New Reference Values to Measure Obesity
In a poster presentation at the American Society for Bone and Mineral Research meeting, researchers from Hologic in Massachusetts has released new acceptable ranges for 10 body composition parameters, including a proposed replacement for Body Mass Index (BMI).
Thomas Kelly and colleagues used data from NHANES, the National Health and Nutrition Examination Survey, to establish reference data for both male and female and for three major racial groups – Whites, African Americans, and Mexican Americans. They also define separate reference values for children and adolescents.
NHANES is a program of studies by the Centers for Disease Control and the National Center for Health Statistics that is designed to assess the health and nutritional status of American adults and children. The program began in the 1960’s and surveys about 5,000 persons each year.
The new parameters include total body fat percentage, lean mass to height squared, total body bone mineral density and content, and subtotal body bone mineral density and content. Two new measures will evaluate the level of abdominal obesity, known to be a greater health risk than fat mass stored in the peripheral areas of the hips and thighs. These measures are called ratio of percent fat in trunk to percent fat in legs, and trunk-limb fat mass ratio.
Kelly also proposed a new obesity measure, the Fat Mass Index or FMI, which measures a ratio of fat mass to height squared. This measure is proposed to be a better indication of obesity than the currently used BMI, which merely indicates higher than normal weight-for-height, and may not be an accurate measure of actual body fat. In addition, BMI does not account for gender or ethnicity differences.
Data from NHANES show substantial differences in excess body fat for males versus females. The current BMI scale does not differentiate between genders, although it is normal for women to carry more fat than men. A Danish study published in the American Journal of Epidemiology proposed an alternative to the current BMI thresholds for obesity, listing separate normal values for women versus men.
In a study presented at the Endocrine Society’s 91st annual meeting in Washington DC in June 2009, researchers found that the conventional methods to measure obesity, BMI and waist circumference, did not accurately measure total fat mass and abdominal fat status in African-Americans. DEXA-measurements of 93 adults, 53 of which were African-American, found that body fat is likely to be lower in blacks than in whites of the same weight and height.
The FMI values proposed will include eight new categories of weight ranges: normal, deficit (mild, moderate and severe levels of being underweight) and obesity (excess fat or overweight and obesity classes 1, 2, and 3). Normal will be defined as 3 to 6 kg/m2 for men and 5 to 9 kg/m2 for women.
The new obesity measure does have limitations, in that it was based on prevalence data and not disease risk. Body Mass Index has used research that links body composition with health risks at each stage to determine values for normal, underweight and overweight/obesity. In addition, Body Mass Index is an easy tool for health screenings, while the new proposed Fat Mass Index requires specialized x-ray equipment to measure fat to lean body tissue.
Primary Sources: American Society for Bone and Mineral Research and Centers for Disease Control.
Written by Denise Reynolds RD
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