Since ancient times, humans have been natural explorers, constantly investigating mysteries around us. When we gaze at the starry sky, we wonder where the universe ends; when we look down at the earth, we seek to understand how life began; we dive into oceans to discover unknown species; we break down matter to understand atomic structure.
Among all these explorations, one question has always followed us: "What exactly are we, these thinking, feeling, creating beings, made of?"
This seemingly simple question carries profound philosophical and scientific significance. Ancient Egyptians viewed the body as a vessel for the soul, developing sophisticated mummification techniques and gradually learning about human anatomy through repeated practice. Ancient Greek philosophers proposed the "four elements theory" [1], believing that all matter, including the human body, consisted of fire, air, water, and earth. Ancient Chinese physicians used the five elements—metal, wood, water, fire, and earth—to understand the human body [2], applying the same framework to explain both the natural world and the human form.
These early explorations, though deeply philosophical, all pointed to the same fundamental question: What are we made of? As anatomy, chemistry, and physics advanced, this ancient question evolved from speculation to empirical science. We no longer just theorized, we began measuring and analyzing, gradually understanding this existence so familiar yet mysterious to us. How do modern scientists precisely measure the fat, muscle, and bone in our bodies? This millennia-old question found revolutionary answers over the past century.
The Foundation of Modern Measurement
In 1896, scientists discovered they could determine fish oil content by measuring fish density. Lower density meant higher oil content. This seemingly minor observation became the foundation of modern body composition analysis.
By 1942, scientist Behnke applied this principle to humans, inventing "underwater weighing." People would completely submerge in water to have their weight measured, then Archimedes′ principle would calculate body density. Since fat has lower density (about 0.9 kg/L) while muscle has higher density (about 1.1 kg/L), body fat percentage could be estimated [3].
Early body composition analysis simplified the complex human body into just two components: fat tissue and fat-free mass. This model dominated the field for over forty years. It was simple, practical, and reasonably accurate given the technology available. However, this two-compartment model relied on a key assumption: that all people have identical fat-free mass composition, specifically 73.2% water, 20% protein, and about 7% minerals. Reality, of course, is far more complex. People of different ages have varying body water content; men and women have different muscle proportions; different ethnicities have varying bone densities.
Evolution Toward Precision
In the 1960s, scientists developed the "three-compartment model," adding total body water measurement. Subjects would drink water containing special isotopes (like deuterium), allowing researchers to track how this water distributed throughout the body.
By the 1980s, dual-photon absorptiometry emerged, later becoming what we know as DXA, enabling scientists to measure bone mineral content. This gave birth to the "four-compartment model," dividing the human body into fat, total body water, protein, and minerals.
As technology advanced, methods for measuring body composition became increasingly sophisticated:
Neutron activation analysisrepresents perhaps the most precise approach. It analyzes body composition using radiation produced when neutrons collide with atomic nuclei, measuring eleven major elements in the human body: oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus, potassium, and others. It essentially treats the human body as a chemical sample, achieving extremely high precision. However, the expensive and complex equipment limits its use to specialized research centers.
Dual-energy X-ray absorptiometry (DXA) became widespread in the 1990s. It measures bone density while distinguishing between fat, muscle, and bone, all with relatively low radiation exposure. Today, DXA is primarily used in hospitals for bone density testing.
In 1995, air displacement plethysmography[4] opened new possibilities for body composition measurement. This technology applies Boyle′s Law, precisely calculating body density by measuring the air volume a person displaces in a sealed chamber. Subjects simply wear tight-fitting clothing and sit quietly in the chamber briefly. Compared to underwater weighing, this method offers a much more accessible option for people who fear water, elderly individuals, children, pregnant women, and those with mobility limitations.
The Four Levels of Understanding
Modern science examines body composition across four distinct levels, each telling its own story:
Atomic Level
At the most fundamental chemical level, the human body consists primarily of 11 elements, with oxygen, carbon, hydrogen, and nitrogen making up the vast majority.
Molecular Level
Moving up from atoms, we find the molecular realm, including fats, proteins, water, minerals, and more. This represents the analytical framework scientists use most frequently and the language we most easily understand.
Cellular Level
Intracellular and extracellular spaces each support different biological processes. Understanding this level reveals the secrets of metabolism.
Tissue and Organ Level
Tissues and organs represent our most familiar view of the body, showing the actual size and shape of muscles, bones, and internal organs. They provide the most intuitive picture of body composition.
Whether underwater weighing, neutron activation analysis, or DXA scanning, these precise measurement techniques function like archaeological tools for the body, being both accurate and detailed. However, they require expensive equipment and complex procedures, creating barriers for ordinary people wanting to understand their body composition. This led scientists to seek alternatives: Could there be a more accessible, simpler way to unlock the body′s secrets?
The answer lay in a phenomenon already discovered but not yet fully exploited—the electrical properties of the human body. In our next article, we′ll explore how bioelectrical impedance analysis quietly revolutionized the field.
Reference
[1] Stanford Encyclopedia of Philosophy. (n.d.). Empedocles. Stanford University. Retrieved June 12, 2025, from https://plato.stanford.edu/entries/empedocles/
[2] GingerChi. (2024, November 15). What are the five elements. GingerChi Blog. https://gingerchi.com/blogs/journal/what-are-the-five-elements
[3] Pietrobelli, A., Heymsfield, S. B., Wang, Z. M., ＆ Gallagher, D. (2001). Multi-component body composition models: Recent advances and future directions. European Journal of Clinical Nutrition, 55, 69-75.
[4] Measurement Toolkit. (n.d.). Air displacement plethysmography. Retrieved June 12, 2025, from https://www.measurement-toolkit.org/anthropometry/objective-methods/plethysmography