Gender Differences in Relationship between Fat-Free Mass Index and Fat Mass Index among Korean Children Using Body Composition Chart

The recent increase in the prevalence of childhood obesity has been well-established around the world, including South Korea and concerns about the excess adiposity in childhood are increasing.1-3

Body mass index (BMI) is widely used as a surrogate marker of fatness.4, 5 The statistical definition of BMI is that it adjusts weight (WT) for variability in height (HT)6:

BMI = WT/HT² (equation 1)

However, it has limitations and should be used with caution when assessing children as the change in BMI does not reflect the change in adiposity during childhood.7-9 The simplest model of body composition divides WT into fat mass (FM) and lean mass (LM) components:

WT=LM+FM (equation 2)

VanItallie, et al.10 proposed that equations 1 and 2 could be combined in order to divide BMI into its fat and lean components:

BMI=fat-free mass index (FFMI)+fat mass index (FMI) (equation 3)

In general, lean mass index (LMI) and FFMI can be used to mean the same thing. Body composition is difficult to measure in children because accurate techniques require participant compliance and are only available in specialized research institutions. Recently, more practical methods such as bioelectrical impedance analysis (BIA) for epidemiologic studies and clinical practice are available.11 Although increases in FM are generally observed in boys and girls, increases in BMI throughout childhood are driven primarily by increases in FFM, particularly in boys.12 The relationships between FFMI and fat mass index (FMI) can be delineated on the body composition chart. Hattori13 have devised a chart, based on FFMI and FMI on the x and y axes, respectively. It has advantage of expressing both aspects of body composition in common units.14 The effects of variation in FFM and FM on BMI can be explored using body composition chart, and various combinations of FFMI and FMI can result in a similar BMI.15

The objective of this study was to determine the gender difference in the relationship between FFMI and FMI with increasing age by applying body composition chart analysis on Korean elementary school children.

The gender difference in the relationship between FFMI and FMI was demonstrated in this study. Girls are generally known to have relatively more fat. In contrary to such expectation, however, this study proved to be not always true, especially in children. Girls did not have higher levels of fat mass or percentage body fat than boys did.

Although boys showed higher BMI, FM was not different between boys and girls. The higher values of FFM in boys could explain the difference of BMI in this age group. Even though body fat percent, FFMI and FMI were not different between boys and girls, the relationships between FFMI and FMI were different by gender using body composition chart (Fig. 1).

In subgroup analysis by grade, there was a significant gender by FFMI interaction, indicating that the boys gain FFM more rapidly compared to girls during 6th grade period. In subgroup analysis by gender, grade by FFMI interaction was significant in boys, indicating that FMI vs. FFMI relationship differed according to the grade of the boys. FFM gain was more accelerated in 6th grade boys, indicating that the magnitude of this gender difference in elementary school children increased with grade.

This study result also showed the limitation of BMI in children, that has already been known,7-9 and the needs for FMI and FFMI chart plotting.

Over the last two decades, the importance of body composition measurement according to nutritional status has increasingly been recognized, especially in pediatric age group.15 Tracing body composition in children is not only to detect excessive fatness but also concerns in the acquisition of FFM. FFMI and FMI are used to express both aspects of body composition in common units. It should be kept in mind that the same BMI value can result from various combinations of FMI and FFMI, and the range of variability in the levels of both FM and FFM is wide in growing children.12, 16 The accuracy of BMI among children varies, depending on according to the degree of fatness: BMI fits for relatively heavy weight children (e.g., BMI-for-age≥85th P) but it does not for light weight children.17

Previous studies indicated the importance of FMI and FFMI, however, there was no result on the difference between boys and girls.18, 19 FFMI in boys and girls, gender difference and year in year out these were very diverse between childhood and adolescent.20-22

In the present study, we found definite differences in FFMI and FMI relationship between boys and girls as they grew. We also found the time of change in body composition through slope variation, a marked change occurring in 6th grade boys (Table 2).

Our results have several limitations. In the result of this research, difference by the grade was marked and as the Table 1 suggests average grade age rarely goes out the actual school year age. Moreover, in order to find the overall general differences between children, pubertal stage should be included, however, the present study was limited within a single elementary school.

In summary, this study demonstrates that body composition chart containing FFMI and FMI is appropriate for expressing both aspects of body composition in children. Gender difference in the direction of change of FFMI and FMI relationship is evident in children. Thus, the use of this chart should highly be encouraged, and extensive studies to establish gender and age specific cut-off values that are valid for the health risk-based definitions of pediatric obesity should follow. This task is important not only for the research/public health, but also imperative in clinical settings.