BMI can be calculated in both children and adults. However, the interpretation of the results in children is done in a different way. Overweight and obesity among children are an increasingly common problem. Thus, there is a growing interest in methods of assessing the nutritional status of children. One of them is BMI. Is BMI a good method of assessing the correct body weight in children? What are percentile grids for? What are the BMI norms in children?
BMI(body mass index) is a universal indicator that suggests the nutritional status of the body - from underweight to obesity of various degrees. BMI is very easy to find by dividing your weight in kilograms by your height in meters squared.
BMI=body weight [kg] / height2 [m2]
How to calculate BMI in children?
BMI can be calculated in both children and adults, but the interpretation of the results is completely different. For adults over the age of 20, there are ranges of BMI values that indicate whether a person's body weight is normal.
Even at this level, the BMI can prove confusing because it does not take into account body shape, e.g. musculature, high bone mass, low bone mass in the elderly, fluid retention in women due to hormonal changes, etc.
Even in adults, BMI is only preliminary information on the nutritional status of the body, and the assessment of overweight or underweight requires more advanced measurement methods.
In children and adolescents, the BMI itself is completely unreliable information, because body weight during intensive development depends not only on height, but also age and gender. In most sources, specialists do not even provide the correct numerical ranges of BMI values for children and adolescents, but refer to percentile grids, which allow for a much more precise interpretation of the nutritional status.
Thus, BMI in children is calculated according to the same formula as in adults, but the result is interpreted usingpercentile grids appropriate for the child's gender, separate for boys and girls.
There are numerous BMI calculators for children on the Internet, but their use should rather be treated as a curiosity, and not as a source of reliable information about the correctthe nutritional status of the child.
It is uncertain according to which algorithm the child's data are analyzed, whether all the necessary values are taken into account and to which statistical data the results are related. A definitely more reliable method is to use percentile grids for the Polish population, and if a parent has doubts about the interpretation of the result, it is best to ask the pediatrician for help.
What are percentile grids?
Percentile grids are characteristic charts that are created as a result of examining the parameters of a given population. The most common percentile grids for children and adolescents concern:
- height,
- body weight,
- head circumference
- and the circumference of the chest.
Increasingly, BMI percentile grids are also used. Each of these parameters relates to age, so the horizontal axis of the chart includes age, and the vertical axis - the analyzed parameter.
Curved lines for key percentiles are plotted on the graphs, corresponding to changes in the value of a given feature with age.
The lines on the percentiles are the 3rd, 10th, 25th, 50th, 75th and 97th percentiles, and for the BMI 5th, 85th, 90th and 95th percentiles
The percentile nets are prepared separately for boys and girls, as well as for premature babies and children with selected diseases(always distinguishing between sex).
What do the percentiles mean?
For example, the 25th percentile says that in 25% of children from a given population, the analyzed parameter has a lower value. If we analyze the child's body weight and it is in the 60th percentile, it means that 60% of his peers have a lower body weight.
Centiles allow us to determine how a particular analyzed trait of a given child fares against the population, whether it is among the average results, in the upper or lower norm, or is it extremely low or extremely high in relation to the studied population.
How to use the percentile grid?
To take advantage of the percentile grid, you need to choose the right one for your child in terms of gender, date of delivery (prematurity) and diseases (e.g. Down's syndrome).
- To read BMI on the percentile grid, first calculate your child's BMI - divide the weight in kg by the square of height in meters.
- Find the child's age on the horizontal axis of the chart and draw a straight vertical line.
- Find the BMI value on the vertical axis and draw a straight, horizontal line.
- The intersection of the two lines shows which percentile the child is in terms of BMI.
How to interpret the BMI result on the gridpercentile?
Calculating the BMI and relating the result to the percentile grid is considered reasonable from the age of 2. At an earlier age of the child, the measurement criteria are body weight and length in relation to the child's age.
The upper limit for using percentile grids is the age of 20, but different he alth organizations do not treat this limit rigidly. Sometimes it is referred to as 18, sometimes 19.
When calculating the BMI of a teenager (or young adult), you need to take into account what stage of development they are at. For not all of them complete puberty at the same time. Adult BMI values can be used by the age of 18 in some people, and as late as in the age of 20 for others. This is especially true for boys who mature later than girls.
Most countries use percentile grids prepared on the basis of their own population surveys, as differences in the growth rate of children and adolescents in individual countries are visible. WHO has developed percentile grids for international use, but the range of standards used in them is broad - wider than in most countries.
This means that fewer children are classified as underweight or overweight percentiles for the same body weight that will already be considered underweight or overweight in some countries based on country statistics.
In Poland, percentile grids developed on the basis of research on Warsaw children and newer percentile grids developed in the OLAF and OLA studies in 2007-2012 are commonly used.
According to the Polish guidelines, the results are read as follows:
- If your child's BMI is above the 85th percentile, it is overweight.
- If your child's BMI is above the 95th percentile, this indicates obesity.
- If your child's BMI is below the 5th percentile, this indicates underweight.
The norm of body weight among children and adolescents is not what is objectively considered the he althiest, but what is common in a given society. The more obese the societies, the wider the norms are.
For example, in Great Britain, the correct weight of a child is between the 2nd and 91st percentile, and children over the 98th percentile are considered obese. According to WHO, obesity occurs when a child is above the 97th percentile. This shows how ambiguous and difficult it can be to interpret the correct weight of a child.
It is worth comparing the results from the percentile grids also for height and weight in order to have a more complete picture of the nutritional status of the child.Ideally your height and weight should be in the same rangepercentile, i.e. between the 50th and 75th percentile . A problem with being overweight or underweight is indicated by values for height and weight, which are wider than in the two percentile bands, e.g. height at the 10th percentile and weight at the 75th.
BMI results, even if read from the percentile grid, will not always give a real picture of the nutritional status of a child. For example, children of very tall and thin parents will probably be below the 5th percentile of BMI, i.e. underweight. However, if the increase in body length and weight is proportional and the biochemical results are normal, there is no reason to be concerned about the malnutrition of the child.
A similarly high BMI, suggesting overweight, with a large bone mass and greater than average muscle mass can be misleading.
BMI is a very imperfect body mass index that does not account for many of the parameters that are responsible for actually underweight or overweight. The very value of BMI in the case of children and adolescents does not give any information about the nutritional status of a child.
Only the use of BMI percentile grids shows where the child is compared to the population and helps to assess whether he is overweight or underweight.
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