DXA: A New Way to Monitor Weight Loss

By Mary K. Oates, MD, and Steven M. Petak, MD, JD, FACE, FCLM

Has your doctor ordered a DXA (dual energy x-ray absorptiometry) scan to test your bones for osteoporosis? The same machine that is used to evaluate your spine and hips for brittle bones also takes a full-body image of your fat, muscle and bone. It is called DXA Total Body Composition Analysis (DXA-TBCA). DXA-TBCA can precisely monitor how much fat you lose during weight loss or how much muscle you gain during a physical activity program. Better yet, it can show all of these changes at one time, or over the course of a weight management program.

For this test, you simply lie fully clothed on a padded x-ray table for 5-12 minutes. The x-ray exposure is minimal – less than the amount of natural radiation you receive each day from the environment. The test reports your total body fat and the amount of muscle and bone in your body. It also gives you a dramatic, very personal, full-body x-ray picture. It can clearly show you and your doctor if you have excess fat around your waist. This type of fat is known to lead to heart disease, diabetes, high cholesterol, and high blood pressure.

There are older, less sophisticated methods to assess body composition that are still widely used today. They are readily available at gyms, offices and even at home and on the Internet.

  • The digital scales that give a percentage fat reading are actually a form of Bioelectric Impedance Analysis, known as BIA. These scales give total body fat readings, but cannot show you the fat distribution in your arms, legs or waist like a DXA-TBCA scan can.
  • Skinfold measurements, or the “pinch test,” measure the fat underneath the skin at various sites with calipers. These measurements do not estimate the deep fat around the waist.
  • Your doctor may also use a measuring tape to measure your waist and hip circumference. The waist to hip ratio (WHR), which is the circumference of the waist divided by the circumference of the hips, has been used as a simple method for determining body fat distribution. Health risks have been shown to increase with increasing WHR, although the standards for risk vary with age and gender.

If you have been tested by any of these methods, realize that the values are not the same. DXA-TBCA gives you the most valuable information because it gives you fat, muscle and bone values.

DXA-TBCA reports provide fat mass, muscle mass, and bone mineral content for the whole body, waist and hip region, arms, legs and trunk. Absolutely no other body composition method can provide these regional values for body fat distribution. The DXA-TBCA waist and hip region is particularly important, because we can measure abdominal fat, which is recognized as an important predictor of the health risks of obesity. This is the analogy of the “apple and pear” body shape. People with an apple shape have more fat in the waist, and they have a higher risk for high blood pressure, type 2 diabetes, high cholesterol, heart disease and early death. People with a pear figure have more fat in the hip and thigh area, and have a lower health risk. Researchers have found that DXA-TBCA is able to determine cardiovascular and metabolic syndrome risks by looking at percent total body fat and regional values. Our research has shown that healthy, active men will have an average body fat of about 20% and women will average about 30%. Contrast this to some of the contestants on the NBC reality weight loss show The Biggest Loser. These contestants started the show with body fat in the 50% range. We used DXA-TBCA to follow their dramatic weight loss (Figs. 1, 2). On the other end of the spectrum, serious athletes, like male track and field athletes average 13% total body fat, and swimmers average 16%.

DXA-TBCA can be found today in many medical practices, especially those that help people lose weight. Endocrinologists, internists and rheumatologists that already offer DXA for osteoporosis testing also may offer DXA-TBCA. Executive wellness programs and insurance physicals sometimes bundle these procedures with nutritional consultations and physician-directed weight loss programs. You may also want to ask if they do resting metabolic rate (RMR) testing. RMR is the amount of calories your body burns in a day when it is at rest. This can be helpful to identify a daily calorie limit to induce weight loss.

DXA-TBCA scans can be a powerful motivational tool for patients in weight-loss programs. One advantage of using DXA-TBCA in monitoring weight loss is that we now can see that if you lose 20 pounds, it could be from losing 20 pounds of fat, or from losing 30 pounds of fat and gaining 10 pounds of muscle. This is especially helpful as patients reach plateaus on the scale. DXA-TBCA can show that in reality they are continuing to lose fat and adding muscle.

Mary K. Oates, MD, graduated from Ohio State University College of Medicine in 1990 and the University of California, Irvine residency program in Physical Medicine and Rehabilitation in 1994. She has been specializing in the diagnosis and treatment of Osteoporosis since 1997 and is a Certified Clinical Densitometrist. Both Dr. Mary Oates and Dr. David Oates are faculty advisors to FORE (Foundation for Osteoporosis Research and Education). Together, they provide instruction for the limited license school for DXA technologists at their Santa Maria office.

Steven M. Petak, MD, JD, FACE, FCLM, has been an associate at the Texas Institute for Reproductive Medicine and Endocrinology since 1989 and is also the Director of the Bone Densitometry Unit and Osteoporosis Center with the Institute. Dr. Petak received his MD degree from the University of Illinois Medical School in Chicago. He completed three years of training in general internal medicine at the University of Texas Medical School in Houston where he also completed a three-year Fellowship in Endocrinology and Metabolism. Dr. Petak is the President of the American College of Endocrinology (ACE).