Q: The doctor told my 14-year-old daughter today at her well child visit to keep a watch on her BMI as it was creeping up. What does that mean?
A: Body Mass Index, or BMI for short, has been used for many decades as a means to determine body fat. So everyone’s weight is different, but what does that weight mean for that person is based on that individuals height. Hence the ideal weight for a shorter individual will be less than for a taller individual.
Scientists came up with this idea several decades ago and it has been used extensively across the world by doctors, public health officials and insurance agencies, as it is clear that obese individuals, or those with high BMI’s, are at greater risk of developing “metabolic” illnesses. More about this in the next column.
So how is it measured? The number is obtained by dividing the weight in kilogram by the height in meters and again dividing that value by the height in meters. Hence the number is measured as kg/mÂ². You don’t really have to worry too much about doing the math as there are several BMI calculators available on the internet.
The BMI gives us an estimate of how much fat is in the body. It is a relatively accurate assessment for most individuals, but in some it may overrepresent or underrepresent the fat. For example, in a body builder or an athlete, the muscle mass would be a big component of the weight. So even though the BMI maybe high, the muscle makes up more of the weight than the fat. But these individuals are in the minority.
It is also possible that people from different regions of the world have different “normal” range BMI. Some experts feel, that for Asians, the currently used BMI range may be higher than desired – this means that for the commonly accepted BMI range, Asian individuals may have a higher risk for developing complications of being overweight or obese than their Caucasian counterparts. More research is needed in this area, but some scientists are of the opinion that for the Asian population a BMI of more than 25 kg/mÂ² should be considered as obese.
The BMI range for adults is shown below and are constant numbers because their height is now constant. However, in children, it is different. Children grow in height and hence the normal ranges of the BMI vary with age. It also differs for boys and girls. Hence, BMI in children is more often expressed in percentiles. But again you need not worry – BMI calculators and graphs for different age groups are easily available on the internet (CDC is a good website to visit for this, cdc.gov/healthyweight/bmi/calculator.html).
For the adult population, BMI ranges include:
Underweight: BMI under 18.5 kg/m Â²
Normal weight: BMI range from 18.5 to 24.9 kg/m Â²
Overweight: BMI range from 25 to 29.9 kg/m Â²
Obesity: BMI greater than 30 kg/m Â²
Coming back to your question – watch the BMI. This could mean that your daughter’s BMI is moving across percentile lines – ideally it should stay in the same band if she is a well child. Now moving up the percentile lines is definitely more common than moving down. Moving up could mean that she is going from the normal range to over-weight range (between the 85th-95th percentile for that age) or moving into the obesity range (>95th percentile for that age). Moving down the BMI would mean the opposite.
In today’s world where overweight and obesity is rampant, it would be good to calculate the BMI twice a year to ensure that our children stay in the normal range and do not cross percentiles into the overweight or obese category. Trends in the BMI can more accurately predict for the large majority of children if they are becoming too heavy for their height rather than just measuring their weights as they grow taller.Â
More next time about how BMI trends are changing in our country and what does it imply.
Pankaj Vohra is a Pediatric Gastroenterologist at UNM. Please send your questions to firstname.lastname@example.org.