Obesity assessment from a medical perspective: Clinical application of body composition analysis and bioelectrical impedance analysis
II. Medical Evaluation Methods for Obesity
The application of body composition analysis in weight loss is based on the fact that the human body is composed of water, protein, fat, carbohydrates and minerals. Under different diseases and nutritional states, the water, protein, fat and minerals of tissues and organs will change accordingly. Therefore, it is necessary to use the method of detecting body composition to accurately reflect the changes of various body components and evaluate the nutritional status of individuals and populations. (1) The origin and development of body composition analysis: It is generally believed that body composition measurement began with the research of the German chemist Jusstus von Liebig. In 1850, using chemical analysis, it was discovered that human tissues contain high concentrations of K' and low concentrations of Na; while body fluids contain low concentrations of K and high concentrations of Na'. In 1942, the famous Archimedes principle in physics was applied to the study of body composition, and the fat mass (FM) and fat-free mass (FFM) of the human body were measured, and a two-component model of the human body was established. At present, there are many methods for measuring body composition, including the isotope-labeled double-labeled water method, MRI/CT, dual-energy X-ray absorptiometry (DEXA) and bioelectrical impedance analysis. The double-labeled water method with isotope labeling is currently the gold standard for measuring energy expenditure and can also be used for body composition analysis. However, its clinical application is limited due to the cumbersome process and the involvement of isotopes. MRI/CT scans use imaging techniques to obtain images of various sections of the human body, displaying structures such as bones, subcutaneous fat, and visceral fat. Software is used to calculate the content and distribution of fat and muscle, with relatively accurate calculations for visceral fat. Dual-energy X-ray absorptiometry (DEXA) is commonly used for bone mineral density measurement. In recent years, due to advancements in measurement technology and software, it is currently the gold standard for measuring muscle mass. Bioelectric impedance analysis (BIA) is based on Ohm's law, assuming the measured area is a uniform cylindrical conductor. It utilizes the difference in resistivity between adipose tissue (a poor conductor of current) and non-adipose tissue (primarily composed of water, a conductor) to estimate body fat content. BIA has become a widely used method for measuring body fat composition due to the inexpensive equipment, simple operation, short testing time, and non-invasive nature of the procedure. (2) Main parameters of body composition analysis: Taking the more common BIA as an example, the main parameters used in weight management are: intracellular fluid, extracellular fluid, protein, inorganic salts, body fat, and muscle mass. The relationship between the main indicators is: body water content = intracellular fluid + extracellular fluid; lean body mass = intracellular fluid + extracellular fluid + protein + inorganic salts; muscle mass = lean body mass - bone content; body weight = lean body mass + body fat. (3) Application of body composition analysis in weight loss populations: 1) Muscle and fat analysis: By comprehensively analyzing the weight, skeletal muscle and body fat, the body shape and muscle and fat reserves can be determined. The normal weight range of a general body composition analyzer is calculated according to the standard value of BMI (22 kg/m² for adult males >18 years old, 21 kg/m² for adult females): standard weight = BMI × height², the standard weight range is 85% to 115% of the standard weight, and the corresponding BMI is 18.523 kg/m². 1) Skeletal muscle mass is calculated from the muscle mass of the limbs, with standard values of 47% (men) and 42% (women) of ideal body weight, and a standard range of 90% to 110% of the standard value. 2) Obesity analysis: Muscle and fat data can provide three parameters: BMI (kg/m²), body fat percentage (%), and waist-to-hip ratio. For two people with the same BMI, a higher body fat percentage indicates greater obesity. A higher waist-to-hip ratio indicates higher abdominal fat content, thus more visceral fat and a higher risk of chronic diseases. 3) Limb muscle balance: The muscle mass of the left and right upper limbs, trunk, and left and right lower limbs is compared with the ideal muscle mass at ideal body weight to determine the evenness of muscle distribution. This indicator is crucial for monitoring the effectiveness of weight loss programs. Generally, during weight loss program implementation, the emphasis is on reducing fat mass while maintaining muscle mass as much as possible to avoid lowering the basal metabolic rate, thus contributing to the effectiveness of the weight loss program. 4) Visceral fat area: The visceral fat area (VFA) is also included in the body composition analysis report. If VFA > 100 cm², it indicates increased visceral fat; if VFA > 150 cm², it indicates a significant increase in visceral fat. The greater the increase in visceral fat, the higher the risk of developing chronic diseases such as fatty liver, type 2 diabetes, hypertension, and other cardiovascular diseases. Therefore, this indicator can help patients understand the urgency and necessity of weight loss.
Anthropometry and Obesity (1) Broad Concept of Anthropometry: Anthropometry is one of the basic methods for understanding the various changes in human characteristics during phylogenetic and individual development by measuring data and using statistical analysis methods. It can help people understand the similarities and differences in the physical structure of different races and ethnicities in ancient and contemporary times, as well as the changing patterns of the human body under different living conditions. At present, anthropometry is also widely used in medicine to evaluate nutritional status related to clinical practice, conduct nutritional surveys, and track and monitor diseases. Therefore, the methods of anthropometry are not only of great significance to anthropological and human body theory research such as human evolution, but also have practical application value in fields such as industry, national defense, medical and health care, forensic medicine, education, sports, art, and sculpture. In addition, the content of anthropometry is constantly changing with the times. Initially, the laws of human evolution were found by measuring and observing ancient human fossils at different evolutionary stages. Later, anthropometry and comparative analysis of different races and populations were conducted to find the differences and variation patterns of humans. Anthropological methods were introduced into the field of child and adolescent health to carry out research on growth and development and reveal the laws of human growth and development. In sports science, anthropometry is used to select athletes and guide training; in the arts, anthropometry is used to guide sculpture and painting; in maxillofacial surgery, facial biometry is used for orthopedic and cosmetic surgery; in forensic medicine, anthropometry is used for individual identification and cranial measurement for facial reconstruction; in ergonomics, anthropometry is used more extensively, such as in machine manufacturing, furniture design, weaponry, cockpits, houses, and spacesuits, where basic data provided by anthropometry is applied; in the medical field, anthropometry is used to study the risk tendency of certain diseases, determine the composition of the human body, and evaluate health. (2) Anthropometry and obesity: Anthropometry plays a role in judging obesity, including measuring height, weight, skinfold thickness, muscle strength, body composition, physical strength, and physiological and metabolic functions. Taking skinfold thickness measurement as an example, skinfold thickness is a simple measurement indicator for measuring the body fat content. By measuring the skinfold thickness of different parts, the body fat content can be roughly understood, and nutritional status can be evaluated. Studies show a positive correlation between skinfold thickness and total body fat. Clinically, skinfold thickness can be used to roughly estimate fat loss. Commonly used skinfold thickness measurement sites include the triceps, biceps, inferior angle of the scapula, and suprailiac crest, with the triceps skinfold being the most frequently used. Due to the influence of age, muscle mass, and the tester's technique, measurement errors are relatively large. Besides mastering the testing technique, each site needs to be tested three times, and the average value taken. Taking the triceps skinfold thickness test as an example: ① Locate the test site-the midpoint of the line connecting the right acromion to the olecranon process of the ulna; ② Use your left hand to lift the skin and subcutaneous tissue of the area to be measured; ③ Use your right hand to measure the thickness of the skinfold below the lifting point with a skinfold gauge. Note that the muscle should not be grasped along with the skin when lifting it, and the value should be accurate to one decimal place. Currently, there are no reference values from population surveys in my country; comparisons can be made between patients before and after treatment. The landmark for biceps measurement is 1 cm above the midpoint of the line connecting the acromion and the olecranon process, roughly level with the nipple. The subscapular measurement is taken 1 cm below the inferior angle of the right scapula. The upper iliac crest measurement is taken at the intersection of the anterior axillary line extending downwards and the upper iliac crest. To determine obesity by skinfold thickness, the subscapular fat thickness in the subscapular and triceps areas is first measured, and then the two measurements are added together. For individuals with uniform obesity, the degree of obesity determined by subscapular fat thickness is roughly the same as that determined by BMI. Measuring subscapular fat thickness can reflect the body's fat content to some extent. However, this measurement method itself has certain limitations.