How long should you fast to improve cardiometabolic health? Original paper

In this randomized controlled trial in participants without apparent health conditions, alternate-day fasting, 16/8 time-restricted eating, and 20/4 time-restricted eating had similar effects on cardiometabolic risk factors.

The effects of different intermittent fasting protocols on cardiometabolic risk factors.

The primary outcome was the change in BMI. The secondary outcomes were anthropometrics (e.g., weight, waist-to-hip ratio) and body composition, blood lipids, C-reactive protein, glucose tolerance (measured using an oral glucose tolerance test), glycemic control (e.g., average glucose level, glycemic variability; measured using a continuous glucose monitor), and various blood markers (e.g., red blood cell count, white blood cell count, hemoglobin, serum platelets).

25 participants (average age of 26; average BMI of 25; 16 men, 9 women) without apparent health conditions.

In this randomized controlled trial, the participants completed a 4-week run-in period, and then they were assigned to 1 of 3 intermittent fasting protocols for 8 weeks:

• Alternate-day fasting (ADF): the participants fasted for 24 hours and ate ad libitum for the subsequent 24 hours
• 16/8 time-restricted eating: the participants fasted for 16 consecutive hours and ate ad libitum during the 8 remaining hours
• 20/4 time-restricted eating: the participants fasted for 20 consecutive hours and ate ad libitum during the 4 remaining hours

During the fasting periods, the participants were only allowed to consume water. In the 16/8 and 20/4 groups, the participants were instructed that almost 50% of the fasting duration should be overnight; no other instructions were given with respect to adhering to specific time frames during the fasting period.

To monitor diet, the researchers had the participants report their daily dietary intake in an app.

There were no significant differences between groups.

Compared to baseline, there was a decrease in BMI (−0.60), weight (−1.90 kg), fat-free mass (−1.36 kg), and white blood cell count in the ADF group; a decrease in waist-to-hip ratio in the 16/8 and 20/4 groups; and an increase in total cholesterol and LDL-C in the 16/8 and 20/4 groups.

Dietary adherence did not significantly differ between groups, but it was highest in 20/4 (93%) and lowest in ADF (78%).

Due to the small sample size, the study had limited statistical power to detect significant differences between groups. Also, the researchers did not adjust for multiple comparisons, which increases the risk of false-positive results. As such, the results should be considered exploratory.