GNeuro: Assessing vitamin C status

Discuss some of the biochemical procedures for assessing vitamin C status. Which are better measurements of dietary intake? Which are better measurements of cellular stores?

Vitamin C has been postulated to have an important role in reduction in oxidative stress and prevention of cancer and heart disease, and in boosting the immune system (1). Vitamin C deficiency, as is well known, causes scurvy which is typically seen after 1 month of dietary deficiency largely due to impaired collagen biosynthesis (1).

The USDA Dietary Reference Intakes for Vitamin C indicate that the Recommended Daily Allowance for males 19+ years old is 90 mg and females of the same age range is 75 ug/d. Of note, for smokers, there is an additional 35 mg/day recommended. Food sources of vitamin C include fruits and vegetables, especially citrus, tomatoes, potatoes, red and green peppers, kiwifruit, broccoli, strawberries, Brussels sprouts, and cantaloupe. According to the 2001–2002 National Health and Nutrition Examination Survey (NHANES), mean intakes of vitamin C are 105.2 mg/day for adult males and 83.6 mg/day for adult females, meeting the currently established RDA for most nonsmoking adults (1).

Vitamin C exists in the body as (1) Ascorbic Acid, (2) Ascorbate, and (3) Dehydroascorbic acid (3).

Under neutral serum pH, Vitamin C exists predominantly as Ascorbate. As cells absorb Ascorbate, it is oxidized and, in the cell, exists as in an OXIDIZED state as Dehydroascorbic Acid. So you see a flow of electrons from REDUCED state in Ascorbic Acid to OXIDIZED state inside the cell. The OXIDIZED form is then hydrated to Diketogluconic Acid which is excreted in urine (3).

As such it follows that measuring Vitamin C level can come from any of the 3 pools of vitamin C in the body: (i) Urine, (ii) Plasma, (iii) and Tissues, including leukocytes and platelets.

In the Plasma, the most sensitive and selective method for measuring vitamin C uses high-pressure liquid chromatography (HPLC) coupled to an electrochemical (EC) detector. This can be problematic as vitamin C is unstable and requires extra steps be taken including continuous storage at -80^o C. Total plasma vitamin C is measured as ascorbic acid is easily oxidize to dehydroascorbic acid and indeed from the serum, dehydroascorbic acid may be completely produced after the sample is taken and transported to the lab. An UV HPLC technique has been described which requires minimal transport and costs relative to the typical expensive HPLC (3).

In the tissues, vitamin C can also be measured in the leukocytes or platelets and may be more accurate. The levels in urine and plasma vary quickly with intake, vitamin C from tissue may best measure body stores. Studies suggest three different normal reference ranges in tissue, from 1500 to 5,000 mg (20, 22, 32 mg per kg of body weight). The half-life of vitamin C in tissues varies depending on the literature data, from 16 to 20 days. Assuming a tissue level of 5000 mg, a lack of vitamin C in the diet for 16 days would reduce the tissue store to about 2500 mg; in 32 days it would be about 1250 mg; in 44 days it would be 625 mg and in 64 days it would be about 313 mg and clinical signs of scurvy should start to develop (2).

As many recognize from supplementing with vitamin C, a significant amount appears in urine soon after ingesting. The serum appears to have a saturation point for vitamin C and excess of this water-soluble vitamin excreted by kidney. The renal threshold of plasma vitamin C is ~1.4 mg/dL and further intake results in a rapid urine clearance. Vitamin C can be excreted in the urine unchanged as ascorbic acid, as dehydroascorbic acid, 2,3-diketogluconic acid and oxalic acid (2).

An at home method has been produced to assess urinary Vitamin C levels: Vita-Chek-C (http://store.riordanclinic.org/product/VPLCStrips.html). The VitaCheck-C color chart reflects levels of vitamin C of 0, 5, 10, 20, 40, 50, and 100 mg/dL.

In a study using the Vita-Check-C and Plasma levels, these were the results (2):

Note that many with Plasma levels below the renal threshold of 1.4 still showed significant amount in urine. Therefore this test is a screen and generally over 40 is considered “normal” without clinical suspicion of deficiency (2).