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Body composition may be analyzed in terms of molecular type e.g., water, protein, connective tissue, fats (or lipids), hydroxylapatite (in bones), carbohydrates (such as glycogen and glucose) and DNA. In terms of tissue type, the body may be analyzed into water, fat, muscle, bone, etc. In terms of cell type, the body contains hundreds of different types of cells, but notably, the largest number of cells contained in a human body (though not the largest mass of cells) are not human cells, but bacteria residing in the normal human gastrointestinal tract.
Contents
Major, minor and trace elements
Almost 99% of the mass of the human body is made up of 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 11 are necessary for life. The remaining elements are trace elements, of which more than a dozen are thought on the basis of good evidence to be necessary for life. All of the mass of the trace elements put together (less than 10 grams for a human body) do not add to the body mass of magnesium, the least common of the 11 non-trace elements.
Other elements and questionable human-required elements
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 toxics, depending on amount (cadmium, mercury, radioactives). The possible utility and toxicity of a few elements at levels normally found in the body (aluminium) is debated. Functions have been proposed for trace amounts of cadmium and lead, although these are almost certainly toxic in amounts very much larger than normally found in the body. There is evidence that arsenic, an element normally considered a toxic in higher amounts, is essential in ultratrace quantities, even in mammals (rats, hamsters, goats).
Some elements (arsenic, silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Bromine is used abundantly by some (though not all) lower organisms, and opportunistically in eosinophils in humans. One study has found bromine to be necessary to collagen IV synthesis in humans. Fluorine is used by a number of plants to manufacture toxins (see that element) but in humans only functions as a local (topical) hardening agent in tooth enamel, and not in an essential biological role.
Elemental composition list
The average 70 kg (150 lb) adult human body contains approximately 7027700000000000000♠7×1027 atoms and contains at least detectable traces of 60 chemical elements. About 29 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, mainly due to differences in the proportion of fat, muscle and bone in their body. Persons with more fat will have a higher proportion of carbon and a lower proportion of most other elements (the proportion of hydrogen will be about the same). The numbers in the table are averages of different numbers reported by different references.
The adult human body averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males. Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. Because of water content, the human body contains more oxygen by mass than any other element, but more hydrogen by atom-fraction than any element.
The elements listed below as "Essential in humans" are those listed by the (US) Food and Drug Administration as essential nutritients, as well as six additional elements: oxygen, carbon, hydrogen, and nitrogen (the fundamental building blocks of life on Earth), sulfur (essential to all cells) and cobalt (a necessary component of vitamin B12). Elements listed as "Possibly" or "Probably" essential are those cited by the National Research Council (United States) as beneficial to human health and possibly or probably essential.
*Iron = ~3 g in men, ~2.3 g in women
Most of the elements needed for life are relatively common in the Earth's crust. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is harmful in large amounts. Transferrins can bind aluminium.
Other elements
Of the 94 naturally occurring chemical elements (which now include Neptunium and Plutonium) 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body. For some of these elements, numbers for concentrations in various tissues or organs is available, typically from studies involving small population sample sizes.
Noble Gases
Concentration of noble gases in whole blood.
* hypothetical value for radon based on 10 Bq/m3 and 0.4 blood/air partition coefficient.
Lanthanides
Of the seventeen rare earth elements (REEs), fifteen belong to the lanthanide series. The other two, scandium and yttrium, are listed in the table above, as are three lanthanides: lanthanum, cerium, and samarium. Of the remaining twelve lanthanides, eleven are listed below. No information is available regarding the remaining lanthanide, promethium.
Concentration of rare earth elements / lanthanides in blood serum.
Platinum group metals (PGMs)
Concentration of platinum group metals in the blood.
Essential elements on the periodic table
Periodic table highlighting dietary elements
Composition by molecule type
The composition can also be expressed in terms of chemicals, such as:
The composition of the human body can be viewed on an atomic and molecular scale as shown in this article.
The estimated gross molecular contents of a typical 20-micrometre human cell is as follows:
*The amount of water is highly dependent on body composition and amount of fat. In adults in developed countries it averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males. DNA: A human cell also contains mitochondrial DNA. Sperm cells contain less mitochondrial DNA than other cells. A mammalian red blood cell normally contains no nucleus in adulthood, and thus no DNA. However, nucleated red blood cells, or NRBCs, are present in the fetal and neonatal circulations, and may appear in mature mammals when disease is present.
Materials and tissues
Body composition can also be expressed in terms of various types of material, such as:
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 humans are called commensal organisms.