Chapter 071. Vitamin and Trace Mineral Deficiency and Excess (Part 6)

Chia sẻ: Thuoc Thuoc | Ngày: | Loại File: PDF | Số trang:5

Thêm vào BST

Báo xấu

78
lượt xem 7
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

Toxicity The safe upper limit for vitamin B6 has been set at 100 mg/d, although no adverse effects have been associated with high intakes of vitamin B 6 from food sources only. When toxicity occurs, it causes a severe sensory neuropathy, leaving patients unable to walk. Some cases of photosensitivity and dermatitis have also been reported. Folate, Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid are biologically active. Actions of vitamin C include antioxidant activity, promotion of nonheme iron absorption, carnitine biosynthesis, the conversion of dopamine to norepinephrine, and the synthesis of many peptide hormones. Vitamin...

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Chapter 071. Vitamin and Trace Mineral Deficiency and Excess (Part 6)

Chapter 071. Vitamin and Trace Mineral Deficiency and Excess (Part 6) Toxicity The safe upper limit for vitamin B6 has been set at 100 mg/d, although no adverse effects have been associated with high intakes of vitamin B 6 from food sources only. When toxicity occurs, it causes a severe sensory neuropathy, leaving patients unable to walk. Some cases of photosensitivity and dermatitis have also been reported. Folate, Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid are biologically active. Actions of vitamin C include antioxidant activity, promotion
of nonheme iron absorption, carnitine biosynthesis, the conversion of dopamine to norepinephrine, and the synthesis of many peptide hormones. Vitamin C is also important for connective tissue metabolism and cross- linking (proline hydroxylation), and it is a component of many drug-metabolizing enzyme systems, particularly the mixed-function oxidase systems. Absorption and Dietary Sources Almost complete absorption of vitamin C occurs if 1 g. Enhanced degradation and fecal and urinary excretion of vitamin C occur at higher intake levels. Good dietary sources of vitamin C include citrus fruits, green vegetables (especially broccoli), tomatoes, and potatoes. Consumption of five servings of fruits and vegetables a day provides vitamin C in excess of the RDA, 90 mg/d for males and 75 mg/d for females. In addition, approximately 40% of the U.S. population consumes vitamin C as a dietary supplement in which "natural forms" of vitamin C are no more bioavailable than synthetic forms. Smoking, hemodialysis, pregnancy, and stress (e.g., infection, trauma) appear to increase vitamin C requirements.
Deficiency Vitamin C deficiency causes scurvy. In the United States, this is seen primarily among the poor and elderly, in alcoholics who consume
Diets high in vitamin C have been claimed to lower the incidence of certain cancers, particularly esophageal and gastric cancers. If proved, this effect may be due to the fact that vitamin C can prevent the conversion of nitrites and secondary amines to carcinogenic nitrosamines. However, one intervention study from China did not show vitamin C to be protective. Toxicity Taking >2 g of vitamin C in a single dose may result in abdominal pain, diarrhea, and nausea. Since vitamin C may be metabolized to oxalate, it is feared that chronic, high-dose vitamin C supplementation could result in an increased prevalence of kidney stones. However, this has not been borne out in several trials, except in patients with preexisting renal disease. Thus, it is reasonable to advise patients with a past history of kidney stones to not take large doses of vitamin C. There is also an unproven but possible risk that chronic high doses of vitamin C could promote iron overload in patients taking supplemental iron. High doses of vitamin C can induce hemolysis in patients with glucose-6-phosphate
dehydrogenase deficiency, and doses >1 g/d can cause false-negative guaiac reactions as well as interfere with tests for urinary glucose.