Measuring obesity

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Obesity is a condition associated with having an excess amount of body fat. The condition may be due to genetic, hormonal, behavioural, and environmental factors that are difficult to control when attempting to lose weight. It is one of the most pervasive chronic medical diseases that is currently in need of new strategies for prevention and treatment.

Obesity significantly increases a person’s risk of diseases and health problems such as sleep apnea, heart disease, high blood pressure, diabetes, and certain cancers. Consequently, even modest weight loss through appropriate dieting and exercise can make a difference to a person’s length of life (longevity) as well as quality of life. Prescription medications and weight-loss (bariatric) surgery have also been required for persons with severe obesity.



Because of the increased health risks associated with obesity, health promotion efforts have customarily focused on reducing the incidence and prevalence of obesity throughout the population. How we measure obesity, however, has been in flux lately.

There are simple measures, such as pinching up the fat at the underside of a person’s upper arm, and if the fat raised is above 2”, then that person is obese.

The girth of the abdomen has also served as a relatively easy method of determining obesity, whereby waist circumference above 40” in men and above 35” in women is determined to be obese.

A more scientific calculation which involves the body metabolic index (BMI) that utilizes height and weight in the formula kg/m₂, where weight is in kilogrammes and height is in metres squared, has been in recent use. A BMI over 25 is considered overweight, and over 30 is classified as obese.



However, new research has indicated that a relative fat mass (RFM) equation, based on height-to-waist circumference ratio, better predicts the percentage of whole-body fat in men and women than does the BMI. This RFM equation has resulted in fewer instances of mis-classification into the obesity categories (mild, moderate, severe and morbid obesity) in both sexes and in all the ethnic groups tested.

Although used to assess body fatness, BMI has been found to have limited accuracy in estimating body fat percentage, and so simple and low-cost alternatives to BMI that have better diagnostic accuracy for obesity in both sexes are now considered to be of greater value and importance.

Consequently, in the population that was researched and the findings published in the journal Scientific Reports, RFM was found to be more accurate than BMI in estimating whole-body fat percentage among women and men, and improved on the mis-classification of body fat-defined obesity among adult individuals of European, African, and Mexican ethnicity in the USA.

The researchers, however, recommended that it is crucial for doctors to consider what effect any excess weight might have on a patient’s health.



To develop and validate RFM, the researchers used two sets of data from the National Health and Nutrition Examination Survey in the USA. Percentages of whole-body fat were measured using dual-energy X-ray absorptiometry. They considered over 350 anthropometric measures (relating to the measurements, and proportions of the human body) in order to arrive at a simple, anthropometric linear equation that was more accurate than standard BMI.

The height/waist equation (relative fat mass) was the final model selected because of its simplicity. Use of this new equation (RFM) more accurately classified men and women in terms of the various categories of obesity across ethnic groups, and among young, middle-age, and older adults who were involved in the research.

As a high body fat percentage has been associated with increased mortality (risk of death), these findings have been considered very important for health promotion across all populations.



Another valuable consideration in regards to health is the actual distribution of fat over the body. Fat deposited centrally in the body has been associated with diabetes, fatty liver disease, and several metabolic abnormalities — to a greater degree than body fat that has been accumulated peripherally.

For the relative fat mass to be reliable however, a person’s waist circumference has to be accurately measured. This is best done by someone trained and experienced in doing the measurement, since huge differences may occur depending on what is measured as waist circumference, and how and where the measurement is taken.

In the end, the level of health care needed will depend on the clinical indicators of disease, and the RFM calculation may well turn out to be a great epidemiological tool in predicting and managing major metabolic diseases like obesity and its sequelae.


Dr Derrick Aarons MD, PhD, is a consultant bioethicist and family physician; a specialist in ethical issues in health care, research, and the life sciences; the health registrar and head of the health secretariat for the Turks & Caicos Islands; and a member of UNESCO’s International Bioethics Committee.

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