Ascorbic Acid Status
Most of the approaches to the study of an individual's nutritional status with respect to ascorbic acid involve an attempt to study the degree of tissue saturation. There is little diffculty in the estimation of blood or urine levels, but such data alone often do not truly reflect the condition in the tissues. It has been pointed out that the level in the white cells may be more significant than that of the plasma for diagnostic purposes. The so-called saturation tests give additional information. The blood, and urine levels may well be affected by the intake during the day previous to the time the samples are tested, but the degree of tissue saturation would not be markedly altered unless the intake were very high. The tissue saturation tests involve a study of urinary ascorbic acid excretion in response to a test dose.
In the individual with low reserves the vitamin is held in the tissues, and the total excretion is below normal. In one of the tests 5 mg of ascorbic acid per pound is given orally, and if 50 per cent is excreted during the ensuing 24 hours, the patient is said to have been saturated. Excretion of less than this amount indicates unsaturation. In a severely deficient patient the excretion may be from almost none up to 20 per cent of the test dose. Human blood plasma ascorbic acid levels of 0.7 to 1.2 mg per 100 ml are considered to be within the normal range. Many investigators now consider that levels of 0.4 to 0.7 mg per cent indicate a mild deficiency, and those below 0.4 mg a severe deficiency. On high dietary intakes the plasma level may rise to 2.0 mg per cent. In controlled studies of men in the Canadian Air Force on known vitamin C intakes good correlation was found between the white cell level and the amount consumed daily. Thus on intakes of 8, 23, and 78 mg ascorbic acid daily the white cell levels were found to be 11.9, 12.9, and 24.2 mg per 100 g white cells.
The plasma level of ascorbic acid except on constant intake probably reflects the recent consumption of the vitamin more than it does the degree of bpdy saturation and thus may tell one less about vitamin C status of an individual. In older people the blood level may decline. Thus, Berlina found the low average level of 0.13 mg per 100ml in 20 healthy aged men and women on intakes of 30 to 35 mg of ascorbic acid per day. Urinary excretion ranged from two thirds to nearly all of the vitamin C intake.
Distribution of Ascorbic Acid
Vitamin C is distributed rather widely in nature. Important dietary sources for man include many vegetables and fruits. Canning, cooking, and other processing result in various degrees of vitamin C loss. Fresh vegetables such as broccoli, kale, parsley, and turnip greens have a high content of the vitamin, but these foods are generally not eaten in the raw state. The citrus fruits are also excellent sources and are consumed largely without processing, although for economic reasons they are not an important vitamin C source for the overall population. Certain vegetables constitute important sources by virtue of the amount eaten rather than due to a high level of the vitamin. In this class are potatoes, beans, and peas. Most animal products contain only small amounts of the vitamin. Because the vitamin of milk is largely destroyed in pasteurization or in the evaporation process, another source of vitamin C is regularly supplied to infants. By definition 1 IU or 1 USP unit is equivalent to 0.05 mg of Lascorbic acid. This makes 1 mg equal to 20 USP units.
