Health and weight

Measuring weight - The Body Mass Index

Weight and size matter because they can affect a person’s health and wellbeing now and later in life. Most children are weighed at school so that doctors can help them and their parents ensure that they are a healthy weight. Here we will talk about what young people and their parents said about the Body Mass Index or BMI which is a measure used by doctors for deciding if someone is a healthy weight. 

What is BMI?
The body mass index is a calculation of body fat that takes into account your age, weight and height. The result is given as a number. For adults the BMI categories are:
  • Under 18.5: Underweight
  • 18.5-25: Normal weight
  • 25-30: Overweight
  • 30-35: Obese
  • 35 and over: Morbidly Obese

In children and teenagers (below the age of 18), weight is compared with other people of their age, height and gender. BMI is then given as a percentage or ‘centile’ of this group.
Some people think that Body Mass Index (BMI) is not a good way of deciding if a person is overweight or obese. It is especially difficult to say if a child or young person is overweight or obese because they are still growing and each child grows at a different rate. Most of the young people we talked to accepted the BMI as a measure, as long as it was only used in a medical sense (by doctors or other health professionals). Many felt unhappy about the words used with the BMI and didn’t like to think of themselves as being ‘obese’.
The Body Mass Index explained 
Among the people we spoke to, there was a lot of confusion about how BMI is measured in adults and children. A few people said they think that BMI is not a good way of measuring whether a person is overweight. Some young people understood it and said they accepted they were ‘obese’ but others rejected their BMI measurement.
Some parents we talked to admitted they knew very little about the BMI, or about how their children’s weight was categorised. 
Feelings about the Body Mass Index 
Lots of people, including parents, really disliked the terms ‘obese’ and ‘morbidly obese’. They said obese was a ‘horrible’ word and that once you were given that label it was difficult to shake it off. Obese had unpleasant associations with words such as ‘grease’ and ‘fat’; to one mother it also suggested that the person was ‘fat and thick’.
A few did not mind the word ‘obese’ being used as a general description.

Some young people said they knew they were morbidly obese and  they found the use of those words scary and worrying. Several found the labels discouraged them from losing weight; they wondered why they would bother just to get from ‘morbidly obese’ to ‘obese’.
Most parents preferred to focus on their child’s good points. They didn’t want to damage their son or daughter’s self-confidence even when they knew s/he had become much larger than their friends and siblings. Not everyone felt like this; one mother said she believed children and their parents needed to be ‘shocked’ into taking drastic action over eating, exercise and health.

Last reviewed July 2017.
Last updated February 2012.


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