- Intermittent fasting (IF), or limiting food intake to a specific time period, appears to offer a range of health benefits, including for weight loss.
- A new study comparing fasting styles found that a longer fasting time isn't necessarily better.
- Data showed a four-hour and six-hour eating window both led to moderate weight loss by helping dieters cut an average of 550 calories a day while still eating what they wanted.
- Visit Insider's homepage for more stories.
There's a growing body of evidence that intermittent fasting could help you lose weight and improve your health without counting calories, just by limiting your meals to a specific time period each day.
But fasting styles vary. Advocates suggest anything from a 10-hour fasting period from bedtime to breakfast, to entire days-long fasts, making it difficult for newcomers to know where to begin.
If you're looking to lose weight, a longer fasting period may not necessarily be better, according to a study published July 15 in Cell Metabolism. The study found that both an 18-hour and 20-hour fasting window worked equally well for weight loss by helping participants eat less without counting calories.
Eating all your meals in a six hour window could boost weight loss
Researchers from the University of Illinois, Chicago, compared 58 adults with obesity selected to be on one of three diets for a 10-week period. One group only ate between 3 pm and 7 pm, fasting the remaining 20 hours day; the second ate between 1 pm and 7 pm, fasting 18 hours; and the final group ate normally.
The researchers found that at the end of the 10 weeks, both of the groups who fasted lost a moderate amount of weight (about 3% of their starting body weight).
There didn't appear to be a significant difference between the two groups in terms of weight loss.
This suggests an 18-hour period could be plenty to provide the benefits of fasting, according to Krista Varady, professor of nutrition at the University of Illinois, Chicago and co-author of the study.
"Until we have further studies that directly compare the two diets or seek to study the optimal time for fasting, these results suggest that the 6-hour fast might make sense for most people who want to pursue a daily fasting diet," Varady said in a statement.
This shorter fasting period closely resembles one of the most popular styles of intermittent fasting known as 16/8, in which participants eat during an 8-hour window each day.
But there's also evidence that an even shorter fasting period of just 10 hours a day could provide benefits, so more research is needed to directly compare these styles of fasting for weight loss and health effects.
Fasting can reduce calories without restricting food groups
Regardless of the style of fasting, this most recent study suggests that fasting works to facilitate weight loss simply by reducing calorie intake.
During the eating windows, participants were allowed to eat as much of any food they chose. They had to stick to only water and other zero-calorie beverage during their fasting period.
Researchers estimated that both fasting groups ate about 550 fewer calories each day as a result of fasting. Again, the two-hour longer fasting period did not appear to make a significant difference.
Participants did report some initial side effects of fasting such as fatigue, headaches, and dizziness, but those dissipated after three weeks, indicating it can take a little time for the body to adjust to a new fasting diet.
The fasting groups also showed improvements in their insulin resistance and oxidative stress after the 10 weeks —more promising evidence that fasting could improve overall metabolic health and even resistance to chronic disease.