What Happens to the Nitrogen When Amino Acids Are Used for Energy Purposes

Amino acids are different from carbohydrate and fat because they contain nitrogen (N). This creates an additional consideration for the body if it wishes to use amino acids for energy or to make fat (in an overfed state) or glucose (in a fasting or exercise state). Thus an important step in using amino acids for any of these purposes is to remove the nitrogen-containing portion of the molecule. Once removed the nitrogen portion of amino acids becomes ammonia (NH4+), which is potentially toxic to the brain. Thus it must be removed from the body before it builds up in the blood.

The most prevalent way to rid the body of the nitrogen removed from amino acids is as urea (Figure 6.5). Urea is made by the liver and released into the blood, circulates to the kidneys and is subsequently lost from the body in urine. Each molecule of urea allows for the efficient removal of two nitrogen atoms from our body.

The nitrogen from amino acids used for energy is mostly removed from the body as urea.

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