Intermittent fasting (IF) means alternating periods of eating with periods of not eating, rather than changing what you eat. It has become popular for weight loss and is often promoted with big claims about longevity and metabolism. The research is promising in some areas but more modest than the hype.
Common methods
- Time-restricted eating (16:8 or 14:10): eating within an 8- or 10-hour window each day, for example from noon to 8 p.m.
- 5:2 diet: eating normally five days a week and limiting intake to about 500 to 600 calories on two non-consecutive days.
- Alternate-day fasting: alternating normal eating days with very low-calorie days.
What the research shows
- Weight loss: intermittent fasting helps many people lose weight, mainly because they eat less overall. Several trials comparing it to regular daily calorie restriction found similar weight loss with either approach.
- Blood sugar and insulin: some trials found improvements in insulin sensitivity and blood sugar, especially in people with excess weight. Eating earlier in the day (early time-restricted eating) may have extra benefits.
- Heart health markers: modest improvements in blood pressure and cholesterol have been seen, often linked to weight loss.
- Longevity: the strong effects reported in animal studies have not been demonstrated in humans.
The Harvard T.H. Chan School of Public Health’s Nutrition Source reviews this evidence in plain language.
Why some people like it
For some, a simple rule (“no food after 8 p.m.”) is easier to follow than counting calories. It can reduce late-night snacking.
Possible downsides
- Hunger, irritability, headaches and low energy, especially at first.
- Overeating during the eating window, which cancels the benefit.
- Difficulty fitting it with family meals, social life or training.
- Getting enough protein and nutrients in a shorter window.
Who should not fast without medical advice
- People with diabetes, especially those taking insulin or other blood sugar-lowering drugs.
- Pregnant or breastfeeding women.
- Anyone with a history of eating disorders.
- Children and teenagers.
- People with certain medical conditions or on medications that must be taken with food.
Talk to a doctor or registered dietitian before starting.
If you try it
Start gradually (12 hours overnight, then 14), drink water, and focus on quality food in the eating window: protein, vegetables, whole grains and healthy fats. If it makes you miserable, a regular balanced diet works just as well.
Practical eating habits that help with weight are covered in filling, low-calorie snack ideas and how to eat more mindfully.
