Which of the B-complex vitamins can be synthesized from tryptophan?

^9 What is the general function of each member of the

B complex?

Vitamin C, or ascorbic acid. Ascorbic acid is a crystalline compound that contains six carbon atoms. Chemically, it is similar to the monosaccharides (fig. 18.15). Vitamin C is one of the least stable of the vitamins in that oxidation, heat, light, or bases destroy it. However, vitamin C is fairly stable in acids.

Ascorbic acid is necessary for the production of the connective tissue protein collagen, for conversion of fo-lacin to folinic acid, and in the metabolism of certain amino acids. It also promotes iron absorption and synthesis of certain hormones from cholesterol.

Although vitamin C is not stored in any great amount, tissues of the adrenal cortex, pituitary gland, and intestinal glands contain high concentrations. Excess vitamin C is excreted in the urine or oxidized.

Individual requirements for ascorbic acid may vary. Ten mg per day is sufficient to prevent deficiency symptoms, and 80 mg per day saturate the tissues within a few weeks. Many nutritionists recommend a daily adult intake of 60 mg, which is enough to replenish normal losses and to provide a satisfactory level for cellular requirements.

Ascorbic acid is fairly widespread in plant foods; particularly high concentrations are found in citrus fruits and tomatoes. Leafy green vegetables are also good sources.

Prolonged deficiency of ascorbic acid leads to scurvy, which occurs more frequently in infants and children. Scurvy produces abnormal bone development and swollen, painful joints. Because of a tendency for cells to pull apart in scurvy, the gums may swell and bleed easily, resistance to infection is lowered, and wounds heal slowly. If a woman takes large doses of ascorbic acid during pregnancy, the newborn may develop symptoms of scurvy when the daily dose of the vitamin drops after birth. Table 18.7 summarizes the water-soluble vitamins and their characteristics.

H What factors destroy vitamin C?

^9 What is the function of vitamin C?

^9 Which foods are good sources of vitamin C?

Q What are symptoms of vitamin C deficiency?


Carbohydrates, lipids, proteins, and vitamins are all organic compounds. Dietary minerals are inorganic elements that are essential in human metabolism. These elements are usually extracted from the soil by plants, and humans obtain them from plant foods or from animals that have eaten plants.

Characteristics of Minerals

Minerals are responsible for about 4% of body weight and are most concentrated in the bones and teeth. The minerals calcium and phosphorus are very abundant in these tissues.

Minerals are usually incorporated into organic molecules. For example, phosphorus is found in phospho-lipids, iron in hemoglobin, and iodine in thyroxine. However, some minerals are part of inorganic compounds, such as the calcium phosphate of bone. Other minerals are free ions, such as the sodium, chloride, and calcium ions in the blood.

Minerals are present in all body cells, where they comprise parts of the structural materials. They also assist enzyme molecules, contribute to the osmotic pressure of body fluids, and play vital roles in conduction of nerve impulses, contraction of muscle fibers, coagulation of blood, and maintenance of pH. The physiologically active form of minerals is the ionized form, such as Ca+2.

Homeostatic mechanisms regulate the concentrations of minerals in body fluids. This ensures that excretion of minerals matches intake.

99 How do minerals differ from other nutrients? Q What are the major functions of minerals? H Which are the most abundant minerals in the body?

Major Minerals

Calcium and phosphorus account for nearly 75% by weight of the mineral elements in the body; thus, they are major minerals (macrominerals). Other major minerals, each of which accounts for 0.05% or more of body weight, include potassium, sulfur, sodium, chlorine, and magnesium. Descriptions of the major minerals follow:

1. Calcium. Calcium (Ca) is widely distributed in cells and body fluids, even though 99% of the body's supply is in the inorganic salts of the bones and teeth. It is essential for nerve impulse conduction, muscle fiber contraction, and blood coagulation. Calcium also decreases the permeability of cell membranes and activates certain enzymes.

The amount of calcium absorbed varies with a number of factors. For example, the proportion of calcium absorbed increases as the body's need for calcium increases. Vitamin D and high protein intake promote calcium absorption; increased motility of the digestive tract or an excess intake of fats decreases absorption. Consequently, the amount of dietary calcium needed to supply cells

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