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

ToxicityThe 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, promotionof nonheme iron absorption, carnitine biosynthesis, the conversion of dopamine to norepinephrine, and the synthesis of many peptide hormones.Vitamin...
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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 noadverse effects have been associated with high intakes of vitamin B 6 from foodsources only. When toxicity occurs, it causes a severe sensory neuropathy, leavingpatients unable to walk. Some cases of photosensitivity and dermatitis have alsobeen reported. Folate, Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid arebiologically active. Actions of vitamin C include antioxidant activity, promotionof nonheme iron absorption, carnitine biosynthesis, the conversion of dopamine tonorepinephrine, 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-metabolizingenzyme systems, particularly the mixed-function oxidase systems. Absorption and Dietary Sources Almost complete absorption of vitamin C occurs if 1g. Enhanced degradation and fecal and urinary excretion of vitamin C occur athigher intake levels. Good dietary sources of vitamin C include citrus fruits, green vegetables(especially broccoli), tomatoes, and potatoes. Consumption of five servings offruits and vegetables a day provides vitamin C in excess of the RDA, 90 mg/d formales 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 formsof vitamin C are no more bioavailable than synthetic forms. Smoking,hemodialysis, pregnancy, and stress (e.g., infection, trauma) appear to increasevitamin C requirements. Deficiency Vitamin C deficiency causes scurvy. In the United States, this is seenprimarily among the poor and elderly, in alcoholics who consume Diets high in vitamin C have been claimed to lower the incidence of certaincancers, particularly esophageal and gastric cancers. If proved, this effect may bedue to the fact that vitamin C can prevent the conversion of nitrites and secondaryamines to carcinogenic nitrosamines. However, one intervention study from Chinadid 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 fearedthat chronic, high-dose vitamin C supplementation could result in an increasedprevalence of kidney stones. However, this has not been borne out in several trials, except in patientswith preexisting renal disease. Thus, it is reasonable to advise patients with a pasthistory of kidney stones to not take large doses of vitamin C. There is also an unproven but possible risk that chronic high doses ofvitamin C could promote iron overload in patients taking supplemental iron. Highdoses of vitamin C can induce hemolysis in patients with glucose-6-phosphatedehydrogenase deficiency, and doses >1 g/d can cause false-negative guaiacreactions as well as interfere with tests for urinary glucose.