BMI Properties

BMI, or body mass index, is a number calculated from the weight and height of a given person. This number has gotten a lot of attention over the course of the 2000s, since insurance companies, government agencies and doctors all use it to determine whether or not someone is overweight. However, the BMI may not be as accurate as some claim. Knowing how the BMI is calculated and what health risks are associated with high or low body mass can help people make smarter decisions about their weight and health.

Significance

The concept of BMI as a single numeric measurement of fatness or thinness makes many calculations, such as those performed by insurance companies, much easier. Many companies and organizations use BMI as an indicator of health. This means that a person's body mass index can affect not only her health, but also how much she pays for particular services or whether she is eligible for them at all. BMI can also serve as a warning indicator for some conditions, such as heart disease.

Accuracy

BMI does not work to diagnose obesity accurately in all people. According to the National Institutes of Health, in a study of 13,601 adults, BMI measurements indicated obesity in 21 percent of men and 31 percent of women. However, when the study measured obesity by body fat percentage, 50 percent of men and 60 percent of women scored as obese. BMI was less predictive in older people and for people in intermediate weight ranges. BMI can also be inaccurate in the other direction. According to the Mathematical Association of America, athletes and body builders often score as obese according to BMI standards.

Misconceptions

Many people, including licensed medical doctors, believe that BMI is a direct indicator of a person's health. According to the Mathematical Association of America, some doctors recommend that their patients lose weight whenever their BMI is over the prescribed number, regardless of that patient's muscle mass, genetic background, level of physical fitness or other health indicators. While BMI can be a useful indicator for health risk, using it as a strict rule can lead to problems.

Risks

According to the National Heart, Lung and Blood Institute, BMI is best used as a risk indicator in conjunction with some other factors. For instance, people with a larger waist size have a higher risk of diseases like heart disease and diabetes, regardless of their actual BMI. Other factors that could indicate the risk of a problem include high levels of LDL cholesterol or triglycerides and low levels of HDL cholesterol, high blood pressure, high blood sugar and a family history of heart disease. Smokers and inactive people are also at a much higher risk.

Considerations

High BMI has been correlated with an increased risk of death, but this is not consistent in all cases. For instance, according to the Journal of the American Medical Association, underweight and obese people had an increased rate of death when compared to people who had a normal BMI. However, people in the overweight category -- BMI between 25 and 30 -- actually had a reduced rate of death when compared to people with normal body mass index.