Dr Macklin Explains Percentiles and Obesity Categories

On this season of X-Weighted, we are dealing with families and children, which means that the term "percentiles" comes up on every episode. Here Dr Macklin explains why we use percentiles when talking about childrens' weight, and what the differing categories of obesity mean in regard to adults.

When you are determining the health and weight of someone that is under 18, it is most informative to discuss them in terms of where they fall as a percentile: Meaning how they would compare with 1000 randomly selected children their height and age.

Through very good population studies, we've been able to determine that a child at the 85th percentile is overweight and anyone above the 95th percentile is considered obese. When we are talking about obesity in children, really we are talking about their relative risk of disease and other complications that result from weight. These complications could include social stigmatization, poor quality of life and adverse health outcomes. You can pretty much predict what will happen to that child depending on which percentile they are in.

Sometimes it is very difficult for parents to hear that their child is obese, but it is important for them to hear that. Awareness is the first step for someone to be driven to make the changes needed to prevent all those adverse outcomes.

Because kids generally aren't concerned or thinking about their future health risks, it is more instructive to give them a relative idea of where they sit in terms of their peers, instead of simply telling them that they are morbidly obese (which translates as being at risk of disease because of their weight). 

By comparing them to their peers, the message hits home. As we've seen on the show, sitting down and telling an obese kid that if you took a random sample of 1000 kids their age and height, then 997 would be lighter than them and only 2 in that 1000 would be heavier, that really affects the child and their parents in a more clarified way than just labeling them morbidly obese.

When it comes to adults, we use categories to explain the extent of their obesity. Using the Body Mass Index (BMI) which is simply a measurement of weight over height, is a very accurate method of determining someone's relative risk of disease and quality of life due to their weight. If someone has a BMI of 30, they are determined to be obese, and until 20 years ago that is the only label that people were given.

However, these days the biggest people are getting bigger so we've had to create new sub-classes that we didn't need before: So when someone reaches a BMI of 35, then we'll start calling them class 2 obese and then if they reach a BMI of 40 we call them class 3, which is what we call morbid obesity. But in my practice I see individuals with class 4 (a BMI of above 45) and class 5 obesity (a BMI of above 50) all the time.

Dr David Mackin is X-Weighted's obesity expert and will be featured throughout season 4.