Body composition may be analyzed in terms of molecular type e. In terms of tissue type, the body may be analyzed into water, fat, muscle, bone, etc. All 11 are necessary for how many enzymes are found in the human body. Not all elements which are found in the human body in trace quantities play a role in life. 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, in mammals such as rats, hamsters, and goats. One study has found bromine to be necessary to collagen IV synthesis in humans. 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. The numbers in the table are averages of different numbers reported by different references. This varies substantially by age, sex, and adiposity. 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. Of the 94 naturally occurring chemical elements, 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body.
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