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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Keywords:©2011The American College of Sports Medicine
EXERCISE CAPACITY; OBESITY; YOUTH; PROSPECTIVE STUDY