Body mass index (BMI) is a basic measure of body fat based on one’s height and weight and is usually used as a screening tool to identify possible weight problems among kids, although adults may use BMI measurements as well. For children and teens, BMI is age- and sex-specific because body fat changes with age and body fat differs between girls and boys. When used for kids, this is often referred to as BMI-for-age.
It’s widely debated as to how useful a general BMI calculation is or is not, mainly because BMI does not measure body fat directly and because it’s such a general scale. Still, much research shows that BMI measurements do correlate to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry.
Pros of BMI measurements
- A good way to measure and identify obesity and body fat in large populations or groups of people. For example, the average BMI of a large group (say in a state) can be useful when compared to other groups.
- Research shows that BMI is a reliable indicator of body fatness for most children and teens, making it a useful starter tool for parents.
- The CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight and obesity in children beginning at 2 years old.
- Fast. With a BMI measurement, you can quickly get a general idea about if your child is obese, overweight, a healthy weight, or underweight.
- Non-intrusive. For kids this is a big deal. Parents can measure a child’s BMI at home, before visiting a doctor.
- Easy-to-perform method of screening for higher than average weight categories that could lead to health problems.
Cons of BMI measurements
- Does not differentiate between fatty and lean tissue.
- BMI is not a clinical diagnostic tool, it only gives you an idea about your child’s body. To really determine if excess fat or too little fat is a health problem, a health care provider would need to perform more in-depth, further assessments, such as skinfold thickness measurements, evaluations of diet, physical activity, family history and other health screenings.
- If you measure or weigh your child wrong, the BMI results can be incorrect.
- BMI may overestimate body fat in very athletic kids or kids with a muscular build.
- For adults, age and gender are not taken into account. Some argue that you naturally gain or lose weight with age, depending on who you ask.
- BMI thresholds change at times, which some people find annoying.