Article 11: Multiple Methods for Measuring Body Fat Content

The potassium content in non-fat tissues of the human body is constant, while the potassium content in adipose tissue is extremely low. Based on this physiological characteristic, the total potassium content of the body can be measured, the weight of non-fat tissues can be calculated, and then the amount of fat can be estimated.

The average potassium content in the non-fat portion of a normal adult is 68.1 mmol/L. Therefore, fat mass = body weight (kg) - total potassium content mmol/L / 68.1.

The measurement can be performed using a whole-body scan with potassium-4 or potassium-42 isotopes to determine the total potassium content, thereby indirectly estimating the amount of fat. This method works under the assumption that the intracellular and extracellular potassium concentrations in lean body tissues are relatively constant, but the results will be inaccurate when the extracellular potassium concentration is abnormal.

In addition to the large accumulation of subcutaneous fat, weight gain can also be caused by water retention (such as edema, ascites, etc.) or the development of muscles throughout the body. Therefore, measuring weight is not a very accurate method for detecting obesity. Measuring the thickness of subcutaneous fat is a more accurate method.

To measure the thickness of subcutaneous fat, specialized measuring calipers must be used. The skin thickness gauge used to measure the thickness of skin folds is a specially designed curved-leg caliper; commonly used models include the American-made Lange type, the British-made Harpenden type, and domestically produced skin thickness gauges.

When measuring, pinch the skin fold with your thumb and forefinger, but do not pinch the muscle. Then use calipers to hold the skin fold for 2-3 seconds and read the pointer reading in millimeters. Measure each area twice.

Measurement sites are generally chosen for the back of the upper arm, back (at the lower angle of the scapula), abdomen, waist (iliac crest), and front of the thigh. During measurement, the area being measured should be loose; the skin layer should be pinched up with the index finger and thumb; the thickness should be measured using skin fold calipers.

Subcutaneous fat accounts for about two-thirds of human body fat, and its thickness is parallel to the overall degree of obesity. Therefore, measuring subcutaneous fat thickness can roughly reflect the amount of body fat. Because the measurement method is simple, it is widely used.

In many parts of the body, subcutaneous fat is loosely attached to the skin tissue, so the skin folds can be pinched together with the thumb and forefinger, and the thickness of the skin folds can be measured with a special skin thickness gauge.

Since there is very little individual variation in skin thickness, the thickness of skin folds can represent the thickness of subcutaneous fat, and the total amount of body fat can be calculated from this.

The best locations for measuring skin folds are still debated. The most commonly used location is the posterior border of the triceps brachii muscle belly in the left upper arm (the midpoint between the acromion and the radial head of the left upper arm). Other commonly used locations include the area below the inferior angle of the right scapula, 5 cm to the left of the navel, and the anterior mid-thigh.

The skinfold thickness measurement only measures the thickness of subcutaneous fat. To determine whether someone is thin or obese, it is necessary to calculate the body fat percentage. Body fat percentage can generally be calculated using a formula or obtained from a table.

(1) Formula method

1) Calculate body density

Let X = inferior angle of scapula + thickness of triceps brachii skin fold

Males aged 15-18 years: Body density = 1.0977 - 0.00146X

Body density for individuals aged ≥19 years = 1.0913 - 0.00116X

Females aged 15-18 years: Body density = 1.0931 - 0.00160X

Body density for individuals aged ≥19 years = 1.0897 - 0.00133 X

2) Calculate body fat percentage using the Brozek modified formula.

Body fat percentage (F%) = (4.570 / body density - 4.142) × 100%

(2) Table lookup method: Skin folds in certain areas can reflect the distribution of subcutaneous fat, which can be used as a standard for weight classification. Domestic experts have formulated a standard for judging body fat based on subcutaneous fat thickness (Table 1-7).

Bioelectrical impedance analysis (BIA) is a novel method for measuring body composition, invented in recent years. This method utilizes the principle that adipose tissue is less conductive than lean tissue, and infers body fat content by measuring the body's electrical impedance.

The bioelectrical impedance method for measuring human body fat content is more accurate than anthropometric methods; it is also more convenient, simple, and faster than body density and total body water determination methods, and is suitable for both individual measurements and group health checks.

(1) Ultrasonic method: By utilizing the principle that different tissues of the body have different echoes when ultrasonic waves strike them, the changes in the morphology and density of the tissues can be vividly displayed, and the content of adipose tissue can be further inferred.

(2) Infrared method: Infrared rays are delivered to certain parts of the body (e.g., the biceps brachii). Fat absorbs infrared rays, and the more light absorbed, the higher the fat content. The fat content in the biceps brachii is significantly correlated with the total body fat content. If the fat content in the biceps brachii is known, the total body fat content can be further calculated.

(3) Neutron activation method: When the human body is in front of a neutron emitter, atoms in the body, such as nitrogen atoms, are activated and can produce radioactivity. Since the percentage of nitrogen atoms in proteins is basically constant, the total nitrogen content of the body can be estimated by detecting radioactive nitrogen atoms, and then the total protein, lean body tissue, and fat tissue content of the body can be obtained.

(4) Computed Tomography (CT): This method utilizes the different attenuation properties of X-rays on bodies of different densities to form a two-dimensional image at the X-ray scanning site, and then uses computer analysis to determine the local fat content.

(5) Photon Absorption Method: This method uses 153Gd (gadolinium) double photon (light quantum) absorption (DPA). When a photon beam passes through tissue, its energy is partially absorbed. By measuring the change in the energy of the photon beam, the body fat content can be calculated.

(6) Double-dose X-ray absorption method (DEXA): Due to the decay of the gadolinium source, the accuracy of DPA will decrease. Therefore, DPA has been further improved, and a new method has been developed that uses a stable X-ray generator instead of a decaying radionuclide source, which is the double-dose X-ray absorption method. This method has a high degree of consistency with the volume density method.