Intermittent fasting (IF) improves metabolic health in both rodent and human models. However, it is unclear whether continuous energy restriction or IF produces optimal health outcomes, or whether weight loss is necessary for the beneficial health effects of IF.
Healthy women who were overweight (n=88, 50.2±0.9 y, BMI 32.2±0.5 kg/m2) were randomly assigned to 1 of 4 groups for 8 weeks in a 2:1 intervention: control ratio. Participants were provided with all foods at 70% (IF70 and DR70), or 100% (IF100 and control) of calculated energy requirements. IF groups fasted for 24h from 0800h on 3 non-consecutive days per week. Weight, body composition, insulin sensitivity (by hyperinsulinaemic-euglycaemic clamp [60mU/m2/min]), HOMA-IR and plasma/serum markers of health, were assessed at baseline and 8-weeks after a 12h overnight fast, and again 3-7 days later after a 24h fast (in IF groups only).
The IF70 group lost more weight and body fat compared with all other groups (vs IF100 and control all P<0.0001; vs DR70 both P<0.05), despite prescription of a similar energy deficit to the DR70 group. The IF100 group lost more weight vs. control (P<0.0001). Fasting insulin and HOMA-IR were improved in IF70 (both P<0.0001) and DR70 (both P=0.01) groups vs the IF100 group. There was no effect of group on insulin sensitivity by clamp. A 24h fast reduced insulin sensitivity (P=0.02), however this was not different between IF70 and IF100 groups. Improvements in plasma lipids and liver enzymes were not different between groups. Fasting non-esterified fatty acids (NEFAs) were reduced in IF70 vs DR70 and control groups (both P<0.05).
Intermittent fasting, with prescribed energy restriction is a legitimate and effective alternative to daily energy restriction, and may be superior, at least in the short term.