The Disadvantages of BMI

The World Health Organization predicts that about 2.3 billion adults, worldwide, will be overweight and over 700 million will be obese by 2015. Increased consumption of fat and sugar and decreased physical activity are partly to blame. Body mass index (BMI) is used to indicate if a person is overweight or obese. BMI is defined as weight in kilograms divided by height in meters squared (kg/m2). A person is considered overweight if his BMI is 25 to 29.9 and obese if it is 30 or more. Despite its popularity, there are disadvantages to using BMI to indicate trends.

Age

• Readings are similar among males and females during childhood, but then females jump ahead of males in adolescence. And, BMI does a roller coaster ride regarding age, increasing during the first year of life then decreasing until six years of age then increasing again on through adolescence. This variation within children makes it hard to interpret the meaning of a certain BMI, compared with adults. In addition, older people tend to have higher BMIs than younger adults.

Gender

• In adults, even if they have the same BMI, women are more likely to have more body fat than men. Including approximate body compositions would help explain how different sexes compare regarding BMI, obesity and death. A study appearing in Twin Research and Human Genetics suggests that genetic differences between men and women may influence BMI variation as well. Considering sex-specific effects when pinpointing susceptible obesity genes could help remedy this problem.

Height

• Height is a varying factor, including relative leg length. BMI doesn't take into account varying skeletal sizes that could have a bearing on the relationship between BMI and amount of body fat. For example, a long-legged person would weigh more and have a higher BMI than a person of the same height but with shorter legs, because legs weigh more than the trunk.

Total Body Weight

• BMI includes total body weight -- both fat and muscle tissue. Since fat mass percentage and fat distribution of different populations vary, BMI meanings will, again, vary. For example, at the same BMI, whites tend to have more fat than blacks. Also, just because BMIs have increased in the general population, it does not necessarily mean that the percentage of fat has increased substantially while lean body mass has decreased. In addition, people with muscular builds may get a mistakenly high body fat reading.