Plasma ascorbic acid

The possible involvement of free radicals in the development of hypertension has been suspected for a long time. In 1988, Salonen et al. [73] demonstrated the marked elevation of blood pressure for persons with the lowest levels of plasma ascorbic acid and serum selenium concentrations. In subsequent studies these authors confirmed their first observations and showed that the supplementation with antioxidant combination of ascorbic acid, selenium, vitamin E, and carotene resulted in a significant decrease in diastonic blood pressure [74] and enhanced the resistance of atherogenic lipoproteins in human plasma to oxidative stress [75]. Kristal et al. [76] demonstrated that hypertention is accompanied by priming of PMNs although the enhancement of superoxide release was not correlated with the levels of blood pressure. Russo et al. [77] showed that essential hypertension patients are characterized by higher MDA levels and decreased SOD activities. 

One report has described low plasma ascorbic acid concentrations during indometacin treatment, and a case of hyperglycemia has been reported (489). 

Khaw K, Bingham S, Luben R, et al. Relation between plasma ascorbic acid and mortality in men and women in EPIC-Norfolk prospective study a prospective population study, Lancet 2001 357 657-663. 

Disposition in the Body. Readily absorbed after oral administration the proportion of a dose absorbed tends to decrease with increasing dose it is widely distributed in the body tissues. The concentration of ascorbic acid is higher in leucocytes and platelets than in erythrocytes and plasma. Ascorbic acid is metabolised to dehydroascorbic acid, 2,3-diketogulonic acid, oxalate, and carbon dioxide some conjugation with sulphate occurs to form ascorbate-3-sulphate. Ascorbic acid in excess of the body s requirements is rapidly eliminated in the urine. About 85% of an intravenous dose, given to subjects previously saturated with the vitamin, is excreted in the urine in 24 hours, with about 70% of the dose excreted unchanged and 15% as dehydroascorbic acid and diketogulonic acid. The amount normally present in the body is in excess of 1.5 g. 

A trial in 868 children showed that in those with high plasma ascorbic acid concentrations, the duration of upper respiratory infection was greater than in those with low concentrations (13). This finding, which contrasted with the optimistic expectations of other workers, requires confirmation. 

Blood and Animal Tissues. The most commonly used and practical procedure for evaluating vitamin C nutritional status is the measurement of serum (plasma) levels of ascorbic acid (87). Low plasma levels of ascorbic acid do not necessarily indicate scurvy, although scorbutic patients invariably have low or no plasma ascorbic acid, but continued low levels of plasma ascorbate of less than 0.10 mg/100 mL would eventually lead to signs and symptoms of scurvy. In general, serum ascorbic acid concentrations are usually more reflective of recent intakes rather than of total body stores (88). 

The plasma ascorbic acid concentration is low at the time of ovulation, whereas that of folate is unaffected by the menstrual cycle. Serum CK activity may be slightly reduced at the time of ovulation, but the activities of other enzymes appear to be unaffected by the menstrual cycle. 

Ascorbic acid (C) Enlargement and keratosis of hair follicles, impaired wound healing, anemia, lethargy, depression, bleeding, ecchymosis Plasma ascorbic acid Gl disturbances, kidney stones, excess iron absorption with excess intake smokers need 35 mg/day more than nonsmokers... 

Wang S, et al. Plasma ascorbic acid in patients undergoing chronic haemodialysis. Eur J Clin Pharmacol 1999 55 527-532. 

A similar increased degradation may be the action of the antimetabolite, co-methylpantothenic acid (P12). The plasma ascorbic acid level fell in guinea pigs fed this compound, although the intake remained constant. 

I. Plasma Ascorbic Acid Levels. The measurement of plasma levels of ascorbic acid in populations has been used to provide more reliable data on average intakes than that obtained from the incidences of scurvy or from nutritional histories. It must be emphasized, however, that scurvy develops only in those individuals with plasma levels below those which can be accurately measured. Extensive studies have provided empirical correlations of expected plasma and white blood cell levels with different ascorbic acid intakes (L24, M24, S23). Other studies have also shown that both the ascorbic acid and the dehydroascorbic acid content of foods were equally effective in raising the plasma level of ascorbic acid (D8). 

Tests measuring the plasma ascorbic acid levels a few hours following the test dose were more useful for rapid evaluation of tissue storage of ascorbic acid (W7, S26, RIO). The clinical usefulness of such a test was reported (D23) in a trial in which nine scorbutic patients had serum concentrations less than 0.25mg/100ml three hours after an oral dose of 15 mg ascorbic acid/kg body weight. Some individuals with normal intakes and low initial levels also had equally small rises. But rises to levels greater than 0.25 mg/100 ml were not consistent with scurvy. 

Wu, K., Helzlsouer, K. J., Alberg, A.J., Comstock, G.W., Norkus, E.P., and Hoffman, S.C., A prospective study of plasma ascorbic acid concentrations and breast cancer (United States), Cancer Causes Control, 11,279, 2000. 

Tribble, D.L., Giuliano, L.J., and Fortmann, S.P., Reduced plasma ascorbic acid concentrations in nonsmokers regularly exposed to environmental tobacco smoke, Hw. J. Clin. 

As has been pointed out, Dodds et al, (D6) in 1950 indicated that the plasma ascorbic acid concentration in healthy young women on a supplementary daily intake of 100 mg ascorbic acid was higher than that in males. Reports on changes during the menstrual period have been conflicting (R2). Using a small number of subjects on a dietary intake of 150 mg of ascorbic acid, Rivers and Devine (R2) reported that fasting plasma levels of ascorbic acid and total ascorbic acid were... 

Pharmacokinetics During administration of high doses of ascorbic acid, plasma ascorbic acid concentrations do not rise in proportion to the dose, because of saturable absorption and extensive renal elimination aided by saturable tubular reabsorption [3 ]. For example, when the daily dose of ascorbic acid is increased from 200 to 2500 mg (from 1.1 to 14 mmol) the mean... 

Prospective cohort studies were not able to relate vitamin C with lens opacities. In the Baltimore Longitudinal Study on Aging, plasma ascorbic acid was not inversely associated with risk, but vitamin E was (Vitale et al., 1993). In the Women s Health Study and the Physicians Health Study of Harvard University, Boston, vitamin C intake did not show a meaningful relationship to risk (Hankinson et al., 1992 Seddon et al., 1994). A modest reduction in risk was found with multivitamin use in men. Also the Italian-American Cataract Study Group (1991) did not find a significant reduction in risk for any nutrient studied, including vitamin C. 

Sinha, R., Block, G., and Taylor, P. R., 1992, Determinants of plasma ascorbic acid in a healthy male population. Cancer Epidemiol. Biomarkers Prev. 1 297-302. 

Ascorbic acid is an important essential nutrient for the health maintenance of the population. Nevertheless, its importance must not be overestimated. The status assessment of the vitamin in well-nourished populations showed only a small percentage of subjects with an insufficient plasma ascorbic acid concentration. It has been documented that risk factors for the development of an ascorbic acid deficiency exist (e.g., smoking, alcohol use, pregnancy, certain pathophysiological conditions). These deficiencies can, however, be overcome without any problems when the current recommendations on vitamin C intake are fulfilled by the intake of either... 

Loh, H. S., 1972, The relationship between dietary ascorbic acid intake and buffy coat and plasma ascorbic acid concentrations at different ages, Int. J. Vitam. Nutr. Res. 42 80-85. 

---