Chemistry - Matter cycles
Atoms are neither created nor destroyed in chemical reactions. The
chemical processes occurring in your body ensure that you incorporate C, H,
O, N and other elements into your body from the food you eat.
However, you are also releasing elements from your body all the time, e.g.
in exhaled carbon dioxide, water vapour and excreted urea.
The two main matter cycles that influence the production and consumption
of food are the carbon cycle,  (also referred to as the carbon-oxygen
cycle) and the nitrogen cycle. 
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