<P> 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 . </P> <P> 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 up to the body mass of magnesium, the least common of the 11 non-trace elements . </P> <P> 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, in mammals such as rats, hamsters, and goats . </P>

99 of all is found inside the cells of the body