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Chapter: Medical Physiology: Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals

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Ascorbic Acid (Vitamin C)

Ascorbic Acid Deficiency Weakens Collagen Fibers Throughout the Body.

Ascorbic Acid (Vitamin C)

Ascorbic Acid Deficiency Weakens Collagen Fibers Throughout the Body. Ascorbic acid is essential for activating theenzyme prolyl hydroxylase, which promotes the hydrox-ylation step in the formation of hydroxyproline, an inte-gral constituent of collagen. Without ascorbic acid, the collagen fibers that are formed in virtually all tissues of the body are defective and weak. Therefore, this vitamin is essential for the growth and strength of the fibers in subcutaneous tissue, cartilage, bone, and teeth.

Ascorbic Acid Deficiency Causes Scurvy. Deficiency of ascor-bic acid for 20 to 30 weeks, which occurred frequently during long ship voyages in the past, causes scurvy. One of the most important effects of scurvy is failure ofwounds to heal. This is caused by failure of the cells todeposit collagen fibrils and intercellular cement sub-stances. As a result, healing of a wound may require several months instead of the several days ordinarily necessary.

Lack of ascorbic acid also causes cessation of bonegrowth. The cells of the growing epiphyses continue toproliferate, but no new collagen is laid down between the cells, and the bones fracture easily at the point of growth because of failure to ossify. Also, when an already ossified bone fractures in a person with ascor-bic acid deficiency, the osteoblasts cannot form new bone matrix. Consequently, the fractured bone does not heal.

The blood vessel walls become extremely fragile in scurvy because of (1) failure of the endothelial cells to be cemented together properly and (2) failure to form the collagen fibrils normally present in vessel walls. The capillaries are especially likely to rupture, and as a result, many small petechial hemorrhages occur throughout the body. The hemorrhages beneath the skin cause purpuric blotches, sometimes over the entire body. To test for ascorbic acid deficiency, one can produce such petechial hemorrhages by inflating a blood pressure cuff over the upper arm; this occludes the venous return of blood, the capillary pressure rises, and red blotches occur on the forearm if the ascorbic acid deficiency is sufficiently severe.

In extreme scurvy, the muscle cells sometimes frag-ment; lesions of the gums occur, with loosening of the teeth; infections of the mouth develop; and vomiting of blood, bloody stools, and cerebral hemorrhage can all occur. Finally, high fever often develops before death.


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