Is Body Mass Index Accurate For Measuring Fat?

By Marisa Ramiccio. May 7th 2016

For years, Body Mass Index, or BMI, has been used to determine whether or not a person is overweight. Many sports coaches and dance instructors use BMI to decide whether or not a child is of the right weight to make the team. But many people feel that BMI is not an accurate way to calculate weight. In fact, BMI calculations may show that certain people are morbidly obese when they really aren’t.

What Is Body Mass Index?

Body Mass Index is a simple, cost-effective way to measure fat and is used by many physicians and schools. BMI compares a person’s weight to that of the general public. It is calculated from a person’s height and weight. There are two formulas based on the different systems of measurement:

  • The metric system: To calculate BMI, square the height in meters and then divide weight in kilograms by that number.
  • The English system: To calculate BMI, square the height in inches and then divide weight in pounds by that number. Then multiply that number by 703 to convert it to a number comparable to that of the metric system.

The final result will show which category that person falls into on the standard weight chart, which looks like this:

  • Below 18.5 – Underweight
  • 18.5- 24.9 – Normal
  • 25- 29.9 – Overweight
  • 30 and above – Obese

Although the formula is the same for both adults and children, the interpretation is a bit different. Both age and gender are factors in interpreting a child’s BMI. Once their BMI is calculated, it is plotted on a CDC BMI-for-age growth chart, which shows which percentile the child’s BMI falls into. The percentile chart looks like this:

  • Less than the fifth percentile – Underweight
  • Fifth percentile to less than the 85th percentile – Normal
  • The 85th percentile to less than the 95th percentile – Overweight
  • The 95th percentile and above – Obese

Children ages two to 19 are assessed by the BMI percentile while adults 20 years of age and older are assessed by the standard BMI chart.

Is Body Mass Index Accurate?

Although BMI has been used for years, many people have been claiming that it’s not as accurate as was first thought. Critics of the BMI calculation say that it fails to take certain factors into account such as:

  • Muscle mass
  • Age
  • Race

BMI may be particularly inaccurate for athletes, who tend to be more muscular than the general population. Several professional and student athletes have been categorized as overweight or obese by the BMI, but they are actually just muscular – not overweight.

At the other end of the spectrum are those who have lost muscle mass, whether from being ill or from getting older. Since the standard chart is used for adults 20 and above, people in their 20s and people in their 80s are being judged by the same standard. Therefore, many older people can easily be incorrectly classified as being underweight.

The question of BMI accuracy for all ages has already been studied by researchers at Michigan State University and Saginaw Valley State University. After measuring the BMI of 400 students, they found that the majority of BMIs were incorrect, that they did not accurately measure body fat percentage. At least one of the researchers believes that using the same criteria for all ages could be the reason for the inaccuracy.

But age isn’t the only factor that the BMI may overlook. Some people feel that race should be taken into account when measuring body fat. Researchers at the Baylor College of Medicine and the University of Houston found that African-Americans tend to have a lower percent of body fat than Caucasians while Asians, Asian-Indians and Hispanics tend to have a higher body fat percentage than Caucasians. This means that the BMI chart is inaccurate for these groups and can misrepresent their weight category by stating that they are overweight when they are not (African-Americans) or are normal weight when they are not (Asians, Asian-Indians, Hispanics).

Other Ways To Measure Fat

According to the CDC, BMI should not be used as a diagnostic tool and is, instead, an alternative tool to measure body fat. That means that, although a person may have a high BMI, they should use other methods to determine whether that excess fat poses a threat to their health. Other tests include:

  • Skinfold thickness measurements with calipers
  • Bioelectrical impedance
  • Dual-energy X-ray absorptiometry
  • Isotope dilution
  • Underwater weighing
  • Evaluations of diet, physical activity, family history, etc.

Some of these test can be quite expensive and not as readily available to the public as the simple BMI formula, hence its widespread use. But BMI seems to be excluding important factors, such as muscle mass, age and race. By doing so, it’s skewing the weight evaluations of many people and is putting many people in a category where they don’t belong.


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