Keywords
obesity - fitness - children
Introduction
Obesity and low cardiorespiratory fitness (CRF) are independently associated with
unhealthy status and premature death [1]
[2]
[3], and their combination may exaggerate
the problem [4]. Along this line, studies
have shown that men with overweight or obesity and low CRF present a greater risk
for early death due to cardiovascular diseases than men with normal weight and high
CRF [5]
[6]. In addition, individuals who are unfit
presented a higher risk of mortality, regardless of their body mass index (BMI),
compared to normal weight-fit individuals [4].
The prevalence of the unhealthy phenotype, with the coexistence of obesity and low
CRF, is unknown in children and adolescents. What is known is that the prevalence of
childhood obesity is growing globally, and this is associated with physical and
mental health abnormalities [3]
[7]
[8]. The World Health Organization (WHO) is alerting societies and
policymakers on the obesity epidemic, and is calling for actions to combat pediatric
obesity in order to ensure the health and wellbeing of societies in the long term
[9]. Along this line, evidence
suggests that children and adolescents with obesity are five times more likely to
become adults with obesity [10]. Obesity
in adulthood, in turn, may affect the quality of life negatively and elevate the
risk of cardiovascular diseases [7].
Regarding CRF, data show an accelerating decline with age in children and adolescents
[11]. Most importantly, the rate of
CRF decline is higher in children who are less fit (those in the lowest
percentiles), making them vulnerable to obesity-related diseases later in life [11]. The later study of 65,139 children and
adolescents reported that performance in the 20-m shuttle run test, which assesses
CRF, declined by about 2.9% from 2005–2009 to 2015–2020+ [11]. The benefits of a high CRF in youth
have been well established. Studies show that a high CRF is associated with better
body fat distribution with less central obesity [12], better cardiovascular [13]
[14], and metabolic function
[15]
[16]. In the IDEFICS study (Influence of
physical fitness on cardio-metabolic risk factors in European children), 1,635
children were followed for 2 years and the results showed that a poor CRF was
associated with a worse cardio-metabolic profile [16].
The complications of obesity and low CRF could be exaggerated with the duration of
their existence [17]. Accordingly, studies
on the stability or tracking of obesity and CRF are important to understand the
potential impact of this unhealthy phenotype on health. Evidence suggests that
tracking of obesity is moderate to strong in adulthood for children and adolescents
with obesity [10]. Additionally, tracking
of abdominal obesity, which is independently associated with metabolic dysfunction
[18], seems to be high [19]. No study so far has examined the
stability of the unhealthy phenotype of being an individual who has overweight or
obesity and low CRF in youth.
Therefore, the aim of this study was two-fold: 1) to examine the coexistence of
obesity and unhealthy cardiorespiratory fitness in a cohort of children and
adolescent boys, and 2) to investigate the stability (tracking) of this phenotype
(having overweight or obesity and unhealthy CRF) in a sub-group of participants.
Materials and Methods
Statement of human rights
The study was approved by the university’s ethical committee (ERSC_2023_2492;
23/4/2023), and was conducted in accordance with the principles of the Helsinki
declaration. Signed informed consent was obtained from the participants’
parents.
Participants
1,754 boys, aged 8 to 14 years, participated in this study; all participants were
Caucasians of European origin from the same school complex. The tests were
conducted as part of the fitness assessment program.
Study design
Baseline measurements were taken between the last week of September and the end
of October every year from 2004 to 2021. All measurements were taken in the
morning hours during the school program. For those boys who had more than one
assessment in the specified period, only the first assessment was considered for
the cross-sectional analysis (aim 1). To fulfil aim 2 of this study (tracking),
the data from a sub-group of 353 children were used. These 353 children (aged 8
to 11 years) had at least two assessments performed at least 36 months apart
during the study period. All assessments were conducted following the same
procedure as the one at baseline, and assessments were always performed in the
period between the last week of September and the last week of October.
Assessments
Body height was recorded without shoes to the nearest 0.1 cm using a stadiometer
(SECA, Hamburg, Germany). Body mass was taken on the same day, wearing light
clothes, to the nearest 0.1 kg using a digital scale (SECA, model 770, Hamburg,
Germany). Body mass index was calculated and participants were classified as
having normal, overweight, or obesity, using the age-related cut points for boys
[20]. The cut points to classify a
boy as having overweight or obesity are shown in [Table 1]
[20].
Table 1 Cut points for BMI (kg/m2) to classify a
boy as having overweight or obesity [20].
|
Age (years)
|
Overweight
|
Obesity
|
|
8
|
18.44
|
21.60
|
|
8.5
|
18.76
|
22.17
|
|
9
|
19.10
|
22.77
|
|
9.5
|
19.46
|
23.39
|
|
10
|
19.84
|
24.00
|
|
10.5
|
20.20
|
24.57
|
|
11
|
20.55
|
25.10
|
|
11.5
|
20.89
|
25.58
|
|
12
|
21.22
|
26.02
|
|
12.5
|
21.56
|
26.43
|
|
13
|
21.91
|
26.84
|
|
13.5
|
22.27
|
27.25
|
|
14
|
22.62
|
27.63
|
Cardiorespiratory fitness was assessed using the 20-m shuttle run test [21]. Participants started running at
8.0 km/h, and the speed was gradually increased by 0.5 km/h every minute until
volitional fatigue. The pace was recorded by an audio signal. Children were
always made to run between two lines 20 m apart. The actual score of the test
was the last stage completed before the boys quit. The test is used worldwide
[16]
[22]
[23], and it is suggested to be a valid tool for the estimation of
maximum oxygen uptake [VO2max; 21]. In a previous study with 55
participants of similar age and socioeconomic background to the present study,
we have reported a mean difference of 0.13 stages for repeated shuttle-runs with
95% confidence intervals (CI) between − 0.13 and +0.39 stages [19]. Based on the test performance,
children were classified as having an unhealthy or healthy CRF, using the cut
points previously suggested [24]. The
suggested cut point for cardiorespiratory fitness to avoid cardiovascular
disease risk is equal to a VO2max of 41.8 ml/kg/min in 8- to
18-year-old boys [24], and this is the
threshold we used in the present study. This cut point corresponds to one stage
completed during the 20-m shuttle run for 8-year-old boys, two stages completed
for 9- and 10-year-old boys, three stages for 11-year olds, four stages for
12-year olds, and five stages for 13- and 14-year-old boys [24]. In the meta-analysis of Ruiz and
colleagues [24], it was reported that
boys with low CRF (estimated VO2max below 41.8 ml/kg/min) had a 5.7
times greater likelihood of having CVD risks compared to those with higher
CRF.
Statistical analysis
In the cross-sectional part of this study (aim 1), the participants were
classified into four groups based on their BMI and CRF: 1) normal BMI with
unhealthy CRF (N/UN), 2) normal BMI with healthy CRF (N/H), 3) boys with
overweight or obesity with unhealthy CRF (OO/UN), and 4) boys with overweight or
obesity with healthy CRF (OO/H). Overweight and obesity were combined together
for analyses due to lower sample sizes in the groups. Chi-square tests were used
to compare the observed and expected distributions across the groups, and odds
ratios (OR) and 95% CI were calculated for the exposed group (OO/UN: boys who
had overweight or obesity and unhealthy CRF). Whenever needed, the Yates’
continuity correction was used in the chi-square test. We assumed the existence
of an association, if the 95% CI range did not include a value of 1, and
interpreted an OR 1.22 ≤ OR < 1.86 as small, 1.86 ≤ OR <3.00 as medium,
and OR≥3.00 as large [25]. For the
tracking analyses, a four-level Cohen’s weighted Kappa statistic was used. The
proportion of agreement was calculated, i. e. the proportion of children that
remained in their class between evaluations. The observed proportions of
agreement were compared with the proportions expected given no tracking.
Weighted Kappa analyses were performed using a set of weights that are based on
the squared distance between categories. We used the guidelines by Muñoz and
Bangdiwala [26] to interpret the Kappa
coefficients. Benjamini and Hochberg’s [27] procedure was applied for multiple testing corrections, and the
false discovery rate (FDR) adjusted p-values were reported. FDR-adjusted
p-values lower than 0.05 were assumed to be statistically significant. All
statistical analyses were conducted using R statistical software (version 4.2.1,
R Foundation for Statistical Computing, Vienna, Austria).
Results
The mean age of the participants was 11.4±1.7 years and their BMI was 19.86±3.30
kg/m2 (mean±SD; [Table
2]). The prevalence of normal body mass index, overweight, and obesity in the
sample was similar to the national data (65.7% of the boys in this sample had normal
BMI, and 34.3% had overweight or obesity) [28]. The averages for the 20-m shuttle run are between the
40th and 50th percentile for 8-, 9-, 10-, 11-year-old
boys, between the 50th and 60th percentile for 13- and 14-year
olds, and between the 60th and 70th percentile for 12-year
olds, based on normative values for European boys [22]. The odds that a boy had overweight or
obesity and an unhealthy profile for CRF compared to the normal body weight category
are 2.67 (13-year old) to 6.74 (9-year old) times higher ([Table 3]).
Table 2 Participants’ characteristics during the first
assessment.
|
Age
|
N
|
BMI (kg/m2)
|
20-m shuttle run (stages)
|
|
Total
|
1,754
|
19.9±3.30
|
4.6±2.50
|
|
8
|
202
|
18.1±2.61
|
2.8±1.59
|
|
9
|
200
|
19.3±3.12
|
3.2±1.71
|
|
10
|
352
|
19.2±2.94
|
3.6±1.74
|
|
11
|
210
|
20.2±3.10
|
4.4±2.23
|
|
12
|
513
|
20.2±3.31
|
6.0±2.41
|
|
13
|
160
|
21.4±3.71
|
5.9±2.35
|
|
14
|
117
|
21.9±3.07
|
6.7±2.58
|
BMI: body mass index.
Table 3 Distribution of children and adolescents in our sample in
the four groups (N/H: normal BMI with healthy CRF; N/UN: normal BMI with
unhealthy CRF; OO/H: boys with overweight or obesity with healthy CRF;
OO/UN: boys with overweight or obesity with unhealthy CRF) and odds
ratios for the exposed group (OO/UN: boys with overweight or obesity
with unhealthy CRF).
|
Age
|
n
|
Distribution, n (%)
|
χ2
|
FDR-adjusted p
|
Odds ratio (95% CI)
|
|
N/H
|
N/UN
|
OO/H
|
OO/UN
|
|
Total
|
1,754
|
963 (54.9)
|
189 (10.8)
|
374 (21.3)
|
228 (13.0)
|
100.55
|
<0.001
|
3.11 (2.48–3.90)
|
|
8
|
202
|
131 (64.9)
|
6 (3.0)
|
61 (30.2)
|
4 (2.0)
|
0.04
|
0.845
|
1.43 (0.39–5.26)
|
|
9
|
200
|
103 (51.5)
|
11 (5.5)
|
50 (25.0)
|
36 (18.0)
|
28.29
|
<0.001
|
6.74 (3.17–14.34)
|
|
10
|
352
|
210 (59.7)
|
20 (5.7)
|
87 (24.7)
|
35 (9.9)
|
24.17
|
<0.001
|
4.22 (2.31–7.72)
|
|
11
|
210
|
106 (50.5)
|
23 (11.0)
|
49 (23.3)
|
32 (15.2)
|
12.09
|
<0.001
|
3.01 (1.60–5.67)
|
|
12
|
513
|
290 (56.5)
|
70 (13.6)
|
86 (16.8)
|
67 (13.1)
|
32.51
|
<0.001
|
3.23 (2.14–4.88)
|
|
13
|
160
|
70 (43.8)
|
37 (23.1)
|
22 (13.8)
|
31 (19.4)
|
8.29
|
0.005
|
2.67 (1.36–5.24)
|
|
14
|
117
|
53 (45.3)
|
22 (18.8)
|
19 (16.2)
|
23 (19.7)
|
128.46
|
0.008
|
2.92 (1.33–6.39)
|
BMI: body mass index; CRF: cardiorespiratory fitness; FDR, false discovery
rate. Values in bold are significant.
The mean time interval between measurements varied from 2.97 to 6.06 years, with a
mean of 3.79±0.83 years (Median 4.01 years, IQR = 1.05). The average BMI and CRF
were 19.12±3.01 kg/m2 and 3.30±1.68 stages in assessment 1 and 21.50±3.34
kg/m2 and 6.27±2.17 stages in assessment 2 for these boys,
respectively. 56.6% of the participants remained in the same category, whereas 28.0%
of them had ≥1 category (BMI or CRF) changing for the worse ([Table 4]). The Cohen Kappa between the
first and last measure was 0.557 (p<0.001), indicating a substantial agreement,
meaning that a substantial number of people maintained their position in the second
measurement.
Table 4 Number of participants (n) and percentages (%) in each
group considering the first and the second assessment (mean duration
between assessments was 3.79±0.83 years with a range from 2.97 to 6.06
years, n= 353).
|
|
Second assessment
|
|
First assessment
|
|
Normal BMI with heathy CRF
|
Normal BMI with unhealthy CRF
|
Boys with overweight or obesity with healthy CRF
|
Boys with overweight or obesity with unhealthy CRF
|
|
Normal BMI with heathy CRF (n=204)
|
n
|
132
|
48
|
20
|
4
|
|
%
|
64.7
|
23.5
|
9.8
|
2.0
|
|
Normal BMI with unhealthy CRF (n=20)
|
n
|
8
|
5
|
1
|
6
|
|
%
|
40.0
|
25.0
|
5.0
|
30.0
|
|
Boys with overweight or obesity with healthy CRF (n=89)
|
n
|
26
|
4
|
43
|
16
|
|
%
|
29.2
|
4.5
|
48.3
|
18.0
|
|
Boys with overweight or obesity with unhealthy CRF (n=40)
|
n
|
6
|
1
|
13
|
20
|
|
%
|
15.0
|
2.5
|
32.5
|
50.0
|
BMI: body mass index; CRF: cardiorespiratory fitness.
Discussion
The main finding of this study is that the odds ratios for boys having unhealthy
profiles of combined excessive body weight and low CRF ranged from 2.67 (medium) to
6.74 (large). This unhealthy phenotype shows a substantial stability within an
average follow-up period of 3.79 years in this sample. This means that a substantial
number of boys maintained their position in the second measurement.
To our knowledge, this is the first study reporting the rate of this unhealthy
profile in youth. Assuming that these cut points for the 20-m shuttle run test
estimated CRF apply to all populations [24], on average 13% of the children and adolescents in our sample had
overweight or obesity and unhealthy CRF ([Table
3]). We can assume that this unhealthy profile may undermine the physical
and mental health of these individuals should this condition remain for a long time
[3].
The medium to large odds for the presence of the unhealthy phenotype in the present
study ([Table 3]) is an additional alarm
for families, school administrators, and policymakers. The WHO declared obesity as a
major public health problem and a global epidemic. The medium to large odds ([Table 3]) and the substantial stability of
this unhealthy profile ([Table 4]) pose
an additional risk for future complications in health when these children become
adults. Indeed, previous studies show that children with obesity have a high chance
of becoming adults with obesity [29]. In
our study, 20 out of 40 boys who had overweight or obesity and unhealthy CRF (50%)
remained in the same category in the follow-up ([Table 4]). Interestingly, 7 out of 20 who
shifted categories (35%) moved to the normal BMI category, whereas 13 out of 20
(65%) improved the CRF category while maintaining the BMI category. In another study
with 374 participants aged 7 to 18, who were followed for 22 years, it was reported
that about 83% of youth who had overweight remained in this category as adults [29]. In addition, the odds of an individual
having overweight as an adult were 6.2 times greater in children who had overweight
compared to children who had normal body weight [29]. Besides its presence, the duration of obesity will impact the health
outcomes [17]. The analysis of the
National Health and Nutrition Examination Survey data in the USA shows that
individuals, who were inactive and had overweight or obesity for longer periods, had
an increased risk of poor metabolic health compared to those who were active and had
overweight. In addition, individuals who had overweight or obesity for longer
durations presented an elevated risk of poor metabolic health [17]. A positive message derived from our
data is that only a few boys who had normal BMI and healthy CRF at baseline become
individuals who have overweight or obesity and unhealthy CRF in the second
assessment (4 out of 204, which is less than 3%; [Table 4]). Overall, our results highlight
the role of prevention in body composition and healthy CRF in boys.
The potential beneficial mechanisms explaining the impact of a high CRF on health are
related to a better body fat distribution, reduced inflammation and improved
metabolic profile [3]
[30]. Regarding the body fat distribution,
data using magnetic resonance imaging showed that aerobically fit children aged 10.6
years had lower visceral adipose tissue compared to children who are unfit of
similar age [31]. Interestingly, children
who had overweight or obesity and high CRF presented lower central and total
adiposity compared to children in the same BMI category with low CRF [12]. Regarding the improved metabolic
profile, we have previously reported improved insulin sensitivity, assessed with the
oral glucose tolerance test, after 3 months of aerobic training in youth with
overweight or obesity [15]. Interestingly,
this improvement in the metabolic profile was associated with an elevation of CRF by
18.8%, without any change in body fat and body mass, suggesting an independent role
of a high CRF on metabolic health [15].
Implications of our findings
The implications of our findings in the context of the existing literature
discussed above are multiple. Prevention of adoption of the unhealthy profile is
key. The achievement of normal BMI and the elevation of CRF should be the
primary targets and all stakeholders (families, school administrators, and
policymakers) should work towards achieving these goals [32]
[33]. The elevation of CRF will be achieved by spending more time in
vigorous physical activities.
Limitations
The present study has certain limitations. Obesity was assessed via BMI and CRF
via performance in the shuttle run. Both assessments are considered as indirect,
and direct measures of body composition and CRF could prove insightful. Our
study represents an observational study of a single cohort of European boys. Big
scale, representative studies including girls, will be of great value and will
allow the generalizability of the findings. The low number of participants in
the tracking analysis and the short follow-up period should be considered when
interpreting these findings. The maturity stage of these boys was not assessed,
and this could also have affected the results. It is acknowledged that CRF is
affected by genetics too [34], and we
cannot exclude the influence of common genes in body composition and CRF. This
would also affect the interpretation of these findings.
Conclusion
The odds of boys having an unhealthy profile of combined excessive body weight and
low cardiorespiratory fitness was medium to large. Of note, this unhealthy phenotype
presents a substantial stability in the short-term. Future studies should
investigate the stability of this profile in the longer term.
