<P> Proteins are the basis of many animal body structures (e.g. muscles, skin, and hair). They also form the enyzmes which control chemical reactions throughout the body . Each molecule is composed of amino acids which are characterized by inclusion of nitrogen and sometimes sulphur (these components are responsible for the distinctive smell of burning protein, such as the keratin in hair). The body requires amino acids to produce new proteins (protein retention) and to replace damaged proteins (maintenance). As there is no protein or amino acid storage provision, amino acids must be present in the diet . Excess amino acids are discarded, typically in the urine . For all animals, some amino acids are essential (an animal cannot produce them internally) and some are non-essential (the animal can produce them from other nitrogen - containing compounds). About twenty amino acids are found in the human body, and about ten of these are essential, and therefore must be included in the diet . A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important in some situations: during early development and maturation, pregnancy, lactation, or injury (a burn, for instance). A complete protein source contains all the essential amino acids; an incomplete protein source lacks one or more of the essential amino acids . </P> <P> It is possible to combine two incomplete protein sources (e.g. rice and beans) to make a complete protein source, and characteristic combinations are the basis of distinct cultural cooking traditions . Sources of dietary protein include meats, tofu and other soy - products, eggs, grains, legumes, and dairy products such as milk and cheese . A few amino acids from protein can be converted into glucose and used for fuel through a process called gluconeogenesis; this is done in quantity only during starvation . The amino acids remaining after such conversion are discarded . </P> <P> Dietary minerals are the chemical elements required by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen that are present in nearly all organic molecules . The term "mineral" is archaic, since the intent is to describe simply the less common elements in the diet . Some are heavier than the four just mentioned--including several metals, which often occur as ions in the body . Some dietitians recommend that these be supplied from foods in which they occur naturally, or at least as complex compounds, or sometimes even from natural inorganic sources (such as calcium carbonate from ground oyster shells). Some are absorbed much more readily in the ionic forms found in such sources . On the other hand, minerals are often artificially added to the diet as supplements; the most famous is likely iodine in iodized salt which prevents goiter . </P> <P> Many elements are essential in relative quantity; they are usually called "bulk minerals". Some are structural, but many play a role as electrolytes . Elements with recommended dietary allowance (RDA) greater than 200 mg / day are, in alphabetical order (with informal or folk - medicine perspectives in parentheses): </P>

Where do animals get the nitrogen for proteins like muscle