The primary objective of this longitudinal study was to determine the association between cardiorespiratory fitness and the risk of overweight status in youth. To accomplish this aim we analyzed data from annual school-based surveys of cardiorespiratory fitness and anthropometry conducted between 2004 and 2006. The first analysis was performed on a cohort of 902 youth aged 6–15 years followed for 12 months to assess the association between cardiorespiratory fitness levels determined from a graded maximal field test and the risk of becoming overweight. The second analysis was conducted on a cohort of 222 youth followed for 2 years to assess the continuous association between annual changes fitness and weight gain. Children with low cardiorespiratory fitness were characterized by higher waist circumference and disproportionate weight gain over the 12-month follow-up period (P < 0.05). Within the entire cohort, the 12-month risk of overweight classification was 3.5-fold (95% confidence = 2.0–6.0, P < 0.001) higher in youth with low cardiorespiratory fitness, relative to fit peers. A time series mixed effects regression model revealed that reductions in cardiorespiratory fitness were significantly and independently associated with increasing BMI (r = −0.18, P < 0.05) in youth. Accordingly, low cardiorespiratory fitness and reductions in fitness over time are significantly associated with weight gain and the risk of overweight in children 6–15 years old. An assessment of cardiorespiratory fitness using a common field test may prove useful for the identification of youth at risk of overweight and serve as a potential target for obesity prevention.
Childhood overweight is a growing public health concern in industrialized countries and affects 25–35% of youth (6–18 years) in North America (1,2). Overweight in childhood is a multifactorial condition that is often attributed to genetic background, nutritional habits, and physical activity patterns (3). In addition to these traditional risk factors, cardiorespiratory fitness has emerged as an independent determinant of weight status in youth (4,5,6,7). More specifically, cross-sectional studies demonstrate that low cardiorespiratory fitness is a characteristic feature of overweight youth (8,9,10,11) and independently associated with adiposity (5,6,7,12). Additionally, average cardiorespiratory fitness levels in American youth have declined in parallel with the rising prevalence of childhood obesity (13,14). Longitudinal studies of cardiorespiratory fitness and weight gain would provide much needed insight into the temporal nature of the association between cardiorespiratory fitness and the risk of becoming overweight in childhood.
Within this context, we conducted a school-based longitudinal study of cardiorespiratory fitness and overweight risk in youth 8–16 years old. We hypothesized that a dose–response association exists between cardiorespiratory fitness and the 12-month risk being overweight. A secondary hypothesis was that annualized changes in cardiorespiratory fitness during childhood would be significantly associated with weight gain.
Methods and Procedures
Beginning in the spring of 2004, an annual survey of anthropometrics (height, weight, waist to hip ratio), cardiorespiratory fitness and systolic blood pressure was conducted in a school-based sample of youth 6–18 years attending schools in the Black Gold School District, an urban and rural district surrounding Edmonton, Alberta, Canada. Between 2004 and 2006, two separate cohorts were available for analysis. The first cohort consisted of 902 youth aged 8–16 years who participated in at least two consecutive waves of the survey, which provided the ability to assess the association between cardiorespiratory fitness and the 12-month risk of overweight classification. The second cohort consisted of 222 youth with serial measurements of cardiorespiratory fitness and anthropometry in each of the three study years. The second cohort was used to test for an association between longitudinal changes in cardiorespiratory fitness and weight gain over a period of 2 years.
Exclusion criteria included the inability to perform a shuttle run test of cardiorespiratory fitness, self-reported presence of diabetes or hypertension, and/or the use of medications to control blood pressure, glucose or lipid metabolism. Written informed consent was obtained from parents, and verbal assent was obtained from children prior to the investigation. The Health Research Ethics Review Board in the Faculty of Medicine and Dentistry at the University of Alberta approved the study protocol. Recruitment was performed in schools and at community meetings held within the Black Gold School District following approval by provincial and local administrative bodies.
Data collection methods were identical in all three study years. Satellite laboratories were created within individual schools where data was collected over a period of 2 days. On study Day 1, anthropometric and systolic blood pressure data were collected. On study Day 2, cardiorespiratory fitness was assessed. The data presented in this article is restricted to anthropometric and cardiorespiratory fitness data, whereas data from a cohort of 354 youth who had measurements of blood pressure and weight gain over this time frame have been published elsewhere (15). The different sample sizes for these two studies can be explained by a number of youth who did not have complete blood pressure or cardiorespiratory fitness measurements in either of the study waves. The schools provided access to a gymnasium for the collection of cardiorespiratory fitness data and private rooms for the collection of anthropometric data.
Cardiorespiratory fitness. The Leger 20 m shuttle run was used to quantify cardiorespiratory fitness in this study (16,17). This is a validated and reliable field test that is frequently used in physical education curriculum to track cardiorespiratory fitness levels in youth. The shuttle run was performed in groups of 8–10 youth of the same age under the supervision of two or three trained research assistants. To ensure appropriate pacing and to encourage participants to complete as many stages as possible, a research assistant concurrently performed the protocol. Using validated regression formulas, VO2 peak was predicted from the final exercise stage achieved (16,17). Age- and sex-specific standards established by Leger, were used to classify children into one of five cardiorespiratory fitness categories: (i) excellent, (ii) very good, (iii) average, (iv) acceptable, (v) needs improvement (Table 1). The standards reflect the age- and sex-specific quintiles of shuttle run test scores obtained from a sample of >6,000 children surveyed in the province of Quebec between 1977 and 1980 (16). For bivariate analyses, we grouped youth with “acceptable” and “needs improvement” into a low cardiorespiratory fitness category and the remaining three groups into the average-high cardiorespiratory fitness category.
Table 1. Example of thresholds used to categorize youth into fitness classes for boys and girls 9 and 12 years old
Anthropometric data. Children were weighed in their school clothing bare footed. Height and weight were measured in duplicate to the nearest 0.1 kg and 0.5 cm using a standard digital scale (SECA 880; SECA, Ontario, CA) and portable stadiometer (SECA Road Rod Portable; SECA). Waist and hip circumference were measured in duplicate according to standards established by McCarthy et al. (18). International age- and sex-specific BMI standards established by the International Obesity Taskforce were used to classify children as overweight (19). For bivariate analysis youth considered obese or overweight according to these standards were classified collectively as “overweight.” BMI Z-scores were calculated according to age- and sex specific algorithms provided by the Centers for Disease Control for application within SAS statistical software.
Data are presented as mean ± s.d., unless otherwise stated. In Table 2, group wise comparisons of normally distributed variables were tested using an ANOVA model with Bonferroni correction for post hoc analyses. When variables were not normally distributed, Kruskal–Wallis test with Dunns' method for post hoc analysis was used to test for group wise differences. In Tables 3 and 4, cardiorespiratory fitness and overweight status were converted to binary outcomes; (i) high and low cardiorespiratory fitness and (ii) healthy weight and overweight, respectively. The lower two categories from Leger's classification were used to categorize youth as “low cardiorespiratory fitness” for two reasons: (i) these values reflect “below average cardiorespiratory fitness” for youth for a given age and sex and (ii) dividing youth at this level also allowed an even distribution of youth in each comparison group.
Table 2. Participant characteristics of the 1 year follow-up
Table 3. Odds of being classified as overweight after 1 year follow-up in lean and overweight youth
Table 4. Odds of becoming or remaining overweight after 1 year follow-up in children stratified according to fitness level and overweight status
The odds of being overweight after 12 months of follow-up (Tables 3 and 4) were calculated using Cochrane–Mantel–Haentzel statistics with age, BMI and sex entered into the model as covariates. The dose–response association between cardiorespiratory fitness classification and the odds of being overweight presented in Table 5 was determined using a binary logistic regression. In Table 5, an autoregressive multifactorial mixed linear regression with fixed (age, gender, previous year's BMI) and random (change in cardiorespiratory fitness score) effects was used to determine the continuous association between changes in cardiorespiratory fitness and BMI in the subset of 222 children followed for 2 years. All data were analyzed using SPSS for Windows, version 14.0 (SPSS, Chicago, IL) and SAS (SAS Institute, Cary, NC).
Table 5. The dose–response association between fitness and the 12-month risk of being classified as overweight in youth
Participant characteristics are provided in Table 2 stratified according to baseline cardiorespiratory fitness and overweight status. On average, healthy weight children were significantly (P < 0.01) younger (11.0 ± 3 vs. 11.5 ± 2 years), displayed a lower BMI (18 ± 2 vs. 24 ± 4 kg/m2) and a lower waist circumference (64 ± 8 vs. 80 ± 12 cm), than overweight peers. Cardiorespiratory fitness levels were not different between healthy weight and overweight youth at baseline (48 ± 6 vs. 44 ± 6 ml/kg/min, P = 0.60). When healthy weight and overweight youth were further stratified according to baseline cardiorespiratory fitness levels, children in the high cardiorespiratory fitness group tended to be older, taller and display significantly lower waist circumference (Table 2). BMI was not different between the high cardiorespiratory fitness and low cardiorespiratory fitness groups at baseline. As previously reported (15), youth lost to follow-up were generally older, heavier and displayed lower cardiorespiratory fitness levels than those who were retained for follow-up.
On average, BMI increased 0.74 ± 1.72 kg/m2 for all children followed for 1 year, being slightly higher in boys than girls (0.77 ± 1.7 vs. 0.71 ± 1.7 kg/m2 respectively, P < 0.01). On average, children in the low cardiorespiratory fitness groups gained significantly more weight (4.6 ± 5.1 vs. 3.8 ± 3.3 kg, P < 0.05) and waist circumference (2.3 ± 8.2 vs. 1.2 ± 5.8 cm, P < 0.05) than children in the high fit group, after controlling for baseline differences in age, gender and waist circumference. The differences in weight gain and waist circumference between high fit and low fit children remained significant after stratification into healthy weight and overweight groups. No differences were observed in the change in BMI Z-score between the groups (0.03 ± 0.4 vs. 0.08 ± 0.05 in high fit and low fit healthy weight youth respectively; −0.1 ± 0.4 vs. −0.06 ± 0.3 in high fit and low fit overweight youth, respectively).
The 12-month risk of overweight, in youth stratified according to cardiorespiratory fitness classification, is presented in Tables 3 and 4. Overall, low cardiorespiratory fitness was independently associated with a 3.5-fold increased 12-month risk of overweight classification (P < 0.01; Table 3). Despite a similar BMI at baseline, the 12-month risk of becoming overweight was significantly higher in healthy weight children with low cardiorespiratory fitness relative to their fit peers, independent of age and gender differences between the groups (P < 0.01; Table 4). Similar trends were observed for overweight children, where a classification of “high cardiorespiratory fitness” was associated with a 75% reduced odds of remaining overweight (P < 0.01; Table 4). The odds of becoming or remaining overweight increased 48% for every decrement in cardiorespiratory fitness strata (P < 0.01; Table 5 and Figure 1).
A significant negative continuous association existed between the changes in cardiorespiratory fitness and BMI over the 2-year follow-up (Table 6), independent of age, baseline BMI and sex. For every unit decline in cardiorespiratory fitness (shuttle run score) over 2 years, BMI increased 0.18 kg/m2 (P < 0.01, Table 6).
Table 6. A time-series linear multivariable regression model of the determinants of the 2-year change in BMI in youth
Three important findings relevant to the public health challenge of childhood obesity emerged from this school-based longitudinal study of youth. First, low cardiorespiratory fitness is strongly and independently associated with the risk of becoming or remaining overweight. Second, reductions in cardiorespiratory fitness over time are significantly and independently associated with weight gain over a period of 2 years. Finally, high cardiorespiratory fitness is associated with reduced central adiposity and a reduced age-related change in waist circumference during childhood.
Recent population-based surveys reveal that fit youth tend to be leaner than their unfit peers, independent of habitual physical activity patterns (5,6,7). The data presented here provide a unique temporal layer to this association by demonstrating that the 12-month risk of becoming overweight is elevated in healthy weight youth with low cardiorespiratory fitness, independent of their age and sex. More importantly, overweight youth with low cardiorespiratory fitness displayed a 3.5-fold increased risk of remaining overweight, relative to peers with above-average cardiorespiratory fitness. In the absence of intervention studies, these studies suggest that having above-average cardiorespiratory fitness is associated with a modest protection from becoming or remaining overweight in youth. A number of longitudinal studies have demonstrated that low cardiorespiratory fitness in adolescence is associated with weight gain and cardiovascular risk factor clustering in adulthood (20,21,22,23,24). Our data extend these observations by demonstrating that low cardiorespiratory fitness is also associated with short-term risk of disproportionate weight gain during childhood/adolescence.
The observed effect size of low cardiorespiratory fitness on the risk of overweight was similar to a previous school-based longitudinal study of youth that reported a 3.3-fold (95% confidence interval: 2.0–5.5) increased risk of overweight in girls considered “underfit” for their age (25). In the investigation of Kim et al. (25) “low fitness” was classified as a composite score from five tests of musculoskeletal health, making it difficult to interpret the data and identify one aspect of fitness associated with weight gain in youth. The data presented here support this observation and extend it in several important ways. First, using a common, validated field test to assess cardiorespiratory fitness we were able to demonstrate a dose–response association between cardiorespiratory fitness and overweight risk in youth. Second, by following a cohort for a period of 2 years we were able to demonstrate that reductions in cardiorespiratory fitness over time were positively associated with weight gain. Collectively, a score from a common field test of cardiorespiratory fitness provided valuable information regarding the likelihood of disproportionate weight gain in school-aged children.
An increased waist circumference is associated with an increased risk of cardiometabolic risk factor clustering in youth (26,27) and cardiovascular morbidity in adults (28). Increasing physical activity levels preferentially reduces visceral fat and likely contributes to the cardioprotective effects associated with exercise (29,30,31). The data presented here support the concept that cardiorespiratory fitness may be a mediating variable involved in this association (31,32). Specifically, healthy weight and overweight youth with low cardiorespiratory fitness for their age were characterized by elevated waist circumference and experienced disproportionate increases in waist circumference over time. Importantly, these associations were observed independent of BMI, reinforcing the value of a measurement of waist circumference when assessing cardiometabolic disease risk (33,34,35). It is possible that similar to adults (32), increased cardiorespiratory fitness or habitual moderate–vigorous physical activity attenuates visceral obesity in youth, potentially attenuating cardiometabolic risk.
A major limitation of this study is that physical activity was not determined concurrently with measures of cardiorespiratory fitness. As a result, it is unclear if children with low cardiorespiratory fitness levels are less active than their fit peers. A significant portion of a child's cardiorespiratory fitness is determined by the time spent engaged in moderate–vigorous physical activity (5,7). Therefore, it is possible that the increased risk of overweight in youth with low cardiorespiratory fitness may reflect reduced daily moderate–vigorous physical activity; however, intervention studies are needed to confirm this observation. The second limitation of this study is the lack of self-reported dietary records. As these instruments are questionable in younger age children (36) and that baseline anthropometrics were comparable between the groups, it is unlikely that differences in dietary intake would have dramatically attenuated or negated the strength of the observed relationship. Finally, puberty is a well-known determinant of weight gain in youth. In an effort to control for the potential confounding effect of puberty on the outcome of weight gain, data were reanalyzed within subgroups of youth 6–9 years and 10–13 years (data not shown) and the risk of overweight remained significantly elevated in those with “low fitness” (odds ratio: 2.5–4.5, P < 0.001) within each subgroup, suggesting that puberty was not a confounding variable in the observations presented here. A similar effect size was observed in adolescents (14–17 years), however due to a limited sample size (n = 198), it was not statistically significant.
The fact that this study was conducted in a school setting using a common, maximal, field test increases the translational potential of these findings. The Leger shuttle run test is part of the Canadian Standardized Test of fitness which is still commonly used within physical education curriculum in elementary schools in Canada. The observation that a Leger shuttle run score below the 40th percentile for age and sex is associated with an increased risk of overweight provides preliminary evidence for a target for physical education practitioners committed to preventing weight gain in youth.
In conclusion, cardiorespiratory fitness and the change in fitness over time are powerful independent predictors of overweight status and weight gain in children and adolescents. The inclusion of cardiorespiratory fitness testing within physical education curriculum in schools could prove useful for the determination of overweight risk status in youth.
We are deeply indebted to the students, parents, teachers, and administrators in the Black Gold School District for their support of this project. We also thank the expert technical support of Lynn Bonnah, Cameron McKnight, Michael and Charles Metcalfe, and Amanda Smith for their help with data collection and handling. Financial assistance for this project was provided by the Alberta Initiative for School Improvement (Province of Alberta); the Canadian Diabetes Association and the Alberta Centre for Child, Family & Community Research. J.M.M. is a Canadian Diabetes Association Scholar and was supported by TORCH and Target Obesity Fellowships provided by the Canadian Institutes of Health Research, the Heart and Stroke Foundation of Canada and the Canadian Diabetes Association during the collection of this data. B.D.T. was supported by a graduate studentship from the Canadian Hypertension Society.