A research team from Michigan State University and Saginaw Valley State University measured the BMI of more than 400 college students, where some of them were athletes and some not. It was found that in most cases the student's BMI did not accurately reflect his or her percentage of body fat.
The research is published in the March issue of Medicine and Science in Sports and Exercise, the official journal of the American College of Sports Medicine.
BMI is determined using the equation of the weight divided by the square of the height.
An ACSM guideline for BMI is less than 25 for everyone over the age of 20. For Health Promotion Board guidelines (2005), recommended healthy range is 17.9. Generally, a person with BMI higher than 25 is considered as overweight and if BMI exceeds 30, he or she is considered to be obese. A person with higher BMI is often associated with higher risk of heart disease, diabetes and other weight related problem.
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The main issue for using BMI is that regardless of gender and age, the same criteria is used to determine whether one is normal, overweight or obese. However, BMI should be used carefully when determining whether one is obese or not, especially when classifying different categories of people.
BMI does not really accurately tell how much fat a person really has; this is due to the nature that it cannot really differentiate between body fat and muscle mass. This is especially true when BMI were used to measure athletes, a large percentage of them were often considered to be obese, where in actual fact they are not. Many athletes tend to have high BMI due to muscle mass being built up and not due to body fat.
Taking for instance, two people with the same height and same weight, where one is an elite athlete who do a lot of strength training with large muscle mass and the other who hardly exercise. Both will have the same BMI, based on the equation alone and both will be considered as falling into the same category, but we know that the elite athlete is definitely not obese.
If BMI cannot accurately differentiate between muscle and fat, then how do we know whether one has too much body fat or muscle mass? A more accurate measurement is to look at body composition instead. The body composition is made up of total body water percentage, body fat, bone mass and muscle fat.
Such measurement can be done using skin fold measurement, bioelectrical impedance, photometry and water weighing. Most of the above mentioned required specialize equipment or skilled personnel to do the measurement, which often is not easily available for normal users for measurement.
However, there are now body compositions monitor available in the market for home users, which uses bio-electrical impedance which can quite accurately measure the bone mass, muscle mass, body water percentage and body fat percentage.
How the body composition monitor works is like this, it passes safe, low-level electrical signals through the body via the footpad of the monitor platform, where it is easy for the signal to flow through fluids in the muscles and other body tissues but meets resistance as it passes through body fat, since it contains little fluid. This resistance is called impedance and the impedance readings are then entered into medically researched mathematical formulas to calculate the body composition. From these reading, we can then better analyze whether one is having too much or too little body fat.
According to ACSM guidelines, for females 15 to 25 percent of body fat is considered optimal, 25 to 30 percent is considered fair and 30 percent and above is considered obese.
For males, 10 to 20 percent body fat is considered optimal, 20 to 25 percent is considered fair and 25 percent and above is considered obese.
In conclusion, BMI can be used as a quick reference or guideline to see which range one belongs. However, one has to take into consideration their physical activities level as well as their body type.
A more accurate measurement is still to look at the body compositions. With constant self-monitoring of body composition, one can better understand their own physical conditions and also to adjust their diet, training intensity and frequency accordingly. It also serves as a motivational tool for monitoring progress in their training.