The data from the European Association for the Study of Obesity inform that rapid increases in the prevalence of overweight schoolchildren are being seen in most of EU countries for which data are available (7). Although the prevalence of overweight and obesity in youth is lower in Nordic countries than in other European countries (e.g., Scotland, Portugal, or Spain), the figures are still high. The most recent Swedish data, involving more than half a million of adolescent boys (82% of the Swedish male adolescent population), indicate that both moderate and morbid obesity have increased dramatically in males during the last 30 yr (28). National representative data on overweight and obesity in Estonian children are scarce and mainly based on self-report data. Nevertheless, the available information indicates high levels and rising trends in the prevalence of overweight and obesity among child populations in Eastern Europe and suggests that the economic costs for the transition countries are likely to be substantial (18). Recent data support that obesity in adolescence is related to increased rates of mortality, regardless of smoking status (27). Because the health risks and disease-related costs increase disproportionately in overweight and obese people, it is important from a public health and economical point of view to identify determinants for overweight/obesity at early stages in life.
Physical fitness, particularly cardiorespiratory fitness (fitness), is a well-known predictor of health status and survival. A recent meta-analysis found that a 1-MET (corresponding to 3.5 mL·min−1·kg−1 of oxygen consumption) increment in fitness can reduce all-cause mortality by 13% (19). Compelling evidence indicates that fitness is a marker of health, already in young people (29), and levels of physical fitness at these ages are associated with future cardiovascular health (33). Low fitness is strongly associated with overweight/obesity in adult people (40). Such a relationship has also been observed in young people, yet most of the available information comes from cross-sectional studies (29). In the present study, we followed a group of normal-weight children during a 6-yr period to identify factors in childhood that determine the development of overweight/obesity in adolescence. We focused on fitness, demographic factors, parental overweight, and parental education, using data collected on children from Estonia and Sweden. We also examined whether changes in fitness from childhood to adolescence are related to the risk of becoming overweight/obese in adolescence.
Study sample and design.
Both the Estonian and Swedish samples were originally part of the European Youth Heart Study in 1998-1999 (baseline). Study design, selection criteria, and sample calculations have been reported elsewhere (41). In 2004-2005 (follow-up), the participants were invited to complete the same examination as in 1998-1999; the median (25th-75th percentile) follow-up period was 5.97 (5.73-6.00). Measurements at baseline and follow-up were made by essentially the same group of trained investigators and using the same protocols, which allows us to perform combined analyses (21). The follow-up assessment in the Estonian cohort was carried out as part of the longitudinal Estonian Children Personality Behaviour and Health Study (15). A total of 583 Estonian children (9 yr) and 561 Swedish children (9 yr) participated in the baseline examination (1998-1999). Six years later, 483 Estonian (dropout rate = 17%) and 281 Swedish (dropout rate = 49.9%) participants took part in the follow-up examination. After excluding those children with missing data in body mass index (BMI) at baseline or follow-up (n = 6), a total of 758 children participated in the present study (63.7% Estonian). We observed no significant difference between participants and nonparticipants in the study outcome, i.e., BMI (P = 0.51). The study protocol was performed in accordance with the ethical standards laid down in the 1961 Declaration of Helsinki (as revised in 2000) and approved by the Research Ethics Committees of the University of Tartu (No. 49/30-199), Örebro County Council (No. 690/98), and Huddinge University Hospital (No. 474/98). Children and adolescents gave verbal assent after procedures were explained, and one parent or legal guardian provided written informed consent.
Height (cm) and weight (kg) were measured by standardized procedures, and BMI (kg·m−2) was calculated. The age- and sex-specific BMI international cutoffs were used to classify the children as underweight, normal weight, overweight, or obese (2,3). Sexual maturation status was assessed by a trained researcher of the same sex as the participant, according to Tanner and Whitehouse (37) (pubertal stages from I to V). At baseline, only 11 children were at Tanner III, and none were at Tanner IV or V. Children at Tanner III were considered at Tanner II in all the analyses.
Fitness was determined by an incremental maximal cycle ergometer test (829E Ergomedic; Monark, Vansbro, Sweden) (14). The criteria for exhaustion were at HR ≥185 beats·min−1 and a subjective judgment by the test leader that the adolescent could no longer keep up, even after vocal encouragement. The "Hansen formula" was used for estimating maximal oxygen consumption (V˙O2max) in milliliters per minute = 12W + 5 × weight (kg) (W = power output calculated) (14). The test used to assess V˙O2max has been previously validated in young people (14,31).
The highest level of education was reported by the mothers and fathers and coded as 0 = below university or 1 = university. Parents also reported their own height and weight. Parents' BMI was calculated, and the overweight status according to international cutoff for adults was determined (≥25 kg·m−2 = overweight and ≥30 kg·m−2 = obesity). The validity of BMI based on self-reported weight and height in adults has been documented elsewhere (35).
All statistical analyses were performed using Predictive Analytics Software (formerly SPSS), version 18.0 (SPSS, Inc., Chicago, IL). The level of significance was set at <0.05 for all the analyses. The characteristics of the study sample are presented as means and SD or as frequencies and percentages, as appropriate. To answer the main study question, we focused the analyses on the group of normal-weight children at baseline and examined the factors that predicted incident overweight/obesity in adolescence. For this purpose, only normal-weight children at baseline were included in the main analyses, and we conducted binary logistic regression models that included country, sex, age, and sexual maturation (basic model) as predictors and weight status at follow-up (dichotomized as overweight/obesity vs otherwise) as outcome. Fitness or parental factors (maternal and paternal education and baseline maternal and paternal weight status) were added to the basic model to examine which of them predicted the development of overweight/obesity, after controlling for the covariates mentioned above. Moreover, the association between changes in fitness and risk of becoming overweight/obese was also studied using binary logistic regression, with the difference between V˙O2max at baseline and at follow-up as the main predictor. The effect of adjusting for baseline BMI was also examined, and potential interactions with country and sex were studied in every model.
Baseline characteristics of the study sample are shown in Table 1. Table 2 shows the weight status distribution at baseline by country and gender. Percentages of overweight/obese children in Estonia and Sweden were 11.6% and 14.9%, respectively (chi-squared, P = 0.31). The corresponding figures for girls and boys were 12.2% and 13.6%, respectively (chi-squared, P = 0.83). Sixty-seven percent of the overweight children at baseline were also overweight at follow-up (data not shown).
Table 3 shows the percentage of normal-weight children that developed overweight/obesity (n = 46, 7.7%). Nine percent of the normal-weight Estonian and 5.0% of the normal-weight Swedish children became overweight/obese 6 yr later (chi-squared, P = 0.06). Six percent of the normal-weight girls and 10.0% of the normal-weight boys became overweight/obese 6 yr later (chi-squared, P = 0.06).
Predictors in childhood of incident overweight/obesity 6 yr later.
Tables 4 and 5 show how several factors measured in a sample of normal-weight children predict overweight/obesity in adolescence, after controlling for age, sexual maturation, sex, and country. After additional adjustment for baseline BMI, being male was related with higher odds of becoming overweight/obese 6 yr later, i.e., odds ratio (OR) = 2.65 (95% confidence interval (CI) = 1.18-5.96) (Table 4). Swedish children were less likely to become overweight/obese during the 6-yr follow-up, compared with Estonian children, i.e., OR = 0.32 and 95% CI = 0.14-0.70 (Table 4). This association was not affected after further adjustment for maternal (OR = 0.31 and 95% CI = 0.14-0.69) or paternal (OR = 0.32 and 95% CI = 0.14-0.72) educational level. Boys were at a higher risk than girls of becoming overfat/obese when skinfold thicknesses were used as an indicator of adiposity, instead of BMI (see Supplemental Digital Content 1, http://links.lww.com/MSS/A85 for further information about methods and results related to this extra analysis).
In childhood, fitness level, maternal weight status, and sexual maturation status were related to the risk of developing overweight/obesity during the 6-yr follow-up (Tables 4 and 5). Higher fitness at childhood was associated with lower risk of becoming overweight/obese at adolescence, OR = 0.89 and 95% CI = 0.84-0.95. Also, children with overweight mothers had more than two times higher odds of being overweight/obese in adolescence, OR = 2.45 and 95% CI = 1.28-4.70. No significant relationship was observed between parental education and the risk of incident overweight/obesity. After controlling for baseline BMI, the associations of fitness, maternal overweight, and sexual maturation status with incidence of overweight were attenuated and became nonsignificant (Table 5). No significant interaction with sex or country was observed in any of the predictors studied (all P > 0.2), and stratified analyses by sex did not provide any significant association. Baseline BMI strongly predicted (OR = ∼4) incident overweight/obesity, controlling for any other covariate.
Changes in fitness and incident overweight/obesity.
Table 6 shows the associations between changes in fitness from childhood to adolescence and risk of becoming overweight/obese, after adjustment for a set of confounders. A significant association was found between fitness and incident overweight/obesity with and without adjustment for baseline BMI. The risk of developing overweight/obesity was reduced by 10% every increment of 1 mL·kg−1·min−1 of V˙O2max (OR = 0.90 and 95% CI = 0.84-0.95) after additional adjustment for baseline BMI. This association was consistent in Estonian and Swedish participants, as well as in boys and girls. The results were also consistent when using skinfolds instead of BMI as indicator of adiposity, i.e., OR = 0.91 and 95% CI = 0.87-0.96, after additional adjustment for baseline skinfolds (see Supplemental Digital Content 1, http://links.lww.com/MSS/A85 for further information about methods and results related to this extra analysis).
Five important findings relevant for health promotion and obesity prevention emerged from this prospective study in youth. First, boys are nearly three times more likely to become overweight/obese than girls. Second, Estonian children have a 70% higher risk of developing overweight/obesity from childhood to adolescence, compared with Swedish children. Third, BMI in childhood is a major predictor of incident overweight/obesity. Fourth, a low fitness level in childhood increases the likelihood of becoming overweight/obese in adolescence, independently of many confounders, but this association is attenuated after controlling for baseline BMI, remaining significant only in girls. The rest of potential predictors studied do not predict the development of overweight/obesity after controlling for baseline BMI. Finally, improvements in fitness from childhood to adolescence were associated with a reduction in the risk of incident overweight/obesity, regardless of baseline BMI and in both girls and boys.
Gender as predictor of incident overweight/obesity.
In accordance with our results, others have observed that more boys (48.3%) than girls (23.5%) became overweight or obese from childhood to young adulthood (8). Likewise, a 20-yr retrospective longitudinal study concluded that normal-weight girls were less likely (probability = 7% vs 20% for girls and boys, respectively) to become overweight or obese, regardless of their baseline BMI (4). Moreover, a recent study conducted on Greek children followed from 7 to 18 yr old observed an increase in the prevalence of (parent-reported and self-reported data) overweight in boys (16.1%-19.1%) and a decrease in girls (19.2%-8.0%) during the study period (39). The reasons why girls might be at a lower risk than boys of developing overweight/obesity are not fully understood but might be related with more frequent dieting and weight control behaviors in girls.
Country as predictor of incident overweight/obesity.
The differences observed by country are of social and political interest. To the best of our knowledge, this is the first time that data on incident overweight in pediatric population from an Eastern (low to middle income) country are directly compared with a Western-Northern (high income) country. We observed that Estonian children are at a higher risk of becoming overweight/obese compared with their Swedish peers during the study period. The association was not explained by differences in maternal or paternal educational level.
European statistics suggest that mortality rates (related to ischemic heart disease, cerebrovascular disease, or to all causes) are dramatically higher in Estonia than in Sweden (9,10,25) (for more information about European statistics, see http://data.euro.who.int/hfadb/). It has been suggested that these two countries are genetically similar (26), so it is reasonable to think that the differences observed in morbidity and mortality between these two countries are largely explained by differences in lifestyle. The findings presented in this study provide unique information suggesting that the differences in lifestyle between countries with different socioeconomic situations might be present very early in life and that they may result in a more or less healthy development.
BMI in childhood as predictor of incident overweight/obesity.
The evidence consistently and strongly indicates that a high BMI in childhood is a major predictor of overweight/obesity later in life (8,17,32). In addition, we observed that two-thirds of the overweight/obese children were so in adolescence, a tracking coefficient similar to those previously reported (8,23). A retrospective longitudinal study with multiple measurements on a sample of US children supports that being overweight/obese in childhood (at all ages over 3 yr old) is strongly related to a higher risk of obesity in young adulthood (42). This is also confirmed by data on British children (22). Obesity prevention strategies should partially be directed to promote a healthy BMI in childhood, which would well track into adolescence and adulthood.
Fitness in childhood as predictor of incident overweight/obesity
We observed that a high fitness level in childhood decreases the likelihood of becoming overweight/obese in adolescence, yet this association is attenuated when baseline BMI is taken into account, which concurs with previous studies (17). This finding suggests that people with a better fitness have a lower BMI in childhood (29) and that lower baseline BMI reduces the risk for later development of overweight/obesity. In accordance with our results, Kim et al. (17) observed that the association between fitness and later BMI was attenuated after adjustment for baseline BMI, yet in their study, it remains significant in girls. The authors followed a large sample (N = 2927) of children initially aged 5-13 yr during 1 yr, and the reasons for this gender difference remain unexplained. McGavock et al. (24) followed 902 children and adolescents aged 6-15 yr during 1 yr and observed that fitness was an independent predictor of overweight status and weight gain in children and adolescents.
Change in fitness as predictor of incident overweight/obesity.
Our data suggest that the change in fitness level from childhood to adolescence is a stronger predictor than baseline fitness level (i.e., at childhood) of becoming overweight/obese in adolescence. This finding was confirmed when using skinfolds instead of BMI as an indicator of adiposity, which strengthens the study conclusions (see Supplemental Digital Content 1, http://links.lww.com/MSS/A85 for further discussion on this extra analysis). Dwyer et al. (6) came to an identical conclusion in a sample of 647 Australian adults (33 yr old) who had been assessed 20 yr earlier in their childhood (12 yr old). Similarly, McGavock et al. (24) concluded that a decrease in fitness during a 2-yr follow-up period independently predicts incident overweight/obesity in children and adolescents (N = 222), in line with our results.
Change in fitness in an individual is strongly correlated with a change in daily energy expenditure and physical activity undertaken during leisure time (36). Data on change in fitness from one time point to another are a good marker of change in physical activity (16). Therefore, the inverse association observed between changes in fitness and risk of incident overweight/obesity might be reflecting the beneficial effects of physical activity enhancement on weight status. This notion is supported by Kvaavik et al. (20), who observed that adolescents who increased their leisure time physical activity level had a lower risk of overweight in adulthood than those with a stable low-activity level (N = 485).
Reverse causation should always be considered when interpreting observational data, including longitudinal data. Changes in body weight could also lead to changes in fitness. Nevertheless, well-designed randomized controlled trials support the notion that physical training can improve fitness and reduce fatness. As an example, Gutin et al. (12) showed that fitness of adolescents was significantly improved by physical training, as well as both visceral and total body adiposity, as measured by accurate techniques, i.e., V˙O2max directly measured by a gas analyzer, dual-energy x-ray absorptiometry, and magnetic resonance imaging. It is important to note, which Gutin et al. (12,13) as well as other researchers (5,34) have repeatedly observed in their observational studies and randomized controlled trials, that vigorous rather than moderate physical activity might be more beneficial for fitness enhancement and fatness reduction.
Limitations and strengths.
Several limitations must be acknowledged. First, approximately half of the Swedish participants from the baseline examination did not participate in the follow-up, yet no differences in the main outcome (i.e., BMI) were observed between dropouts and nondropouts. We conducted a comprehensive dropout study on the Swedish sample and concluded that the dropouts did not differ from the nondropouts in relation to physical activity, fitness, and anthropometric indices (11). On the other hand, we observed that children with a higher maternal educational level were more likely to participate in the follow-up, as observed in previous longitudinal studies (1). In fact, it is a general assumption that subjects from lower socioeconomic groups are underrepresented in epidemiological studies (30,38). In our analyses, further adjustment by parental education did not affect the results to any extent. We used a dichotomic variable for parental education (university or below university), and it is unknown if the results would have changed if a more precise measure would have been used, e.g., a three-category variable (primary, secondary, and university). Second, data on physical activity and dietary report were not available. Third, the genetic background of the participants was not taken into account in this study. Fourth, a direct measure for parental weight and height, instead of self-reported, would have improved the quality of the data.
The age characteristics of the study sample and the follow-up period used were carefully designed in advance. For the present study, an age-homogenous cohort of children was selected, i.e., 90% of the participants at baseline were aged 9-10 yr. The sample was mostly prepubescent at baseline, i.e., 79% and 20% belonged to the Tanner stages I and II, respectively, and it was mostly mature (postpubescent) at the follow-up, i.e., 67% of the participants belonged to the Tanner stages IV or V. Therefore, the current study design provides useful information about the maturation process from childhood to adolescence and about relevant predictors of developing overweight/obesity during this critical period of life. In addition, the inclusion of two European countries and the lack of interactions with country observed indicate that the study findings are consistent in children from countries with different socioeconomical characteristics. The objective measurements of fitness both at baseline and at follow-up in more than 700 children over a 6-yr follow-up period, together with the inclusion of important covariates, are notable strengths of this study.
The present longitudinal data, together with previous randomized controlled trials, suggest that improvements in fitness from childhood to adolescence are associated with the risk of becoming overweight/obese in adolescence. Change in fitness was a stronger predictor of incident overweight/obesity than childhood fitness, parental overweight, and parental education. The current findings highlight the importance of promoting fitness through physical exercise from early stages in life, as a promising strategy to fight against overweight and obesity. Gender and country differences observed in this study require social and political attention.
This study was supported by grants from the Estonian Ministry of Education and Science (0180027 and 0942706) and the Estonian Science Foundation (6932 and 6788). The study was also supported by grants from the Stockholm County Council, the Spanish Ministry of Science and Innovation (EX-2008-0641, RYC-2010-05957), the Swedish Council for Working Life and Social Research, and the Swedish Heart-Lung Foundation (20090635).
The authors thank the Estonian and Swedish participants and families, as well as the European Youth Heart Study and Estonian Children Personality, Behavior and Health Study fieldwork teams.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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