16:8 or 5:2 fasting? In 68 women, the scale moved with both — and so did muscle mass

Two popular versions of intermittent fasting arrange the week very differently. The 16:8 approach leaves an eight-hour eating window every day. The 5:2 approach keeps five ordinary eating days and sharply restricts food on two non-consecutive days. A randomized trial published in Obesity on 28 October 2026 directly compared them for four weeks in 68 Korean women with overweight or obesity.

The 16:8 group chose an eating window and kept to it daily. The 5:2 group received an approximately 800-kcal packed meal, salad and protein drink on each fasting day, with no prescribed restriction on the other five days. Both groups kept daily food records, received brief nutrition counselling at weekly measurements and were encouraged to follow a healthy eating pattern. Neither group was told to count calories every day, yet reported intake fell by about 533 kcal a day with 16:8 and 548 kcal with 5:2.

By week four, estimated weight loss was 2.19 kg in the 16:8 group and 3.13 kg in the 5:2 group. Fat mass fell by 1.23 kg and 2.10 kg respectively. The difference emerged mainly in the later weeks; muscle mass fell modestly in both groups, by 0.56 kg with 16:8 and 0.59 kg with 5:2, with no meaningful difference between them. Across everyone, waist circumference dropped 3.2 cm and systolic blood pressure 6.7 mmHg, but those changes did not differ by fasting pattern.

This is evidence about an early four-week response, not which pattern works best for a year. The trial was small, open-label and limited to metabolically healthy women; food intake was self-reported and physical activity was not measured. Weekly contact, supplied fasting-day meals and the large fall in energy intake could all have helped. Three participants did not reach the final measurement, and the study reported no specific adverse events.

The practical reading is that timing may be a structure for eating less, not a substitute for food quality or a guarantee of keeping muscle. The two schedules produced different week-by-week curves, but both combined fasting with a substantial energy deficit. Longer trials that include men, measure activity and test whether people can keep the pattern are needed before declaring one timetable the winner.