Doubts About Body mass index

Science marches on, which sometimes causes embarrassment when new information emerges. This might be the case with body mass index or BMI, which is generally taken to be the best feasible way of measuring the risk and degree of obesity in children.

A downloadable PDF file from Shape Up America! describes how a screening program should be set up and implemented. (Aside from any other factors, frequent repetition of the word “surveillance” is likely to ruffle the feathers of some citizens.) The topmost authorities try to get everybody to test every kid within reach, according to uniform criteria. Every bureaucracy is bribed and/or coerced into following what seems like a perfectly reasonable policy. They are also urged to make accurate and timely reports, so the situation can be assessed on a national scale.

This record-keeping can be an uphill endeavor because many school districts, rural health clinics, and other facilities simply do not have the resources to do these things. With luck, the ponderous machinery of government funding is geared up, perhaps supplemented or replaced by foundation grants, and eventually, one way or another, conditions are created where everybody is pretty much on board.

But then… various factions start to question the supremacy of BMI as a universal language of obesity. As Childhood Obesity News has noted before, Elizabeth Waters in Preventing Childhood Obesity: Evidence Policy and Practice wrote:

The lack of consensus about obesity-related illnesses is evidenced by differences between studies in terms of the range of co-morbidities included. Studies vary in terms of the BMI cut-off points used to define obesity, as well as the perspective employed from which to measure costs.

Also, different research projects look at things from different perspectives. When studies of any social phenomenon are compared, a plus-or-minus factor is built into the informed guesses.

Nature’s way

The UK’s Daily Mail obligingly provides a bullet-pointed summary at the beginning of a story. A recent one began with the headline, “Quarter of obese children missed by BMI tests could be at risk of diabetes and heart disease, scientists warn,” which pretty much says it all. The problem, as described here, is that what really should be measured are the person’s waist circumference and percentage of body fat.

The waist measurement appears to be the physical statistic most directly relatable to diabetes and heart disease. To borrow a line from an old pop song, “It’s nature’s way of telling you something’s wrong,” in the most simple and direct way.

Now, here’s the clincher:

Scientists at the Mayo Clinic say 25% of obese children could be missed by BMI tests because they appear to be a normal weight but have high body fat.

Lizzie Parry explains in more detail how researchers wanted get a handle on the diagnostic abilities of the BMI measurement, as compared to other methods. Published in Paediatric Obesity, the meta-study accessed data on 53,521 individuals between the ages of 4 and 18, collected by a total of 37 different studies. Parry quoted Dr Lopez-Jimenez, who warns:

[W]e need additional research in children to determine the potential impact of having high fat in the setting of normal BMI to recognize this issue and perhaps justify the use of body composition techniques to detect obesity at an early stage.

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