity, insulin resistance, and type 2 diabetes.
Fung emphasizes that fasting allows insulin levels to decline, encouraging the body to use stored fat for energy.
His approach focuses on when we eat, not simply how many calories we consume.
Although fasting can improve metabolic health, clinical trials generally show weight-loss results comparable to conventional calorie restriction.
Fung's emphasis on insulin is important, but his broader explanations of obesity remain debated.
Eric Westman: Is Fasting Necessary?
Dr. Eric Westman, a longtime researcher and clinician specializing in low-carbohydrate diets, considers intermittent fasting optional.
He emphasizes carbohydrate restriction as the primary approach to improving insulin resistance, obesity, and type 2 diabetes.
Westman observes that people following low-carbohydrate diets often experience less hunger and naturally eat less frequently.
His advice is to eat when hungry rather than follow a prescribed fasting schedule. Fasting can be added if helpful, but it is not necessary for low-carbohydrate eating to be effective.
The Bottom Line
Intermittent fasting can help with weight management and certain metabolic risk factors.
However, its benefits are generally comparable to conventional calorie restriction.
For older adults, the goal should be better metabolic health without sacrificing muscle, strength, or adequate nutrition.
Learn More
2025 Review of 99 randomized trials finding that fasting and conventional calorie restriction produce similar weight loss.
A 12-month trial finding no significant additional weight-loss benefit from restricting eating hours.
A randomized trial finding modest improvements in blood glucose control with an 8- to 10-hour eating window.
Books and resources on intermittent fasting, insulin resistance, and type 2 diabetes.
Reviews clinical research comparing intermittent fasting with calorie restriction and discusses fasting alongside low-carbohydrate diets.
Health Curious provides educational information, not individualized medical advice. People with diabetes, those taking medications that can cause hypoglycemia, and those who are underweight or frail should consult their healthcare professional before beginning fasting.