Adults Featured Kids Seniors — 15 January 2013

By: Jodie Shield, RD

Just when everyone was making their New Year’s resolution to lose weight, a new government study came out reporting that people who are moderately heavy (30 or so pounds above a healthy weight) have a 6% advantage of living longer than people who are not overweight. Confused?  You’re not alone.  Even I had to scratch my head over this news.  But hold your fork. I did some research and discovered that once again, BMI seems to be fueling the fire, especially in this study.  Here’s my take on the study and the truth about BMI.

What is BMI? BMI stands for Body Mass Index. Your BMI is basically a number that you calculate using your height and weight (Click here to calculate your BMI).  Although BMI doesn’t directly measure body fat, the number does reflect how much your weight affects your risk for developing various weight related problems such as heart disease or diabetes.  For adults, there’s no difference in BMI weight ranges for age. That’s because studies have found that the health risks seem to be the same regardless of gender or age.  So men and women can use the same BMI classifications:

  • Underweight: Below 18.5
  • Normal: 18.5 – 24.9
  • Overweight: 25.0 – 29.9
  • Obese: 30.0 and above

Why do you calculate BMI differently for children? When BMI is used to evaluate weight in an adult, the BMI number alone is sufficient to place the individual in a weight category (underweight, healthy, overweight or obese).  However, you can’t use a BMI chart for kids because their body fatness changes as they grow.  Also, girls and boys differ in body fatness, especially as they mature, so you need to use gender-specific criteria for evaluating BMI in kids. To calculate BMI for children between the ages of 2 and 20, you need to use BMI growth charts which will help you determine their BMI-for- age percentile (click here to calculate your child’s or teen’s BMI). Here’s how to interpret their BMI:

  • Underweight: BMI is less than the 5th percentile.
  • Healthy Weight: BMI is between the 5th and 84th percentiles.
  • Overweight: BMI is between the 85th and 94th percentiles.
  • Obese: BMI is equal to or greater than the 95th percentile.

How accurate is BMI? For most people, the BMI is a fairly reliable indicator of body fatness.  As I mentioned earlier, even though you don’t measure body fat directly, researchers have found that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA) – the gold standards.  However, BMI is not perfect. Since muscle and bone weigh more than fat, if you or your youngster are very athletic and muscular, your BMI may be too high because of the extra muscle mass, not body fat.

Should you be concerned about your BMI?  Absolutely!   BMI is best used for screening potential weight-related health problems.  I always caution people: one BMI number doesn’t tell the whole weight story – it should never be used to make a final diagnosis. If you, or a family member, has a BMI above or below healthy weight, consider it a red flag.  I highly recommend you check with your doctor for additional testing.

Speaking of weight stories, let’s get back to the study I mentioned earlier. There was a major flaw in the study design. Unfortunately, the healthy weight group included a mix of lean and active people along with: heavy smokers, patients with cancer and other medical conditions that cause weight loss, and frail elderly folks who had lost weight due to rapidly declining health. Because the overweight and obese study groups were compared to this mix of healthy and ill persons who have a very high risk of death, this led to the false conclusion that being moderately overweight is beneficial.  Also, what most news stories failed to highlight was that people who were 60 or more pounds over healthy weight (extremely obese) had a 29% greater risk of dying early compared to the healthy weight folks.

Bottom Line: For your health’s sake, it’s important to know your BMI and to make sure it;s in the healthy weight range.


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About Author


Jodie Shield, M.Ed., R.D., L.D.N. Jodie Shield has been a consultant and spokesperson in the field of nutrition for over two decades. A former national media spokesperson for the American Dietetic Association (1989-1995), she has worked extensively with the Rush-Presbyterian-St. Luke’s Medical Center and taught nutrition and medical dietetics at the University of Illinois. Currently she is a complemental faculty member of the College of Health Sciences in the Department of Clinical Nutrition at Rush University in Chicago.

(1) Reader Comment

  1. A BMI calculator uses this equation to come up with a number index. 1st, divide bodyweight in bodyweight through height in inches squared and after that multiply by a conversion factor of 703. Right before the inception of the BMI calculator as an acceptable entire body type measurement, several professionals were guided only simply by the measurement of what somebody weighed when stepping on a scale.

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