Five-level model of the human body

Figure 1. The five-level model of human body composition (adapted from Wang et al 1992, colours added)
The five level model of human body composition

Introduction The five level model of the body composition divides the human body into five levels of increasing complexity. The levels are

  1. Atomic
  2. Molecular
  3. Cellular
  4. Tissue-System
  5. Whole Body

The five level model was designed a tool to organise biological information about the human body. Each level has clearly defined components that form the building blocks of the level above.

The model also provides a clear conceptual framework for teaching and learning about health and nutrition. To understand how the human body functions, we need to understand what that the body is made of:

Learning how the body’s building blocks are obtained from the diet provides a clear rationale for consuming a wide variety of nutritious foods.

Knowing how those building blocks are absorbed and excreted from the body provides a clear rationale for maintaining an active lifestyle.

Understanding how the five levels combine to form a whole living person fosters a deep appreciation for the complexity of the human body.

Background The five-level model of human body composition was conceived in 1992 by Professor Zimian Wang, Professor Richard Pierson and Professor Steven Heymsfield and published in the American Journal of Clinical Nutrition.

Level 1: Atoms

Of the 118 elements, approximately 50 are found in the tissues of the human body. Twenty elements are essential for life. Fluorine is not considered essential, however, fluoride ions play an important role in maintaining dental health and possibly bone health.

Figure 2. Human body composition at the atomic level (Adapted from Wang et al 1992)
List of essential and non-essential elements in the human body

Six elements (oxygen, carbon, hydrogen, nitrogen, calcium and phosphorous) make up ~99% of the human body. The remainder are present at trace levels.

Table 1 below lists the essential elements in order of their contribution to the total weight in an average 70 kg adult.

The non-essential elements are listed in order of increasing atomic number (the number of protons contain in the nucleus of the element’s atoms).

Table 1. Atomic composition of the human body – essential elements

Total in 70kg
adult (g)
* Fluorine has an important biological role in dental health, however, it is generally not considered essential for life

Table 2. Atomic composition of the human body – non-essential elements

Non-essential elementsSymbolAtomic
Total in 70kg adult (g)
BoronB5< 0.02
GoldAu79< 0.0098
BismuthBi83< 0.0005
Source data: Snyder et al 1975 (except where indicated) with additional data from 1: Emsley 2011
RDI and daily balance of essential elements

Level 2: Molecules

The human body is made of more than 100,000 different chemical compounds.

Measuring each of these substances is impossible so body composition researchers group them into categories of closely related molecular species instead. The four most abundant kinds of molecules in the human body are 1) water, 2) lipids, 3) protein, 4) minerals and 5) glycogen, the storage form of carbohydrate in animal tissues.

Vitamins are essential molecules that the body cannot make, which fall into two categories, a) the water soluble vitamins and b) the lipid (fat) soluble vitamins.

The nucleic acids, DNA and RNA, are vital compounds at the molecular level, however, their contribution to total body weight is minimal.

Figure 3. Human body composition at the molecular level (Adapted from Wang et al 1992 with additional data from Piovesan et al 2019, Müller et al 2002)
1) Water
2) Lipids

Lipids are a family of chemical compounds, including fats and oils, that do not dissolve in water but do dissolve in non-polar solvents. This is reflected in the old saying, “oil and water don’t mix”.

Lipids include:

  • Fats and oils (triglycerides)
  • Phospholipids
  • Steroids
  • Waxes

Fats and oils

Fats and oils are both triglycerides, which is the substance stored inside fat cells. Fats are solid at room temperature whereas oils are liquids at room temperature.

Most animal triglycerides are solid at room temperature, whereas most plant triglycerides are liquid, hence the term animal fat versus vegetable oil.

There are exceptions to this simple rule of thumb. For example, fish are animals that produce triglycerides which tend to be liquid at room temperature, hence the term fish oil. Coconut, palm and palm kernel oil are all solid at room temperature, however, these are still referred to as oils. The confusion melts away when we learn at the molecular structure of a triglyceride.

Triglycerides are made of three fatty acid molecules and one glycerine molecule. Every glycerine molecule can make a chemical bond with up to three fatty acids. When a single fatty acid binds with glycerine, the resulting molecule is called a mono-glyceride. When two fatty acids bind with glycerine, a di-glyceride is formed. It is not possible for four fatty acids to bind to glycerine so there are no tetra-glycerides.

Fatty acids are produced by plants fatty acids, such as the essential omega-3 and omega-6 fatty acids which humans cannot synthesise and therefore must eat.

One fatty acid chemically bound to one glycerine molecule form a substance called mono-glyceride. Two fatty acids bound to one glycerine molecule form a di-glyceride. Three fatty acids bound to one glycerine form triglyceride.

3) Protein
4) Mineral

5) Carbohydrates (mostly glycogen)
6) Nucleic Acids (DNA, RNA)
Level 3: Cells
Figure 4. Human body composition at the cellular level (Adapted from Wang et al 1992 with additional data from Bianconi et al 2013, Janssen et al 2000 and Sender et al 2016)

The human body consists of at least 412 different kinds of cells (Vickaryous and Hall 2006). The total number of cells in an adult body is approximately 37,000,000,000,000 i.e. 37 trillion cells (Bianconi et al 2013).

The human microbiome consists of an additional 38,000,000,000,000 (38 trillion) bacterial and archaea cells, with a combined weight of approximately 0.2kg. The number of bacteria in the microbiome outnumbers the archaea by a factor of 100 to 1000. (Sender et al 2016).

Level 4: Tissues
Figure 5. Human body composition at the tissue-systems level (adapted from Wang et al 1992 with additional data from Fazeli et al 2013, Kaess et al 2012, Snyder et al 1975)
Level 5: Whole Body
Figure 6. Human body composition at the whole body level (adapted from Wang et al 1992)
Level Up: Extending the model for science education

The five-level model of human body composition conceived by Wang et al can be extended to include all the matter in the universe.

Adopting the visual metaphor of a high-rise building, the extended thirteen-level model of the universe acquires three basement levels and five additional storeys above the whole-body.

Figure 7. The extended thirteen-level model of the composition of the Universe

Level B1, Protons, neutrons, electrons Atoms are made of smaller parts, called protons, neutrons and electrons. Protons and neutrons occupy the nucleus at the centre of atoms. Electrons orbit the nucleus in a form of motion that obeys the laws of quantum physics.

Protons carry a positive electric charge, electrons carry an equal but opposite negative electric charge and neutrons are electrically neutral (ie they carry no electric charge).

Neutrons are about 0.1% heavier than protons. Protons and neutrons are 1836 times heavier than electrons. Therefore, nearly all of the mass of an atom comes from its protons and neutrons.

An isolated proton is stable, however, an isolated neutron will spontaneously decay into a proton and an electron within an average of approximately 15 minutes. This process is called beta decay and is responsible for beta radiation (β radiation), which is one of the three forms of ionising radiation (the two other forms are alpha and gamma radiation).

Level B2, Quarks, leptons, bosons Electrons have no known internal parts but protons and neutrons are made of smaller particles called quarks. Electrons are members of a family of particles called leptons. Bosons include the particles that make up visible light and all electromagnetic radiation. These three particle families make up the Standard Model of Physics.

Level B3, Unknown Nobody knows if quarks are made of even more fundamental particles. Therefore, basement 3 level of the model may or may not exist.

Level 6, Earth includes all humans, animals, plants and microbes and the natural and built environment. This level therefore includes all human relationships, societies, politics and economics, ecosystems, weather systems as well as the Earth’s finite natural resources.

Level 7, Solar System includes all the other planets, comets, asteroids and the sun, which provides all of the energy for photosynthesis and weather.

Level 8, Galaxy The sun is just one of 100 billion stars in our galaxy, which is called the Milky Way. Our nearest star is Alpha Centauri, a triple star system approximately 4 light years from Earth (i.e. the photons that our eyes receives from Alpha Centauri departed the system approximately four years ago).

Level 9, Universe Our galaxy is just one of 100 or possibly up to 2000 billion galaxies in the universe.

Level 10, Unknown Nobody knows what, if anything, lies beyond the known universe. Therefore, level 10 of the model may or may not exist. The many worlds interpretation of quantum mechanics holds that there are many worlds which exist in parallel to our own, produced by the bifurcation of our universe every time a quantum event occurs one way, which could have occurred in another way. A different theory called the multiverse hypothesis posits the existence of multiple universes, each with different conditions to our own.

Dark matter and dark energy are not depicted in the model but are believed to make up most of the mass of the universe. Dark matter is believed to make up approximately 27% of the universe. Dark energy makes up a further 68% and everything else, including the Earth and all other planets, stars and space dust accounts for less than 5% of the total mass of the cosmos. Nobody knows what dark matter and dark energy are made of so, for now, the name is best interpreted as an acronym for Don’t Actually Really Know.


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