Carbohydrates

A well-nourished adult has about 440 g of carbohydrate in the body, most of it in three places: about 325 g of muscle glycogen, 90 to 100 g of liver glycogen, and 15 to 20 g of blood glucose.

Sugars function as a structural component of other molecules including nucleic acids, glycoproteins, glyco-lipids, ATP, and related nucleotides (GTP, cAMP, etc.), and they can be converted to amino acids and fats. Most of the body's carbohydrate, however, serves as fuel—an easily oxidized source of chemical energy. Most cells meet their energy needs from a combination of carbohydrates and fats, but some cells, such as neurons and erythrocytes, depend almost exclusively on carbohydrates. Even a brief period of hypoglycemia4 (deficiency of blood glucose) causes nervous system disturbances felt as weakness or dizziness.

Blood glucose concentration is therefore carefully regulated, mainly through the interplay of insulin and glucagon (see chapter 17 and later in this chapter). Among other effects, these hormones regulate the balance between glycogen and free blood glucose. If blood glucose concentration drops too low, the body draws on its stores of glyco-gen to meet its energy needs. If glycogen stores are depleted, physical endurance is greatly reduced. Thus it is important to consume enough carbohydrate to ensure that the body maintains adequate stores of glycogen for periods of exercise and fasting (including sleep).

Carbohydrate intake also influences the metabolism of other nutrients. Excess carbohydrate is converted to fat

4 hypo = below normal + glyc = sugar + emia = blood condition

Saladin: Anatomy & I 26. Nutrition and I Text I © The McGraw-Hill

Physiology: The Unity of Metabolism Companies, 2003 Form and Function, Third Edition

Chapter 26 Nutrition and Metabolism 989

Table 26.1

Nutrient Classes and Their Principal Functions

Nutrient

Daily Requirement

Representative Functions

Water

2.5 L

Solvent; coolant; reactant or product in many metabolic reactions (especially hydrolysis and

condensation); dilutes and eliminates metabolic wastes; supports blood volume and pressure

Carbohydrates

125-175g

Fuel; a component of nucleic acids, ATP and other nucleotides, glycoproteins, and glycolipids

Lipids

80-100g

Fuel; plasma membrane structure; myelin sheaths of nerve fibers; hormones; eicosanoids; bile

salts; insulation; protective padding around organs; absorption of fat-soluble vitamins; vitamin D

synthesis; some blood-clotting factors

Proteins

44-60 g

Muscle contraction; ciliary and flagellar motility; structure of cellular membranes and extracellular

material; enzymes; major component of connective tissues; transport of plasma lipids; some

hormones; oxygen binding and transport pigments; blood-clotting factors; blood viscosity and

osmolarity; antibodies; immune recognition; neuromodulators; buffers; emergency fuel

Minerals

0.05-3,300 mg

Structure of bones and teeth; component of some structural proteins, hormones, ATP,

phospholipids, and other chemicals; cofactors for many enzymes; electrolytes; oxygen transport

by hemoglobin and myoglobin; buffers; stomach acid; osmolarity of body fluids

Vitamins

0.002-60 mg

Coenzymes for many metabolic pathways; antioxidants; component of visual pigment; one hormone

(vitamin D)

and conversely, fat is oxidized as fuel when glucose and glycogen levels are too low to meet our energy needs. This is why the consumption of starchy and sugary foods has a pronounced effect on body weight. It is unwise, however, to try to "burn off fat" by excessively reducing carbohydrate intake. As shown later in this chapter, the complete and efficient oxidation of fats depends on adequate carbohydrate intake and the presence of certain intermediates of carbohydrate metabolism. If these are lacking, fats are incompletely oxidized to ketone bodies, which may cause metabolic acidosis.

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