Intermediary metabolism

Carbohydrate and fat form the immediate and long-term energy stores of the body, and protein constitutes the active (functional) cell mass and is also an energy source but, normally, a relatively minor one. All three macronutrients are interrelated. Proteins are synthesized from amino acids derived from ingested protein. Glucose and fat provide energy via adenosine triphosphate. The brain and red cells can only obtain their energy from glucose. Glucose is oxidized via the glycolytic and the tricarboxylic acid (Krebs) cycle pathways. Fatty acids are metabolized by the process of beta-oxidation, whereby two carbon fragments are cleaved from the fatty acid chain and enter the Krebs cycle. Amino acids are deaminated to keto acids and the nitrogen moiety excreted in the urine mostly as urea. The keto acids enter the metabolic pathways at various points, mostly in the Krebs cycle. Glucose can be synthesized from lactate, glycerol and amino acids (gluconeogenesis), but not from fatty acids.