A comprehensive guide to obesity assessment: obesity calculation, BMI application, and body fat percentage measurement explained.

Assessing the degree of obesity requires scientific quantitative indicators. The simplest method is to calculate the "degree of obesity," which is (actual weight - ideal weight) / ideal weight × 100%. According to this formula, a weight below -20% is considered underweight; below -10% is considered slightly underweight; and between +/- 10% is considered normal to moderately obese. A weight above 10% is considered overweight; 20%–30% is considered mildly obese; 30%–50% is considered moderately obese; and a weight above 50% is defined as severely obese. While this indicator is intuitive, it has certain limitations because it ignores the muscle-to-fat ratio.

Body Mass Index (BMI = weight / height²) is currently the most widely recognized and used assessment standard. The following criteria are suitable for Asians: a BMI of 18.5–23.9 is considered normal; 24.0–27.9 is overweight; and over 28 is obese. More specifically, a BMI of 28–35 is mild obesity; 35–40 is moderate obesity; over 40 is severe obesity; and over 50 is extremely obese. Because height and weight are very accurate and readily available, BMI has become a core tool in public health for assessing obesity levels.

However, obesity is essentially caused by excessive accumulation of body fat, therefore body fat percentage (body fat rate) is the truly reliable indicator. The normal body fat percentage range is 10%–20% for men; above 20% is considered overweight or obese. For women, it is 15%–25%; above 25% is also considered overweight or obese. Specific diagnostic criteria are as follows: for men, a body fat percentage of 25%–29.9% is mild obesity, 30%–34.9% is moderate obesity, 35%–39.9% is severe obesity, and above 40% is morbid obesity. For women, the corresponding standards are generally raised by 5 percentage points; above 45% is considered morbid obesity.

There are various methods for determining body fat percentage. Skinfold thickness measurement is simple and economical; it involves measuring the skinfold thickness at the inferior angle of the scapula and the midpoint of the outer side of the upper arm, then substituting the measurements into a formula to calculate body density, and finally determining body fat percentage. Dual-energy X-ray ablation (DEXA) provides the most accurate results, but the equipment is expensive. Other methods, such as underwater weighing and impedance methods, are greatly affected by testing conditions and require strict control of variables to obtain accurate data. When developing a weight loss plan, both BMI and body fat percentage should be considered together.

Human body weight is composed of two parts: fat mass and lean mass. Fat mass is the product of body fat percentage and body weight; bone, muscle, and non-fat components are collectively called lean mass. Under normal circumstances, the two should maintain a scientific ratio, which reflects the true level of bone and muscle development. Obese patients often have an imbalanced ratio. Athletic ability largely depends on the proportion of lean mass (mainly skeletal muscle) to total body weight. Regular physical exercise is the core means of increasing lean mass. For adolescents, they should not blindly pursue low weight, but rather maintain a balance between fat mass and lean mass to ensure that growth and development are not interfered with.

In addition, waist circumference (WC) and waist-to-hip ratio (WHR) are key indicators for assessing abdominal obesity (central obesity). Obesity is defined as a waist circumference of 85 cm or more for men and 80 cm or more for women. Regarding the waist-to-hip ratio, a ratio greater than 0.9 for men and greater than 0.85 for women indicates a higher risk of metabolic diseases. Only by considering these diverse indicators can an accurate diagnosis of an individual's obesity status be made.