Department of Economics and Social Sciences Dalarna University
D level essay in statistics, 2008
“Defining obesity for children by using Body Mass Index”
Authors
Asrar Hussain Mahdy1 Iram Bilal2
SUPERVISED By
Johan Bring
Jun 12, 2008.
1 asrar_statistician@hotmail.com (800425-T057)
2 udasmausam@hotmail.com ()
Content
ABSTRACT... 4
Objective: ... 4
Method: ... 4
Results:... 4
1. INTRODUCTION ... 5
1.1 Background ... 5
1.2 The use of Body Mass Index (BMI) ... 5
1.3 Complete history of methods used for childhood obesity and overweight... 7
1.4 Limitations for the percentiles growth curves developed in year 2000 ... 10
1.5 The role of WHO ... 11
1.6 Comparison of the centile based references and the ISO –BMI cut-off values... 12
1.7 Objectives of the study... 13
2. MATERIALS AND METHODS: ... 14
2.1 Data ... 14
2.2 The LMS method ... 14
2.3 The ISO-BMI... 15
2.4 Comparison of growth curves ... 15
3. RESULTS ... 17
3.1 Tabulation of ISO-BMI references (17.5, 25, 30, 35) for NCHS 2000, WHO 2007 and Swedish data 2001 ... 17
3.2 Percentile based BMI curves (5th, 85th, 95th, 99th) and ISO-BMI Curves (17.5 kg/m2, 25 kg/m2, 30 kg/m2, 35 kg/m2)... 19
3.3 Tabulation of Ranges of Percentile based growth curves and ISO-BMI based growth curves. ... 23
3.4.Cumulative Range Plots for Variation in Growth Curves ... 24
3.5.Interpretation of results ... 26
DISCUSSION: ... 29
CONCLUSION:... 33
REFERENCES: ... 34
Web References ... 35
List of Tables
Table 1.2.1: ... 7
Table 1.2.2: ... 7
Table 3.1.1: ... 17
Table 3.1.2: ... 18
Table 3.1.3: ... 18
Table 3.3.1: ... 23
Table 3.3.2: ... 23
List of Figures Fig 1.3.1: ... 8
Fig1.3.2: ... 10
Fig 1.6.1: ... 12
Fig 3.2.1 (a):... 19
Fig 3.2.1 (b):. ... 19
Fig 3.2.2 (a):... 20
Fig 3.2.2 (b): ... 20
Fig 3.2.3 (a):... 21
Fig 3.2.3 (b): ... 21
Fig 3.2.4 (a):... 22
Fig 3.2.4 (b): ... 22
Fig 3.4.1 : ... 24
Fig 3.4.2 : ... 24
Fig 3.4.3 : ... 25
Fig 3.4.4:. ... 25
Used abbreviations
BMI = Body Mass Index
CDC = Center for Disease Control and prevention NCHS = National Center for Health Statistics
NHANES = National Health and Nutrition Examination Survey IOTF = International Obesity Task Force
ABSTRACT
Objective: Body Mass Index (BMI) is recommended for assessing body fatness in adults.
For children there is a problem with BMI, since there are no universally accepted cut-off limits.
However, often, percentiles or multiples of standard deviations are used. Bring and Forslund have suggested using ISO-BMI instead, which is a way of standardizing the BMI-values. We have compared ISO-BMI references against BMI centile references.
Method: Growth curve references values of BMI for age published by CDC 2000, WHO 2007 and by Swedish data were used for comparing time to time variation within the percentile based BMI reference curves and cutoff value based ISO-BMI (Swedish data).
Results: The ISO-BMI cutoff value based references are founded to be more precise as compared to the percentile based BMI references. ISO-BMI was found to be less population or time dependent.
Keywords: BMI, ISO-BMI, Growth Curve,
1. INTRODUCTION
1.1 Background
The obesity epidemic is a worldwide phenomenon (Popkin and Doak CM, 1998). Obesity, according to the medical terminology, is a chronic condition describing excess body weight in the form of fat. It results when the size or number of fat cells in a person's body increases.It plays a major role in the risks of the chronic diseases such as hypertension, type II diabetes, coronary artery disease, dyslipidemia (high cholesterol problems), heart attacks, stroke, gallbladder diseases, osteoarthritis, sleep apnea, respiratory problems and some types of cancers (eudiometrical, breast and colon)(CDC, 2008).
1.2 The use of Body Mass Index (BMI)
In the past decades, National Health and Nutrition Examination Survey (NAHNES) suggested different anthropometrics1 methods to measure the body fatness (NHANES, 1988). It involved different types of measurements i.e., arm fold, skin fold waist, hip and many others. The anthropometrical measures were not implemented due to the complexity. Despite of the anthropometrics measures, Body Mass Index (BMI) is recommended by the International Obesity Task Force (IOTF)(Dietz and Bellizzi, 1994). BMI is used worldwide to measure the fatness of the body. With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared.
) (
) (
2 m Height
kg Weight BMI =
Since height is commonly measured in centimeters, thus an alternate calculation formula, dividing the weight in kilograms by the height in centimeters squared, and then multiplying the results by 10,000, can be used. For adults Word Health Organization (WHO) have developed
1 “ Technique that deals with the measurement of the size, weight, and proportions of the human body”
three different levels of obesity (obesity degree I = BMI 30, obesity II (or severe obesity) = BMI 35, obesity III (or very severe obesity) BMI 40. They have also developed 3 different degrees of underweight; underweight BMI 18.5, severe underweight BMI 17 and very severe underweight BMI 16.(WHO, 2008) (Table1).
However for determining the child obesity, WHO did not provide the standard cut - off values.
The children have a different growth pattern as their environment, lifestyle and nutrition affects their body composition and they don’t have any definite shape. Instead of the age-specific BMI standards, growth curves with percentile are used for defining obesity in the children.
In order to determine the obesity of children, National Center for Health Statistics (NCHS) has provided percentile based reference ranges. According to NCHS children’s BMI value less than 5th percentile are assumed to be underweight, and similarly, 5th percentile to less than 85th percentile are healthy weight, 85th to less than the 95th percentile are at risk of overweight, and equal to or greater than the 95th percentile are overweight. (Table 2)
As compare to the past few decades, there is an increase in the obesity prevalence of children and adolescents across the world. According to facts and figures by WHO in 2005, it was indicated that globally there were at least 20 million children under the age of 5 years and approximately 1.6 billion adults (age 15+), were overweight and at least 400 million adults were obese. WHO further projects that by 2015, approximately 2.3 billion adults will be overweight and more than 700 million will be obese (WHO, 2008).
Different investigators from all over the world focused on the dramatic increase in the prevalence of obesity in different parts of the world. The prevalence of obesity in children is much higher than that of adolescents. For instance, the prevalence of obesity among children has increased to 21% in USA till 1998 (Strauss and Pollack, 1998). In Sweden, the prevalence of overweight among the boys and girls (aged 10-16 years) was reported between 21.7 %and 13.3 % in 2001 while the prevalence of obesity was between 2.9 % and 6.2 % respectively (Ekbolm et al., 2001).
Table 1.2.1: The International Classification of adult underweight, overweight and obesity according to WHO BMI cut-offs.
Classification BMI(kg/m2) Principal cut-off points Under weight <18.50
Severe thinness <16.00
Moderate thinness 16.00 - 16.99
Mild thinness 17.00 - 18.49
1ormal range 18.50 – 24.99
Overweight ≥25.00
Pre obese 25.00 - 29.99
Obese ≥30.00
Obese class I 30.00 - 34-99
Obese class II 35.00 - 39.99
Obese class III ≥40.00
Source: Adapted from WHO, 1995, WHO, 2000 and WHO 2004.
These standards cut off BMI values are independent of age and are same for both sexes.
Table 1.2.2: BMI-for-age weight status categories corresponding to percentiles.
Weight Status Category Percentile Range
Underweight Less than the 5th percentile
Healthy weight 5th percentile to less than the 85th percentile At risk of overweight 85th to less than the 95th percentile
Overweight Equal to or greater than the 95th percentile Source: Adapted from NCHS.
1.3 Complete history of methods used for childhood obesity and overweight
In 1977, National Centre for Health Statistics (NCHS) developed different growth charts to assess the adequacy of the child growth. These growth charts were only based on the US population and were developed with the help of the percentiles curves. They were widely used as a clinical tool by the pediatricians and health professionals at that time. The 1977 growth charts were also adopted by World Health Organization for international use. The growth curves weight
for age curves, weight for height curves and many others (CDC, 2008). For example, figure 1 depicts the 1977 growth charts weight for age charts for girls from birth to 36 months.
Fig 1.3.1: Weight for age growth charts for the girls from (0 to 36 months of age) developed in 1977 by the National Centre of Health Statistics (NCHS)
In year 1994, the international Obesity Task Force (IOTF) was established and summarized that BMI offers a reasonable measure of fatness in children, and to provide a consistent assessment for the obesity across the life span, IOTF has suggested that, the cut point should be selected to identify obesity in the children and should be agree with that used to identify obesity in the adults (Dietz and Bellizzi, 1994).
In the year 2000, Centre for Disease Control and prevention (CDC) published growth charts for the children (aged 2 to 19 years) based on the US population and by revising some previous growth charts developed by CHS in 1977 with an additional chart of BMI for age for both boys and girls. These growth curves were generated with the z scores (from – 2 SD to 2 SD) and
percentiles by using the LMS method. The final LMS model for generating the centile curves was developed by (Cole and Green, 1992) and is briefly described in methodology of the essay. The formula of the LMS method is as followed
z L
S L M BMI
1
)
*
* 1
= ( + ……….. 1
Some examples from the NCHS 2000 BMI for age growth charts are described as follows.
For a 10 year old boy having values such asL=2.765,M =16.646, S =0.120 and Z =−1.64, substituting these values in equation 1 which givesBMI =14.22, which is at 5 percentile on the th curve. Similarly, for a 6 year old girl, L=3.225,M =15.213,S=0.093and Z =1.64 gives BMI=18.83, that lies at the 95th percentile on the growth curves.
Fig1.3.2: BMI for age growth charts for boys from (2-20 years of age) in 2000 developed by Center for Disease Control and prevention (CDC) based on the US Population
1.4 Limitations for the percentiles growth curves developed in year 2000
The percentiles growth curves developed in the year 2000 had certain limitations. Firstly, the LMS values and the related percentiles were only based on the US population, so could not implement to the children from the different parts of the world. Secondly, the results of BMI growth curves presented by CDC were different from the latest NHANES III surveys in USA, which showed that the children in USA were more obese and overweight than presented in the
2000 growth curves. The CDC growth curves required very large data sets and were obtained by combining several older studies (NHES II, NHANES I and II). The main drawback of combined data was the older data shows less obesity. That lowered the CDC curves performance at the highest percentiles (Kuczmarski et al, 2000).
Some examples of abnormal BMI values at the highest percentiles are as follows: For a 10 year old boy having values such asL=2.765,M =16.646, S =0.120 and Z =3, substituting these values in equation 1 which givesBMI =129.52, which lies at 99.86th percentile, is an abnormal value. For a 11 year old boy having values such asL=2.5905,M =17.2008, S =0.1267 and
=3
Z , substituting these values in equation 1 which givesBMI =86.89, which is at 99.86th percentile on the curve.
1.5 The role of WHO
World health organization has gathered the expert pediatricians from different parts of the world and proposed a Multi-centre Growth Reference Study (MGRS), with objective to assess the growth of the infants and the children and to provide the new international references (Onis et al., 1990). The MGRS expert committee has worked on all the aspects of the child growth in different parts of the world like, the baseline characteristics of the child growth involving breastfeeding and the non smoking habits of the mothers (WHO MGRS, 2006), development of WHO growth references for school aged children and adolescents using the LMS method (Onis et al, 2007) and many others etc. The information of the related surveys have been gathered from different parts of the world , namely India, Oman, USA, Brazil, Ghana and Norway.
In 2007, World Health Organization presented the new growth references for the child growth by summarizing the workings of MGRS, and published the data BMI for agewith percentiles curves and the LMS values. The new references were also based on the LMS method (Cole and Green, 1992).
Some examples of BMI values for the WHO 2007 references are as follows using (1). For a 7 year old boy having values such asL=−1.246,M =15.4832, S =0.0906 and Z =−2.326, substituting these values in equation 1 which givesBMI =12.8, which is at st1 percentile on the curve.
The main limitations regarding the new growth references WHO 2007 are, time to time variation in the distribution of the BMI values and the distribution of the BMI has different meanings in different countries.
1.6 Comparison of the centile based references and the ISO –BMI cut- off values
The BMI growth curves passing through the cut off values like 25kg/m2, 30kg/m2 at age 18 years are usually termed as ISO-BMI growth curves. The concept of ISO-BMI references is basically a way of standardizing the BMI cut off values at age 18 years. The idea of ISO-BMI was firstly developed in establishing a standard definition of childhood obesity worldwide (Cole TJ et al., 2000). They proposed the cut off values of BMI for children based on the standard cut off points of 18 kg/m2 and 25 kg/m 2 based on the centile curves from an international survey of six large nationally representative cross sectional growth studies (figure 3)
Fig 1.6.1: ISO-BMI at 25 kg/m2 cut-off for age growth charts for boys and girls (0-20 years of age) at age 18 of six large data bases across the world.
After the initial concept of ISO-BMI, Bring and Forslund (2003) proposed a study titled “The use of ISO-Body Mass Index as a marker for nutritional status in children, youth and adults” (in progress), which was based on ISO- BMI, cut off values rather than the percentiles that were often used in the previous. ISO-BMI cut-off values were basically the cut off values of BMI to define obesity, over weight, normal weight and under weight defined by WHO for the adults.
These cut off values were taken standard at age 18 as these ISO-BMI cut-off values were not associated with any type of statistical data i.e. obese (ISO-BMI ≥30), overweight (ISO-BMI
≥25), normal weight (ISO-BMI ranged 18 to 25) and underweight (ISO-BMI 18≤ ). By using ISO-BMI cut off, Bring and Forslund were linking the known cut offs for adults and children.Their BMI reference lines were derived based on Swedish data (Karl berg et al 2001).
Firstly, they derived the percentile-value corresponding to each BMI-value at age 18. Then calculations were made on the BMI-value corresponding to a specific percentile each age, e.g.
the percentile corresponding to a BMI of 25kg/m2at age 18 corresponded to a BMI of 19.5 for a boy at age 11. Hence, the ISO-BMI OF 19.5 for a 11 year old boy corresponds to ISO-BMI 25.
They concluded that advantage of using ISO-BMI values over the percentile based BMI values was the ISO-BMI values were not affected by the continuous changes in the prevalence of obesity worldwide. Hence Bring and Forslund have suggested ISO-BMI cut off points for the assessment of different degrees of underweight and obesity for children.
1.7 Objectives of the study
The aims and objectives of this study are as follows.
• Whether the percentile based growth references are varying with respect to time or not?
• Whether the percentile based growth references are varying with respect to different populations or not?
• Out of percentile based BMI and ISO method based BMI, Which one method reveals higher Variation?
2. MATERIALS AND METHODS:
2.1 Data
The data used for the comparison of centile based references and ISO-BMI references for BMI Body mass index was published by the Centre of Disease Control (CDC) 2000 and World Health Organization (WHO) 2007.
The CDC 2000 data comprises values for L, M and S and the information of the following percentiles (5th, 85th, 95th and 99th) from 5 years to 18 years of age for both males and females.
The data published by WHO (2007) also includes the L, M, S values and the information on the percentiles, (5th, 85th, 95th, and 99th) from 5-18 years of age. Moreover, the WHO (2007) also published the information on the multiples of the standard deviation related to the z scores regarding the same LMS values for the same ages. The data sets are available on the internet pages for both the organizations.
Besides the above mentioned data, the Swedish data (2001) provided by STATISTICON AB, is also recruited in our study for the comparison with the WHO 2007 references. The Swedish data (2001) was available from age 0 to 18 years but due to the limitation of the study only 5 years to 18 years males and females data was used. It includes the information for the ISO-BMI (ISO-BMI 17.5, ISO-BMI 25, ISO-BMI 30 and ISO-BMI 35) and the percentiles (5th, 95th, 85th and 99th) for all the ages.
For the comparison of percentile based references and ISO-BMI references from three different sources i.e. CDC 2000, WHO 2007 and the Swedish data (2001) , the data on the some specific percentiles (5th, 85th, 95th and 99th ) and ISO-BMI values (17.5, 25, 30 and 35) is tabulated.
2.2 The LMS method
The LMS summarizes the distribution of the BMI age and sex in terms of three curves namely L for lambda curve, M for mu curve and S for sigma curve. The M curve is median BMI by age, the S curve is the coefficient of variation of BMI whereas L curve represent age dependent skewness of the BMI distribution. The values of the LMS values were tabulated for a series of ages and are presented in the Appendix. The assumption underlying the LMS method is that after the Box Cox transformation, the data at each age is normally distributed. The points on the centile curves are defined by the formula in (1) where L, M and S are the values of the fitted
curves and the Z indicate the Z score corresponding to the required centile. The values of L, M and S vary with age and sex. (Cole TJ et al. 2000a & 2007 b)
The values of the body mass index can also be converted to the exact z scores from the L, M and S values with the formula
............2
* 1 S L M BMI Z
L
−
=
2.3 The ISO-BMI
The idea with ISO-BMI can be seen as a prediction of a child’s adult BMI. ISO-BMI is the BMI Value that a child will have if one stays at same percentile until 18 years old. By ISO-BMI, the adults BMI are linked to children BMI. For determining ISO-BMI based curves, the calculations of percentiles corresponding to different cut off points (ISO-BMI 17.5, ISO-BMI 25, ISO-BMI 30, ISO-BMI 35) at age 18 , for the different data sets (CDC 2000,WHO 2007,Swedish 2001) were made. The concept behind ISO-BMI method is also well explained by some examples.
Take an example for an 8 year old boy (Swedish data 2001) where the corresponding percentile related to BMI of 25 kg/m2 at age 18 yields BMI 17.9.It means that BMI of 17.9 for an 8 year old Swedish boy corresponds to ISO-BMI 25.
Take another example of 9 year old girl (WHO reference) , the corresponding percentile related to BMI of 35kg/m2 at age 18 yields BMI value 25.5. It means that BMI of 25.5 for a 9 year old girl (WHO reference) corresponds to ISO-BMI 35.
2.4 Comparison of growth curves
For the comparison purposes, it is hard to find an appropriate statistical measure that can describe the variation between different growth curves. Among different choices of order statistics, e.g. range, mean, variance, standard deviation, maximum, minimum, range is selected in the present study as a medium of variation between the different growth curves of the percentile based method ( 5th,85th,95th,99th ) and ISO-BMI based method (ISO-BMI 17.5, ISO- BMI 25, ISO-BMI 30, ISO-BMI 35).
Different ranges describing variation of different growth curves by two different methods are tabulated for the ages (5 to 18 years), for both males and females. Moreover, besides the growth curves at some points by the two methods briefly described earlier, the cumulative range plots describing the overall difference between some points are also generated. The cumulative range plots are generated in pairs with the values of the percentile based method and the ISO-BMI values for both males and females aged 5 to 18 years, i.e. the cumulative range plot of 5th percentile &ISO-BMI 17.5, 85th percentile & ISO-BMI 25, 95th percentile & ISO-BMI 30 and 99th percentile & ISO-BMI 35
3. RESULTS
The result section of this study is divided in following sub sections.
Tabulation of ISO-BMI references (17.5, 25, 30, 35) for NCHS 2000 ,WHO 2007 and Swedish data 2001
Percentile based BMI curves (5th, 85th, 95th, 99th) and ISO-BMI curves (17.5 kg/m2, 25 kg/m2, 30 kg/m2, 35 kg/m2)
Tabulation of Ranges describing the variation of growth curves mentioned in 1.
Cumulative Range Plots for Overall Variation in Growth Curves
3.1 Tabulation of ISO-BMI references (17.5, 25, 30, 35) for NCHS 2000, WHO 2007 and Swedish data 2001
Here are the tables generated for the ISO-BMI based references for the year 2000 and 2007 Table 3.1.1: ISO-BMI references (NCHS-2000) for age for males and females
Age (Year)
BMI 17.5
BMI 25
BMI 30
BMI 35
Male Female Male Female Male Female Male Female 5 13.50 13.5 16.6 16.6 18.3 18.2 19.7 19.5 6 13.40 13.4 16.7 16.8 18.9 18.7 21.0 20.4 7 13.40 13.4 17.0 17.3 19.7 19.6 22.8 21.5 8 13.40 13.5 17.5 18.0 20.8 20.6 25.0 22.9 9 13.60 13.s7 18.2 18.7 22.0 21.7 27.1 24.3 10 13.80 14.0 18.9 19.5 23.1 22.8 29.0 25.7 11 14.10 14.3 19.6 20.4 24.3 24.0 30.6 27.1 12 14.50 14.8 20.4 21.2 25.4 25.1 31.9 28.4 13 14.90 15.2 21.2 22.0 26.3 26.1 32.8 29.7 14 15.40 15.7 22.0 22.8 27.2 27.0 33.5 30.9 15 15.90 16.2 22.8 23.4 28.0 27.9 33.9 31.9 16 16.50 16.7 23.5 24.0 28.7 28.7 34.2 33.0 17 17.00 17.1 24.3 24.6 29.3 29.4 34.5 34.0 18 17.50 17.5 25.0 25.0 30.0 30.0 35.0 35.0
Table 3.2.2: ISO-BMI references (WHO-2007) for age for males and females
Age (years)
BMI 17.5
BMI 25
BMI 30
BMI 35 Male Female Male Female Male Female Male Female 5 13.1 13.3 16.7 17.0 18.5 19.0 20.1 20.9 6 13.2 13.3 16.8 17.1 18.8 19.4 20.6 21.6 7 13.3 13.3 17.1 17.4 19.3 20.0 21.4 22.6 8 13.4 13.5 17.5 17.9 20.1 20.9 22.6 23.9 9 13.6 13.8 18.0 18.5 20.9 21.9 24.0 25.5 10 13.9 14.1 18.6 19.2 21.9 23.0 25.8 27.2 11 14.2 14.6 19.3 20.0 23.1 24.1 27.6 28.8 12 14.6 15.2 20.1 21.0 24.3 25.4 29.5 30.4 13 15.1 15.8 20.9 22.0 25.5 26.7 31.2 31.9 14 15.6 16.3 21.9 23.0 26.7 27.8 32.6 33.1 15 16.2 16.8 22.8 23.7 27.8 28.7 33.6 34.1 16 16.7 17.2 23.7 24.3 28.7 29.4 34.3 34.6 17 17.1 17.4 24.4 24.7 29.4 29.8 34.8 34.9 18 17.5 17.5 25.0 25.0 30.0 30.0 35.0 35.0
Table 3.1.3: ISO-BMI references (Swedish Data 2001) for age for males and females Age
(years)
ISO-BMI 17.5
ISO-BMI 25
ISO-BMI 30
ISO-BMI 35 Male Female Male Female Male Female Male Female
5 13.5 13.1 17.30 17.4 19.40 20.0 21.30 22.3
6 13.3 13.0 17.20 17.5 19.50 20.4 21.60 23.2
7 13.3 12.9 17.30 17.8 19.90 21.1 22.30 24.6
8 13.3 13.0 17.90 18.3 20.50 22.2 23.50 26.5
9 13.5 13.1 18.10 18.9 21.50 23.4 25.10 28.8
10 13.7 13.4 18.70 19.7 22.60 24.8 27.20 31.2
11 14.0 13.7 19.50 20.5 23.95 26.2 29.70 33.4
12 14.3 14.1 20.30 21.4 25.40 27.4 32.30 34.9
13 14.7 14.5 21.10 22.2 26.70 28.3 34.60 35.9
14 15.2 15.05 22.00 23.0 27.90 29.1 36.10 36.3
15 15.7 15.6 22.80 23.6 28.90 29.6 36.80 36.4
16 16.3 16.2 23.60 24.2 29.50 30.0 36.60 36.1
17 16.9 16.85 24.35 24.7 29.80 30.1 35.90 35.8
18 17.5 17.5 25.00 25.0 30.00 30.0 35.00 35.0
3.2 Percentile based BMI curves (5th, 85th, 95th, 99th) and ISO-BMI Curves (17.5 kg/m2, 25 kg/m2, 30 kg/m2, 35 kg/m2)
Fig 3.2.1 (a): Comparison of 5th percentile-BMI with ISO-BMI cut-off 17.5, for growth references NCHS-2000, WHO-2007 & Swedish data for boys.
Fig 3.2.1(b): Comparison of 5th percentile-BMI with ISO-BMI cut-off 17.5, for growth references NCHS-2000, WHO-2007 & Swedish data for girls.
Fig 3.2.2 (a): Comparison of 85th percentile-BMI with ISO-BMI cut-off 25, for growth references NCHS-2000, WHO-2007 & Swedish data for boys.
Fig 3.2.2 (b): Comparison of 85th percentile-BMI with ISO-BMI cut-off 25, for growth references NCHS-2000, WHO-2007 & Swedish data for girls.
Fig 3.2.3 (a): Comparison of 95th percentile-BMI with ISO-BMI cut-off 30, for growth references NCHS-2000, WHO-2007 & Swedish data for boys.
Fig 3.2.3 (b): Comparison of 95th percentile-BMI with ISO-BMI cut-off 30, for growth references NCHS-2000, WHO-2007 & Swedish data for girls.
Fig 3.2.4 (a): Comparison of 99th percentile-BMI with ISO-BMI cut-off 35, for growth references NCHS-2000, WHO-2007 & Swedish data for boys.
Fig 3.2.4 (b): Comparison of 99th percentile-BMI with ISO-BMI cut-off 35, for growth references NCHS-2000, WHO-2007 & Swedish data for girls.
3.3 Tabulation of Ranges of Percentile based growth curves and ISO-BMI based growth curves.
Table 3.3.1 Ranges of Percentile based BMI references and ISO-BMI references (for boys)
Age of boys (years)
5th percentile
ISO-BMI 17.5
85th percentile
ISO-BMI 25
95th percentile
ISO-BMI 30
99th percentile
ISO-BM 35
5 0.7 0.4 0.4 0.7 0.3 1.1 0.8 1.6
6 0.5 0.2 0.2 0.5 0.5 0.7 1.7 1.0
7 0.4 0.1 0.3 0.3 1.1 0.6 3.1 1.4
8 0.3 0.1 0.6 0.4 1.6 0.7 4.5 2.4
9 0.2 0.1 0.7 0.2 2.0 1.1 5.7 3.1
10 0.3 0.2 1.0 0.3 2.3 1.2 6.4 3.2
11 0.2 0.2 1.1 0.3 2.4 1.2 6.8 3.0
12 0.2 0.3 1.1 0.3 2.5 1.1 6.9 2.8
13 0.1 0.4 1.2 0.3 2.5 1.2 6.5 3.4
14 0.1 0.4 1.2 0.1 2.3 1.2 6.0 3.5
15 0.1 0.5 1.1 0.0 2.3 1.1 5.6 3.2
16 0.1 0.4 1.0 0.2 2.3 0.8 5.4 2.4
17 0.4 0.2 1.0 0.1 2.3 0.5 5.1 1.4
18 0.7 0.0 1.1 0.0 2.4 0.0 5.4 0.0
Table 3.3.2. Ranges of Percentile based references and ISO-BMI references (for girls)
Age of girls (years)
5th percentile
ISO-BMI 17.5
85th percentile
ISO-BMI 25
95th percentile
ISO-BMI 30
99th percentile
ISO-BM 35
5 0.6 0.4 0.2 0.8 0.3 1.8 1.6 2.8
6 0.4 0.4 0.1 0.7 0.7 1.7 2.5 2.8
7 0.4 0.5 0.4 0.5 1.2 1.5 3.4 3.1
8 0.3 0.5 0.6 0.4 1.6 1.6 4.2 3.6
9 0.2 0.7 0.9 0.4 1.9 1.7 4.9 4.5
10 0.2 0.7 1.1 0.5 2.2 2.0 5.4 5.5
11 0.1 0.9 1.2 0.5 2.4 2.2 5.8 6.3
12 0.1 1.1 1.2 0.4 2.7 2.3 6.3 6.5
13 0.2 1.3 1.3 0.2 2.8 2.2 6.9 6.2
14 0.2 1.3 1.3 0.2 3.0 2.1 7.5 5.4
15 0.3 1.2 1.3 0.3 3.1 1.7 8.4 4.5
16 0.4 1.0 1.4 0.3 3.4 1.3 9.5 3.1
17 0.8 0.6 1.4 0.1 3.7 0.7 11.0 1.8
18 1.3 0.0 1.5 0.0 4.1 0.0 12.8 0.0
3.4 Cumulative Range Plots for Variation in Growth Curves
Fig 3.4.1 : Comparison of cumulative ranges of 5th percentile and ISO-BMI 17.5
Fig 3.4.2 : Comparison of cumulative ranges of 85th percentile and ISO-BMI 25
Fig 3.4.3 : Comparison of cumulative ranges of 95th percentile and ISO-BMI 30
Fig 3.4.4: Comparison of cumulative ranges of 99th percentile and ISO-BMI 35.
INTERPETATION OF THE RESULTS
In the first section of the results, the ISO-BMI references for the three different data sets i.e.
NCHS 2000, WHO 2007 and Swedish data 2001 are tabulated. All the values are calculated by the method proposed by Bring and Forslund. These tables give different ISO-BMI values for the different data for the same ages 5 to 18 years.
After the tabulation of ISO-BMI references, these values are plotted on a graph and thus the growth curves of the percentile based (5th, 85th, 95th and 99th ) and ISO-BMI based (ISO-BMI 17.5, ISO-BMI 25, ISO-BMI 30, ISO-BMI 35) are generated with the help of software SPSS version 16. All the growth curves basically compare the variation of percentile based references and ISO-BMI based references, which method reveals the least variation among the curves.
Here, the variation depicts the efficiency of the LMS method (Cole TJ,1992) and the ISO-BMI method (Bring and Forslund , in progress) among growth curves can be interpreted in both with respect to time and population. The variation within the growth curves can be interpreted both with respect to time and population.
Firstly, for the variation of growth curves with respect to time, ISO-BMI curves at different points having lesser time variation than in the percentile based curves in most of the curves. For example, the variations in the ISO-BMI 35 curves (for both boys and girls) are closer to each other than 99th percentile growth curves (see Figure 3.2.4). The exceptional high variation in ISO-BMI growth curves appear in case of ISO-BMI 17.5 for the girls (see Figure 3.2.1 (b)). This variation means that percentile based method works efficiently in the lower percentile i.e., in 5th percentile for the girls.
Secondly, for the variation with respect to different population, it is a fact that US population has a high prevalence of obesity compared to the other parts of the World. This fact is also depicted in all the growth curves. In most of the percentile based growth curves, the US population curves have been varying from the growth curves for the world and the Sweden, i.e. WHO 2007 and Swedish data 2001 at the same points. For example, in 95th percentile, we can see clearly, the US growth curves are higher than the other two growth curves for both boys and girls. But, on the other hand, in case of ISO-BMI 30, three different populations are quite closer to each other despite of the fact that US population has a high prevalence of obesity in the adjacent curves of the mentioned figure (see Figure 3.2.3). Similar is the case with 85th and 99th percentile, the
curves of ISO-BMI 25 and ISO-BMI 35 are closer to each other for the different populations than the percentile based references mentioned for both the sexes (see Figure 3.2.2 & 3.2.3)).
On the whole, all the plotted growth curves for the comparison of variation of the two methods depict that ISO-BMI references of three different data are closer to each other i.e. Having lesser variation with respect to time and different populations, as compared to percentile based BMI growths curves (see Section 3.2). As ISO-BMI curves are drawn for both the international references published in 2000 and 2007, this closeness at all different point taken (ISO-BMI 17.5, ISO-BMI 25, ISO-BMI 30 and ISO-BMI 35) concludes that ISO-BMI method gives more precise and less varying results within long life span.
For statistical comparison of the growth curves for both the methods in the previous section, the ranges of the different points for the two methods are tabulated (see Section 3.3). The range at different points of the two methods compares the variation occurring within the points as well as with contrasting points of the two methods. For example, in 95th percentile & ISO-BMI 30, the 17 year old boy has range 2.3 and 0.5 respectively. These figures tell us that at age 17 in the growth curves of the 95th percentile and ISO-BMI 30 (see Figure 3.2.3(A)), there exists a difference of 2.3 and 0.5 respectively between different data used. Moreover, the ranges depicts that the ISO-BMI method is more précised than the percentile based method at this specific age (see Table 3.3.1).
Consider another example, in case of 85th percentile & ISO-BMI 25, the 10 year old girl has range 1.1 and 0.5 respectively. These figures, like the previous example, tell at age 10, in the growth curves of the 85th percentile and ISO-BMI 25 (see Figure 3.2.2(B)), there exists a difference of 1.1 and 0.5 respectively between different data used. Moreover, the ranges depicts that the ISO-BMI method is more précised than the percentile based method at this specific age (see Table 3.3.2).
As far as these ranges are concerned, it becomes too difficult to interpret the overall variation of the growth curves by the two methods. For this, the cumulative ranges are plotted for the better explanation of the overall variation of all the points taken by both the percentile based and ISO- BMI method. These plots are drawn by taking the ranges in pairs for the two methods. i.e., the cumulative ranges of 5th percentile &ISO-BMI 17.5, 85th percentile &ISO-BMI 25, 95th percentile
& ISO-BMI 30 and 99th percentile &ISO-BMI 35 are plotted on a same graph separately for girls and boys aged 5 to 18 years. In all the cumulative range plots for both and boys, it is depicted
that ISO-BMI method has less variation than the LMS method for the percentile based references (see Section 3.4).
Besides different variation explained in the objectives, it is interesting to know that the overall variation regarding same points of the percentile based references and ISO-BMI references also differ with respect to gender. This gender difference in the overall variation is well explained with the following examples.
Consider the cumulative range plot in case of 5th percentile and ISO-BMI 17.5 for girls, (see Figure 3.4.1), it shows that ISO-BMI method has a higher cumulative variation than the percentile based method. Starting from 5 years of age, the ranges of both the methods were very closed till age 7, and the difference become quite high in case of ISO-BMI till the age 18. On the other hand, in the same cumulative plot for boys, ISO-BMI method has less variation than the percentile based method. The variation of the percentile based method is high from age 5 to 15 years, and ranges for the two methods become same at age 17 and again the variation in percentile based gets higher at age 18
By considering another example of cumulative range plot of 99th percentile and ISO-BMI 35 for both genders (see Figure 3.4.4), it is clear that the variation of boys and girls are quite different from each other. The cumulative ranges of the girls were the same by both the methods for the age group of 5-14 year, and after that, the variation in the percentile based method become higher than that of the ISO-BMI based method. On the other hand, the ranges of the boys get close to each other only for age group of 5-7 year, and then the difference in the ranges get extremely high till the age 18.
In this study, we have used different national and international growth references and data. The first data i.e., CDC 2000 was based on USA population, Second data set used was collected from different parts of the world named as WHO 2007, and third data set was taken from Sweden in 2001. Here, the explanation of the different data means that by the results including growth charts, cumulative ranges plots and tabulated ranges at certain points of percentile based method and ISO-BMI, we have checked the validity of ISO-BMI method for different parts of the world including USA, Sweden and others. In all the cases, the ISO-BMI growth curves are more precise than percentile BMI growth curve references. Thus ISO-BMI reference growth curves used for assessment for child growth are less population and time dependent. We have also used Swedish data for the comparison that whether the ISO-BMI is comparatively less population dependent
than percentile –BMI growth curve references. Thus we conclude that all the percentile based method should only be used for setting the standard at national level of different parts of the world but not international level. Hence the results obtained in this study proves that ISO-BMI results gives the evidence about the ISO-BMI method suggested by Bring and Forslund has more ability to set the world wide growth curve references, so this method should be considered as preferred method as compare to percentile BMI previously used.
DISCUSSION:
Much has been written about the child growth and increasing epidemic of child obesity in the world, but still the international standards available for assessing child growth seems inadequate to solve the problem. Sometimes, it is difficult for child specialists all over the world, to give sufficient answer to the parents of children regarding their child growth. May be they don’t know the exact answer and they are confused with biological and statistical interpretation of the percentile based BMI references and are still misinterpreting these references.
In the past decades, in 1977, National Centre for Health Statistics, formally World Health Organization developed some growth charts by using percentile curves for assessing the child hood obesity based on the US population. At that time, the growth charts included weight for age charts, weight for height charts for the assessment of child growth. An example of weight for age chart is being quoted in the paper (Figure 1.3.1). In 1988, from the results of different surveys (NAHNES, 1988), despite of the percentile curves, some anthropometric measures, i.e. skin fold thickness, waist hip ratio, arms length and many other measures were in common use for the assessment of the child obesity. It was summarize by IOTF (International Obesity Task Force) in 1994 that Body mass index should also be used for assessing child hood obesity in the same way as it has been used for adults (Dietz and Bellizzi, 1994).
In 2000, an American organization Centre for Disease Control and Prevention (CDC) published BMI for age growth charts curves based on the US population (Figure 1.3.2). These percentiles were based on the LMS method (Cole TJ and Green, 1992). The were certain limitations in CDC 2000 growth references as these references were only based on the US population and they used 85th and 95th percentile to define overweight and obesity in the world. The problem arose that why the world should use the reference only based on the US population and why only 85th and 95th percentiles are selected?
WHO proposed a Multicentre Growth Reference Study (MGRS) where expert committee from all over the world, had worked on different aspects of child growth by involving the data sets from various international surveys in different parts of the world. In 2007, the new BMI for age growth references were published by WHO, involved the data sets from different parts of the world, and based on the percentile curves and the LMS method suggested by Cole TJ and Green, 1992.
According to NCHS Classification about the percentile based references (see Table 1.2.2) , children’s BMI less than 5th percentile are assumed to be underweight, and similarly, 5th percentile to less than 85th percentile are healthy weight, 85th to less than the 95th percentile are at risk of overweight, and equal to or greater than the 95th percentile are overweight.
According to statistical interpretation of percentile based BMI, it should depend on the sample taken from some population while biologically, BMI is a measure to assess fatness in the body regardless of any specific population. World Health Organization has taken samples of children from 6 different parts of the world and set the growth standards based on the collected samples, it means percentile based BMI growth charts (WHO, 2007) show the statistical distribution of BMI of the particular samples. The MGRS study gives good idea about how the selected sample grew up, but it does not indicate how an individual child should grows up. The limitation concerning 2007 growth curves references is the variation exists in the distribution of BMI with respect to time and with respect to different populations. With the passage of time, a continuous change occurs in the prevalence of overweight and obesity worldwide.
It is fact that children are getting fatter day by day (Strauss and Pollack, 1998). The percentile based references are not valid, as after some years if WHO will conduct the same study in the same countries with new sample, that sample will yield percentiles on the higher BMI reference values as compared to present. If we remain using percentile based BMI growth curve references
“which are comparatively more population and time dependent than the method of ISO-BMI suggested by Bring and Forslund” then after a particular time period we need to re-set the older one references growth curves with new one, so it will be a continuous process, it means we need to give limited time and specific population for which the standards are valid, which is not possible. Thus we should use some alternative method which should be comparatively more precise, less population dependent, and do not vary significantly with long time span. This type of method can help for setting standards.
