Download Using the BMI-for-Age Growth Charts and more Summaries Nutrition in PDF only on Docsity!
Using the BMI-for-Age Growth Charts
INTRODUCTION
The Body Mass Index (BMI)-for-age charts for boys and girls aged 2 to 20 years are a major addition to the new CDC Pediatric Growth Charts. For the first time there is a screening tool to assess risk of overweight and overweight in children and adolescents. This module presents the rationale behind the decision to include the BMI-for-age charts, discusses characteristics of the BMI-for-age charts, and provides an opportunity for practical application of calculating BMI, and plotting and interpreting BMI-for-age through case studies.
OBJECTIVES
- Describe the advantages of using the BMI-for-age charts as a screening tool to evaluate overweight and underweight in children and adolescents
- Calculate BMI using the metric and English methods
- Demonstrate an understanding of the use of the BMI-for-age chart by accurately plotting and interpreting BMI-for-age on the appropriate chart
TABLE OF CONTENTS
- What is BMI?
- Advantages to using BMI-for-age as a screening tool for overweight and underweight
- Recommendations for using BMI to assess overweight
- Characteristics of BMI-for-age
- Recommended BMI-for-age cutoffs
- Calculating BMI
- Other methods to obtain BMI
- Visual assessment versus calculation of BMI
- Accuracy of measurements
- Interpretation
- Summary
- Steps to plot and interpret BMI-for-age
- References and resources
Appendix
1. WHAT IS BMI?
- An anthropometric index of weight and height
Body Mass Index (BMI) is an anthropometric index of weight and height that is defined as body weight in kilograms divided by height in meters squared (Keys et al., 1972).
BMI = weight (kg) / height (m)≤
BMI is the commonly accepted index for classifying adiposity in adults and it is recommended for use with children and adolescents.
Like weight-for-stature, BMI is a screening tool used to identify individuals who are underweight or overweight. BMI is NOT a diagnostic tool (Barlow and Dietz, 1998).
For example, a child who is relatively heavy may have a high BMI for his or her age or high weight-for-stature. To determine whether the child has excess fat, further assessment would be needed and that might include skinfold measurements. To determine a counseling strategy, assessments of diet, health, and physical activity are needed.
For children, BMI is gender specific and age specific (Hammer et al., 1991; Pietrobelli et al., 1998). Because BMI changes substantially as children get older, BMI-for-age is the measure used for children ages 2 to 20 years.
FOR CHILDREN, BMI DIFFERS BY AGE AND GENDER
Because adiposity varies with age and gender during childhood and adolescence, BMI is age and gender specific. As illustrated on this growth chart for boys, in a growth pattern established along the 95th percentile, BMI-for-age reached a minimum at 4 years of age and then increased with increasing age.
The tracking of BMI that occurs from childhood to adulthood is clearly shown in data from a study by Robert Whitaker and colleagues (Whitaker et al., 1997). They examined the probability of obesity in young adults in relation to the presence or absence of overweight at various times during childhood. For example, in children 10 to 15 years old, 10% of those with a BMI-for-age < 85th percentile were obese at age 25 whereas 75% of those with a BMI-for-age > 85th percentile were obese as adults and 80% of those with a BMI-for-age > 95th percentile were obese at age 25. (The sample size for the study was 854.) From this study, it is clear that an overweight child is more likely than a child of normal weight to be obese as an adult.
Other studies have shown this same trend of tracking occurring from childhood to adulthood (Guo et al., 1999; Guo et al., 1994; Garn and LaVelle, 1985).
- BMI-for-age relates to health risks.
o BMI-for-age correlates with clinical risk factors for cardiovascular disease including hyperlipidemia, elevated insulin and high blood pressure. Freedman and colleagues used data from the Bogalusa Heart Study and found that approximately 60% of 5 to 10 year-old children who were overweight had at least one biochemical or clinical risk factor for cardiovascular disease such as those just mentioned, and 20% had two or more risk factors (Freedman et al., 1999).
o BMI-for-age during pubescence is related to lipid and lipoprotein levels and blood pressure in middle age (Must et al., 1992). Risk factors in children can become chronic diseases in adults.
- BMI-for-age compares well with both weight-for-stature measurements and measures of body fat.
o A study completed by researchers at the CDC (Mei et al., 2002) compared the performance of BMI-for-age and weight-for-stature with fatness measured by dual energy x-ray absorptiometry (DXA), a direct measure of adiposity.
- NHANES III data were used to test how well BMI-for-age predicts underweight (below the 15th percentile) and overweight (above the 85th percentile) relative to the traditional weight-for-stature in children 2 to 19 years old.
- Both BMI-for-age and weight-for-stature performed equally well in screening for underweight and overweight among children 3 to 5 years of age.
- For school-aged children (6 to 11 and 12 to 19 age groups), BMI-for-age was slightly better than weight-for-stature in predicting underweight and overweight.
- Ratios of weight relative to stature such as BMI-for-age and weight-for-stature may be used as indirect measures of overweight that correlate with direct measures.
- CDC recommends the use of BMI-for-age for children aged 2 years and older. However, weight-for-stature performs equally well in pre-school aged children and can be used in this age group.
- BMI-for-age is significantly correlated with subcutaneous and total body fatness in adolescents (Barlow and Dietz, 1998).
3. RECOMMENDATIONS FOR USING BMI TO ASSESS OVERWEIGHT
Because of the numerous advantages of using BMI-for-age to assess overweight in children and adolescents, expert committees and advisory groups have recommended BMI-for-age as the accepted measure.
- In 1994, an expert committee on Clinical Guidelines for Overweight in Adolescent Preventive Services was convened by the Maternal and Child Health Bureau (MCHB), American Academy of Pediatrics and the American Medical Association with support from the Centers for Disease Control and Prevention, to advise Bright Futures: National Guidelines for Health Supervision of Infants, Children and Adolescents and Guidelines for Adolescent Preventive Services (GAPS) on the criteria for the identification of adolescent obesity. The committee recommended that BMI-for-age be used to routinely screen for overweight in adolescents (Himes and Dietz, 1994).
- In 1997, a consensus panel recommended that BMI for age be used routinely to screen children for overweight. They also recommended cutpoints of between the 85th and 95th percentiles to identify children and adolescents as at risk of overweight and at or above the 95th percentile to identify children and adolescents as overweight. (Barlow and Dietz, 1998).
- Also, in 1997, an international conference convened by the International Obesity Task Force concluded that BMI is a reasonable measure for assessing overweight in children and adolescents worldwide. (Dietz and Bellizzi, 1999; Bellizzi and Dietz, 1999).
Dr. William Dietz discusses the rationale for the BMI cutpoints, the limitations and sensitivity of BMI-for-age. Dr. Dietz is the Director of the Division of Nutrition and Physical Activity, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, and was a member of the various expert committees on obesity. Full text is included in the Appendix.
Here you see a section of the BMI-for-age chart for boys enlarged to show the shape of the curve in more detail. After 4 to 6 years of age, BMI-for-age begins a gradual increase through adolescence and most of adulthood. The rebound or increase in BMI that occurs after it reaches its lowest point is referred to as "adiposity" rebound (Whitaker et al., 1998; Rolland-Cachera et al., 1991; Rolland-Cachera et al., 1984). This is a normal pattern of growth that occurs in all children.
Recent research has shown that the age when the "adiposity" rebound occurs may be a critical period in childhood for the development of obesity as an adult (Whitaker et al., 1998). An early "adiposity" rebound, occurring before ages 4 to 6, is associated with obesity in adulthood. In the example shown here, adiposity rebound occurred at around age 3. BMI reached the lowest point at 32 months (2 years 8 months) and then began to increase.
However, studies have yet to determine whether the higher BMI in childhood is truly adipose tissue versus lean body mass or bone. Additional research is needed to further understand the impact of early adiposity rebound on adult obesity. (Note that we put the word adiposity in quotations when using it in this context since we do not know if it is truly adipose tissue.)
5. RECOMMENDED BMI-FOR-AGE CUTOFFS
The expert committeesí recommendations are to classify BMI-for-age at or above the 95th percentile as overweight and between the 85th and 95th percentile as at risk of overweight (Himes and Dietz, 1994).
"Overweight" rather than obesity is the term preferred for describing children and adolescents with a BMI-for-age equal to or greater than the 95th percentile of BMI-for-age or weight-for-length.
The 85th percentile is included on the BMI-for-age and the weight-for-stature charts to identify those at risk of overweight.
The cutoff for underweight of less than the 5th percentile is based on recommendations by the World Health Organization Expert Committee on Physical Status (World Health
Organization, 1996).
> 95th percentile Overweight
85th to < 95th percentile Risk of overweight
< 5th percentile Underweight
Classifications of Overweight and Underweight for Adults
Classification of overweight and underweight is different for adults than it is for children and adolescents. For adults, overweight and underweight categories are defined by fixed BMI cutpoints derived from morbidity and mortality data. Adults with low and very high BMIs are at a higher relative mortality risk compared to those with BMIs of 18.5 or greater and less than 30.0 (Strawbridge et al., 2000). For adults, BMI is not age- or gender-specific as it is for children and adolescents.
Clinical guidelines established in 1998 by the National Heart, Lung, and Blood Institute are as follows:
Performance of BMI-for-Age As A Screening Tool
"The validity of selected cutoff points to identify adolescents with the highest percentage of body fat has been investigated. In general, common cutoff points for BMI and relative weight have low sensitivities but high specificities. For example, BMIs > 85 th^ percentile has a sensitivity of 29% and 23% for identifying adolescent males and females, respectively, who are above the 90 th^ percentile for percentage body fat; corresponding specificities are 99% and 100% (Himes and Bouchard, 1989). In screening for adolescent overweight, specificity may be more important than sensitivity. Maximizing specificity minimizes the proportion of adolescents who will be incorrectly considered overweight by the screen" (Himes and Dietz, 1994).
Recently it has been shown that cardiovascular risk factors are associated with the established BMI-for-age cutoffs. Freedman et al., (1999) found that approximately 60% of 5 to 10 year-old children with BMI-for-age values > the 95 th^ percentile had at least one biochemical or clinical risk factor for cardiovascular disease such as hypertension, elevated insulin levels, and hyperlipidemia. Twenty percent of children had two or more risk factors.
6. CALCULATING BMI
BMI can be calculated using either the metric system or the English system.
BMI less than 18.5 underweight
BMI of 18.5 through 24.9 normal
BMI of 25.0 through 29.9 overweight
BMI of 30.0 or greater obese
When using English measurements, ounces (oz) and fractions must be changed to decimal values. Then, calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.
- When using a hand-held calculator, if your calculator has a square function, divide weight (lbs) by height (in) squared, multiply by 703 and round to one decimal place.
- If your calculator does not have a square function, divide weight by height twice as shown in the calculation above, multiply by 703 and round to one decimal place.
Calculations for BMI can be completed as a continuous equation. Note that the formula for the latter calculation is on the CDC Clinical Growth Charts and will be the calculation used in this module.
Example: Letís calculate Samís BMI using the English numeric system. His weight is 37 pounds and 4 ounces and his height is 41 1/2 inches.
Convert ounces and fractions to decimals:
Weight of 37 lbs and 4 oz = 37.25 lbs (16 ounces = 1 pound so 4 oz/16 oz = 0.25).
Height = 41.5 in.
(37.25 lbs / 41.5 in / 41.5 in) x 703 = 15.
Practice calculating BMI using the English system
Complete the following two calculations, rounding to one decimal place.
Calculation 1: Georgiaís weight is 36 Ω pounds and her height is 39 inches. What is Georgiaís BMI?
Answer: Convert fractions to decimals: Weight of 36 Ω lbs = 36.5 lbs
(36.5 lbs / 39 in / 39 in) x 703 = 16.
Calculation 2: Joseís weight is 40 º pounds and his height is 40 æ inches. What is Joseís BMI?
Answer: Convert fractions to decimals: Weight of 40 º lbs = 40.25 lbs Height of 40 æ in = 40.75 in
(40.25 lbs / 40.75 / 40.75) x 703 = 17.
Note: There is a difference of 0.1 between BMI calculations when using the metric system (17.1) versus the English system (17.0). This is due to the conversion factor.
7. OTHER METHODS TO OBTAIN BMI
- CDC Table for Calculated Body Mass Index Values for Selected Heights and Weights for Ages 2 to 20
In addition to calculations, BMI can also be determined by looking it up on the CDC Table for Calculated Body Mass Index Values for Selected Heights and Weights for Ages 2 to 20 in which BMI has been calculated_._
Example:
Using the metric system, find the BMI for a child with a weight of 17.0 kg and a height of 105.4 cm.
Using the English system, find the BMI for a child with a weight of 37.5 lb and a height of 41.5 in.
See BMI TABLE in Appendix.
- Epi Info 2000 has an anthropometric data component called NUTSTAT that can be used to look at population-based data or group data.
- SAS program to calculate BMI and percentiles for all indices.
8. VISUAL ASSESSMENT VERSUS CALCULATION OF BMI
- How accurately can you screen for risk of overweight in children by looking?
It has been said that "few medical conditions can be diagnosed as confidently by untrained individuals as gross obesity." Yet it is very difficult to distinguish children who are at risk of overweight from normal children. In childhood, the distinction is made more difficult by age- related physiological variations (Poskitt, 1995).
Take a look at three photos of preschool aged children to see how accurately you can screen for risk of overweight in children solely by looking at them. We want you to try to identify children with a BMI-for-age equal to or greater than the 85th percentile and less than the 95th percentile.
SECOND EXAMPLE
Letís assess for risk of overweight by calculating BMI.
Mindyís height is 41.9 inches and her weight is 35.5 pounds.
BMI = (weight / height≤) x 703
Mindyís BMI: (35.5 lbs / 41.9 in / 41.9 in) x 703 = 14.
Now, plot her BMI on the appropriate chart for her age and interpret the findings.
Here is Mindy, a 4 year-old girl. Does she appear:
**1. Underweight
- Normal
- At risk of overweight
- Overweight**
Plotted on the BMI-for-age chart for girls, Mindy's BMI-for-age falls just above the 10th percentile indicating that her BMI is within normal range.
THIRD EXAMPLE
Letís assess for risk of overweight by calculating BMI.
Lisaís height is 39.2 inches and her weight is 38.6 pounds.
BMI = (weight / height≤) x 703
Lisaís BMI: (38.6 lbs / 39.2 in / 39.2 in) x 703 = 17.
Now, plot her BMI on the appropriate chart for her age and interpret the findings
The point of this exercise is to demonstrate the difficulty of making a consistently accurate visual assessment of at risk of overweight. To make a consistently accurate determination of at risk of overweight, BMI must be determined and plotted on the appropriate growth chart for the appropriate age.
Lisa is another 4 year-old girl. Does she appear:
**1. Underweight
- Normal
- At risk of overweight
- Overweight**
Plotted on the BMI-for-age chart for girls, Lisa's BMI-for-age falls on the 94th percentile so she would be classified as at risk of overweight.
10. INTERPRETATION
Interpretation of the plotted BMI-for-age is based on the established cutoff values previously shown.
These percentiles indicate the rank of BMI in a group of 100 children of the same gender and age. For example, in a group of 100 children:
- 5 children will have a BMI-for-age that is at or above the 95th^ percentile
- 10 will have a BMI-for-age that is between the 85 th^ and 95 th^ percentiles
- 5 will have a BMI-for-age less than the 5 percentile, and
- 80 children will have a BMI-for-age that is within the normal range.
> 95th percentile Overweight
85th to < 95th percentile Risk of overweight
< 5th percentile Underweight
Inaccurate measurement
If her weight were measured or recorded inaccurately at 60.0 pounds (2.0 pounds above her actual weight of 58.0 pounds), her BMI-for-age would be 18.7 and fall on the 85 th percentile (shown by the red dot) and thus Maria would be considered at risk of overweight. An error in weight could occur if scales are not adequately maintained, standard protocol is not followed for weighing, or a recording error is made.
When assessing physical growth, it is desirable to have a series of accurate measurements to establish an observed growth pattern. Having a series of measurements takes into consideration short- and longer-term conditions and provides a context for individual measurements in interpretation.
Growth patterns that fall outside the established parameters, the 5th and 95th percentile for any given anthropometric indices, suggest the need to recheck measurements, plots, and calculations and make any necessary corrections or adjustments. If measurements are correct, further evaluation is needed to determine the cause.
11. SUMMARY
- BMI-for-age is the method recommended for screening overweight and underweight in children and adolescents from 2 to 20 years of age.
- BMI-for-age is a screening tool that may lead to further assessment to diagnose a specific health condition.
- For children, BMI is age and gender specific and nutritional status is identified based on percentiles. For adults, BMI is neither age nor gender specific and nutritional status is defined by fixed cut points.
- Periodic, accurate measurements and growth records are important elements of growth screening. An accurate interpretation of growth depends on the accuracy of weighing and measuring.
12. STEPS TO PLOT AND INTERPRET BMI-FOR-AGE
The six steps outlined here to plot and interpret BMI-for-age are similar to those used for a general growth assessment.
Step 1: Obtain accurate weights and height measurements Step 2: Select the appropriate growth chart (based on the age and gender of the child being weighed and measured) Step 3: Record the data Step 4: Calculate BMI Step 5: Plot measurements Step 6: Interpret the plotted measurements
An instruction sheet on Using and Interpreting the CDC Growth Charts contains detailed instructions for the above steps.
Example Case Study
point. Then find his BMI on the vertical axis and visually draw a horizontal line across from that point. The point where the two intersect represents Samís BMI-for-age.
When plotted on the growth chart, Samís BMI-for-age falls just below the 25th percentile curve.
Step 6. Interpret the plotted measurements
Since Samís BMI-for-age falls just below the 25th percentile curve, this means that of 100 children with the same gender and age as Samís, fewer than 25 children have a BMI-for-age lower than his and more than 75 have a BMI-for-age higher than his. Sam is not overweight, underweight or at risk of overweight.
When a childís plotted measurement falls between the 5th and 85th percentiles it is considered to be within the normal range. Samís BMI-for-age is in the normal range based on this measurement.
PLOTTING AND INTERPRETING BMI-FOR-AGE USING THE ENGLISH SYSTEM
A Case Study of ëSamí
Step 1. Obtain accurate weights and height measurements
Date of Birth (DOB): 9/15/ Date of Visit (DOV): 4/4/ Weight: 37 pounds 4 ounces Height: 41 1/2 inches
Step 2. Select the appropriate growth chart
Because Sam is a normal 3.5 years old, a standing height was obtained. The appropriate chart to use is the ëBoys 2 to 20 BMI-for-ageí chart.
Step 3. Record the data
On the data entry table found on the clinical growth chart, information relevant to the growth chart is recorded. Here data has been recorded for the date of visit, the childís age, weight and height at the visit.
Date Age Weight Stature BMI
4/4/98 3.5 yrs 37.25 lbs 41.5 in
Step 4. Calculate BMI
Calculate BMI at the time of the clinic visit.
Convert ounces and fractions to decimals:
Weight of 37 lbs and 4oz = 37.25 lbs (16 ounces = 1 pound so 4oz/12 oz = 0.25).
Height = 41.5 in
BMI = (weight / height / height) x 703
BMI: (37.25 lbs/41.5 in/41.5 in) x 703 = 15.
Then the BMI of 15.2 is entered on the data entry table.
Date Age Weight Stature BMI
4/4/98 3.5 yrs 37.25 lbs 41.5 in 15.
Step 5. Plot measurements
All the necessary information is recorded and Samís BMI can be plotted. On the BMI-for-age chart, find Samís age on the horizontal axis and visually draw a vertical line up from that point. Then find his BMI on the vertical axis and visually draw a horizontal line across from that point. The point where the two intersect represents Samís BMI-for-age.
When plotted on the growth chart, Samís BMI-for-age falls just below the 25th percentile curve.
