Thursday, December 26, 2013

Much Ado about BMI (Body Mass Index)

I learnt that the standard measure most health practitioners use is the body-mass index or BMI, which states a "healthy weight" falls within the 18.5-24.9 range of the index. Then again, I also learnt that the BMI has its own limitations as it doesn't take into much account body composition, body shape and lifestyle changes, as mentioned below.
"The formula used to calculate body mass index is flawed and skewed in favor of shorter people, says Oxford University mathematician Nick Trefethen. BMI is the standard tool used to assess healthy weight and the measure by which almost all obesity statistics are calculated.
The BMI formula — developed by a Belgian scientist, Adolphe Quetelet, in the 1830s — uses weight and height to calculate ideal weight. Those who fall within the 18.5-24.9 range of the index are considered to be at a healthy weight. A BMI of under 18.5 is considered underweight, a BMI of 25-29.9 overweight and a BMI of 30 and above obese.
The BMI has received quite a bit of criticism and scrutiny since coming into widespread use for both statistical analysis and as a proxy for health status. It’s that latter part that some take issue with — though the tool was never intended to be used in isolation for determining individual health status.As blogger Phoebe Maltz Bovy commented on Facebook:
“My impression is that BMI is about population-level trends, not whether any of us personally will get heart disease, let alone some kind of official statement about whether we do or do not look fat in those pants.”
In other words, while a BMI over 30 may be broadly associated with high body fat, obesity and/or poor health, not every individual with a BMI over 30 looks or is obese or unhealthy. As many critics note, the formula fails to take into account body composition, and there’s a big health difference between pounds of muscle and pounds of fat."
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Similarly, this website below said:

"2. BMI does not differentiate between apples and pears

For over 60 years, we have known that independent of how heavy a person is, the distribution of their body weight, or more generally the shape of their body is a key predictor of health risk. It is now well established that individuals who deposit much of their body weight around their midsection, the so called apple-shaped, are at much greater risk of disease and early mortality in contrast to the so called pear-shaped, who carry their weight more peripherally, particularly in the lower body. Thus, two individuals with a BMI of 32 kg/m2 could have drastically different body shapes, and thus varying risk of disease and early mortality.

Fortunately, a very simple measure allows you or your physician to decide whether your elevated BMI is of the apple or pear variety: waist circumference. Current thresholds suggest that a waist circumference above 88 cm in women and 102cm in men denotes abdominal obesity. Interestingly, for the same BMI level, those individuals with an elevated waist circumference have a greater risk of diabetes, cardiovascular disease, mortality, and numerous other health outcomes. Thus, as studies from our laboratory have consistently suggested, waist circumference may be a more important measure of obesity and health risk than BMI

Currently, most researchers would agree that waist circumference should be measured along with BMI to adequately classify obesity-related health risk. 
3. BMI does not always budge in response to lifestyle change

Given the number of papers my supervisor, Dr. Ross, and I have published on the topic, I would argue this is the biggest drawback of using BMI: it doesn’t always change even though you may be getting healthier. This is particularly so if you adopt a physically active lifestyle, along with a balanced diet, but are not necessarily cutting a whole lot of calories. This lack of change in BMI or body weight is all too often interpreted as a failure, resulting in the disappointed individual resuming their inactive lifestyle and unhealthy eating patterns.

However, as we have argued most recently in a paper in the Canadian Journal of Cardiology, several lines of evidence suggest that weight loss or changes in BMI are not absolutely necessary to observe substantial health benefit from a healthy lifestyle. Thus, an apparent resistance to weight-loss should never be a reason for stopping your healthy behaviours.

First, it is well established that increasing physical activity and associated improvement in cardiorespiratory fitness are associated with profound reductions in coronary heart disease and related mortality independent of weight or BMI. Second, exercise (even a single session) is associated with substantial reduction in several cardiometabolic risk factors (such as blood pressure, glucose tolerance, blood lipids, etc.) despite minimal or no change in body weight. Third, waist circumference and abdominal fat (arguably, the most dangerous fat) can be substantively reduced (10-20%) in response to exercise with minimal or no weight loss. In fact, significant reductions in fat mass often occur concurrent with equal increases in muscle mass in response to physical activity – equal but opposite (and beneficial!) changes which are not detected by alterations in body weight on the bathroom scale, and thus BMI.

So in the end, while BMI surely has its strengths in ease of use and pretty good reliability in large populations, on an individual basis, the greater focus should be on healthy behaviors: physical activity and a healthy diet. And if you must measure something, check your waist circumference."

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