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MU researcher's formulas more
If you’re an exercise fan, you may be in for a surprise the next time the trainer at the gym takes your measurements: You may find yourself a little fatter than the last time.
Don't blame those holiday treats just yet.
Your trainer may be using a new equation to estimate body fat from the skin-fold test.
Steve Ball, a University of Missouri Extension fitness specialist, developed new formulas for men and women after finding that the old equations underestimated body fat by about 3 percent.
Skin-fold assessments measure the thickness of fat folds at various points on the body to estimate the percentage of body fat.
"Body fat percentage is a better predictor of overall health status, compared to the commonly used Body Mass Index, which is only for a given height and weight," Ball said.
Men with body-fat percentages of 25 percent or more, and women with percentages of 35 or more, would be considered overly fat. Those percentages generally would put such individuals at risk for diseases and complications associated with obesity, he said.
Generally, Ball said, the healthy range for men is 10 percent to 20 percent body fat. For women, the healthy range is about 17 percent to 28 percent.
Having a very low body-fat level is generally reserved for athletes interested in performance, he said.
"It is important to note that having a percentage that's too low also can put people at risk," Ball said. "Essential body fat, or the amount of body fat needed to function, is 5 percent for men and 12 percent for women."
To develop the new equations, Ball used a relatively new technology in the health and fitness field, dual energy X-ray absorptiometry. He compared DXA scans, which measure fat, lean mass and bone, to the Jackson-Pollack skin-fold equations trainers have used for more than 20 years.
Ball found that the body-fat percentages were 4.6 percent higher for women and 3 percent higher for men using DXA than the Jackson-Pollack equations.
The difference, Ball said, is that the old formulas were devised from hydrostatic, or underwater, weighing, which just measures fat and everything else, Ball said.
Because the DXA-based formulas take bone density into account, there is less room for error, Ball said.
"We've got our equation error down to about plus-2 percent, so that's a significant improvement," he said. "The more things you measure the more accurate you can be."
Ball's research was published in the December 2006 issue of Measurement in Physical Education and Exercise Science, a journal for fitness instructors and trainers.
In addition to athletes, Ball said more accurate assessment could be important to people in the military and other careers, where physical condition is an important factor in promotions.
For the average person, Ball said, the real value of skin-fold assessments is tracking changes in body composition.
"I tell people to look in the mirror and see how their clothes fit. It can tell you a lot," he said. "But people always want to know what the number is. It's always better to be as accurate as possible, and that's why these new equations should be used."
Source: Steve Ball, 573-882-2234
Last update: Tuesday, May 05, 2009