Body mass index (BMI) may not be an accurate measure of body fat content or fitness because it doesn’t take into account important factors such as:
- Muscle mass
- Bone density
Learn more about the drawbacks of using BMI to measure overall health.
6 potential drawbacks of BMI as a measure of body composition
1. Muscle mass
Since BMI is only based on your height and weight, it is too simplistic a measure to determine your health. It does not take into account how much of your weight is muscle mass. When you have more muscle, your BMI will be higher, even if you are fit and healthy.
2. Bone density
Some people have denser bones and larger frames than others. However, people with large frames will have higher BMIs, even though they may be healthy and not overweight.
Even if body weight remains constant, aging causes considerable changes in body composition, including a significant decrease in muscle mass and an increase in visceral fat. Visceral fat is more dangerous, but BMI does not use this factor in its calculation.
Studies in the past found that women had a higher prevalence of obesity than men, especially women over 50. Visceral or subcutaneous fat gain is linked to fat accumulation, which increases body mass. These changes differ in men and women because aging women experience a redistribution of abdominal fat.
However, this physiological difference in body fat percentage between males and females is not properly measured by BMI.
Aside from age and gender, large ethnic variations in body composition have been found. Individuals from certain ethnic backgrounds may have a higher BMI but still may be healthy due to higher non-fat mass.
In addition to fat distribution, hypertension risk, and type II diabetes risk, there are also disparities in muscularity, bone mass, and leg length among different ethnicities. This increases the ethnic-specific connections between body fat and BMI.
These ethnic disparities in body fat are the result of genetically determined differences in body composition and metabolism, as well as a variety of risk factors caused by social and environmental factors.
6. Self-reported data
Because of the lower costs and ease of fieldwork, nutritional epidemiology mainly depends on self-reported data to measure health. However, the credibility of self-reported data is questionable. The same is true for BMI.
According to many studies, body weight is strongly under-reported and height is significantly over-reported, with severe miscalculations of BMI categories. Miscalculation of the various BMI categories can obscure the underlying links that exist between obesity and chronic health conditions, such as heart disease, asthma, diabetes, high blood pressure and sleep disturbances.
What are the limitations of BMI in children?
The same concerns about using BMI to measure body fat in adults apply to children and adolescents. In children, other factors such as puberty change the link between BMI and body fat.
While BMI may still be a good predictor of excess body fat in obese children, elevated BMI levels in overweight children can be caused by an increase in either fat or non-fat mass. Similarly, disparities in BMI among relatively thin children are often related to differences in fat-free mass.
Dhurandhar EJ. The downfalls of BMI-focused policies. Int J Obes (Lond). 2016 May;40(5):729-30. https://www.nature.com/articles/ijo201618
American College of Cardiology. Body Mass Index Fails to Accurately Measure Body Fat. https://www.cardiosmart.org/news/2015/7/body-mass-index-fails-to-accurately-measure-body-fat
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