Assuring Pediatric Nutrition in the Community

FREQUENTLY USED GUIDELINES:
Assessing Nutritional Status - Stature and Weight Proportionality

Assessing stature and weight proportionality
Body mass index (BMI)
Using BMI
Factors that affect BMI

How do I assess stature and weight proportionality?
Weight for length or height and Body Mass Index (BMI) are useful in evaluating weight to stature proportionality. Growth charts for 0-36 month olds include percentiles for weight for length. BMI for age is available for children over the age of 2 years. BMI provides two indicators: a percentile ranking similar to current weight for height, and a number that may be useful for monitoring changes.

What is Body Mass Index (BMI)?
Body Mass Index (BMI) is an indicator of weight and stature proportionality. Weight and stature are routinely measured, equipment is not expensive, and measurers can be quickly trained. Calculation of BMI is not difficult, and nomograms exist to aid in computation. Thus far, most published reports focus on the use of BMI to identify obesity. Of several height- and weight-based indices, BMI appears to be the more accurate predictor of body fatness.

BMI is calculated by the following equation: kg/m2, where kg = weight in kilograms and m = height in meters. For example, if an 8 year old boy weighs 26.8 kg (50-75th percentile for age) and is 130.4 cm tall, his BMI would be calculated as follows:

26.8/(1.304)(1.304) = 15.8

How do I use Body Mass Index (BMI) in a pediatric assessment?
BMI should be used in combination with other assessment tools. It is important to be aware of factors that influence BMI and use professional judgement in determining what influences these factors have on an individual’s BMI.

As with all anthropometric indicators of growth, BMI over time will reflect a child’s status better than a single value. Presently, children with BMIs above the 85th percentile for age are categorized as at risk for overweight, while children with BMIs for age above the 95th percentile are categorized as overweight. BMI’s power to identify underweight requires further attention. It will be important to ensure that the public is informed of the differences between BMI for adults and BMI for children. The indices are not identical, and a "healthy" adult BMI is often not healthy for a child. It is also important to remember that body composition changes with age and that an acceptable BMI number for a 5 year old may not be acceptable for a 12 year old.

What factors affect Body Mass Index (BMI) in a pediatric population?
Growth and development patterns influence BMI. Because height and weight seldom increase at the same rate, a single BMI measurement will not necessarily reflect a child’s overall, long-term body composition.

The rate of sexual maturation is correlated with BMI. While most school-aged children are not developing sexually, some are. BMI is not able to account for this; thus some children are erroneously labeled obese, while the obesity of other children is ignored.

Sex has an influence on BMI—BMI seems to be a better predictor of obesity in girls than in boys.

The influence of race on BMI (e.g., differences in body composition between blacks and whites) should not be ignored.

BMI does not take fat distribution into account. Abdominal obesity is associated with more risk factors than trunkal obesity, but no differentiation between the two is made by BMI.