Plasma ascorbic acid level

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. 

In vivo studies on the polyol pathway suggest a link to ascorbic acid levels. For instance, certain aldose reductase inhibitors and dietary my -inositol appear to correct plasma ascorbic acid levels in rat models of diabetes mellitus. However, the effect was ascribed to changes in kidney function (urinary excretion), and such studies are complicated by the ability of rats to synthesize ascorbic acid (Yue et aL, 1989). 

It has been found that considerably more than 100 mg. daily is required to saturate body tissues of some individuals, whereas others are apparently saturated at 50 mg. In times of stress such as fevers, the plasma ascorbic acid level drops to low values, and as much as 1000 mg. daily may fail to raise it for a few days. The argument about maintaining a high intake as insurance against such stress periods is prominently featured in relation to ascorbic acid, but in the face of negative evidence (Kark, 1953). 

RS Harapanhalli, RW Howell, DV Rao. Testicular and plasma ascorbic acid levels in mice following dietary intake a high-performance liquid chromatographic analysis. J Chromatogr 614 233-243, 1993. 

The adrenal glands and pituitary glands have the highest tissue concentration of ascorbic acid. The brain, Hver, and spleen, however, represent the largest contribution to the body pool. Plasma and leukocyte ascorbic acid levels decrease with increasing age (152). Elderly people require higher ascorbic acid intakes than children to reach the same plasma and tissue concentration (153). 

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. 

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). 

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... 

Significant correlation was found between plasma and cerebrospinal fluid ascorbic acid levels in demented patients (BarabAs et al. 1995). As a consequence, intravenous infusion of ascorbic acid (2 g), a slow, but marked increase of the concentration in the cerebrospinal fluid was measured by high performance liquid chromatography with electrochemical detection. Ascorbic add level might be an important factor representing the protection of the central nervous system against free radicals. 

It is well established that stress affects the ascorbic acid concentration in various components of the body. What mechanism causes stress is unimportant because similar effects are obtained with prolonged exposure to cold, heat, or X-irradiation, excessive oxygen tension, or even a simple injection of saline. Stress increases the excretion of ascorbic acid in the urine and the plasma levels of ascorbic acid in the blood (in the early stages of stress), and reduces the ascorbic acid level of the adrenal. Increased urinary excretion of dehydroascorbic and diketonic acids usually is associated with ascorbic acid in the urine. 

It appears that a number of antioxidant defenses in diabetic individuals are compromised. These include decreases in cellular vitamin E (Karpen et al., 1984) and decreased plasma levels of glutathione (Illing et al., 1951) and uric acid (Herman et al., 1976). More significantly, for the purposes of this review at least, plasma ascorbic acid (vitamin C) also appears to decrease in diabetes mellitus (Nienhuis, 19SI Som et al., 1981 Jennings r /., 1987 ... 

Although few in number, some studies on ascorbic acid supplementation in diabetic individuals and in diabetic animal models have been reported. In man, ascorbic acid supplementation has been shown to increase cutaneous capillary strength, but to have no effect on retinopathy, until supplementation ceased, when some increases in retinal hemorrhage were observed (Cox et al., 1975). Ascorbic acid supplementation appears to increase plasma ascorbic acid concentration as well as dehydrdoascorbic acid concentration (Sinclair et al., 1991). Correction of ascorbic acid levels is only transient once supplementation is halted ascorbic acid levels return to the presupplementation level (Som et al., 1981 Sinclair et al., 1991). Thus, ascorbic acid supplementation has shown no beneficial effect and on one occasion has proved harmful on withdrawal. It is possible that a sudden reduction in ascorbic acid intake may lead to a dramatic fall in plasma and tissue levels with subsequent adverse effects (Halliwell, 1994). 

In a study of the ascorbic acid level per 100 ml of blood plasma of 48 women and 41 men, the senior author of this book, Audrey H. Ensminger, found 14 subjects, or 15.7%, at the scurvy level. 

RECOMMENDED DAILY ALLOWANCE OF VITAMIN C. Many studies have been conducted to determine the human vitamin C requirements. Consideration has been given to the effect of age, environment, physical exertion, infections, and fevers. Also, various measurements for determining the adequacy of vitamin C have been devised, including (1) the daily intake of vitamin C necessary to prevent the symptoms of scurvy (2) the amount of vitamin C required to saturate whole blood, blood plasma, white blood cells, or body tissue and (3) the intake of vitamin C necessary to maintain blood and urinary ascorbic acid levels within normal range. The results of these studies vary widely. 

---