Physical Fitness and Cardiometabolic Risk in Seven to Nine Years Old Children


 Background: Obesity and lower physical fitness levels in children are related to the development of cardiometabolic risk. The objective of the study was to determine the relationship of cardio-metabolic risk and physical fitness in school children from Santiago de Chile.Methods: Physical fitness was assessed as cardiorespiratory fitness and muscle strength. Fitness was measured using the six-minute-walk-test and muscle strength was assessed by hand grip and standing lung jump. Overweight (≥ 1 SD) and obesity (≥ 2 SD) were determined by body mass index. A cross sectional study was done in 452 children (185 boys and 267 girls), age range (7-9 years). Cardio-metabolic-risk (serum glucose, triglycerides, HDL, insulin) and waist for height ratio was expressed as z score. Data was analyzed using bivariate analysis and multiple and logistic regression.Results: A 21% were obese and 27% overweight. Children with high body fat and low cardiorespiratory fitness and muscle strength had an OR of 6.8-fold (IC 95% 3.4 to 13.9) cardiometabolic risk z-score, compared to those most fit.Conclusion: School children with higher body fat and lower physical fitness had increased cardio-metabolic-risk-score. No significant difference in cardio-metabolic risk in the non-obese children was found independent of their physical fitness.


Background
The prevention and treatment of cardiovascular disease and its risk factors are commonly focused on the adult population. However, in recent years, evidence has revealed that these risk factors can be observed in early life [ 1 , 2 ]. Childhood obesity in Chilean children has tripled in recent decades, reaching 23.9% in 2017 [ 3 ]. Obesity has been shown to be associated with a high prevalence of hypercholesterolemia (27%) and high insulin levels during fasting (42% of all obese children have abnormal values) [ 4 ]. Childhood obesity is also associated with an increased risk of cardiovascular disease in adulthood [ 5 ]. The sum of risk factors, assessed using a cardiometabolic risk (CMR) score, is considered a more appropriate tool for de ning disease risk in children [ 6 , 7 ]. Inappropriate eating patterns and a lack of regular moderate to vigorous physical activity [ 8 ] are important contributors to the early onset of CMR. Sixty percent of the world's population does not have an adequate level of physical activity in terms of preventing noncommunicable disease [ 9 , 10 ]. Chilean children are predominantly sedentary, given that only 20.2% of them meet the physical activity recommended guidelines [ 11 , 12 , 13 ]. Similarly, in northeast England, sedentary behaviour was found in 78% to 82% of 7-to 9-year-old children [ 14 ]. In school children and adolescents, this problem has further consequences if we consider that only 9% of school-age Chilean children have acceptable physical tness levels [ 15 ].
Cardiorespiratory and muscle tness are inversely associated with the risk factors for cardiovascular diseases [ 16 ]. Physical tness assessments are feasible measures that could help improve the monitoring of paediatric health status [ 17 ].The independent biological effect of cardiorespiratory tness and muscle strength on cardiometabolic risk has been demonstrated in adolescents [ 18 , 19 ].
Artero el al, evaluated the relationship between metabolic risk and physical tness in 709 adolescents.
They founded that muscular tness was negatively associated with clustered metabolic risk independent of cardiorespiratory tness (β = −0.249, p < 0.001). Independent of muscular tness, an inverse association was also found between cardiorespiratory tness and clustered metabolic risk (β = 0.264, p < 0.001) [ 20 ] . Steene-Johannessen J, et al, measured in 2818 children 9-and 15-yr-olds physical tness and metabolic risk. They also encountered that muscle tness was negatively associated with clustered metabolic risk, independent of cardiorespiratory tness, and after adjustment for age, sex, and pubertal stage (beta = -0.112, P < 0.001) [ 21 , 22 ] . Additionally, there is scarce information in this group age. The objective of the study is to determine the relationship of cardiometabolic risk and physical tness in school children from Santiago de Chile.

Methods
This study is cross-sectional; the sample was drawn from children enrolled in the "Growth and Obesity Chilean Cohort Study", which assesses the association of early growth and development with adiposity and metabolic risk [ 23 ].
We randomize by using random numbers, included healthy 7-to 9-year-old schoolchildren from the original study, both girls and boys, who had provided a blood sample in 2009. According to the objective of this project, tness tests were performed in 2010. Parents or guardians signed a previously approved informed consent form by the Institute of Nutrition and Food Technology Ethics Committee for research in human subjects. The nal sample size was 452 children (267 girls and 185 boys).
Weight was measured with an electronic scale (Body Composition Analyzer TANITA BC-418, USA) with a 0.1 kg accuracy and 220 kg maximum measurement. Children were measured in underclothing, while placing their feet in the centre of the scale. Height measurements were performed with a portable stadiometer (SECA 222 ®) with a range between 0 and 200 cm and a 1 mm precision using the Frankfurt standard [ 24 ]. Nutritional status was determined using body mass index (BMI) and BMI-Z. Overweight and obesity were de ne as BMI-z ≥1 and 2≥ SD respectively (WHO 2007) [ 25 , 26 ]. Waist circumference was measured with an inextensible tape (SECA ®) [ 27 ]. All variables were measured twice for each child. The measurements agreed within 0.5 cm for height and waist circumference and 20 g for weight. Finally, waist circumference and height were used to calculate the waist-to-height ratio (WHtR), and central obesity was de ned as having a WHtR ≥ 0.5 [ 28 ].
The body composition of a child was classi ed in tertiles of BMI or WHtR. According to the results, body fat mass was classi ed as low, medium and high.
Identi cation of pubertal status (Tanner stages I-II) was assessed by study professionals using the method described by Tanner and Whitehouse [ 29 ].
Blood samples were obtained between 08.30 and 10.30 after an 8-h overnight fast. Glucose, insulin and blood lipids were assessed after 8 h of fasting. Ten millilitres of venous blood were collected, and serum glucose was assessed using a commercial kit by the GOD-PAP (Clinical Chemistry Applied SA) enzymatic colorimetric method. Insulin was measured by RIA (RIA DCP Diagnostic Products Corporation, LA, USA).
The remaining blood was eliminated.
The CMR score was de ned by the sum of the Z score of the WHtR-z and blood lipid related variables: (Glycaemia-z, Insulin-z and Triglycerides-z -HDL-z)/5, as previously reported [ 35 ]. These variables were chosen because they are commonly used in adults as criteria to diagnose metabolic syndrome [ 36 , 37 ].
Physical tness was assessed as cardiorespiratory tness and muscle strength. Cardiorespiratory tness was measured using a sub-maximal test, the six-minute walk test (6MWT) [ 38 , 39 ] and the six-minute walk test divide by height [ 40] The 6MWT, hand grip strength and standing long jump without momentum have been shown to be valid, reliable, and feasible for health monitoring purposes at the population level [34]. The 6MWT results were expressed as the maximal distance (metres) divided by height because leg length was not assessed [ 41 ]. The value was also standardized using the z score.
Muscle strength was assessed using hand grip and standing long jump. Handgrip was measured using an adjustable hand grip digital dynamometer (Baseline 12-0286 ®; 100 g accuracy), and the results were expressed in kg. The hand grip was performed twice by each child, by squeezing the dynamometer as hard as possible, for at least two seconds. One minute of recovery between squeezes was measured (alternating the right and left hands). Finally, hand grip strength was divided by body weight (kg) to take into account body size differences. The best result of two attempts was registered [ 42 ]. The best value was also standardized using the z score.
The standing long jump was performed by starting in a semi-squatting position, then jumping forward.
This test was performed on a 2-m long non-slippery surface, free of obstacles. A cross mark was made to identify the initial position from which the child jumped. A piece of chalk was used to mark the landing point for each of the two attempts; the distance in cm served to register the best jump [ 43 ]. The best value was also standardized using the z score 16 .
The results for muscle strength are expressed as a z score and were calculated based on standard values of hand grip and standing long jump from this study (hand grip/ weight z + standing long jump -z) [ 44 ].
The physical tness z score was de ned as 6MWT/height -z score + (hand grip/ weight z score + standing long jump -z score). The z score data were continuous.

Statistical analysis
The normality of the data for each variable was checked using the Shapiro-Wilk test and graphical methods. Statistical normality was tested using both statistical (Shapiro Wilk test) and graphical methods (normal probability plots). The majority of outcomes were normally distributed except hand grip, hand grip/weight, triglycerides, insulin, HOMA and body mass index.
Based on the distribution of the variables, the Mann-Whitney test or a t test was used to analyse independent samples. Preliminary analyses showed signi cant interactions between sex, physical tness and cardiometabolic variables; therefore, combined association analyses were performed with boys and girls together to increase the statistical power of the results.
The effect of muscle strength and cardiorespiratory tness on the CMR was also studied using two-way ANCOVA by dividing groups by muscle strength and cardiorespiratory tness tertiles. The interaction between CMR-z score and fatness categories (low and high BMI or WHtR) was assessed. The post hoc analysis of differences between groups was performed using a Bonferroni adjustment to test for signi cance and was adjusted by age and maturation stage. The respective physical tness values for age groups were segregated by tertiles since no reference values are available. Finally, the odds ratio (OR) and 95% CI of having a high CMR were examined by combined physical tness variables (i.e., cardiorespiratory tness and muscle strength). A multinomial logistic regression adjusted by age, sex and maturation stage was used. All analyses were performed using SPSS software, version 21.0. Statistical signi cance was set at p < 0.05.

Results
Descriptive statistics for the studied children are shown. Table 1 shows that girls were signi cantly older than boys and that boys had slightly higher blood glucose than girls (p <0.05).    Table 3 shows the physical tness results by nutritional status and sex. Compared to obese children, the normal nutritional status group had better muscle strength and cardiorespiratory tness in both sexes in most tests (p <0.05). Notably, on the hand grip strength test, children with overweight and obesity had better records than normal weight children, but when adjusted for weight, the results were reversed. Boys showed higher values of muscle strength and cardiorespiratory tness compared to girls (p <0.05). The ndings of suggest the time of moderate-intensity physical activity and sedentary time modi es cardiometabolic risk in children.  Figure 1a shows the CMR score tertiles in relation to physical tness and fat categories (BMI and WHtR).
In subjects with a low fat mass tertile, there was no difference in the CMR risk by muscle strength and cardiorespiratory tness tertiles. Whereas in children with a high fat mass tertile, higher muscle strength or cardiorespiratory tness was associated with a lower CMR than that of other groups. The post hoc analysis found signi cant differences between the low and high tertiles (p <0.05), and CMR was lower for the higher tertile.
The post hoc analysis found signi cant differences between the low and high tertiles (p <0.05), and CMR was lower for the higher tertile. Figure 1b shows the CMR score according to fat categories and muscle strength tertiles. In subjects with a low fat mass tertile, there were no differences in CMR according to cardiorespiratory tness tertile. In high-fat tertile children, the group with higher cardiorespiratory tness had lower CMR values than the low tertile cardiorespiratory tness group. The results of Figure 1a and 1b were adjusted by age, sex and maturation status. Table 4 shows the odds ratio (OR) of the relationship between high CMR and the combined association of physical tness (cardiorespiratory tness and muscle strength). The analysis was performed with the high tertile of cardiorespiratory tness and the high tertile of muscle strength as references. The results show that a low tertile of cardiorespiratory tness and a low tertile of muscle strength are associated with a 6.8 times higher CMR (95% CI, 3.4 to 13.9) compared to the high tertiles for both of these factors. In addition, a low tertile of muscle strength and a medium tertile of cardiorespiratory tness had a high CMR (OR=3.5 Finally, a medium tertile of cardiorespiratory tness and a medium tertile of muscle strength resulted in a high CMR (OR=2.6 95% CI, 1.2 to 5.5; p=0.003). The rst multinomial logistic regression analysis revealed that higher muscle strength and cardiorespiratory tness explained lower CMR. In the overweight group of schoolchildren, it was observed that children with better muscle strength and cardiorespiratory tness had a lower CMR compared to other overweight children. These results coincide with other works in adolescents and adults, which were attributed to changes in muscle metabolism, as lower visceral fat mass provides protection [16,54 ]. In similar studies, muscular strength has shown a stronger association with CMR score than cardiorespiratory tness has [20,21]. Similar ndings in previous work in children and adolescents have shown an independent association of muscle strength with insulin sensitivity [15]  Several studies have shown that CMR is higher in overweight children and adolescents with low muscle strength [7,49]. The results of this study suggest that muscle strength in schoolchildren may confer additional bene ts attributed to cardiorespiratory tness Most obese children are reluctant to participate in aerobic training; therefore, power exercise may be more attractive and better tolerated [ 58 ].
Hand grip strength and the standing long jump have proven to be a good validity criterion, compared to laboratory tests [34,59 ], suggesting that they are appropriate measures of muscle strength, especially in the rst years of schooling.
Among the strengths of our study, we con rm that a risk score composed of metabolic risk factors can be used to investigate the cardiorespiratory tness and muscle strength [16,18,30,45]. Such variables indicators may better re ect the health status of participants than the assessment of individual risk factors [ 60 ].

Limitations
However, our results should be interpreted with care. The CMR score is speci c to the sample under study and is based on the assumption that each component is equally weighted in the prediction of CMR. The 6MWT is a submaximal test to assess cardiorespiratory tness, and it is not comparable with the standard aerobic test, i.e., the Course-Navette test. Finally, to evaluate muscle strength, only hand grip strength and standing long jump were measured, and other tests could have been chosen 36,46 . However, these tests are easy to apply and well tolerated in children of young ages, whose results are reported in this study.

Conclusion
Muscle strength and cardiorespiratory tness are associated independently and inversely with CMR score. These results con rm the importance of stimulating physical activity to prevent metabolic risk at an early age, especially in sedentary children or those with high weights. In the future, longitudinal studies are needed to further nd the independent and combined effects of cardiorespiratory tness and muscle strength in the prevention of cardiovascular disease risk.  Cardiometabolic risk z-score by fat tertiles categories (a), cardiorespiratory tness (b) and muscular strength * p<0.05 between high and low tertiles In subjects with low fat, there was no difference in the CMR risk as muscle strength tertiles; whereas in children with high fat, higher muscle strength showed lower CMR compared to other groups with lower muscle strength.