체성분 측정 원리, Principles of body composition measurement

 

 

'헬스장이나 보건소에 널리 설치된 체성분 검사 장치는 흔히 '인바디'라고 불린다. 인바디는 상표명으로 햇반, 스팸처럼 상표가 보통명사처럼 쓰이는 것이다. 정식 검사명은 '생체전기 임피던스 분석법'(BIA)이다. 임피던스는 교류 전기 회로에서 일종의 저항인 물리량이다.

BIA는 인체에 다양한 주파수의 미세 전류를 여러 경로로 흘려보내 신체 각 부위의 생체 저항을 측정해 체성분을 추정한다.


우리 몸은 다양한 이온을 포함한 무기질, 체수분이 조직 특성에 따라 다르게 분포한다. 이에 따라 전기가 흐르는 정도가 다르다. 예를 들어 수분이 많은 근육은 전류가 상대적으로 잘 흐르고 수분이 적은 지방은 전류가 잘 흐르지 않는다.

전문가용 기기는 사지 각각에 2개씩 총 8개의 전극을 활용해 전류를 다양한 경로로 흘려보낼 수 있어 부위별 측정이 가능하다. 전극이 발판 4개만 있는 가정용 측정기나 스마트 워치는 일부 부위만 측정하고 전체 값은 수치 보정을 한다.

BIA 측정법 중 8개의 전극을 사용하는 것이 가장 정확도는 높지만 체수분, 전해질 상황에 따라 오차가 나온다는 한계가 있다.

예를 들어 근육 운동 직후에는 운동 부위에 수분 함량이 일시적으로 높아지는 데 기기에서는 근육량이 늘어난 것으로 인식한다. 또 오래 앉아있다가 측정하면 하체에 체액이 몰려 부위별 측정값에 오차가 발생하기도 한다. 다만 동일 조건을 유지하면 체성분 변화 추이를 정확하게 잡아낼 수 있다.

더 정확한 체성분 측정 방법은 비교적 큰 설비와 높은 비용 문제가 있다.

수중체중법은 부력을 활용한다. 육상에서 체중을 측정하고 몸을 물에 완전히 잠기게 해서 수중 체중을 측정한다. 그리고 폐에 남은 잔여 공기를 빼고 체중을 측정해 보정 값으로 활용한다.

이렇게 나온 육상 체중과 수중 체중을 공식에 넣어 체밀도와 지방 비율을 계산한다. 직접 측정이라는 장점이 있어 오랜 시간 BIA 같은 간접 측정법의 정확성을 확인하는 표준 측정법으로 활용됐다.


수중체중법은 몸 전체를 물에 완전히 담가야 하므로 절차가 복잡하고 특수 장비가 필요하다. 그 대안으로 이중 에너지 엑스선 흡수법(DEXA)이 표준 체성분 측정법으로 쓰인다.

DEXA는 두 가지 에너지 대역의 X선을 이용한다. X선은 에너지 수준이 높으면 투과도가 높다. 또 동일한 에너지의 X선은 피부 같은 저밀도 조직보다 뼈와 같은 고밀도 조직에 잘 막힌다. 이런 차이를 활용해 두 종류 엑스선으로 얻은 정보를 조합하면 체지방량, 제지방량, 골밀도 등 정보를 정확하게 얻을 수 있다.

최근에는 카메라와 적외선을 활용한 '3D 스캐닝 분석'도 상용화됐다. 촬영 결과를 컴퓨터 알고리즘으로 분석해 체성분뿐 아니라 각 부위의 둘레, 자세 같은 눈으로 보이는 특성까지 확인할 수 있는 강점이 있다. 다만 내장 지방량 같은 정보를 얻는 것에는 한계가 있다.

 

 

 

Body composition testing devices installed widely at fitness centers or health centers are commonly called in-body. In-body is a trademark that is used like an ordinary noun such as instant rice and spam. The official test name is Bioelectrical Impedance Analysis (BIA). Impedance is a physical quantity that is a kind of resistance in an alternating current circuit.

BIA estimates body composition by measuring the biological resistance of each part of the body by flowing microcurrents of various frequencies through various paths to the human body.


Inorganic and body moisture containing various ions are distributed differently in our body according to tissue characteristics. The degree to which electricity flows varies accordingly. For example, watery muscles flow relatively well, while low-moisture fat does not flow well.

Professional devices can use a total of eight electrodes, two for each limb, to allow current to flow through various paths, enabling them to measure each part. A home measuring device or a smart watch with only four electrodes measures only some parts and performs numerical correction on the entire value.

Using eight electrodes among BIA measurements is the most accurate, but there are limitations in that errors may occur depending on body moisture and electrolyte conditions.

For example, immediately after muscle exercise, the water content in the exercise area temporarily increases, but the device recognizes that the muscle mass has increased. In addition, if you measure after sitting for a long time, body fluid may concentrate on the lower body, causing errors in the measured values for each part. However, if the same conditions are maintained, the trend of changes in body composition can be accurately detected.

More accurate body composition measurement methods have relatively large facilities and high cost problems.

The underwater weight method utilizes buoyancy. We measure our weight on land, and we measure our underwater weight by completely immersing our body in water. We then remove residual air from our lungs, measure our weight, and use it as a correction value.

The body density and fat ratio are calculated by putting the obtained land weight and water weight into the formula. It has the advantage of direct measurement, so it has been used as a standard measurement method to check the accuracy of indirect measurement methods such as BIA for a long time.


The underwater weight method is complicated and requires special equipment because the entire body must be completely immersed in water. As an alternative, the dual energy X-ray absorption method (DEXA) is used as a standard body composition measurement method.

DEXA uses X-rays of two energy bands. High energy levels are transmittance of X-rays. In addition, X-rays of the same energy are more easily blocked by high-density tissues such as bones than low-density tissues such as skin. By combining the information obtained by the two types of X-rays using this difference, information such as body fat, fat mass, and bone density can be obtained accurately.

Recently, 3D scanning analysis using cameras and infrared rays has also been commercialized. By analyzing the photographing results through a computer algorithm, it is possible to check not only body composition but also visible characteristics such as circumference and posture of each part. However, there is a limit to obtaining information such as visceral fat content.