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The integration of metabolism is a story of supply and demand. Food is ingested to supply energy but must be converted to the carbohydrate, lipid, and amino acid forms the body can use, primarily glucose and fatty acids. Individual cells then convert the fuels to usable energy, adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). The body demands energy to function but individual organs and tissues require particular sources of energy under varying conditions.

To convert consumed food into the needed energy, the body uses a variety of organs, each with unique metabolic profiles, to integrate and regulate the use and storage of energy. Specific regulatory points of biochemical pathways provide immediate control of the usage, conversion, or storage of food energy. Various hormones can also regulate these biochemical pathways to provide longer term control of food conversion and energy usage. Essential to both of these processes is the maintenance of glucose homeostasis. Finally, vitamins and minerals serve important functions as cofactors in many of these metabolic reactions. Their deficiency or excess can lead to numerous disease states.


The first consideration is the major sources of energy that can be used by the body, the nutrients required for their metabolism, and the biochemical pathways that integrate them.

Amino acids (Chapter 1, Figure 10-1) provide several major biochemical functions, including serving as (1) the building blocks of proteins; (2) the precursors of hormones, neurotransmitters, and other important signaling molecules (such as nitrous oxide); and (3) contributors to the purine and pyrimidine components of nucleic acids, co-enzymes [NADH and flavin adenine dinucleotide (FADH2)], and other fundamental biological molecules. Additionally, excess amino acids can enter the citric acid cycle and can be used to generate or store biological energy (Chapter 5). Furthermore, the metabolism of some amino acids can be funneled into glucose synthesis (gluconeogenesis) during food deprivation.

Figure 10-1.

Summary of Amino Acid Metabolism. [Reproduced with permission from Naik P: Biochemistry, 3rd edition, Jaypee Brothers Medical Publishers (P) Ltd., 2009.]

Carbohydrates (Chapter 2, Figure 10-2) perform a fundamental role as the primary energy-production source for the human body. Glycolysis and the subsequent metabolic pathways form the primary energy molecules ATP, NADH, and FADH2 via the oxidation of glucose and other carbohydrates (Chapter 6). Storage of carbohydrates as glycogen offers a readily available source of energy when dietary carbohydrate intake is low (Chapter 2). Carbohydrates are also important in the synthesis of reduced nicotinamide adenine dinucleotide phosphate (NADPH) (Chapter 6) and nucleic acids (Chapter 4).

Figure 10-2.

Transport and Fate of Major Carbohydrates and Amino Acids. [Reproduced with permission from Murray RA, et al.: Harper’s Illustrated ...

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