Dr. Kumar’s Take:
Plasma and white blood cell (leucocyte) vitamin C tell you two different things, and this 1971 British Medical Journal paper is where that distinction was worked out. The authors found that leucocytes can reach a saturation level of ascorbic acid, and that in supplemented people the leucocyte values clustered in a narrow band while plasma values ranged widely.
Their conclusion cuts against a common assumption. Measuring leucocyte vitamin C alone is not a reliable guide to tissue vitamin C status in normal individuals. What leucocyte levels indicate is how much ascorbic acid is available for storage; plasma levels indicate its metabolic turnover rate. You need both, related to each other, to say anything useful.
Actionable tip:
Do not read a single leucocyte or plasma vitamin C value as a verdict on your tissue stores. If you are older or at risk of deficiency, focus on consistent adequate intake rather than chasing a saturation number.
Key Takeaways:
✔ Leucocytes can achieve a saturation level of ascorbic acid.
✔ Leucocyte and plasma ascorbic acid were significantly positively correlated in every group studied.
✔ The relationship between plasma and leucocyte values in the supplemented group differed significantly from the unsupplemented groups, so the same leucocyte reading means different things depending on supplementation.
✔ Leucocyte concentrations measure availability for storage; plasma levels indicate metabolic turnover rate. Neither alone defines tissue status.
Study Overview: Why Measure Vitamin C in Both Blood and Cells?
This 1971 British Medical Journal study set out to compare vitamin C levels in plasma (the fluid part of blood) and in white blood cells (leucocytes) across age groups and supplementation status. The question: how do the two measures relate to each other, and can either one tell you about tissue vitamin C status?
Study Design and Methods
- Who was studied?
- Healthy university students, adult factory employees, and old people not receiving supplementary vitamin C.
- A fourth group: healthy old people receiving 500 mg of vitamin C daily.
- How was vitamin C measured?
- Both plasma ascorbic acid and leucocyte ascorbic acid were measured in each participant.
- Regression lines relating leucocyte to plasma values were compared between groups and between the sexes.
Results: How Plasma and Leucocyte Vitamin C Relate
- Correlation: Significant positive correlations between leucocyte and plasma ascorbic acid were found in all the groups.
- Group differences: The regression lines differed significantly from one another within the sexes. In the unsupplemented groups, the pooled lines for each sex did not differ significantly from each other. The relationship in the supplemented group differed significantly from the pooled unsupplemented groups.
- Range of variation: In the supplemented group there was a limited range of variation in leucocyte values compared with the wide range in plasma values. The unsupplemented groups showed a wider spread in leucocyte values.
- Key finding: That narrowing is the authors’ evidence that leucocytes can achieve a saturation level of ascorbic acid. Once supplementation fills them, they stop tracking intake, while plasma keeps rising and falling.
Why This Matters: What Each Measurement Can and Cannot Tell You
- Leucocyte levels alone are not a tissue-status test. The authors state directly that measurement of leucocyte ascorbic acid concentrations alone does not provide a reliable guide for estimating tissue ascorbic acid status in normal individuals.
- Each measure has its own meaning. Leucocyte concentrations measure the availability of ascorbic acid for storage. Plasma levels indicate its metabolic turnover rate.
- The relationship is the information. When the two values are related to each other, the regression lines carry information about both the storage and the metabolism of ascorbic acid, which neither number gives you on its own.
Related Studies and Research
Vitamin C and the Common Cold: Leucocyte Ascorbic Acid Study Investigates how white cell ascorbate reflects immune function and infection risk.
Vitamin C Physiology: The Goldilocks Review Summarizes the “just right” range of vitamin C in the body.
Vitamin C Dosing, Plasma Saturation, and Urinary Loss Explains how intake impacts circulating and stored vitamin C.
Vitamin C Deficiency: Clinical Signs, Risk Factors, and Treatment Provides context for interpreting low vitamin C measurements in patients.
Frequently Asked Questions
Does this study show I need 500 mg of vitamin C a day?
No. It compared people taking 500 mg daily with unsupplemented groups to work out how the two blood measures relate. It was not designed to establish an optimal intake. For dose-response data, see the plasma saturation and urinary loss work.
Can white blood cells become saturated with vitamin C?
Yes. The narrow range of leucocyte values in the supplemented group, against a wide range of plasma values, is what led the authors to conclude that leucocytes can achieve a saturation level.
Is there any harm in taking higher vitamin C doses?
Vitamin C is generally very safe, but extremely high doses can cause digestive upset in sensitive individuals. Most people tolerate 500 to 1,000 mg/day well.
Should I measure my vitamin C levels?
Routine measurement is not needed for most people. This paper is a good reason to be skeptical of a single number: the authors found that a leucocyte value on its own does not reliably tell you your tissue status.
Conclusion
Plasma and leucocyte vitamin C are correlated, but they measure different things, and neither one alone defines your tissue status. Leucocytes can saturate, which is why their values cluster tightly in supplemented people while plasma values continue to swing with intake and turnover. For most people the practical takeaway is unchanged: a diet rich in fruits and vegetables, with supplementation if you are at risk of deficiency, rather than a target number to test toward.
