A Comprehensive Guide to Obesity Detection Methods: From BMI to Instruments, the First Step to Scientific Weight Loss

Methods for Detecting Obesity

I. Simple Parametric Measurement Method

1. Height and Weight Charts
Many countries have developed ideal weight charts for different genders, ages, and heights through large-scale epidemiological surveys and field measurements. These charts show that ideal weights vary among similar populations in different countries and regions. Establishing such charts is more accurate than using formulas to determine whether an individual is truly obese. Currently, my country does not have such an ideal weight chart.

2. Standard Weight Calculation Formulas

(1) Adult Weight Standards

Three commonly used empirical formulas:
Standard weight = height - 100 (kg)
Standard weight = height - 105 (kg)
Standard weight = (height - 105) × 0.9 (kg)

(2) Infant and Child Weight Standards

Infants (1-6 months): Standard weight = birth weight + age in months × 600 (kg)
Infants (7-12 months): Standard weight = birth weight + age in months × 500 (kg)
Children over 1 year old: Standard weight = age × 2 + 8 (kg)
If a child's height exceeds the standard, refer to the adult calculation.

3. Body Mass Index (BMI)

BMI = weight (kg) / height² (cm²)

This is the internationally accepted obesity classification standard recommended by the WHO. It is simple and practical, and the body fat percentage estimated based on it has a good correlation with the results measured using the gold standard underwater weighing method. The disadvantages are that it cannot reflect localized fat distribution and is not suitable for children, pregnant women, or people with very developed muscles or whose height cannot be accurately measured.

4. Circumference: This includes waist circumference, hip circumference, thigh circumference, and the waist-to-hip ratio (WHR) and waist-to-hip ratio (WFR) calculated from them. WHR is the most widely used. Waist circumference is the distance between the mid-axillary line and the midpoint between the costal margin and the anterior superior iliac spine. Hip circumference is the distance measured at the most prominent part of the buttocks. A high WHR indicates upper body obesity, and a low WHR indicates lower body obesity. The cutoff varies with age, sex, and race. WHR has been shown to correlate with the intra-abdominal fat area measured by CT. The advantage of WHR is that it reflects localized fat distribution; the disadvantage is that it is not suitable for children, pregnant women, and patients with ascites.

5. Subcutaneous Fat Thickness: This reflects localized fat distribution and can be used to calculate total fat, but there is currently no unified standard.

Skinfold Thickness Measurement: The skin is lifted with the fingers, and the thickness between the two sides is measured. Common measurement sites include the biceps brachii, the middle of the triceps brachii, and the subscapularis. Normal skinfold thickness at the triceps brachii and subscapularis sites is less than 51 mm for men and less than 70 mm for women.

II. Instrumental Measurement Methods

1. Underwater Weighing Method: This is currently the recognized "gold standard" for measuring body fat. Its advantages are high accuracy and good repeatability; its disadvantage is that the operation method is too complex and only suitable for scientific research.

2. Bioelectrical Impedance Analysis (BIA): This method indirectly estimates total body fat content by measuring the body's electrical resistance. Its advantages are low cost, high accuracy, good repeatability, and simple operation, making it suitable for clinical research and epidemiological surveys.

3. Total Electrical Conductivity (TOBEL): This method indirectly estimates total body fat content based on the different responses of fat and water to electromagnetic fields. Its advantages are speed and good repeatability, but it is expensive.

4. Dual-energy X-ray absorptiometry (DEXA): Calculates the mineral content of adipose tissue, non-adipose tissue, and bone by measuring the attenuation of two weak X-ray beams of different energies passing through the body. Advantages include safety, convenience, high accuracy, and standardized procedures; however, it is expensive.

5. Computed Tomography (CT): Currently the most accurate method for assessing visceral adipose tissue, allowing for quantitative analysis of whole-body fat. Specifically, it determines the subcutaneous fat area (S) and visceral fat area (V) from tomographic images at the umbilicus level, thus identifying fat distribution.

6. Magnetic Resonance Imaging (MRI): Can measure both local and total fat content. Advantages include safety, convenience, speed, accuracy, and no damage to the body; however, it is expensive.

7. Ultrasound: Ultrasound examination has unique advantages for obese patients. It can directly measure fat thickness, understand the distribution of fat throughout the body, and plays an important role in the classification of obesity.

The patient is placed in a supine position, and a longitudinal ultrasound scan is performed along the linea alba from the lateral protuberance to the umbilicus. The maximum thickness of preperitoneal fat (Pmax) and the minimum thickness of subcutaneous fat in the abdominal wall (Smin) are measured. The ratio Pmax/Smin is used as the abdominal wall fat index (AFI) to determine fat distribution. AFI correlates with the subcutaneous intraepithelial fat (SIV) obtained from CT.

8. Other methods: Body potassium measurement, isotope dilution, neutron activation, etc., are not commonly used.

Table 2-5 compares the cost, difficulty, accuracy, and ability to estimate distribution of various body fat measurement methods.