Does BMI Underestimate Adiposity in Kids?Tuesday, July 8, 2014
Regular readers are well aware of my reservations regarding the use of BMI as a diagnostic parameter in clinical practice. After all, while BMI may tell us how big someone is, it certainly is not a good measure of how sick someone is.
But to be honest, BMI was never intended as a measure of disease – it was (at best) introduced as a surrogate measure of adiposity (fatness).
Nevertheless, supporters of BMI continue to argue that it is still a good measure of fatness and as such should remain part of standard assessment – even in kids.
Now, a paper by Javed and colleagues, published in Pediatric Obesity, examines how well BMI performs as a means to identify obesity as defined by body fatness in children and adolescents.
The authors conducted a systematic review and meta-analysis of 37 studies in over 53,000 participants assessing the diagnostic performance of BMI to detect adiposity in children up to 18 years.
While the commonly used BMI cut-offs for obesity showed showed a high specificity (0.93) to detect high adiposity, the sensitivity was much lower (0.73) – particularly in boys.
This means that kids who exceed the current BMI cut-offs are indeed very likely to have fatter bodies (for what it’s worth).
On the other hand, relying on BMI cut-offs alone will miss as many as 25% of kids whose body fat percentage exceeds current definitions of adiposity.
Thus, assuming that bod fatness or adiposity is indeed a clinically useful measure of health, the use of BMI alone will ‘underdiagnose’ adiposity in a significant proportion of kids (especially boys) who may well be at risk from excess fat.
A word of caution about fatness is certainly in order – as in adults, much depends on exactly where the fat is located (abdominal or ectopic vs. subcutaneous) and other factors (e.g. cell size, inflammation, insulin sensitivity, etc.).
Thus, even if BMI was a perfect measure of body fat, it would probably still require further examinations and tests to determine exactly whether or not this “extra” fat poses a health risk.
As in adults, a clinical staging system similar to the Edmonton Obesity Staging System may be a fat better indicator of determining which kids may need to worry about their body fat and which don’t.
Hat tip to Kristi Adamo for pointing me to this study
Javed A, Jumean M, Murad MH, Okorodudu D, Kumar S, Somers VK, Sochor O, & Lopez-Jimenez F (2014). Diagnostic performance of body mass index to identify obesity as defined by body adiposity in children and adolescents: a systematic review and meta-analysis. Pediatric obesity PMID: 24961794