The Skinny on BMI


Millions of Americans have eaten way too much this recent holiday season and have put on some additional pounds.  You may be one of us.  Is losing weight one of your resolutions for the New Year?  I’d be willing to bet it is.  And that’s a good goal for most people — more than two-thirds of American adults are overweight, and about a third are considered obese.  Face it, we’re a fat country.  You’ve probably read lots of articles on how to lose weight, so you already know how to do that; we’ll go in a different, but related, direction here.  Let’s look at something called the Body Mass Index, or BMI.

BMI is a measure of the ratio between weight and height.  It’s used as an indicator of the amount of body fat for most people (we’ll see some exceptions later), and it’s a good gauge of one’s risk for certain diseases.  BMI is a screening tool to identify whether an adult is at a healthy weight.  It is measured by dividing a person’s weight in kilograms by the square of height in meters.  A simpler way of calculating your BMI in American terms is to multiply your weight in pounds by 703, divided by your height in inches, squared. Here’s an example: You weigh 180 pounds and you’re 72 inches tall.

180 × 703 = 126,540
72 × 72 = 5,184
126,540 ÷ 5,184 = 24.4 BMI

What does a BMI of 24.4 mean?  It means that a 6-footer who weighs 180 pounds is at a normal weight.  A BMI score below 18.5 indicates underweight, 18.5 – 24.9 means normal weight, 25.0 – 29.9 gets into the overweight area, and one is considered obese with a Body Mass Index of 30.0 and above.

But what about athletes and others who have a muscular build?  Many of them will show a high BMI because of their weight, but their body fat percentage is low.  That’s one of the limits of using only BMI as a measure of health.  Another limit is that BMI numbers for older adults may actually underestimate body fat because of the natural loss of muscle mass as one ages.  The Body Mass Index measurement cannot differentiate between fat and muscle.

Even though the BMI scale is equal for both sexes, there are differences.  Since women tend to have a higher body fat percentage than men, they have more body fat than men with the same BMI number.  As mentioned above, older people have more body fat at the same BMI as younger people, and athletes have less body fat than non-athletes with the same number.

There is a direct correlation between obesity and overall health.

There are other ways to measure body fatness as a contributor to overall health.  Skinfold thickness (a better test for athletes than BMI), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and isotope dilution are all other ways to measure body fat.  But who keeps an isotope diluter next to the bathroom scale?  That’s one of the reasons why the Body Mass Index tool is so useful.  All you need is a scale and a yardstick.  But there’s an even simpler test – waist circumference.  Just grab a tape measure and put it around your waist (at the top of the hip bones).  Your goal is less than 40 inches if you’re a man and less than 35 inches if you’re a woman.  Granted, BMI and waist circumference are indirect ways to assess body composition, but they are pretty good indicators and are readily available — and cheap.

There is a direct correlation between obesity and overall health.  The list of diseases and other health conditions for which obese people are more susceptible is long and serious.  Number one is that you’ll die younger — from any cause.  Obesity increases the workload on the heart, and that contributes to a higher risk for heart disease and stroke.  Add high blood pressure, type 2 diabetes, and bad cholesterol and triglyceride numbers to the list, plus gallbladder disease.  Carrying too much weight puts more load on the hips and knees, with a higher likelihood of osteoarthritis as a result.  It can cause sleep apnea and breathing problems.  Studies are now linking obesity with cancer.  Some experts say that about one-third of many cancers (including endometrial, breast, colon, kidney, gallbladder, and liver) could be prevented just by maintaining a healthy weight.  Being overweight can lead to depression and anxiety, and simply just getting around is more difficult when you are carrying too many pounds.

The risks of obesity are serious, and the benefits of maintaining a healthy weight can be enjoyed every day.

On the other hand, maintaining a healthy weight has big benefits beyond not having to buy bigger clothes.  You’ll realize improved energy, more ability to participate in the activities you love to do, less joint and muscle pain, lower blood pressure, better sleep patterns, better blood glucose and triglyceride levels, plus a lower risk for type 2 diabetes, heart disease, and certain types of cancer.

The Body Mass Index test is easy to do, and it’s a good snapshot that can reveal if your weight is normal for your height or whether you should consider taking off some of those pounds that have accumulated over the years.  The risks of obesity are serious, and the benefits of maintaining a healthy weight can be enjoyed every day.  You can start on that New Year’s weight-loss resolution by finding out your Body Mass Index.

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