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Rapid Responses: Rapid Responses published:
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BM"I" may be a risk factor in obesity but cannot be a measure of obesity in itself.
- C Kevin Connolly (1 October 2006)
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Body Mass Index, Waist Circumference, or Pulse by Mass Index as Risk Factor ?
- Enrique J. Sánchez-Delgado, MD, Professor (2 October 2006)
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MRI not CT
- Philip Smart (10 October 2006)
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C Kevin Connolly, Retired Physician Aldbrough St John Richmond DL11 7 TP
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Dr Han and colleagues state that BMI is traditionally used as a measure of obesity without discussing the reasons. They allow that a BMI of greater than 30 may be acceptable in an athlete without pointing out that in the modern sportsman, who is often tall, that his apparent obesity may be as much due to his stature as his muscle bulk. Mathematically BM“I” is not an index, which is by definition dimensionless. Effectively it has the dimension of length (assuming similar density length3/length2). The index that does measure obesity is ponderal index (mass/length3). BMI is used because it was observed pragmatically to reflect risk better than ponderal index in the populations observed. These were western cohorts whose date of birth put them at risk in the mid 20th century. The implication of this, incidentally supported more recently by the value of absolute waist measurement, is that obesity is more dangerous in these cohorts in the tall than the short, or alternatively, and possibly more likely, that adequate storage fat is more valuable in small people. This problem has been barely recognized let alone explored or explained. As in individuals of the same proportions BMI is proportional to height, the effects are not small. A person six foot (1.8m) tall, but of the same proportions as one five feet (1.5m) tall and an ideal BMI of 25, has a BMI of 30 and so is considered obese. A boy of 4 feet 2 inches (1.25m) and of the same proportions as a very obese man five feet ten inches (1.75m) tall and a BMI of 35 would be assessed as of ideal weight having a BMI of 25. There are conflicting practical implications. On the one hand, as adult height has increased over time, any rise in prevalence of obesity may be overestimated as BMI increases by approximately 0.6 for every inch (2.5 cm) increase in mean height. On the other hand, although the above comparison may not be strictly fair, the use of BMI in childhood may grossly under estimate the implications for obesity in future adult life. BMI may determine the maximum size of elephants in reflecting mass/cross section area of bone and its inverse (surface area/mass) the smallest size of shrews, but it cannot be used as a measure of obesity when comparing populations of differing date of birth, stature, race or social circumstances, however good a measure of risk it may be in some cohorts. Competing interests: None |
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Enrique J. Sánchez-Delgado, MD, Professor, Director for Medical Education. Internist and Clinical Pharmacologist Hospital Metropolitano Vivian Pellas, Managua, Nicaragua
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Dear Editor, Thang S Han et. al., BMJ, 30 Sept. 2006, compare BMI (Body Mass Index)with waist circumference and disease risk. I agree that BMI is the most widely and easily measured proxy for obesity, but other markers can be better outcome predictors. The Pulse by Mass Index (PMI) can also be a good marker as a cardiovascular risk factor as we published in the section Correspondence of Lancet, 13 March 1999. We showed that the PMI (Resting Heart Rate by Body Mass Index divided by 1730 or 72x24) correlates very highly (r= 0.95) with the global cardiovascular risk according to the Framingham Heart Study. When calculated either fasting or over two hours after eating, it reflects not only obesity, but also the oxidative metabolic rate, stress, sympathetic stimulation, hyperinsulinism or insulin resistance, inflammatory activity, physical fitness and side effects of drugs like water retention, potent vasodilatation and tachycardia. It is well known that not only obesity shortens lifespan, but also the higher resting heart rate (in all the studied species). The higher the PMI (it should be less than one), the higher the global cardiovascular risk, and more so when the PMI is over 1.2 to 1.3. I suggest that the BMI should be adjusted to PMI, to have a more exact picture of the global risk. The waist circumference should be compared with the PMI. The calculation of the global risk according to Framingham or the NCEP-ATP III should also be compared to the PMI, to individualize the equation and have a more exact prediction of risk. Prof. Enrique Sánchez-Delgado, MD. Internist and Clinical Pharmacologist. Director for Medical Education. Hospital Metropolitano Vivian Pellas. Managua, Nicaragua. esanchez@metropolitano.com.ni Competing interests: None declared |
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Philip Smart, Associate Specialist in Radiology King's Mill Hospital NG17 4JL
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The cross-sectional images on page 697 are examples of magnetic resonance imaging (not computed tomography as stated in the caption). Competing interests: None declared |
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