A close friend came back from her annual physical agitated and anxious.
"I'm fat!" she wailed. "According to the doctor, my BMI is 29!"
"Wait a minute," I remonstrated, "How can that be? You told me only a few days ago that you rarely go clothes shopping since you are wearing the clothes you bought years ago, because you like them more than the clothes in the stores these days. How can you fit into these same clothes if you are suddenly fat?"
"But that is what the chart said. And if my BMI increases, I am going to be morbidly obese."
I looked at her. I have known her for years, and her size has always been the same with one difference: I was looking down at her, and she used to be taller than me.
"Uh, did your doctor tell you how the BMI is calculated?" I asked.
"It has something to do about my height and weight. I was so upset, I didn't pay much attention."
"OK. I think I know why your BMI is in the fat range." I told her. "Did the doctor measure your height? Have you shrunk?"
I was right. She lost two inches from her 5'4" height over the past 18 months, a considerable loss, but not unusual for a woman of over 70.
"Open the calculator on your phone so you can figure out your BMI yourself. You will see why your height is making it appear as if you are fat."
She weighed 131lbs and her height in inches was 62. She calculated her BMI by:
1. Squaring her height in inches (multiplying her height 62 by itself);
2. Dividing her weight, 131 lbs, by 3844, her height squared; and then
3. Multiplying that number by 703.
Her BMI was 23.9.
"You are not in the fat category," I told her after checking BMI charts on several internet sites. "You would have to have a BMI of 25 to be considered obese. But anyway, that is not the point. Your BMI went up because of changes in your height, not weight. You may have to have your clothes shortened, but certainly not widened."
After she left, feeling a little lighter in spirit (pun intended), it occurred to me that the BMI is certainly an unreliable measurement for people whose height changes due to age, or orthopedic problems that cause that they be bent over, or have spinal fusion. Shrinking into obesity is not the same as gaining weight without losing height.
Should measures other than the BMI be used to determine obesity?
Another common measurement of obesity is the waist/hip ratio. This is considered to be a good indicator of fat, especially around the mid-section and should not be affected by loss of height. I emailed my friend these instructions:
1. Measure your waist circumference. Locate your upper hipbone and put a tape measure (cloth, not metal) around your bare stomach just above the bone. Don't squeeze the skin and breathe out while measuring.
2. Measure your hip circumference. This requires looking in the mirror and locating the widest part of your hips. Put the tape measure at this point and measure around your body.
3. Divide the waist circumference by the hip measurement.
I sent her the chart of waist-to-hip ratio data for men and women, in case her husband was curious about his own measurements.
The chart is simple. A 0.95 or below waist-to-hip ratio for men, and 0.80 or below for women, is considered normal and low risk for obesity-related disease like diabetes.
A ratio of 0.96-1.0 for men and 0.81-0.85 for women is considered a moderate risk. Above that, individuals are at high risk.
She did the calculations and was pleased to note that she was at a low risk but her husband was starting a diet.
The anxiety and concern my friend experienced when told she was obese (according to her BMI) could have been avoided by asking some obvious questions: Has your weight and clothing size increased during the past year? Certainly if the answer had been yes, then the BMI chart or waist-hip ratio measurements would have been useful to see whether she was hovering close to the obesity line.
Moreover, if her weight had decreased (so it appeared that it compensated for her diminished height), it would have been an oversight to ignore the reasons for the lower weight. She might have lost it dieting or through increased exercise. But what if she had lost weight due to a loss of muscle mass and not fat? The BMI chart would show her in a healthy weight category but a shrinking muscle mass would put her at risk for osteoporosis and falling due to lack of muscular strength. Models, for example, whom are forced to live on semi-starvation rations to look two-dimensional in photographs, may have envious BMI scores. But these scores do not reflect their scrawny muscles and fragile bones.
Lesson learned? Charts are useful, but should never replace common sense and simple observation.