Why does age not matter in BMI?
For adults ages 20 years and older, BMI incorporates weight and height, but it does not take age or sex into account. A woman tends to have more body fat than a man with the same BMI. Likewise, an older person tends to have more body fat than a younger person with an equal BMI.
BMI changes substantially with age. After about 1 year of age, BMI-for-age begins to decline and it continues falling during the preschool years until it reaches a minimum around 4 to 6 years of age. After 4 to 6 years of age, BMI-for-age begins a gradual increase through adolescence and most of adulthood.
BMI is a reasonable indicator of body fat for both adults and children. Because BMI does not measure body fat directly, it should not be used as a diagnostic tool. Instead, BMI should be used as a measure to track weight status in populations and as a screening tool to identify potential weight problems in individuals.
BMI calculations are not adjusted for gender (except when they're used for children and teens), so BMI charts are the same for adult men and women.
Body Mass Index is a simple calculation using a person's height and weight. The formula is BMI = kg/m2 where kg is a person's weight in kilograms and m2 is their height in metres squared. A BMI of 25.0 or more is overweight, while the healthy range is 18.5 to 24.9. BMI applies to most adults 18-65 years.
Underweight: BMI is below the 5th percentile age, gender, and height. Healthy weight: BMI is equal to or greater than the 5th percentile and less than the 85th percentile for age, gender, and height. Overweight: BMI is at or above the 85th percentile but less than the 95th percentile for age, gender, and height.
As we age, we typically lose muscle and gain more fat, so our “normal” muscle-to-fat ratio will be higher. A large meta-analysis in 2014 showed adults over 65 actually had lower mortality rates when they had a higher BMI.
|Age Range||25th Percentile BMI||Average BMI|
Poor lifestyle factors are thought to account for an increase in BMI, indirectly affecting the clustering of CHD risk factors. Lifestyle factors such as eating unhealthy foods and physical inactivity may contribute to the development of overweight status in adolescents (Goran, 2001; Hill & Melanson, 1999).
Despite the wide use of BMI among all adults, it may not accurately reflect the health of certain racial and ethnic populations. For example, numerous studies have shown that people of Asian descent have an increased risk of chronic disease at lower BMI cut-off points, compared with white people ( 34 , 35 , 36 ).
Who is BMI not accurate for?
BMI is also not reliable to use on elderly adults, who generally have lost some amount of muscle and bone mass. In this case, an elderly person's BMI could be within a normal range while they might actually be overweight.
BMI (body mass index), which is based on the height and weight of a person, is an inaccurate measure of body fat content and does not take into account muscle mass, bone density, overall body composition, and racial and sex differences, say researchers from the Perelman School of Medicine, University of Pennsylvania.
The BMI is applied to both children and adults; however, it is not necessarily interpreted the same way. Our Alexandria bariatric surgeon explains: The same formula is used for both kids and adults, but when measuring children, age and gender are also factors that are considered in addition to height and weight.
BMI doesn't take body shape or composition into account.
BMI calculations also don't factor in where you carry your weight. Studies show that a higher waist-to-hip ratio is more closely linked to heart attack risk than BMI, whereas having bigger hips does not seem to pose a similar risk.
The Takeaway on BMI and Measuring Body Weight
BMI is one among many screening tools, and should not be used on its own to assess a person's health risk. Having a BMI that's too high doesn't necessarily mean your health is doomed.
And the body-fat scales, while widely available, ranging in size and cost from your domestic bathroom scales to large coin-operated weighing machines found in chemists, are notoriously inaccurate. In contrast, BMI is cheap and easy to calculate, and is therefore easily scalable.
While BMI is a rough estimate of body fat, body fat percentage is a more accurate number. BMI, but not overfat. Likewise, a person of normal weight on the BMI scales may still be overfat and at risk of weight-related diseases.
BMI doesn't account for body composition, which means it misses the difference between muscle mass and fat. Because muscles are more dense and heavier than fat, bodybuilders and other professional athletes like football players are often considered obese or overweight based on their BMI alone.
Yes indeed. Roughly speaking, each person 6 feet tall would lose a point from their BMI reading, and each person 5 feet tall would gain a point.
Know the difference between the BMI of adult and children? As children grow, the amount of body fat changes with age so BMI for children needs to be interpreted in relation to a child's age. There are also gender differences. Therefore, for children and youth, BMI is age and gender specific.
Why is BMI different for kids and adults?
A child's weight status is different from adult BMI categories. Children's body composition varies as they age and varies between boys and girls. Therefore, BMI levels among children and teens need to be expressed relative to other children of the same age and sex.