BMI in Digital Health: Overview and Clinical Relevance
What is Body Mass Index (BMI)?
Body Mass Index (BMI), also known as the Quetelet index, is a widely used international standard for assessing body fatness and overall health status. It is calculated as follows:
Body Mass Index (BMI) = Weight (kg) ÷ Height (m)²
BMI is a globally recognized method for grading the severity of obesity, and the World Health Organization (WHO) also uses BMI to define obesity and overweight.
BMI Value
The Body Mass Index (BMI) was initially designed as a statistical tool for public health research, converting an individual’s height and weight into a BMI value to determine whether there is a linear correlation between this value and disease incidence. However, BMI serves only as a reference; accurate assessment of obesity requires additional scientific methods. Due to its simplicity and ease of use, the role of BMI has gradually evolved, extending from clinical applications to becoming a general weight-management indicator for the public. The classification criteria are as follows:
Based on the three different criteria mentioned above, it is evident that there is no single universal standard for evaluating Body Mass Index (BMI). Although the World Health Organization (WHO) established early guidelines for obesity, these standards were derived from research data on Western populations. The diversity of populations, particularly between Eastern and Western regions, inherently leads to a lack of uniformity in these standards. Furthermore, variations in living environments within the same country result in differing physical development patterns among people in different regions. Therefore, BMI should be interpreted with a dialectical and context-aware approach.
In an article published in the recently released Issue A7 of Popular Medicine, Professor Zhou Beifan from the Department of Epidemiology at Fuwai Hospital, Chinese Academy of Medical Sciences, reported that the Working Group on Obesity in China (WGOC) has proposed, for the first time, obesity criteria suitable for Chinese adults, based on a pooled analysis report of data from the Chinese population in the 1990s.
A body mass index (BMI) of ≥24 is classified as overweight, and ≥28 as obese; abdominal obesity is defined as a waist circumference of ≥85 cm in men and ≥80 cm in women.
BMI Index: Unsuitable Populations
Adolescents Under 18 Years of Age
Pregnant or Lactating Women
Frail or Sedentary Elderly Population
Athletes or individuals engaged in weight training (bodybuilders have a higher muscle mass, which may result in an elevated BMI; however, their body fat percentage is low, and they are not classified as obese)
Diseases Potentially Caused by Excessively High BMI in Adults
BMI ranges demonstrate a certain correlation between body weight and disease or mortality; individuals who are overweight or obese face a significantly increased risk of the following diseases:
Stroke (Zhongfeng)
Coronary Heart Disease (CHD)
Dyslipidemia
Type 2 Diabetes (T2D)
Gallbladder Disease
Primary Hypertension
Osteoarthritis
Sleep Apnea
Spinal Epidural Lipomatosis (Epidural Lipomatosis)
At least a dozen types of cancer, including endometrial cancer, breast cancer, and colon cancer.
Application of the Body Mass Index (BMI)
▌Public Health
BMI can serve as a rough tool for estimating obesity. However, the duality of BMI lies in the fact that, although it is simple and easy to calculate as a rough estimate, its accuracy and relevance need improvement. BMI is suitable for predicting trends among sedentary and overweight populations due to its low calculation error. As early as the early 1980s, BMI had already been established as one of the criteria for diagnosing obesity. The broad correlation of BMI is particularly applicable to consistent data regarding obesity or other conditions.
▌ Clinical Practice
Beyond its inapplicability to athletes, children, the elderly, and frail individuals, the Body Mass Index (BMI) has numerous applications in clinical practice. In the United States, in particular, BMI values are used to assess underweight conditions resulting from eating disorders, such as anorexia nervosa and bulimia nervosa.
▌ Enacting Laws
France, Israel, Italy, and Spain Have Explicitly Banned Fashion Show Models with a BMI Below 18(Israel must not be lower than 18.5), with the aim of curbing the trend of anorexia among fashion models and the general population who blindly pursue a low Body Mass Index (BMI).
Limitations of the Body Mass Index (BMI)
Healthcare institutions and the statistical community have clearly pointed out the limitations of BMI application.
▌ Ignored the scaling law
Since the BMI calculation relies on weight and the square of height, it is not a linear dimensional comparison. That is, a taller person with relatively the same body type and body composition will have a higher BMI.
▌Overlooking Individual Characteristics
For individuals with a large frame, the BMI threshold should be adjusted upward by approximately 10%, whereas for those with a small frame, it should be adjusted downward by 10%. In other words, individuals with a small skeletal frame may have higher body fat yet still present a BMI within the normal range. Conversely, individuals with a large skeletal frame may be classified as overweight by BMI despite having relatively low body fat and being in good health. Furthermore, as age increases and height decreases due to natural shrinkage, BMI fails to accurately reflect the expected health outcomes.
▌Does not distinguish between muscle mass and fat mass
Sometimes, BMI classifies individuals with a healthy physique as obese, while labeling those with a poorer body composition as normal. Why does this occur? A health study involving 13,601 participants revealed that among individuals classified as obese based on BMI (>30), 21% were male and 31% were female. However, when assessed by body fat percentage, 50% of men and 62% of women were found to be obese. The insensitivity of BMI lies in its failure to distinguish between the relative proportions of muscle and fat. For instance, a standard athlete with a body fat percentage of 10%–15% may have an elevated BMI due to high muscle mass, potentially exceeding the BMI values of sedentary individuals who fall within the normal range.
▌Association Between BMI and Disease Incidence
A study published in 2005 by the Journal of the American Medical Association showed that there was little difference in morbidity and disease-specific mortality between individuals classified as overweight and those with normal weight based on BMI, whereas underweight and obese individuals exhibited higher mortality rates. Elevated BMI is associated with type 2 diabetes, but only in patients who have excessive levels of gamma-glutamyl transpeptidase in their serum.
A review of 40 studies encompassing 250,000 individuals found that patients with normal BMI values who suffered from coronary artery disease had even higher mortality rates than those whose BMI fell within the overweight range (BMI 25–29.9). In fact, a 2010 study that followed 11,000 participants for eight years indicated that BMI is not a reliable predictor of the risk of heart attack, stroke, or death.A better measurement method is the waist-to-height ratio.。
Text | Zhou Yanxun
Editor: Mo Renying