An active lifestyle promotes improvements in physical fitness and health (Ekelund et al., Reference Ekelund, Luan, Sherar, Esliger, Griew and Cooper2012; Jermendy et al., Reference Jermendy, Horvath, Littvay, Steinbach, Jermendy, Tarnoki and Osztovits2011). Modification of these variables is among the goals of most physical exercise practitioners (Patel & Abate, Reference Patel and Abate2013; Silva et al., Reference Silva, Petroski, Souza and Silva2012). However, improvement and relief of symptoms in response to exercise vary substantially between individuals, and this interindividual variability is supported, in part, by genotypic differences (Dias et al., Reference Dias, Gowdak and Pereira2011). This makes studies with twins an important tool, since each monozygotic (MZ) pair comes from a single zygote and the twins present genetically identical elements (Neale & Maes, Reference Neale and Maes1992), reducing genotypic differences in phenotypic responses and making it possible to identify variations between individuals caused by physical exercise.
Cardiorespiratory fitness is seen as a variable with significant genetic effects (50%−67%; Bouchard et al., Reference Bouchard, An, Rice, Skinner, Wilmore, Gagnon and Rao1999; Miyamoto-Mikami et al., Reference Miyamoto-Mikami, Zempo, Fuku, Kikuchi, Miyachi and Murakami2018); however, it is also influenced by factors such as age, sex and level of physical activity (Queiroga et al., Reference Queiroga, Barbieri, Ferreira, Luchessi, Silbiger, Hirata and Kokubun2013). There is evidence that the best cardiorespiratory fitness caused by physical exercise exhibits a dose-response relationship, implying improvements in the lipid and glycemic profiles and body composition, as well as generating general health benefits (Dias et al., Reference Dias, Gowdak and Pereira2011; McKinney et al., Reference McKinney, Lithwick, Morrison, Nazzari, Isserow, Heilbron and Krahn2016; Oliveira et al., Reference Oliveira, Sousa, Cabral, Sánchez, Alonso, Dantas and Lemos2014). Queiroga et al. (Reference Queiroga, Barbieri, Ferreira, Luchessi, Silbiger, Hirata and Kokubun2013) demonstrated that MZ twins discordant for cardiorespiratory fitness presented differences in glycemic metabolism, regardless of genetic factors. However, despite the significant number of studies that have analyzed these factors, few studies have used twins discordant for physical exercise and evaluated the effects on metabolic risk factors (Queiroga et al., Reference Queiroga, Barbieri, Ferreira, Luchessi, Silbiger, Hirata and Kokubun2013).
Therefore, the aim of the current study is to investigate differences in the metabolic profile and body composition of adult MZ twins who are concordant and discordant for physical exercise.
Methods
Sample
The nonprobabilistic sample was obtained from twins registered at the I Twin Festival of Rio Grande do Norte, held in 2016 at the Federal University of Rio Grande do Norte, in partnership with the Brazilian Twin Registry (Ferreira et al., Reference Ferreira, Oliveira, Junqueira, Cisneros, Ferreira, Murphy and Teixeira-Salmela2016). Individuals with physical disabilities or musculoskeletal limitations that prevented walking or running, subjects on obesity-related drug treatment, twins of different sexes and individuals with positive responses to the Physical Activity Readiness Questionnaire (PAR-Q; Warburton et al., Reference Warburton, Jamnik, Bredin and Gledhill2011) were excluded from the study.
Given the exclusion criteria, a sample of 92 MZ twins was used (72.5% female and 27.5% male, mean age 25.4 ± 5.69 years).
After signing the Informed Consent Form, an anamnesis questionnaire was applied to analyze health status and pre-existing diseases, and the PAR-Q to analyze readiness to perform physical activities. To determine the practice of physical exercise, the pairs of twins answered a previously prepared questionnaire with information on physical exercise. Twins with more than 1 month of regular practice of any sport or recreational modality (e.g., weight training, running, cycling and working out in gyms) were considered active. Twins considered inactive did not exercise at all or exercised with a frequency below 2 days a week. From this information, the sample was separated into three groups: Active Concordant twins (Concordant A) where both siblings are active; Inactive Concordant twins (Concordant I) where both siblings are sedentary and Discordant Pairs for Physical Exercise (Discordant PE) where one sibling is active and the other sedentary. Zygosity in twins was determined using a validated zygosity questionnaire with 93.3% accuracy (Ooki & Asaka, Reference Ooki and Asaka2004).
Physical Assessment
The subjects were instructed not to drink caffeine or alcohol-based drinks on the day of the tests, to consume a light meal 2 h before the test and to avoid vigorous physical efforts the day before the tests. All tests were applied in the laboratory at a temperature maintained between 20°C and 25°C.
On day 1, subjects underwent a whole-body fitness evaluation, including measures of weight, height and body composition, using Dual-Energy X-ray Absorptiometry (Marfell-Jones et al., Reference Marfell-Jones, Olds, Stewart and Carter2006), through which we evaluated the percentage of android fat (% AF), percentage of gynoid fat (% GF) and percentage of total fat (% TF).
A Cardiorespiratory Exercise Test was also applied, adopting a ramp protocol with an initial speed of 3 km/h for sedentary individuals and 4 km/h for physically active subjects and 0% incline. Speed and incline were gradually increased during the test up to the subject’s voluntary resignation. All tests lasted between 8 and 12 min (Guazzi et al., Reference Guazzi, Arena, Halle, Piepoli, Myers and Lavie2016).
Ventilatory variables were measured using a metabolic gas analyzer (model Metalyzer 3B, MICROMED®), using the breath-by-breath method with a 10-s window for later analysis using the Metasoft program. The Cortex unit was calibrated by the closed-loop method, using calibration gas (original 16% O2 and 5% CO2 cylinder, supplied by the manufacturer), which allowed a new calibration before each test. The characterization of the test as maximum effort was adopted from the acceptance of, at least, three of the following criteria: (a) voluntary exhaustion; (b) maximum heart rate (HR) reached at least 90% of that predicted for age (220 age); (c) respiratory exchange ratio >1.1; (d) maximum oxygen consumption, observed by the plateau concept, when the VO2 stabilizes without a difference of ≤150 ml/kg per min between the values of the last 30 s of the test, or the VO2 peak was considered maximum when the plateau did not occur (Nelson & Asplund, Reference Nelson and Asplund2016; Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013). The ventilatory thresholds of each patient were determined using the V-slope method (Binder et al., Reference Binder, Wonisch, Corra, Cohen-Solal, Vanhees, Saner and Schmid2008).
On day 2, 10 ml blood samples were collected (overnight fasting) to determine the serum fasting glucose (GLU), total cholesterol (COL), high-density lipoproteins (HDL), very low-density lipoprotein (VLDL), low-density lipoprotein (LDL) and triglycerides (TG). All dosages were performed using enzymatic-colorimetric assays and commercial kits (Labtest Diagnóstica-SA®) in Bio 2000 equipment (Bioplus®.Barueri/SP). The concentrations of LDL and VLDL cholesterol were obtained by applying the Friedewald formula (Friedewald et al., Reference Friedewald, Levy and Fredrickson1972).
Statistical Analysis
In the Concordant A and Concordant I groups, the intrapair mean within each group was calculated and the difference between the groups was verified by the analysis of variance test (p < .001). In the discordant PA group, the Wilcoxon test (p < .05) was applied for intrapair analysis, that is, comparing the active twin versus the inactive twin.
To verify data reliability, Spearman’s intrapair correlation of the MZ twins was also performed, separated into concordant and discordant. We used the Cohen parameter (Cohen, Reference Cohen1992), assuming r = .10−.29 (weak correlation), r = .30−049 (moderate correlation) and r = .50−1 (strong correlation).
Cardiorespiratory fitness can be classified into levels, as indicated by the American College of Sports Medicine (ACSM; Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013). In the present study, we classified the sample according to the categories poor, regular and good cardiorespiratory fitness (Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013), after which the chi-square test and Spearman’s correlation were applied between the categorical variables, level of cardiorespiratory fitness and physical exercise (active and sedentary). All analysis was conducted using the statistical program SPSS®, version 20, and the normality and homoscedasticity of the data distribution were verified by the Shapiro–Wilk test.
Results
The results are shown in Table 1, separated by groups (Concordant A, Concordant I and Discordant PE). Intrapair differences were observed in discordant twins (Active twin vs. Inactive twin) and between groups (Concordant A vs. Concordant I) for the variables body composition and metabolic profile, including maximum oxygen consumption (VO2max), oxygen consumption corresponding to the first and second ventilatory thresholds, lipid profile and fasting blood glucose (see Table 2).
Differences in body composition and metabolic variables in pairs of monozygotic twins concordant and discordant for physical activity expressed as mean ± standard deviation
Concordant A, Active Concordant twins; Concordant I, Inactive Concordant twins; Discordant PE, Discordant pairs for physical exercise; % AF, percentage of android fat; % GF, percentage of gynoid fat; % TF, percentage of total fat; VO2L1, Maximum oxygen consumption corresponding to the first ventilatory threshold (ml/kg per min); VO2L2, maximum oxygen consumption corresponding to the second ventilatory threshold (ml/kg per min); VO2Max, maximum oxygen consumption (ml/kg per min); COL, total cholesterol; HDL, high-density lipoproteins; VLDL, very low-density lipoprotein; LDL, low-density lipoprotein; TG, triglycerides; GLU, fasting glucose.
**p ≤ .001; *p ≤ .05.
Intrapair correlations of monozygotic twins for body composition and metabolic profile variables, separated into concordant and discordant, for physical exercise
% AF, percentage of android fat; % GF, percentage of gynoid fat; % TF, percentage of total fat; VO2L1, maximum oxygen consumption corresponding to the first ventilatory threshold (ml/kg per min); VO2L2, maximum oxygen consumption corresponding to the second ventilatory threshold (ml/kg per min); VO2Max, maximum oxygen consumption (ml/kg per min); COL, total cholesterol; HDL, high-density lipoproteins; VLDL, very low-density lipoprotein; LDL, low-density lipoprotein; TG, triglycerides; GLU, fasting glucose.
Note: r (s), Spearman correlation; r = .10−.29 (weak correlation); r = .30−.49 (moderate correlation); r = .50−1 (strong correlation). p ≤ .001.
An association was found between the level of cardiorespiratory fitness and the practice of physical activity (see Table 3). Of the twins who were active, 51.1% presented regular or good cardiorespiratory fitness and 37% regular or poor cardiorespiratory fitness. Of those who were sedentary, 35.8% presented poor or regular fitness.
Association between cardiorespiratory fitness level and physical exercise in monozygotic twins
Note: *Spearman’s correlation (p < .05); N = 92 monozygotic twins.
Discussion
In our findings, the practice of physical exercise was shown to be an important component of interindividual differences for the percentages of body fat (%AF, % GF and % TF), demonstrated by the intrapair differences in the discordant twins for physical exercise and by the difference between the active twin group and the inactive twin group, indicating a causal effect of the practice of physical exercise. Thus, we can infer that genetics does not guarantee equivalence of MZ twins with respect to percentage of body fat as a consequence of physical exercise. Previous studies indicate heritability of body fat measures with significant genetic effects (58%–86% of heritability; Hopkins et al., Reference Hopkins, Stratton, Maia, Tinken, Graves, Cable and Green2010; Mustelin et al., Reference Mustelin, Latvala, Pietiläinen, Piirilã, Sovijãrvi, Kujala and Kaprio2011; Oliveira et al., Reference Oliveira, Sousa, Cabral, Sánchez, Alonso, Dantas and Lemos2014). However, the current study demonstrates that physical exercise is able to generate changes regardless of genetics. This occurs because the practice of physical activities is able to modify the action of the genes responsible for the predisposition to obesity (Graff et al., Reference Graff, North, Monda, Lange, Lange, Guo and Gordon-Larsen2011).
Thus, it is important that physical activity is encouraged in the population to facilitate alterations in the percentage of body fat, as well as to improve other aspects of physical health. Since the distribution of body fat is related to the risk of developing cardiovascular diseases (Despres et al., Reference Despres, Lemieux, Bergeron, Pibarot, Mathieu, Larose and Poirier2008), as well as dyslipidemia and insulin resistance, among other comorbidities that are associated with elevated body fat (Berntzen et al., Reference Berntzen, Jukarainen, Kataja, Hakkarainen, Lundbom, Lundbom and Pietiläinen2018; Ji et al., Reference Ji, Ning, Duan, Kaprio, Zhang, Wang and Silventoinen2014; Lundbom et al., Reference Lundbom, Bierwagen, Bodis, Szendrödi, Kaprio, Rissanen and Pietiläinen2016; Rottensteiner et al., Reference Rottensteiner, Leskinen, Järvelä-Reijonen, Väisänen, Aaltonen, Kaprio and Kujala2016). Studies with pairs of MZ twins discordant for physical activity (Bathgate et al., Reference Bathgate, Bagley, Jo, Talmadge, Tobias, Brown and Galpin2018; Rottensteiner et al., Reference Rottensteiner, Leskinen, Järvelä-Reijonen, Väisänen, Aaltonen, Kaprio and Kujala2016) found that the inactive twins presented markedly higher percentages of body fat and fat mass than their active twins. In addition, a low level of physical activity is associated with high BMI and fat percentage (Silventoinen et al., Reference Silventoinen, Hasselbalch, Lallukka, Bogl, Pietilainen, Heitmann and Kaprio2009). Thus, individuals with a higher genetic risk for obesity benefit from increased physical activity (Hasselbalch et al., Reference Hasselbalch, Heitmann, Kyvik and Sorensen2010; Segal et al., Reference Segal, Feng, McGuire, Allison and Miller2009; Silventoinen et al., Reference Silventoinen, Hasselbalch, Lallukka, Bogl, Pietilainen, Heitmann and Kaprio2009; Waller, Reference Waller2010a).
The VO2 at the ventilatory threshold (threshold 1) did not demonstrate any difference in the discordant PA; that is, the practice of physical exercise was not enough to generate intrapair differences at this exercise intensity. This behavior suggests a possible effect of heritability on this variable, as indicated in previous studies (Feitosa et al., Reference Feitosa, Gaskill, Rice, Rankinen, Bouchard, Rao and Leon2002; Gaskill et al., Reference Gaskill, Rice, Bouchard, Gagnon, Rao, Skinner and Leon2001), where the ventilatory threshold was shown to have a heritability of 54% and 58% for Black and White sedentary individuals, respectively, adjusted for age and body mass; however, this genetic influence does not seem to influence the training of the ventilatory threshold.
At the anaerobic threshold (threshold 2), there was a difference both in the concordant and in the discordant PA groups; that is, the practice of physical exercise generated intrapair differences in individuals genetically equal at this exercise intensity. Threshold 2 is an anaerobic phase of effort and is modified by interventions. In addition, other factors can influence this component, such as the type of physical activity, intensity, duration of effort and types of muscle fibers (Binder et al., Reference Binder, Wonisch, Corra, Cohen-Solal, Vanhees, Saner and Schmid2008; Guazzi et al., Reference Guazzi, Arena, Halle, Piepoli, Myers and Lavie2016; Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013); however, these variables were not controlled in the current study.
The VO2max presented differences in all groups, suggesting that regardless of genetics, this variable can be modified by the practice of physical exercise. Further, as physical inactivity also promotes changes, generating loss in cardiorespiratory fitness, having ‘privileged’ genetics does not guarantee good VO2max levels. Physical inactivity, in addition to all the health hazards (Waller, Reference Waller2010b), will not maintain inherited levels of cardiorespiratory fitness. Therefore, it is essential to stimulate physical activity so that the aerobic component is also developed, as good levels of aerobic conditioning are associated with a reduction of chronic diseases, in addition to promoting healthy cognitive and psychosocial function (McKinney et al., Reference McKinney, Lithwick, Morrison, Nazzari, Isserow, Heilbron and Krahn2016).
Cardiorespiratory fitness demonstrated a significant association with the practice of physical exercise (p = .001), confirming the development of the aerobic component. In our data, of the twins who were active, 37% presented with regular or weak cardiorespiratory fitness, indicating that they were insufficiently active to perform physical activities, with little use of the aerobic component to generate the necessary gains for good cardiorespiratory fitness. The ACSM (Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013) argues that healthy adults should achieve an energy expenditure equivalent to 150 min/week of moderate intensity aerobic activity; 75 min/week of vigorous intensity aerobic activity; or a combination of the two, to obtain substantial health benefits. These guidelines indicate a cause-effect relationship, detailing that additional health benefits are obtained with 300 min/week or more of moderate intensity aerobic activity.
In the present study, the practice of physical exercise was not enough to generate intrapair changes in the lipid profile and fasting glucose, demonstrated in the discordant PA pairs, suggesting that the components inherited or shared between the twins may influence the behavior of the lipid profile and fasting blood glucose. Corroborating these results, Marson et al. (Reference Marson, Delevatti, Prado, Netto and Kruel2016) observed that physical training in general was not associated with a reduction in fasting blood glucose levels, but rather was associated with reductions in fasting insulin levels, a variable not analyzed in the current study. In addition, other lifestyle factors (e.g., age, sex, diet, smoking, drinking and obesity; Bogl et al., Reference Bogl, Mehlig, Intemann, Masip, Keski-Rahkonen, Russo and Johnson2019) are responsible for modifying these variables.
However, concordant A versus concordant I groups presented differences in VLDL and TG, with lower values in the active group, suggesting that physical exercise has an effect on these variables. This behavior was also demonstrated in the study by Bathgate et al. (Reference Bathgate, Bagley, Jo, Talmadge, Tobias, Brown and Galpin2018), where the trained MZ twin presented lower COL, LDL, TG and plasma glucose levels when compared to the untrained MZ twin. Therefore, an active lifestyle and healthy habits may be able to modify the action of the genes responsible for the predisposition to obesity and cardiovascular risk (Graff et al., Reference Graff, North, Monda, Lange, Lange, Guo and Gordon-Larsen2011).
Despite the indications of modifications in the lipid profile and fasting glucose due to exercise (McKinney et al., Reference McKinney, Lithwick, Morrison, Nazzari, Isserow, Heilbron and Krahn2016), the variables COL, HDL, LDL and GLI did not present differences in any group analyzed. As the current work deals with genetically identical individuals, we can infer that heritability or shared genetic factors have an influence on these variables and it is not possible to observe the effect of exercise on them. Corroborating these data, the serum levels of GLU, COL, HDL cholesterol and LDL cholesterol are strongly influenced by genetic factors with 55%–64% of heritability, even after adjustment for sex and age (Borges et al., Reference Borges, de Sousa, Alonso, Juvanhol Lopes, Medeiros, Maciel and Moreira2018; Chen et al., Reference Chen, Ji, Pang, Wang, Hu and Qin2004; Jedrusik et al., Reference Jedrusik, Januszewicz, Busjahn, Zawadzki, Wocial, Ignatowska-Switalska and Januszewicz2003).
The metabolic and body composition differences in the MZ twins from physical exercise, presented in this study, prove that physical exercise stands out from the heritability of these components through positive changes in the cardiorespiratory component and in the percentage of body fat. These alterations can occur in different proportions, which corroborates biological individuality (Thompson et al., Reference Thompson, Arena, Riebe and Pescatello2013), and confirm that the response to exercise can vary substantially between individuals. Therefore, physical training for benefits of body composition and gains in cardiorespiratory fitness needs to be individualized, even in twins, as the environment generates different responses in genetically similar people.
We conclude that based on the practice of physical exercise, MZ twins present differences in % AF, % GF and % TF, as well as in VO2max and in the second ventilatory threshold. However, the practice of physical exercise was not enough to generate alterations in the lipid profile or fasting glucose, regardless of genetics.
Acknowledgments
The authors would like to thank National Council for Scientific and Technological Development (CNPq) and Higher Education Personnel Improvement Coordination (CAPES), Brazil.
Financial support
This study was supported by the National Council for Scientific and Technological Development (CNPq) and Higher Education Personnel Improvement Coordination (CAPES), Brazil. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Conflict of interest
None.
Ethical standards
The study was approved by the Research Ethics Committee of the Onofre Lopes University Hospital. Natal/RN – Brazil, CEP/HUOL, CAAE 35573214.1.0000.5292, according to Resolution 466/12 of the National Health Council.
