Turnover, renal, ascorbate

Kinetic parameters were estimated in nonsmokers and smokers to help elucidate the quantitative ascorbate metabolism in humans. This approach allows calculation of turnover rates at different levels of steady state intakes of ascorbate. Metabolic and renal turnovers were calculated separately. At plasma levels above about 0.7 mg/100 mL the renal elimination increased sharply and the metabolic turnover showed a saturation at a plasma level corresponding to a total turnover of about 60 mg/d. At the tested levels of intake of ascorbic acid the calculated total pool size increased to a level reached at a steady state plasma concentration achieved at an intake of about 90 mg/d. At intakes of this magnitude the absorption is substantially less than 100%. A daily intake of 100 mg of ascorbate for larger populations should be attained. Similar experiments with smokers showed an increase in the metabolic turnover corresponding to a demand of 140 mg/d to reach a similar stage. 

The ascorbate is assumed to be filtered in the glomeruli and reabsorbed in the renal tubuli. The renal turnover suddenly increases at a certain plasma level (Figure 4), indicating a threshold value at which the tubular absorption is exceeded by the glomerular filtration. The renal... 

Figure 4. Renal turnover in relation to plasma concentration of ascorbate (6). (Reproduced, with permission, from Ref, 6. Copyright 1979, American Journal of Clinical Nutrition.)...

The total turnover is the sum of the renal turnover and the metabolic turnover. The metabolic turnover shows a saturation at about 40-50 mg/d (Figure 5), and this saturation occurs at a total turnover of about 60 mg/d. These values imply that up to this total turnover the metabolic rate of ascorbate increases the asymptotic value could reflect the maximum physiological need for ascorbate. 

The main metabolites of ascorbate in the human body are oxalate, dehydroascorbic acid, 2,3-diketogulonic acid, and ascorbic acid 2-sulfate. Of these, oxalate has attracted the most attention because of the potential hazard for renal complications by precipitation of oxalate stones. The above-mentioned metabolic turnover for ascorbate, which appears saturable, indicates that in normal humans the amount of oxalate that can be formed is limited. Therefore, the overall risk of inducing oxalate precipitation by increasing the intake of ascorbate probably is minute. 

The minimum vitamin C requirement to prevent the development of scurvy has been found to be 10 mg/day (Hodges et al., 1971). This supply is not sufficient for the provision of acceptable reserves of the vitamin. The intake of 10 mg ascorbic acid per day reflects a plasma concentration of 7.6-14.1 p,mol/liter (0.13-0.24 mg/100 ml), a concentration that is far from tissue saturation. The renal clearance of ascorbic acid rises sharply at a point of 82 xmol/liter (1.4 mg/100 ml). An adequate plasma level of 44 p.mol/liter (0.75 mg/100 ml) can be maintained by a daily supply of 60-75 mg ascorbic acid. The daily amount of ascorbic acid, which is catabolized by the human body, can be calculated to be 60 mg based on a total body pool of ascorbic acid of approximately 1500 mg and a maximum turnover rate of 4%. These data are the scientific basis for the formulation of the above recommendations to meet the physiological needs. Table IV shows the recommended daily vitamin C intake for different population groups from the different societies that issue such recommendations. 

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