1. Introduction
There are studies to suggest inverse relationships between body fatness and cognitive abilities in early childhood [
1,
2,
3], while other investigations have not found associations between body fatness and cognitive abilities [
2,
4,
5]. For example, Haapala et al. [
3] found a negative relationship between body fat percentage (BF%) with reading fluency and reading comprehension in 6-8-year-old boys. Martin et al. [
2] observed that higher body mass index (BMI) in 3-year-old boys was associated with worse visuospatial skills, yet not with expressive language skills or reasoning skills at 5 years. Flores et al. [
5] reported that obesity was not associated with arithmetical performance among 3-6-year-old children. In addition, previous studies disagree whether higher regional or whole-body fat content predict worse cognitive performance [
1,
6,
7]. Higher abdominal fat deposition was associated with worse relational memory but not with item memory in children with overweight or obesity at 7-9 years. Whole-body fat content was not associated with relational or item memory. Also, no associations between body fatness parameters and memory tasks existed in children with healthy weight [
6]. Likewise, Raine et al. [
7] found that especially in 8-9-year-old children with obesity, the reduction only in visceral adipose tissue within 9 months was associated with increased inhibitory control [
7]. However, higher whole-body and abdominal body fatness predicted inferior executive function, resistance to distraction, and gestalt processing among 7-11-year-old children with overweight [
1]. Therefore, very few observational studies have checked the sex-specificity of these associations, finding negative association between body fatness and cognition among preschool-aged [
2] and first-grade boys [
3] but not in girls. It has been found that the decrease in obesity status had positive associations with some cognitive skills in preschool-aged girls [
2]. Regarding to the mixed results and the paucity of longitudinal studies [
2,
4], current study aims to explore the possible longitudinal associations between body fatness in preschool-aged boys and girls separately with their cognitive skills in school.
There is lack of uniformity in the findings on the associations between physical fitness (PF) and cognition in children and adolescents, where cardiorespiratory fitness (CRF) or muscular fitness (MF) seem to associate with some cognitive skills, but then not with others. For example, in 4-6-year-old children CRF at baseline was related to improvements in attention and not with working memory during 9-month period [
8]. As well, 9-year-old children with higher CRF had better visual discrimination [
9] but not better reaction time compared to children with lower CRF [
10]. Moreover, CFR at 6–9 years did not predict non-verbal cognitive performance two years later in boys or girls [
4]. Regards to MF, neither standing long jump (SLJ) nor handgrip strength at 7-9 years predicted non-verbal cognitive performance further in childhood and adolescence [
11]. Meanwhile, Syväoja et al. [
12] reported that higher MF was indirectly associated with higher math outcomes through visuospatial working memory only among adolescent girls [
12]. In addition to contradictory findings described above, the number of studies on the associations between CRF, SLJ and handgrip strength either addressed separately or as a compound MF score in children is relatively small, therewith lack of longitudinal studies in preschoolers [
8,
13] or primary schoolers [
4,
11,
14]. Accordingly, it is important to study the associations of CRF and MF in preschool-aged boys and girls with their cognitive skills in school.
Our work intends to extend the understanding of the relationships between body fatness and physical fitness of preschoolers with their verbal, conceptual and perceptual skills in school, as cognitive skills me have significant educational, occupational and health outcomes [
15]. The specific aim of the present study was to investigate whether body fatness, cardiorespiratory fitness and muscular fitness in preschool are associated with cognitive skills in first grade at school in boys and girls.
4. Discussion
This study aimed to investigate whether body fatness, and cardiorespiratory fitness and muscular fitness in preschool are associated with cognitive skills in first grade at school. The main findings were: 1) BF% in preschool was not associated with conceptual, verbal and perceptual skills at first grade in school in boys and girls after adjustment for potential confounding factors; 2) CRF in preschool was not associated with conceptual, verbal and perceptual skills in school in boys and girls when adjusted for possible confounding factors; 3) higher MF in preschool was associated with higher verbal skills in school only in boys after adjustment for potential confounding factors. It was also observed that cognitive skills at preschool were frequently positive predictors of cognitive performance at follow-up in first grade at school both in boys and girls, although there were some sex differences. Additionally, higher parental educational attainment at baseline was associated with higher perceptual skills at follow-up in girls. Correspondingly, Riso et al. [
28] concluded that preschoolers from more highly educated families had higher conceptual and verbal skills. Interestingly, younger age in preschool was occasionally associated with higher verbal or perceptual skills in school only among girls.
In unadjusted analysis we found that skinfold thickness derived BF% in preschool was negatively associated with perceptual skills in school among girls. However, once adjusted for confounding factors, such as age, parental education and cognitive skill at baseline, no associations between BF% in preschool and conceptual, verbal or perceptual skills at first grade in school in boys or girls existed. Prior studies in children show rather mixed results between BF% or obesity and cognitive measures [1-4,7,29]. Although Haapala et al. [
3] reported a weak inverse association between dual-energy x-ray absorptiometry (DEXA) measured BF% with reading fluency and reading comprehension among 6-8-year-old boys once adjusted for age and parental education, these sex-specific associations were largely explained by differences in motor performance between boys with lower and higher BF% [
3]. In a longitudinal study, in boys with obesity aged 3 years was associated with worse pattern construction (visuospatial skills), but not naming vocabulary (expressive language skills) or reasoning skills (picture similarity) at 5 years after controlling for several confounding factors. “Growing out” of obesity between 3 and 5 years had a positive association with reasoning skills in girls at 5 years [
2]. In children at 8-9 years, the degree of reduction only in visceral adipose tissue during 9 months was associated with increased inhibitory control. These associations were particularly obvious among obese children [
7]. However, Davis and Cooper [
1] showed that both whole-body and abdominal body fatness were negatively associated with executive function, resistance to distraction, and gestalt processing in children with overweight aged 7-11 years after controlling for race, sex, and primary caregiver's education level [
1]. Consistent with present study, Haapala et al. [
4] demonstrated that bioelectrical-impedance-method-detected BF% at 6-9 years was not associated with non-verbal reasoning skills two years later in either sex after controlling for cognitive skills at baseline. Meanwhile, one study even suggested that not overweight but underweight predicted worse cognitive and academic performance once adjusted for early behavior, cognitive skills and socioeconomic status at early age [
29]. The variability in the results among studies might come from differences in the weight status of studied children, different methods applied to assess body composition, different cognitive tasks involved in the studies, sociodemographic differences across countries, and differences in adjustments for confounders. In current study, body fatness in preschool was not independently associated with cognitive skills in school in boys or girls.
In our study, higher CRF in preschool predicted higher conceptual skills in girls and higher perceptual skills in boys a year later. However, these associations disappeared after controlling for confounding variables, such as age, parental education and cognitive skill at baseline. Higher number of laps in 20 m shuttle run test in preschool was associated with better perceptual skills in school. However, this association disappeared once adjusted for sex and age, and controlling for maternal education and sports club attendance did not change this association [
13]. In general, there seems to be a lack of consistency in the studies investigating the associations between CRF and cognition at young age. A cross-sectional study has shown that CRF was linked to better working memory performance in boys but not in girls aged 8-11 years [
30]. Children at 9 years with high CRF levels on 20 m shuttle run test exhibited superior performance in a visual discrimination task [
9] and performed more accurately the tasks involving inhibition compared to children in the low fitness group, whereas no group differences were observed for reaction time [
10]. Longitudinal research has also demonstrated mixed findings on the relationships between CRF and cognitive skills. Specifically, Chaddock et al. [
14] showed that children with higher peak oxygen consumption at 9-10 years had superior response accuracy in compatibility conditions, yet not in incompatibility conditions on flanker test at baseline and follow-up testing one year later, compared with lower-fit children. In addition, a shorter compatible and incompatible reaction time for higher-fit children was observed [
14]. Niederer et al. [
8] reported that baseline 20 m shuttle run test results we associated with improvements in attention but not with working memory during 9-month period among children aged 4-6 years after controlling for several confounding factors [
8]. In order to explore the association between CRF and cognition, Haapala et al. [
4] applied a Raven's progressive matrices test (RPM) test, that has methodological similarities to non-verbal perceptual reasoning abilities testing applied in our study. Comparably to our results, cycle ergometer-assessed exercise capacity at 6–9 years was not associated with RPM score, nor with the change in RPM score during 2-year follow-up in boys or girls after adjustment for age, study group and baseline RPM score. In accordance with current study, Haapala et al. [
4] observed that cognitive performance at baseline was a strong predictor of cognitive functioning two years later among both sexes.
Differences in CRF and cognition testing methodology, and confounding variables included may modify the associations between CRF and cognitive performance, and hence clear up the observed diversity in the results between the studies. However, longitudinal research on the associations between other fitness components except CRF and cognitive performance is limited. Current study demonstrated that higher MF in preschool was associated with higher verbal skills in school independent of confounders in boys, with no such association observed for females. Syväoja et al. [
12] detected that MF (the sum of abdominal and upper-body muscle strength) had a positive indirect association with math outcomes through visuospatial working memory among girls aged 12-17 years, and not in boys. Additionally, the indirect path from compound fitness z-score (including the six-minute-run, SLJ and the jumping sideways task) via executive functions among 5–7-years preschoolers on mathematical and reading achievement 18 months later in school has been reported [
31]. While higher SLJ relative to fat-free mass in preschool, but not relative handgrip strength parameters were associated with superior perceptual abilities in school [
13]. Lima et al. [
11] found that at 7-9 years from all the child's PF components tested, only speed and agility fitness (50 m shuttle run test) and manual dexterity and upper-limb movement speed (box and block test) were associated with later cognitive performance on RPM at middle childhood and at adolescence, respectively after controlling for child's age, sex, pubertal status, intervention group, maternal age, family income, parity and gestational age at birth. However, 50 m shuttle run and box and block tests were no longer related to cognition in the adjustment model including prenatal, neonatal and child fitness factors. Baseline SLJ or handgrip strength or sit-ups did not predict later cognitive performance [
11].
This study has some limitations. Our study was not a randomized controlled trial; hence, we cannot infer the causal association between variables. Compared with prior studies, our relatively smaller sample size may also explain the non-significant results reported in the current study. However, the number of participants in our study was comparable to that in similar investigations in this area [
3,
5,
30,
31]. Body fat content was measured indirectly by measuring skinfolds, and although Slaughter equation for determining BF% has reasonably strong validity with DEXA in children, still the gold standard for measuring body fat composition in children is DEXA [
32]. Although we adjusted our statistical analysis for some confounding factors, the results may be confounded by other variables, such as genetic, dietary or socioeconomic factors, we cannot exclude the possibility of residual confounding factors. On the other hand, we consider longitudinal design and the application of standardized tests to assess PF and cognitive abilities as major strengths of our study.