COMBINED EFFECTS OF HIGH-INTENSITY INTERVAL TRAINING AND 5:2 INTERMITTENT FASTING ON WEIGHT LOSS, BODY COMPOSITION, AND PHYSICAL FITNESS: A NARRATIVE REVIEW

Abstract

Background: Increased obesity in adults is a global problem of >1b affecting significant cardiovascular and metabolic morbidity. Two dietary strategies capable of changing the body composition and cardio-metabolic health independently are high-intensity interval training (HIIT) and the 5/2 intermittent fasting (IF) diet—both of which are time-efficient because of their suitability for short-term periods of time. The combined effects of these however have not been synthesized systematically, however.

Objective: The aim of the narration review is to assess critically the literature available about the effects of HIIT with 5:2 IF on weight loss, body composition and physical fitness in the adult population with overweight or obesity.

Methods: peer-reviewed literature was synthesized in narrative format, with a preference for systematic reviews and meta-analysis and for studies published in the past five years that were based on RCTs. The databases were searched such as PubMed, Scopus and Web of Science. The evidence was critically appraised, synthesized and contextualized with the mechanistic insights.

Results: The effect of HIIT + 5:2 IF was more beneficial for weight loss (weighted mean difference (WMD): −3.03 kg), fat mass (WMD: −0.93 kg), and waist circumference (WMD: −2.51 cm) than was either HIIT or 5:2 IF alone. Interestingly, HIIT was found to counteract the lean mass loss commonly seen with IF; in one RCT, there was a +3.3% gain in fat-free mass. Cardiorespiratory fitness (VO₂ peak) significantly improves consistently and improving muscular performance (especially explosive power) is even better. Three possible synergistic mechanisms that have been proposed are: Complementary activation of AMPK, PGC-1 alpha expression, which leads to mitochondrial biogenesis, and metabolic flexibility. But small sample sizes, brief intervention times (4 to 12 weeks) and a narrow range of protocols and limited generalizability of the results for males and diverse groups have limited the generalizability of available evidence. Most outcomes have very low certainties of evidence (due to the GRADE) of 0.0.

Conclusion: HIIT+5:2 IF seems to be an effective, safe and feasible approach for enhancing weight loss, body composition and physical fitness in women suffering with obesity. Conducting large, longer-term, properly designed RCTs in different patient groups with standardized protocols is an absolute necessity to come up with any clear clinical recommendation.