Since Horvath’s epigenetic clock was published, we have seen that it is likely genetically regulated as the ticking rate of the epigenetic clock within co-twins of MZ twin pairs seems to be highly correlated [2]. However, the within-pair correlations in MZ twin pairs provide only an upper limit to the heritability, where the relative roles of genetic and shared environmental factors, epigenetic alterations, and complex gene-gene or gene-environment-interactions cannot be teased apart. Within-pair similarity of MZ twins is not only due to shared genetic factors, but may also reflect common fetal or early childhood environmental factors, as the co-twins often share the same early environment. It may also reflect later experiences and exposures, such as lifestyle but also hobbies, and occupational and residential exposures that MZ twins share more often than DZ pairs on average [15]. Both MZ and DZ twin pairs are needed to calculate the relative contributions of genetic, shared environmental, and non-shared environmental factors on variation in DNAm age acceleration. We showed by applying quantitative genetic modeling methods, that genetic factors explain a majority of variance in DNAm age acceleration in young individuals, but that environmental exposures have also a significant age-dependent role in the epigenetic aging process. Based on our results, both genetic and environmental factors seem to have almost equal effect on age acceleration in older age. No effect of the early environment was seen in the adult pairs, suggesting that such effects, if present, are not sustained into adulthood. On the other hand, the power of the twin design to detect common environmental effects is less than the power to detect genetic effects [16].

Epidemiological studies are prone to selection bias caused by genes or other childhood familiar factors while investigating associations between environmental exposure and progress of aging process, or morbidity/mortality. Co-twin-control study is a unique study design, which can be used to investigate the effects of long-term physical activity on epigenetic aging, with both genetic and familial factors standardized. With data from the TWINACTIVE cohort, we were able to investigate if high-volume leisure-time physical activity is one of the environmental factors that affects variation in DNAm age acceleration in older age. In the TWINACTIVE cohort, the mean intrapair difference in leisure-time physical activity (8.8 MET hours/day) during the 32-year follow-up period corresponds to a volume of a light 2-h daily walk. As MZ twin pairs share all their segregating genotypes, it can be hypothesized that any intrapair difference between the co-twins is due to the difference in environmental factors (including physical activity) and possible epigenetic modifications caused by the environmental exposures and experiences. The leisure-time physical activity discordant twin pairs differed by peak exercise capacity, knee extension strength, body composition (bone structure, fat free mass, body fat distribution), structure of the heart, metabolic pathways and profile, liver fat, gene expression in fat and muscle tissue, etc. [17]. These exercise-related positive alterations in body composition and function are known to help in prevention of several cardiovascular and other inactivity-related diseases, which are the main causes of mortality. Despite all phenotypic differences between the inactive and active co-twins, we did not see any differences in DNAm age acceleration, i.e., faster or slower biological aging.

Twin pairs with leisure-time physical activity discordance over three decades were used in this study, resulting in limited sample size, which in turn may reduce the credibility when generalizing our results to the general population. It must also be noted that although the discordance covered a very large age span, we cannot exclude the possibility that early life health habits or the amount of physical activity may have biased our results. We did not have data on the twins’ physical activity patterns in childhood, but in our experience it is difficult to identify MZ pairs discordant for physical activity during childhood and adolescence. Thus, the discordance arises generally only once the twins leave their childhood home. In utero and during infancy and childhood, the organismal growth accompanied with the high number of cell divisions leads to logarithmic ratio of epigenetic age and chronological age, i.e., faster ticking clock compared to adult age [2]. It has been suggested that the ticking rate of epigenetic clock is largely set before adulthood and remains constant thereafter [18]. However, our findings from genetic modeling show that the relative effect of environmental factors is larger in older twins and thus does not support the hypothesis above. Marioni et al. [19] showed recently in six European cohorts that DNAm age increases at a slower rate than chronological age, which may indicate stronger influence on environmental factors, but also survival bias, ceiling effects of age acceleration at older age plateau, or other factors related to the underlying training population of Horvath’s epigenetic clock. In addition, environmental factors may have different effects on epigenetic aging during adult age. Nevalainen et al. observed an association between increased BMI and accelerated epigenetic aging in the blood cells in middle-aged individuals, but not in young adults or nonagenarians [20]. It is clear that larger longitudinal studies are needed to elucidate whether the ticking rate of the epigenetic clock remains constant over an adult life span or whether certain periods of time (puberty, menopause, diseases, etc.) or exposures result in periods of a faster or slower ticking rate.

We identified a fairly high correspondence between DNAm age and chronological age in both age groups of twins that we studied. Similar strong linear relationships with chronological age and DNAm age have been reported earlier in other large cohorts [3, 21]. The relatively high precision with chronological age together with a number of studies showing associations with aging phenotypes and mortality [5, 22] has already demonstrated that DNAm age is a robust biomarker of age. However, it is not known what DNAm age truly measures [2, 22]. Further studies are required to establish whether DNAm actually regulates aging or whether it is just a biomarker that correlates highly with chronological age.

In this study, DNAm age was analyzed using blood samples rather than muscle tissue, which may be a more relevant tissue in terms of physical activity. Although DNAm age in muscle tissue and chronological age has modest to high correlations compared to blood, muscle tissue may have lower correspondence with chronological age [2, 22]. There is also some evidence that DNAm age may vary within the same individual depending on the tissue sampled [2]. Aging of the liver, rather than blood, muscle, or fat tissue, is accelerated in obese subjects [22], and each tissue may have its own aging profile. Long-term physical activity produces numerous adaptive metabolic and structural responses directly to muscle tissue. However, we are not aware of studies that have investigated DNAm age of the muscle tissue in association with physical activity. Future studies examining DNAm age in muscle tissue may enlighten if leisure-time physical activity affects aging of the muscle tissue and subsequently age-related decline in physical functioning.

In conclusion, we provided further evidence for a role of genes and environmental factors in controlling biological aging. The relative contribution of genes versus environment on epigenetic age acceleration exhibited an age dependency with a significantly greater relative impact of the environment among older compared with younger twins. Although the impact of leisure-time physical activity on health, well-being, and older age physical functioning is well documented [23,24,25], our genetically controlled co-twin-control study did not provide evidence that long-term high-volume physical activity in adult age slows down or accelerates biological aging; the relatively small sample size cautions against drawing far-reaching conclusions from these results. While accelerated aging detected by Horvath’s clock is clearly associated with increased mortality risk [3, 4, 26], our results are consistent with the findings that leisure-time physical activity in adult age has little or no effect on mortality after controlling for genetic factors [13]. It is possible that genetic pleiotropy affecting exercise participation might partly explain the frequently observed associations between physical activity and reduced mortality in humans [13]. Whether physical activity affects programming of the epigenetic clock ticking rate in childhood, or younger adulthood or has tissue specific differences in ace acceleration requires further studies.