Association of physical activity with adiposity in preschoolers using different clinical adiposity measures: a cross-sectional study

The recent World Health Organisation (WHO) report about early obesity prevention measures in young children recommends the promotion of physical activity (PA) [1]. In school-aged children and adolescents, higher levels of PA have been shown to be protective against adiposity [2‐4]. However, the association between PA and adiposity in preschool children seems to depend on the outcome measure of adiposity. Indeed, most studies that have used body mass index (BMI) to assess adiposity failed to find an association, whereas studies that used percent bodyfat (BF) as outcome measure for adiposity found evidence for an inverse association [5]. Furthermore, results for sedentary behaviour (SB) and adiposity are somewhat inconsistent [6‐8] with null associations for objectively assessed SB [6], and a positive association of SB with waist circumference (WC) in girls [7]. Therefore, more research is needed to understand the association between PA, SB and adiposity in preschool children.

The use of practice-based and inexpensive clinical measures of adiposity such as BMI, skinfold thickness, and WC is primordial for utilization in larger studies and clinical practice. Most studies have assessed the association between objectively measured PA and BMI, but a few have looked at WC and skinfold thickness. Using BMI as a proxy od adiposity, the majority of studies found no associations between PA and BMI in preschoolers [9‐17]. However, some studies found an inverse association [18‐20] whereas others found a positive association [7, 21, 22] between these two parameters. To the best of our knowledge, four studies investigated the association between PA and WC in preschool children [7, 17, 23, 24]. Two studies found no associations [17, 24], and one found an inverse relationship between moderate-to-vigorous PA and WC [23]. The fourth study found an inverse association between moderate-to-vigorous PA and WC, but only among a subgroup of obese girls (with a WC of ≥90th percentile) [7]. Three studies have investigated the association of objectively measured PA with skinfold thickness in preschool children. They showed either no association [24], or that higher levels of PA were associated with reduced skinfold thickness [23, 25]. However, many studies in preschoolers have used more complex and expensive measures like dual-energy X-ray absorptiometry (DXA) or air-displacement plethysmography (ADP) when assessing the association of PA with body fat [6, 8, 15, 17, 26]. Nevertheless, these new technologies are not always suitable in large studies. Thus, using simple and inexpensive clinical measures of total or central body fat such as skinfold thickness and WC would allow for larger community-based studies in preschool children and thus increase external validity.

It has been suggested that high-intensity PA improves body composition and prevents obesity in children, possibly by reducing stem cell differentiation into fat mass [27]. In accordance with this observation, five studies using DXA or ADP have found an inverse association between vigorous PA and body fat [6, 8, 15, 17, 26]. In addition, young children’s PA is highly intermittent, and is characterized by brief and sporadic bursts of energy [28]. Therefore, the use of short epoch lengths of 15-s intervals of accelerometer recordings have been suggested, especially for higher intensities [29, 30].

The main aim of this study was to determine the cross-sectional associations between physical activity with adiposity in a sample of 2–6-year-old preschoolers using different practice-based clinical adiposity measures: BMI, skinfold thickness and WC. We were further interested in evaluating the impact of different PA intensities and SB on adiposity.

The descriptive characteristics for all children are summarized in Table 1. Mean age was 3.9 ± 0.7 years and 47% of the participating children were girls. Overweight and obesity prevalence rates were 18 and 5% according to WHO criteria [35]. All prevalence rates were similar between girls and boys (p > 0.05).

Age, sex and wear time adjusted associations between PA and SB with BMI, skinfold thickness, and WC are presented in Table 2a. In the total sample, total PA and time spent in light, moderate, and moderate-to-vigorous PA were positively associated with BMI (p < 0.05). For example, increasing total PA by 100 cpm/day or the time spent in moderate PA by 10 min/day were both associated with a 0.1 kg/m² higher BMI. Sedentary behaviour showed an inverse relation with BMI (p < 0.05). In the NW children, total PA and all PA intensities were positively associated and SB was inversely associated with BMI (p < 0.05), while there were no associations in the OW/OB children.

There were no significant associations between the different PA measures and skinfold thickness in the total sample or in the NW children. However, in the OW/OB children, total PA was inversely associated with skinfold thickness (p < 0.05) and this inverse relationship was weak for vigorous PA (p > 0.05).

The associations between PA and adiposity after further adjustments for sociocultural and biological confounders are presented in Table 2b and show a slightly reduced sample size. The following associations remain significant after these adjustments: Moderate and moderate-to-vigorous PA remained positively associated with BMI in the total sample and total PA and all intensities except for vigorous PA remained significant in the NW children (p < 0.05). Also, SB remained negatively associated with BMI in the total population and in the NW children (p < 0.05). Regarding WC, moderate PA remained associated with WC in the total sample (p < 0.05) and in the NW children (p < 0.05).

Most previous studies using BMI to express adiposity in preschoolers found no significant association between objectively measured PA and BMI and/or BMI z-score [9‐15, 17]. Since in the included studies most of the children were NW or overweight rather than obese, it’s possible that misclassification for none or mild excess adiposity obscured the association between PA and BMI. Three studies found an inverse association with BMI [18‐20]. The reported differences between weight groups were often driven by very obese children at the “extreme” spectrum. Similar to our results, 3 studies found a positive association between PA and BMI [7, 21, 22]. For example, Espana-Romero et al. [7] reported a significant positive association between moderate-to-vigorous PA and BMI z-score in boys only with a BMI up to the 50th percentile. The authors suggested that the positive association maybe explained by greater fat-free mass among more active boys. Likewise, the positive association between PA and BMI in our study was only found in the NW children. In these NW children, BMI accuracy to measure adiposity is indeed limited and BMI differences are largely driven by differences in fat-free mass [39]. A study describing the relation of fat-free mass and fat mass to BMI, showed that in NW 5–8-year-old children fat-free mass was a stronger predictor of BMI than fat mass [40]. Thus, BMI reliability is uncertain for this age group to define adiposity, especially in the non-obese.

We found that PA was negatively associated with skinfold thickness. These associations were observed in the OW/OB children. Similarly, several studies using DXA or ADP found an inverse association between vigorous PA with body fat [6, 8, 15, 17, 26]. However, it is not always suitable to use DXA or ADP in large epidemiological studies because of cost, irradiation exposure, and limited availability outside research settings [41]. Alternative methods such as skinfold thickness are more practical, especially as they highly correlate with DXA in children (r = 0.90) [42]. Similarly, two previous studies found an inverse association between objectively measured PA and skinfold thickness in preschool children [23, 25]. Skinfold thickness could thus represent a very useful and simple measure of body fat in community-based studies and clinical practice involving preschoolers.

The particular importance of vigorous PA has been shown in previous studies using DXA. In a large cohort of British preschoolers, vigorous PA, but not other PA measures, was inversely associated with total and abdominal adiposity [6]. Similarly, several studies showed that children with higher vigorous PA had either lower body fat or were less likely to gain body fat compared to children with low vigorous PA levels [8, 20, 26]. Our results showed similar trend using skinfold measures. Therefore, the role of vigorous PA in the prevention of early childhood obesity should be further investigated in future studies to bring more evidence.

As visceral adiposity is related to numerous cardiovascular and metabolic risk factors [43], we evaluated the use of WC as a potential measure of central body fat. We found a positive association between moderate and moderate-to-vigorous PA with WC and a negative association between SB and WC in the total population and in the NW children, while no association was found in the OW/OB children. To our knowledge, four previous studies investigated this association in preschoolers, but children in three of these studies had higher WC measures compared to our sample. In contrast to our results, Leppännen et al. [17] and Metcalf et al. [24] found no association between PA and WC, noting that the former used wrist placement to capture PA. A third study was done in 2-year old toddlers, and found that moderate-to-vigorous was inversely associated with WC [23]. The fourth study did not find an association between PA and WC in the total sample. However, among a subgroup of obese girls (≥90th WC percentile), they found a positive association between SB and WC and a negative association between moderate-to-vigorous PA and WC [7]. These discrepancies in results could be explained by differences in WC or prevalence of OW/OB between studies and/or by differences in the age of the children as the shape of the children and the WC changes in toddlers. The overall results of our data and the previous studies could suggest that WC might be an interesting and distinctive measure of visceral obesity in preschoolers, but only in the presence of obesity. Furthermore, it is often difficult to reliably measure WC in young preschoolers because of frequent small changes in posture, abdominal inflation or deflation, low abdominal musculature, and as they get easily tickled. This might even be more difficult in toddlers.

In conclusion, we observed that in predominantly NW preschoolers PA was positively related to BMI and WC. This association was only observed in the NW, but not in the OW/OB children. Physical activity, particularly total and vigorous PA, were inversely related to skinfold thickness in the OW/OB children. BMI and WC alone do not adequately reflect adiposity in this population, and BMI is possibly even an indicator of fat-free mass in healthy NW preschool children. Therefore, skinfold thickness or other direct measures of body fat are needed for research and clinical practice. Also, more evidence is needed to understand the role of vigorous PA in the prevention of obesity in the early years. Prospective study designs are needed to further examine the association between PA and body composition in young children and to define the best measures to assess both PA and adiposity.