What is body composition?

Overview

Body composition describes what the human body is made of — typically expressed as percentages of fat, bone, water, and muscle. Understanding body composition helps practitioners assess a client’s health status beyond simple body weight.

The Five Levels of Body Composition

Body composition can be analyzed at five distinct levels, each building on the one before it.

Level 1 — Atomic

The body is examined in terms of chemical elements. Of the 106 known elements, 50 are found in the human body, but just six account for over 98% of body weight:

Oxygen
Carbon
Hydrogen
Nitrogen
Calcium
Phosphorus

Level 2 — Molecular

Elements combine to form molecules. Key molecular components of the body include:

Water (hydrogen + oxygen)
Lipids (fat molecules)
Protein

Level 3 — Cellular

Molecules assemble into cells, which allow the body to function. The three components at this level are:

Cells — including fat cells, muscle cells, bone cells, and connective cells
Extracellular fluid — the fluid surrounding cells
Extracellular solids — organic and inorganic compounds such as collagen fibers

Level 4 — Tissue / System

This is the level most familiar to practitioners and clients. Tissue-level body composition breaks the body into components such as:

Adipose tissue (body fat) — e.g., 21% in a reference 70 kg male
Muscle — e.g., ~40% of body weight
Bone — e.g., ~7% of body weight
Blood, skin, and organs — the remainder

Metrics at this level include lean mass, fat mass, and bone mass.

Level 5 — Whole Body

This level considers overall body size, shape, and physical characteristics — the attributes that most clearly distinguish individuals from one another. Whole-body metrics include:

Circumferences
Segment lengths
Skinfold thickness
Body surface area
Body volume

Notes

Practitioners commonly focus on Level 4 (tissue composition) but often overlook Level 5 (whole-body shape and size). A comprehensive assessment should address both.
The most informative measurement devices provide both Level 4 and Level 5 data in a single assessment — for example, collecting lean mass, fat mass, and bone mass alongside circumferences, segment lengths, body surface area, and body volume.
Styku is designed to support both levels: 3D body scanning captures whole-body shape metrics (Level 5), and when paired with compatible body composition tools, can contribute to a full multi-level assessment.

Applies to: All Styku configurations

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