Study: Better Method Needed to Assess Body Fat
< Apr. 04, 2012 > -- Women who calculate their body mass index (BMI) to figure out if they are obese may be missing the mark.
A new study says that BMI may not be accurate in determining obesity in women, particularly after menopause.
Based on BMI alone, "roughly 30 percent of Americans are obese," says study author Eric Braverman, M.D., at the Path Foundation in New York City. "But when you use other methods, closer to 60 percent are obese."
BMI is one of the main ways that doctors calculate whether a person is of normal weight or is overweight or obese. It's an estimate of the amount of body fat you have determined by your height and weight. A normal BMI is 18.5 to 24.9. A BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or greater rates the person as obese.
But experts have long noted that the BMI measurement can be faulty. According to the National Heart, Lung, and Blood Institute, BMI can overestimate the amount of body fat in an athlete or other muscular person, and underestimate it in older people who have lost muscle.
The problem is especially seen among women, because "as women age, they tend to lose bone and replace muscle with fat," Dr. Braverman says.
For the study, the researchers used a special imaging procedure called a dual-energy X-ray absorptiometry (DEXA) scan to measure body fat more accurately. The DEXA scan can also measure muscle mass and bone density and is used as a screening test for osteoporosis.
Dr. Braverman and Nirav Shah, M.D., the health commissioner for the state of New York, collaborated to screen 1,300 people via a DEXA scan. Forty-eight percent of the women who were screened with DEXA had not been classified as obese by their BMI but were in fact obese by actual percentage of body fat.
In contrast, a quarter of the men who went through the screening had been incorrectly rated as obese by their BMI, but were not, according to the DEXA results.
The National Institute of Diabetes and Digestive and Kidney Diseases acknowledges that it's not easy to take an exact measurement of body fat. In addition to the DEXA scan, the only other precisely accurate test is to weigh a person underwater. Both of these tests are too expensive to offer for routine screening.
A different test
But Dr. Braverman argues that a blood test to measure leptin levels could be used instead. Leptin is a hormone that helps regulate appetite and metabolism.
People with leptin levels below 5 nanograms per milliliter (ng/mL) are considered thin. Levels up to 10 ng/mL are considered normal weight. Leptin levels of 10 to 30 ng/mL could be corrected with diet and exercise, Dr. Braverman says.
William O'Neill, M.D., at the University of Miami School of Medicine, says that more research is needed to find out how leptin is linked to obesity.
Whether lowering leptin levels alone will reduce obesity isn't known. "We don't know if leptin can be a primary target -- it may just be a marker of body fat," Dr. O'Neill says.
The study was published in this month's journal PLoS ONE.
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Health Effects of Obesity
Obesity is linked to high blood pressure, type 2 diabetes, heart disease, stroke, osteoarthritis, and high cholesterol levels. It also has these effects:
Obesity can raise cancer risk. The National Cancer Institute has found that obesity and a lack of exercise can increase your risk for cancers of the breast (after menopause), colon and rectum, kidney, esophagus, pancreas, thyroid, gall bladder, and the endometrium, or lining of the uterus.
Obesity can lead to heart attacks in younger adults. Obesity has been linked to a rise in fatal heart attacks in young people. In people ages 15 to 34, the CDC says, the death rate from heart attacks rose 32 percent among women and 10 percent among men during the 1990s.
Obesity can ruin your day. For those who are obese, simple tasks like carrying groceries, walking up stairs, kneeling, and stooping are more difficult. Sleep apnea, which is more prevalent among obese people, is often a cause of lethargy during the day.
Always talk with your health care provider to find out more information.
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CDC - Defining Overweight and Obesity
CDC - Healthy Weight - It's Not a Diet, It's a Lifestyle!
PLOS One - Measuring Adiposity in Patients