Background and objective: Exercise is thought to reduce high-risk body fat, but intervention studies are frequently limited by short follow-ups and observational studies by genetic selection. Therefore, we studied the effects of a physically inactive vs active lifestyle on high-risk (visceral, liver and intramuscular) fat in twin pairs discordant for leisure-time physical activity habits for over 30 years.
Design: A longitudinal population-based twin study.
Subjects: Sixteen middle-aged (50-74 years) same-sex twin pairs (seven monozygotic (MZ), nine dizygotic (DZ)) with long-term discordance for physical activity habits were comprehensively identified from the Finnish Twin Cohort (TWINACTIVE study). Discordance was initially defined in 1975 and the same co-twin remained significantly more active during the 32-year-long follow-up.
Main outcome measures: Magnetic resonance imaging-assessed visceral, liver and intramuscular fat.
Results: In within-pair analyses carried out after the adult life-long discordance in physical activity habits, the physically inactive co-twins had 50% greater visceral fat area compared with the active co-twins (mean difference 55.5 cm2, 95% confidence interval (CI) 7.0-104.1, P=0.010). The liver fat score was 170% higher (13.2, 95% CI 3.5-22.8, P=0.030) and the intramuscular fat area 54% higher (4.9 cm2, 95% CI 1.9-7.9, P=0.002) among the inactive co-twins. All the trends were similar for MZ and DZ pairs. Peak oxygen uptake was inversely associated with visceral (r=-0.46, P=0.012) and intramuscular fat area (r=-0.48, P=0.028), with similar trends in intrapair difference correlations (r=-0.57, P=0.021 and r=-0.50, P=0.056, respectively). The intrapair difference correlation between visceral and intramuscular fat was also high (r=0.65, P=0.009).
Conclusion: Regular physical activity seems to be an important factor in preventing the accumulation of high-risk fat over time, even after controlling for genetic liability and childhood environment. Therefore, the prevention and treatment of obesity should emphasize the role of regular leisure-time physical activity.