Requirements Estimated from the Plasma and Leukocyte Concentrations of Ascorbate

The plasma concentration of ascorbate shows a sigmoidal relationship with intake. Below about 30 mg per day, the plasma concentration is extremely low and does not reflect increasing intake to any significant extent. As the intake rises above 30 mg per day, so the plasma concentration begins to increase sharply, reaching a plateau of 70 to 85 ^mol per L, at intakes between 70 to 100 mg per day, when the renal threshold is reached and the vitamin is excreted quantitatively with increasing intake.

The midpoint of the steep region of the curve, where the plasma concentration increases linearly with increasing intake, represents a state in which tissue reserves are adequate and plasma ascorbate is available for transfer between tissues. This corresponds to an intake of 40 mg per day, and is the basis of the U.K., European Union, and Food and Agriculture Organization/World Health Organization figures shown in Table 13.3. At this level of intake, the total body pool is about 900 mg (5.1 mmol).

Levine and coworkers (1995, 1999) have argued that setting requirements and reference intakes on the basis of the steep part of a sigmoidal curve is undesirable. They suggested that a more appropriate point would be where the plasma concentration reaches a plateau, at an intake of around 100 to 200 mg per day.

The U.S./Canadian reference intakes of 75 mg for women and 90 mg for men are based on studies of leukocyte saturation (Levine et al., 1996; Institute of Medicine, 2000).

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