Composition of the human body
The composition of the human body can be looked at from the point of view of either mass composition, or atomic composition. To illustrate both views, the human body is ~70% water, and water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent. Thus, most of the mass of the human body is oxygen, but most of the atoms in the human body are hydrogen atoms. Both mass-composition and atomic composition figures are given below.
Almost 99% of the mass of the human body is made up of the six elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Only about 0.85% is composed of another five elements: potassium, sulfur, sodium, chlorine, and magnesium. All are necessary to life. The remaining elements are trace elements, of which more than a dozen are thought to be necessary for life, or play an active role in health (e.g., fluorine, which hardens dental enamel but seems to have no other function).
Not all elements which are found in the human body in trace quantities play a role in life. Some of these elements are thought to be simple bystander contaminants without function (examples: caesium, titanium), while many others are thought to be active toxins, depending on amount (cadmium, mercury, radioactives). The possible utility and toxicity of a few elements at levels normally found in the body (aluminum) is debated. Functions have been proposed for trace amounts of cadmium and lead, but these are almost certainly toxic in amounts normally found in the body. There is evidence that one element normally considered a toxin (arsenic) is essential in ultratrace quantities, even in mammals. Some elements that are clearly used in lower organisms and plants (arsenic, silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Two halogens used abundantly by lower organisms (fluorine and bromine) are presently known to be used by mammals only opportunistically. However, a general rule is that elements found in active biochemical use in lower organisms are often eventually found to be used in some way, by higher organisms.
Elemental composition 
The average 70 kg adult human body contains approximately 6.7 x 1027 atoms and contains at least detectable traces of 60 chemical elements. About 24 or 25 of these elements are thought to play an active positive role in life and health in humans.
The relative amounts of each element vary by individual, with the largest contributor due to fat/muscle/bone body composition ratio differences from person to person. The numbers in the table are averages of different numbers reported by different references.
|Atomic number||Element||Percent of Mass||Mass (kg)||Atomic percent||Positive health role in mammals||Group|
|8||Oxygen||65||43||24||Yes (water, electron acceptor) /No (Reactive Oxygen Species)||16|
|6||Carbon||18||16||12||Yes (organic compounds are hydrocarbon derivatives)||14|
|1||Hydrogen||10||7||63||Yes (e.g. water)||1|
|7||Nitrogen||3||1.8||0.58||Yes (e.g. DNA and amino acids)||15|
|20||Calcium||1.4||1.0||0.24||Yes (e.g. Calmodulin and Hydroxylapatite in bones)||2|
|15||Phosphorus||1.1||0.78||0.14||Yes (e.g. DNA and phosphorylation)||15|
|19||Potassium||0.25||0.14||0.033||Yes (e.g. Na+/K+-ATPase)||1|
|16||Sulfur||0.25||0.14||0.038||Yes (e.g.Cysteine, Methionine, Biotin, Thiamine)||16|
|11||Sodium||0.15||0.10||0.037||Yes (e.g. Na+/K+-ATPase)||1|
|17||Chlorine||0.15||0.095||0.024||Yes (e.g. Cl-transporting ATPase)||17|
|12||Magnesium||0.05||0.019||0.0070||Yes (e.g. binding to ATP and other nucleotides)||2|
|26||Iron*||0.006||0.0042||0.00067||Yes (e.g. Hemoglobin, Cytochromes)||8|
|9||Fluorine||0.0037||0.0026||0.0012||Yes/No (topically hardens teeth; toxic in higher amounts)||17|
|30||Zinc||0.0032||0.0023||0.00031||Yes (e.g. Zinc finger proteins)||12|
|82||Lead||0.00017||0.00012||0.0000045||No (?) (toxic in higher amounts)||14|
|29||Copper||0.0001||0.000072||0.0000104||Yes (e.g. copper proteins)||11|
|48||Cadmium||0.000072||0.000050||0.0000045||No(?) (toxic in higher amounts)||12|
|53||Iodine||0.000016||0.000020||7.5e-7||Yes (e.g. thyroxine, triiodothyronine)||17|
|34||Selenium||0.000019||0.000015||4.5e-8||Yes (toxic in higher amounts)||16|
|28||Nickel||0.000014||0.000015||0.0000015||Yes (e.g. urease)||10|
|24||Chromium||0.0000024||0.000014||8.9e-8||Yes (not confirmed)||6|
|25||Manganese||0.000017||0.000012||0.0000015||Yes (e.g. Mn-SOD)||7|
|33||Arsenic||0.000026||0.000007||8.9e-8||Yes (not confirmed). Toxic in higher amounts||15|
|3||Lithium||0.0000031||0.000007||0.0000015||Yes (not confirmed). Toxic in high amounts. Useful medically (mood stabilizer).||1|
|42||Molybdenum||0.000013||0.000005||4.5e-8||Yes (e.g. the molybdenum oxotransferases, Xanthine oxidase and Sulfite oxidase)||6|
|27||Cobalt||0.0000021||0.000003||3.0e-7||Yes (e.g. vitamin B12)||9|
|23||Vanadium||0.000026||1.1e-7||1.2e-8||Yes (not confirmed)||5|
*Iron = ~3 g in men, ~2.3 g in women
The elements needed for life are relatively common in the Earth's crust, and conversely most of the common elements are necessary for life. An exception is aluminium, which is the third most common element in the Earth's crust (after oxygen and silicon), but seems to serve no function in living cells. Rather, it is harmful in large amounts. Transferrins can bind aluminium.
|The four organic basic elements||Quantity elements||Essential trace elements||Function suggested from active handling in mammals, but no specific identified biochemical function|
Composition by molecule type 
The composition can also be expressed in terms of chemicals, such as:
- Proteins – including those of hair, connective tissue, etc.
- Fats (or lipids)
- Apatite in bones
- Carbohydrates such as glycogen and glucose
- Dissolved inorganic ions such as sodium, potassium, chloride, bicarbonate, phosphate
- Gases such as oxygen, carbon dioxide, nitrogen oxide, hydrogen, carbon monoxide, methanethiol. These may be dissolved or present in the gases in the lungs or intestines. Ethane and pentane are produced by oxygen free radicals.
- Many other small molecules, such as amino acids, fatty acids, nucleobases, nucleosides, nucleotides, vitamins, cofactors.
- Free radicals such as superoxide, hydroxyl, and hydroperoxyl.
The estimated gross molecular contents of a typical 20-micrometre human cell is as follows:
|Molecule||Percent of Mass||Mol.Weight (daltons)||Molecules||Percent of Molecules|
Water: Obviously the amount of water is highly dependent on the level of hydration. DNA: A human cell also contains mitochondrial DNA. Sperm cells contain less mitochondrial DNA than other cells. A mammalian red blood cell contains no nucleus and thus no DNA.
Materials and tissues 
Body composition can also be expressed in terms of various types of material, such as:
- Bone and teeth
- Brain and nerves
- Connective tissue
- Blood – 7% of body weight.
- Contents of digestive tract, including intestinal gas
- Air in lungs
Composition by cell type 
There are many species of bacteria and other microorganisms that live on or inside the healthy human body. In fact, 90% of the cells in (or on) a human body are microbes, by number (much less by mass or volume). Some of these symbionts are necessary for our health. Those that neither help nor harm us are called commensal organisms.
See also 
- Hydrostatic weighing
- Dietary mineral
- Composition of blood
- List of human blood components
- Body composition
- Abundance of elements in Earth's crust
- Abundance of the chemical elements
- Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al.. Baltimore : Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI: 
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- Douglas Fox, "The speed of life", New Scientist, No 2419, 1 November 2003.
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