The Beneficial Effects of Physical Activity: Is It Down to Your Genes? A Systematic Review and Meta-Analysis of Twin and Family Studies.

BACKGROUND: There is evidence for considerable heterogeneity in the responsiveness to regular physical activity (PA) which might reflect the influence of genetic factors. The aim of this systematic review was to assess whether the response to a PA intervention for measures of body composition and cardiorespiratory fitness is (i) correlated within twin pairs and/or families and (ii) more correlated in monozygotic twins (MZ) compared to dizygotic twins (DZ), which would be consistent with genetic effects.

METHODS: We performed electronic database searches, combining key words relating to "physical activity" and "genetics", in MEDLINE, CINAHL, EMBASE, SPORTS Discuss, AMED, PsycINFO, WEB OF SCIENCE, and SCOPUS from the earliest records to March 2016. Twin and family studies were included if they assessed body composition and/or cardiorespiratory fitness following a PA intervention, and provided a heritability estimate, maximal heritability estimate, or within MZ twin pair correlation (rMZ ). Data on heritability (twin studies), maximal heritability (family studies), and the rMZ were extracted from included studies, although heritability estimates were not reported as small sample sizes made them uninformative.

RESULTS: After screening 224 full texts, nine twin and five family studies were included in this review. The pooled rMZ in response to PA was significant for body mass index (rMZ  = 0.69, n = 58), fat mass (rMZ  = 0.58, n = 48), body fat percentage (rMZ  = 0.55, n = 72), waist circumference (rMZ  = 0.50, n = 27), and VO2 max (rMZ  = 0.39, n = 48), where "n" represents the total number of twin pairs from all studies. Maximal heritability estimates ranged from 0-21% for measures of body composition, and 22-57% for cardiorespiratory fitness. Twin studies differed in sample age, baseline values, and PA intervention, although the exclusion of any one study did not affect the results.

CONCLUSIONS: Shared familial factors, including genetics, are likely to be a significant contributor to the response of body composition and cardiorespiratory fitness following PA. Genetic factors may explain individual variation in the response to PA.

TRIAL REGISTRATIONS: PROSPERO Registration No CRD42015020056 .