Does BMI Underestimate Obesity in Children?

Sharon L. Mulvagh, MD; Francisco Lopez-Jimenez, MD


August 06, 2014

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Is Childhood Obesity Underestimated?

Sharon L. Mulvagh, MD: I am Dr. Sharon Mulvagh, Director of the Women's Heart Clinic at Mayo Clinic. Today during Mayo Clinic Talks, we will be discussing childhood obesity. I am joined by my colleague, Dr. Francisco Lopez-Jimenez, who is Director of Preventive Cardiology at Mayo Clinic. He has just written a fantastic article[1] that was featured in the Wall Street Journal.[2]

Obesity is an increasing concern in our youth. It is estimated that almost 20% of children aged 6-19 years are obese as classified by body mass index (BMI). But Francisco commented that we may be missing the mark on this by underestimating the number of obese children when we rely solely on the BMI. Can you elaborate? What is the BMI cut-off point for defining obesity in children? Is it different from that in adults?

Francisco Lopez-Jimenez, MD: Yes. In children, obesity is classified according to standardized tables and percentiles. A child is considered obese if he or she falls in the top 5% of BMI. What is considered obese for a 5-year-old boy will be different from the cut-off for a 15-year-old boy. The top 5% differs according to age and sex.

Dr. Mulvagh: Is it like the growth curves?

Dr. Lopez-Jimenez: Yes. We have no formula to tell us which children are obese. Pediatricians have to check the tables.

Dr. Mulvagh: What did you find in your study that raised a concern that we might be underestimating the number of kids who have obesity?

Dr. Lopez-Jimenez: We performed a meta-analysis of data from 37 individual studies testing these same hypotheses, and found that even though the specificity of obesity diagnosed by BMI was very good (meaning that if somebody had a high BMI, the likelihood that the patient had obesity was pretty high), the sensitivity appears to be suboptimal.

Up to 25% of children with normal BMIs will have an excessive amount of fat when measured by other means. According to the dictionary, the root of the word obesity is "excess fat."

However, for convenience, we diagnose obesity according to the BMI, on the basis of epidemiologic studies showing that in the overall population, a high BMI reflects obesity. That is not the same as saying that in a patient with a normal BMI, we can entirely rule out excessive fat when measured with a more advanced or a more sophisticated technique to measure the fat.

Dr. Mulvagh: It's really the body composition that we are looking at here, because the excess fat or adiposity increases the fat-to-muscle content in the body (and particularly in children) that poses later risk for cardiovascular disease, diabetes, metabolic syndrome -- all of those issues. It's similar to what you have found in terms of "normal-weight obesity," or normal-BMI individuals who have increased risk for cardiovascular disease.

Dr. Lopez-Jimenez: That is the main issue. When we talk about obesity, strictly speaking, we are trying to say "excess fat." Similarly, when we use body weight as a measurement of obesity, the problem is that both muscle and fat add to the total body mass and therefore to the total weight. If somebody has a lot of muscle mass but minimal fat, the BMI will still be high, which is the case in a bodybuilder.

However, we don't see enough bodybuilders to be able to say that a high BMI is normal or does not reflect obesity. The main problem is the other way around -- when people have minimal muscle and excessive fat. Because they don't have a lot of muscle, the total weight falls within normal limits, which is the case in this 25% of children who seem to have a normal body weight but still have a lot of fat.

Replace the BMI?

Dr. Mulvagh: This simple formula for BMI, kg/m2, doesn't work because it doesn't take into account body composition.

Should we be looking at other ways of measuring the obesity rate in children, such as evaluating body composition? There are such methods as bioelectric impedance, BOD POD® (Cosmed; Rome, Italy), or dual x-ray absorptiometry (DXA). Should we measure body composition, or can we just do a really good history and physical examination, ask about eating and exercise patterns, and use common sense to say that we need to shift into a higher gear here?

Dr. Lopez-Jimenez: We don't know the best way to assess adiposity in clinical practice. Thirty years ago, investigators were measuring body fat using very sophisticated, complex techniques that obviously cannot be implemented in clinical practice. Now, perhaps the other extreme, in terms of simplicity, will be measuring the waist circumference and assessing the waist-to-hip ratio to determine whether children have central obesity. That would be the simplest way.

We don't believe that the skin-fold technique to measure body fat is very good. Many studies have challenged the accuracy of that technique. However, other techniques, such as bioimpedance or air displacement plethysmography, seem to be accurate and are not necessarily complicated to perform.

In adults, we use the DXA machine, which is the same machine that is used for bone densitometry. That is a very accurate way to assess body fat and muscle mass. We believe that in the future clinicians, including pediatricians, will be more inclined to use those techniques, particularly in children who might seem to be obese, but the weight just doesn't prove it.

Dr. Mulvagh: In your meta-analysis, were there any signals in children that we should be more concerned about? For example, were there any sex or race differences?

Dr. Lopez-Jimenez: The only subgroup that shows some signal was boys: The measurement appears to miss more obese boys. That was a surprise, because I would have anticipated it to be the other way around -- that it would miss more obesity in girls.

Unfortunately, those studies don't provide information on exercise or lifestyle to use those measures as predictors. So the short answer is no, we don't have an easy way to anticipate or predict who has obesity among those with normal weight.

Dr. Mulvagh: Why were you surprised about the difference between boys and girls? In clinic, it seems that if the BMI in a woman is increased, it correlates with excess adipose content. But in a man, if the BMI is increased, it's frequently because he has more muscle mass. Why were you surprised to find this to be reversed in children?

Dr. Lopez-Jimenez: It is for the same reason that males tend to have more preserved muscle mass. The problem that we saw in this particular analysis was that BMI was actually missing people with obesity. The point is that I anticipate that in boys, the problem will be the other way around. Specificity was not an issue. If the BMI was high, the chance of having obesity in both boys and girls was very high: more than 90%.

Dr. Mulvagh: But if the BMI was normal in a boy, he still might be obese. Do you think this is hormonal, or pubertal? Is there anything that we could hypothesize to explain this finding?

Dr. Lopez-Jimenez: I don't think so. It is probably just a measurement issue, or perhaps a random finding. The differences were not so remarkable as to suggest that there is something major going on. The point is that the problem of limited sensitivity applies to both boys and girls.

Dust Off the Measuring Tape

Dr. Mulvagh: How should overweight and obesity be measured in children? What is the general recommendation?

Dr. Lopez-Jimenez: A lot of research needs to be done. Adults who have excessive amounts of fat with normal weight (normal-weight obesity) have a higher prevalence of the metabolic syndrome, abnormal cholesterol, and impaired fasting blood glucose. Women with normal-weight obesity have twice the mortality rate of women with normal weight and normal fat.

The next step is to prove that children who have a normal BMI according to these charts, but excessive amounts of fat according to direct measurements of fat, have a higher prevalence of metabolic syndrome and other alterations in metabolism that will predict cardiovascular disease. Obviously, we cannot do a cardiovascular mortality study, because that would be a long study. But there are ways to measure the prevalence of subclinical atherosclerosis or early signs of endothelial dysfunction, and if those are proved, it will justify the use of other techniques to be more accurate when estimating obesity in children.

Dr. Mulvagh: How about waist circumference? Should we be making that a standard? For both our children and the adult population, we should be measuring waist circumferences. Isn't it a cheap and valuable technique?

Dr. Lopez-Jimenez: Definitely, and it has been demonstrated that by assessing fat distribution (which is what the waist-to-hip ratio does), you can identify a subset of individuals at high risk for diabetes, metabolic syndrome, and cardiovascular disease. It's simple, and takes only a measuring tape. This might be the next step for pediatricians. I don't know whether measuring body fat directly will add more information, but these results suggest that more research is needed to find out about normal-weight obesity in children and how it can affect their health.

Dr. Mulvagh: What is the next step? You identify this, and then what do you do? That is the 1000-pound elephant in the room. It behooves the clinician who is caring for those children to engage the child and the parents. Parents play a huge role in modifying lifestyle, and need to recognize and deal with weight concerns.

Dr. Lopez-Jimenez: The major significance of our findings is precisely that. When parents are told that their children are not obese because of the BMI, but the children are in fact obese according to the level of fat, the parents will not recognize the urgent need to modify behavior, improve nutrition, and encourage activity. Although we wish every child were active and eating a healthy diet, that's not what happens. A child who is very active, who is doing 20,000 steps a day, exercising, playing around, and eating very healthy foods, is very unlikely to become obese.

Dr. Mulvagh: That goes right to Washington, where right now some very important decisions are being made and efforts need to be supported for optimizing health and nutrition for our children in the schools. We applaud those efforts, and this medical evidence shows the importance of continuing those efforts, as well as in our homes every day.

Thank you very much, Francisco, for these very important insights, and thanks to our listeners for tuning in to Mayo Clinic Talks at on Medscape.


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