January 2011 Issue

When Thin Is Fat — If Not Managed, Normal Weight Obesity Can Cause Health Issues
By Sharon Palmer, RD
Today’s Dietitian
Vol. 13 No. 1 P. 14

A baby boomer with a healthy weight may seem like a rare and beautiful thing to a dietitian. After all, just like other age groups, the baby boomer generation has experienced its fair share of obesity. Boomers have a higher rate of obesity compared with previous generations, according to a 2009 analysis by the Trust for America’s Health, which reports that in every state the rate of obesity has grown to be higher among the oldest boomers: 55- to 64-year-olds. Statewide data comparing the increases in obesity with past generations in this age group range from 5.2% in New York to 16.3% in Alabama, where nearly 39% of the oldest boomers are obese. That translates into a new generation of Medicare patients who are obese. So when a normal weight client walks into your office, you can breathe a big sigh of relief, right?

Not so fast. Even people with a normal body mass index (BMI) can fall prey to a condition called normal weight obesity (NWO). It may sound like an oxymoron, but it’s a real health concern, according to a growing body of evidence. It’s possible to fall into the normal weight range and still have weight-related health issues, says Francisco Lopez-Jimenez, MD, a Mayo Clinic cardiologist who is leading this field of research.

New research indicates that healthcare professionals should do more than monitor patients’ BMI to determine their health status; they should also monitor patients’ levels of body fat. While BMI is still considered a very helpful tool for evaluating weight status, it does have its limitations. For example, individuals who are very muscular (eg, athletes) may have a falsely high BMI because the measurement does not account for increased weight due to muscle composition. And BMI does not take into account central adiposity, which has been associated with increased disease risk. And now NWO is another reason to look beyond BMI.

Mayo Clinic researchers, who presented their findings in 2008 at the American College of Cardiology’s Annual Scientific Session in Chicago, characterize NWO as having a normal BMI with a large percentage of body fat—more than 30% for women and 20% for men. The researchers studied 2,127 normal weight adults, equally divided between men and women, relying on data from the Third National Health and Nutrition Examination Survey. They discovered that more than one half of the subjects had NWO. In fact, NWO is so prevalent that Lopez-Jimenez estimates up to 30 million Americans have the condition.

NWO Brings Health Risks
What’s the big deal about NWO? Unfortunately, this condition carries risks similar to overweight or obesity. Lopez-Jimenez explains that in NWO, people have little of the good stuff, such as muscle mass and dense bones, and too much of the bad stuff: body fat. He adds, “Elderly people [who] are normal weight are likely to have NWO as they have lower muscle mass and lighter bones.”

The Mayo Clinic researchers reported their findings in the European Heart Journal in March 2010. They discovered that in the group of normal BMI subjects with the highest tertile of body fat (greater than 23.1% in men and greater than 33.3% in women), the prevalence of metabolic syndrome in subjects was fourfold higher compared with the low body fat group. In addition, this group of NWO subjects had a higher prevalence of dyslipidemia, hypertension in men, and cardiovascular disease in women; in fact, women with NWO showed a 2.2-fold increased risk of cardiovascular mortality compared with the low body fat group.

In another study, published in the January 2007 issue of the American Journal of Clinical Nutrition, researchers found that women with NWO experienced higher biomarkers of inflammation, putting them at risk for early inflammation, cardiovascular disease, and metabolic syndrome.

Managing the Condition
What can healthcare professionals do about NWO and its accompanying risk factors? Lopez-Jimenez suggests that patients in the normal weight range, particularly those who are sedentary or eat a suboptimal diet, be screened for body composition on a routine basis.

“It’s in the intermediate range—when you fall in the normal or mildly elevated BMI—that you should determine if you’re body fat is too high. It’s not necessary for everybody to measure their body fat to see if they have normal weight obesity. If someone is very skinny (below a BMI of 18.5), they don’t need to check body fat because their body fat is likely very low. And if someone is overweight—over 30 to 35 BMI—then we know that they already have high body fat,” says Lopez-Jimenez.

Unfortunately, assessing patients’ body fat percentage is not as simple as having them hop on a scale. But there are some reliable methods for measuring it. Just steer clear of techniques that are either very accurate but impractical or very simple but unreliable. Lopez-Jimenez suggests the following two methods for measuring body fat composition—both reliable and practical:

• Bioelectric impedance or bioimpedance: Utilizing an electrical current to measure body composition levels, this method is becoming popular at clinics and fitness centers because of its ease of use and relative low cost. But people shouldn’t fall for home versions of bioimpedance machines sold in drug stores because they may not be accurate.

• BOD POD or air displacement: The BOD POD, a machine that looks like a plastic space capsule, uses air to displace body volume to evaluate body fat composition. Available in many locations (eg, hospitals, fitness centers), the BOD POD is very reliable and simple to use.

Once a professional has established the presence of NWO, treatment should focus on more than simply instructing a patient to eat a balanced, healthful diet; this condition depends much more on exercise. Dietitians help individuals who are overweight achieve weight loss. In NWO, a patient’s goal is not to lose weight but to change body composition. Lopez-Jimenez says, “If you’re at normal weight and you lose more weight, you may not be changing your body fat percentage. You could be losing muscle, not fat.”

Lopez-Jimenez suggests that professionals encourage a combination of aerobic exercise and strength or resistance training exercise in addition to a healthful diet to alter body fat composition—yet another reason to remind clients how critical exercise is at any age.

— Sharon Palmer, RD, is a contributing editor at Today’s Dietitian and a freelance writer based in southern California.