Public Health Division, The Children's Memorial Health Institute, Warsaw, Poland

Department of Research, The Children's Memorial Health Institute, Warsaw, Poland

Department of Nephrology and Dialysis, Polish Mother's Memorial Hospital Research Institute, Łódź, Poland

Anthropology Division, Department of Paediatrics, The Children's Memorial Health Institute, Warsaw, Poland

MRC Centre of Epidemiology for Child Health, UCL Institute of Child Health, UK

Abstract

Background

The growth of children is an indicator of health and society's wellbeing. Growth references are useful in monitoring a child's growth, which is a very important part of child care. Poland's growth references are not updated regularly. Although several growth reference ranges have been developed in Poland over recent years, sampling was restricted to urban populations of major cities. The aim of this study was to assess how well Polish children match with, or diverge from, regional charts and to compare them with international growth references.

Methods

Four Polish and two international (WHO 2007 and USCDC2000) growth references were used to calculate the height, weight and BMI z-scores in a recent, large, population-representative sample of school-aged children and adolescents in Poland. The distributions of z-scores were analysed with descriptive and inferential statistical methods.

Results

Mean height z-scores calculated with the use of the WHO 2007 and USCDC2000 references were positive and significantly different from zero over the entire age range. The mean height z-score was closest to zero in the Poznan reference for boys (0.05) and Warszawa reference for girls (0.01). Median weight z-scores were positive under all weight references over the entire age range with only the exception of 18-year-old girls' weight z-score calculated relative to USCDC2000. Median BMI z-scores were positive in males in early childhood, decreasing with age. In the case of girls, the median BMI z-score calculated using WHO 2007 and USCDC2000 was close to zero in early childhood, decreased in adolescents and reached minimum values at age 18 years. Median BMI z-scores calculated with the use of the Lodz reference fluctuated between 0.05 and 0.2 over the studied age range.

Conclusions

In this contemporary sample of Polish school-aged children, distributions of height, weight and BMI differed from those of children from the international growth references. These differences should be considered when using the references. There exist certain limitations to the analysis of height, weight, and BMI z-scores when Polish regional references are used.

Background

A child's growth is an indicator of health

Poland does not have its own growth reference data that have been constructed with a representative national sample of children and adolescents and regularly updated, for example, in ten-year intervals. Although several growth references have been developed in Poland in recent years, sampling was restricted to the urban populations of major cities and, in the case of some of the references, sampling was not random. Due to the limited representativeness of Polish local growth charts, the recently updated World Health Organization (WHO) international height, weight, and BMI references for children and adolescents aged 5-19 years (WHO 2007)

Methods

The analyzed data were collected in the course of the OLAF study (PL0080) in which the reference blood pressure ranges were elaborated for Polish children and adolescents. The present analysis was limited to a cross-sectional sample of school children who were examined between November 2007 and March 2009. Study participants (children and adolescents 6.5-18.5 years of age) were randomly selected using two-stage sampling. Primary units (schools) were sampled from an all-schools-in-Poland sampling frame; sampling was stratified by urban/rural area. In the second stage, all pupils in the required age ranges within the sampled schools comprised the sampling frame. Pupils in schools were selected for the survey by stratified random sampling, the stratification variables being classes. All subjects and their parents (in the case of subjects under 18 years of age) gave their informed consent to participate in the study (subjects over 16 years of age and parents gave written consent). Approval of The Children's Memorial Health Institute Ethics Committee to conduct the study was obtained before the study commenced.

The medical history of the study participants, including past and present diseases, as well as medications used, was taken from the parents. The general health status of each subject was assessed by a physician. Height was measured in duplicate (in case of a difference between measurements exceeding 4 mm, a third measurement was taken) using a stadiometer (SECA 214) in the standing position (with no shoes), to the nearest millimeter. Body weight was recorded in light underwear to the nearest 0.05 kg, using a digital, medical scale (Radwag WPT 100/200). Body mass index was calculated as body weight divided by height in meters squared. The exact ages of the participants were calculated from birth and examination dates. There were 13 015 children and adolescents enrolled in the study (response rate 0.70). In our analysis we excluded height measurements from the OLAF study if the child was recorded as having a posture deficiency, genetic syndrome, cancer, or other chronic disease (230 cases). The weight measurements were missing in the OLAF study sample for 1 boy and 3 girls. Thus, 6227 boys and 6558 girls aged 6.5 to 18.5 years from the OLAF study were included in the analysis of height and 6226 boys and 6555 girls in the analysis of weight and BMI. One third (33%) of all of the children and adolescents in the sample lived in rural areas.

Four Polish growth reference ranges were compared with those of the WHO 2007

All six growth references under consideration were used to calculate height z-scores. Only three weight and BMI references: WHO 2007, USCDC2000, and Lodz, were used to calculate z-scores for the OLAF sample, as the Warszawa and Poznan weight and BMI references did not account for skewness of weight and BMI, while Krakow did not provide the L, M, S parameters for the calculation. Z-scores relative to the USCDC2000 were calculated with the SAS code downloaded from the CDC web site

A child was considered stunted (low height-for-age) if the height-for-age z-score was below -2. The means and standard deviations of z-scores of height and percentage of stunted children were calculated separately for each sex and for each height-for-age chart. The normality of distributions of estimated height z-scores were assessed with the Kolmogorov-Smirnov test. Differences from zero of the means of height z-scores were analyzed with Student's t-test for the whole age range (7 to 18 years of age) and separately for each year of age. Differences of height z-score means between reference ranges were analyzed with the paired t-test. Differences in the distribution of stunting according to sex and reference range were tested with the McNemar test. Differences in the prevalence of stunting between genders were tested with the chi square test. Due to the skewed nature of weight and BMI distributions, medians and inter-quartile ranges (IQRs) are presented by sex and age. Data were processed with the MsAccess database and MsExcel spreadsheet. All analyses were conducted with SAS 9.1 for Windows.

Results

Additional file

**Table S1 - Polish children and adolescents (the OLAF study sample) height, weight, and BMI by sex and age**. table provides descriptive statistics of height (mean, SD, min., max.), weight and BMI (median, IQR, min., max.) by sex and age.

Click here for file

Table

Polish children and adolescents (the OLAF study sample) mean height z-scores relative to the six references and statistical testing

**reference**

**N**

**Mean**

**95% CI**

**SD**

**p**

**Min**.

**Max**.

**boys**

**WHO 2007**

6227

0.50

0.47 0.52

0.99

<.0001

-3.03

4.37

**USCDC2000**

6227

0.45

0.43 0.48

0.97

<.0001

-2.79

4.02

**Krakow**

6227

0.17

0.15 0.20

1.02

<.0001

-3.37

3.85

**Warszawa**

6227

0.08

0.05 0.11

1.06

<.0001

-3.63

4.07

**Poznan**

6227

0.05

0.03 0.08

1.09

0.0001

-3.73

4.08

**Lodz**

6227

0.14

0.11 0.16

1.04

<.0001

-3.66

3.88

**girls**

**WHO 2007**

6558

0.34

0.32 0.37

0.95

<.0001

-3.47

4.29

**USCDC2000**

6558

0.33

0.31 0.36

0.94

<.0001

-3.03

3.95

**Krakow**

6558

0.09

0.07 0.12

1.03

<.0001

-3.34

4.33

**Warszawa**

6558

0.01

-0.02 0.03

1.05

0.6338

-3.73

4.19

**Poznan**

6558

-0.06

-0.09-0.03

1.11

<.0001

-3.86

4.57

**Lodz**

6558

0.15

0.13 0.18

1.03

<.0001

-3.49

4.43

The mean height z-score difference from zero was systematically positive over the whole age range (both boys and girls) (Figure

**Table S2 - Polish children and adolescents (the OLAF study sample) 95% CIs of height mean z-scores by age according to the growth chart**. data provided represent 95% confidence intervals of boys' and girls' height mean z-scores according to the six compared growth charts.

Click here for file

Polish children (the OLAF study sample) means of height z-scores relative to six reference growth charts

**Polish children (the OLAF study sample) means of height z-scores relative to six reference growth charts**.

With the exception of z-scores calculated using the Lodz height reference, mean height z-scores were higher in males compared with females for all other tested growth charts. The difference between genders was statistically significant (p < 0.01) for the WHO 2007, USCDC2000, Warszawa, Krakow, and Poznan growth charts.

The differences between mean z-scores for height were statistically significant (p < 0.0001) compared with the WHO 2007 and USCDC2000 charts for each of the Polish growth references and between each pair of Polish reference growth charts.

The use of WHO 2007 and USCDC2000 height-for-age references resulted in an estimated stunting (height-for-age z-score less than -2) prevalence of below 1% among OLAF study participants. Figure

Prevalence of stunting among Polish school children (the OLAF study sample) using the six references

**Prevalence of stunting among Polish school children (the OLAF study sample) using the six references**. Stunting was defined as height z-scores <-2.

The distributions of weight and BMI z-scores calculated relative to the WHO 2007 and Lodz growth references were right-skewed, whereas the distributions of the OLAF study sample weight and BMI z-scores calculated relative to the USCDC2000 growth references were close to normal with a negative skew tendency. The WHO 2007 does not provide weight reference values for children older than 10 years, thus z-scores were calculated for ages from 7 to 10 years only. Table

Polish children and adolescents (the OLAF study sample) median weight and BMI z-scores relative to the WHO 2007, USCDC2000, and Lodz growth references

**reference**

**weight z-score**

**BMI z-score**

**N**

**Min**.

**Median**

**Max**.

**IQR**

**N**

**Min**.

**Median**

**Max**.

**IQR**

**boys**

**WHO 2007**

1837

-3.08

0.52

5.92

1.72

6226

-4.18

0.11

5.91

1.65

**USCDC2000**

6226

3.71

0.30

3.67

1.38

6226

-5.03

0.06

2.96

1.51

**Lodz**

6226

-3.04

0.10

3.71

1.31

6226

-3.93

0.03

3.64

1.34

**girls**

**WHO 2007**

1778

-3.17

0.29

5.06

1.57

6555

-3.90

-0.07

4.54

1.41

**USCDC2000**

6555

-4.36

0.10

3.27

1.26

6555

-4.39

-0.06

2.73

1.32

**Lodz**

6555

-4.40

0.18

3.94

1.33

6555

-4.24

0.14

3.62

1.30

Under all three weight references, over the entire age range, median weight z-scores were positive, with only the exception of 18-year-old girls' weight z-score relative to the USCDC2000. In both sexes, the median weight z-score calculated using the WHO 2007 reference was higher compared with the USCDC2000 and Lodz weight references and, in contrast with the USCDC2000 and Lodz, the median weight z-scores were increasing through childhood (Table

Polish children (the OLAF study sample) median weight z-scores calculated using three weight-for-age references

**Polish children (the OLAF study sample) median weight z-scores calculated using three weight-for-age references**.

Median BMI z-scores were positive in males in early childhood, decreasing at later ages, and crossing into negativity at the age of 10 and 13 years in the case of z-scores calculated using the Lodz, WHO 2007, and USCDC2000 references, respectively (Figure

Polish children (the OLAF study sample) median BMI z-scores calculated using three BMI-for-age references

**Polish children (the OLAF study sample) median BMI z-scores calculated using three BMI-for-age references**.

Discussion

Periodic health examinations of children and adolescents have been a part of preventive child health programmes in many countries, including Poland. At the examination, it is required to assess centiles of height, weight, and BMI for a child. Screening children's height seems to be cost-effective in detecting height-related disorders and for the detection of other undiagnosed conditions

Our findings demonstrate differences between Polish regional growth charts and the charts based on samples from the USA. The WHO 2007 reference for children aged 5-19 years is based on the 1977 National Center for Health Statistics NCHS/WHO data set, which is used in part in the USCDC2000 reference, but the CDC reference includes some more recent data sets as well. For this reason, the mean z-scores shown in Figure

The observed differences may be attributed to both socioeconomic and genetic factors

Experts agree that children and adolescents grow similarly when exposed to similar external conditions of growth. The influence of genetic factors might be postulated to some degree as well

Secular trends, body composition, and sexual maturation are critical determinants in the interpretation of anthropometric measures. These constitute the main obstacles in the development of both local and universal growth charts. The OLAF study was designed closer to WHO guidelines to be more applicable for the Polish population. Some of the shortcomings mentioned above cannot be ruled out, however. The data from the Czech Republic reported by Vignerova showed that country-wide growth charts should be updated every 10 years with no significant changes in Body Mass Index or weight-for-height normal values

It is noteworthy that the use of local references constructed with mean and SD only, without accounting for weight and BMI skewness, will give misleading results. For example, the calculation of BMI z-scores using the Warszawa (the most popular in Poland), Poznan and Krakow references resulted in several values greater than six (data not presented in this paper), which are considered implausible by others

In girls from the contemporary sample of Polish children and adolescents, median BMI z-scores calculated using international BMI-for-age references, were closer to zero than with the Lodz standard. In boys, all three compared BMI standards revealed an interesting pattern of median BMI z-score changes from childhood into adolescence. In the age range from 7 to 9 years, median z-scores were positive (more so in the case of the international reference compared with the Polish reference), decreasing into adolescence (age from 13 to 18 years). Much the same pattern of BMI change from childhood to adolescence has recently been reported for three historical British national birth cohorts

Conclusions

In this contemporary sample of Polish school-aged children and adolescents, distributions of height, weight and BMI differed from those of children in the international growth references. These differences should be taken into consideration when the references are applied. The analysis of height, weight, and BMI z-scores using Polish regional references has certain limitations. Determining the validity and applicability of an existing growth reference and developing a reference based on a national representative sample are important public health functions.

Abbreviations

BMI: body mass index; BCPE: The Box-Cox power exponential; CDC: Centers for Disease Control and Prevention; CI: confidence interval; IQR: inter-quartile range; max.: maximum; min.: minimum; NCHS: National Center for Health Statistics; SD: standard deviation; WHO: World Health Organization;

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

ZK conceived the study, conducted field examinations, did statistical analyses, and drafted the manuscript. ML conceived the study and drafted the manuscript. MT conducted field examinations and drafted the manuscript. AR conceived the study, conducted field examinations and drafted the manuscript. KB conceived the study, conducted field examinations and did statistical analyses. AG conducted field examinations, did statistical analyses. AS conducted field examinations and drafted the manuscript. BG collected, assembled and processed data, and did statistical analyses. EN collected, assembled and processed data. HP conceived the study, did statistical analyses and drafted the manuscript. All authors contributed to the drafting and revisions of the manuscript. All authors read and approved the revised manuscript.

Acknowledgements

This publication used the OLAF study data. The OLAF study was supported by the European Economic Area Financial Mechanism, the Norwegian Financial Mechanism, and the Ministry of Science and Higher Education of Poland; grant number: PL0080. The following investigators participated in the OLAF study team and conducted study procedures in the regions: Białystok: Agnieszka Rybi-Szumińska, Michał Szumiński, Katarzyna Taranta-Janusz, Edyta Tenderenda, Anna Wasilewska; Bydgoszcz: Beata Jasińska; Gdańsk: Piotr Czarniak, Dominik Świêtoń, Przemysław Szcześniak; Kraków: Monika Miklaszewska, Anna Moczulska, Katarzyna Wilkosz, Katarzyna Zachwieja, Iwona Ogarek; Lublin: Marek Majewski, Aleksandra Sobieszczańska-Droździel, Izabela Szlązak, Paweł Szlązak, Małgorzata Zajączkowska; Łódź: Anna Półtorak-Krawczyk, Monika Pawlak-Bratkowska; Opole: Danuta Gmyrek; Poznań: Tomasz Krynicki, Jolanta Sołtysiak; Toruń: Roman Stankiewicz, Sława Zbucka; Warszawa: Robert Pietruczuk, Jan Szpor; Wrocław: Jacek Kleszczyński, Magdalena Naleśniak, Anna Wawro, Irena Wikiera-Magott, Danuta Zwolińska; Zabrze: Piotr Adamczyk, Tatiana Augustyn-Iwachów, Beata Banaszak, Omar Bjanid, Katarzyna Broll-Waśka, Aurelia Morawiec-Knysak.

Special thanks to the WHO Anthro Team for providing SAS code for the height, weight and BMI z-scores calculations.

Pre-publication history

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