Monday, November 7, 2011

BMI... What you need to know

One term we hear a lot about recently is "Body Mass Index", or BMI. It seems to be important, but many people are not sure what it is, what it means, or why they should be concerned about it.
What does BMI tell you?
Body mass index (BMI) is a number that evaluates an individual’s weight status in relation to his or her height. BMI generally is used as the first indicator in assessing body fat and has been the most common method of tracking weight problems and obesity among adults. Although this is an effective screening tool for children and adolescents, it is important to remember that it is not a diagnostic tool.  BMI provides a guideline based on weight and height to determine weight issues. As children grow, their body fat changes. BMI decreases during the preschool years, and then increases as the child grows older. This pattern is shown better on the BMI-for-age charts. 
How do you calculate BMI?
Two methods are used to calculate BMI – the metric formula and the English formula. Both provide the same information, and neither is more accurate than the other.
Metric Formula
BMI = Weight in kilograms / [Height in meters] x 2 
English Formula
BMI = [Weight in pounds / Height in inches 2] x 703
How do you calculate BMI for age?
The guidelines for interpreting BMI are listed below. It is critical that accurate measurements are obtained. It is common for caregivers to be concerned about their child’s weight and where he or she is on the growth chart.
  • Underweight BMI-for-age < 5th percentile 
  • At risk of overweight BMI-for-age > 85th percentile 
  • Overweight BMI-for-age > 95th percentile 
BMI is particularly helpful for identifying children and adolescents who are at risk for becoming significantly overweight as they get older. In older children and teens, there is a strong correlation between BMI and the amount of body fat. Those with high BMI readings - and probably high levels of fat – might be likely to have weight problems when they are older. If health care providers can identify these at-risk children early on, they can monitor their body fat more carefully and potentially prevent adult obesity through changes in eating and exercise habits.
BMI is not perfect by any means, it is a screening tool. For example, it's very common for kids to gain weight quickly and see the BMI go up during puberty. Your child's doctor can help you figure out whether this weight gain is a normal part of development or whether it's something to be concerned about. If you think your child may be gaining or losing weight too fast, talk to your child's doctor. A child can also have a high BMI because he or she has a large frame or a lot of muscle, not excess fat. By the same token, a person with a small frame may have a normal BMI but might have too much body fat.
Also, it's important to look at the BMI numbers as a trend instead of focusing on individual numbers. Any one measurement, taken out of context, might give you the wrong impression of your child's growth. The real value of BMI measurements lies in viewing them as a pattern over time. That allows both doctor and parents to watch the child's growth and determine whether it's normal compared with that of other children the same age. BMI is an important additional tool that can be used as an indicator that your child is growing and developing in a healthy way.  If you have concerns about your child's growth or eating, discuss it with their pediatrician and ask for a referral to a pediatric dietitian if it is warranted.

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