Vitamin C: For optimal therapeutic benefit

Vitamin C, well known for its immune support action, forms a major part of most Australian diets, leading to presumptions that clients are meeting nutritional requirements. While this essential vitamin has a plethora of other health benefits, more Australians may be deficient than we realise with the re- emergence of scurvy in modern day populations.

Vitamin C (also known ascorbic acid, AA) is a hydrophilic molecule that is vital for optimal functioning. Humans cannot synthesise vitamin C endogenously as they do not make the enzyme L-gulono-1,4 lactone oxidase which is essential for the biosynthesis of vitamin C.¹Instead, dietary vitamin C must be obtained to ensure daily adequate intake.

Measurement of vitamin C

Intake and storage of vitamin C may be measured by both plasma and leucocyte levels.
Leucocytes hold 50-100-fold higher vitamin C versus plasma.
Plasma is reflective of recent intake and does not reflect tissue and body stores reliably when compared to leucocyte ascorbic acid.³

The reference interval for vitamin C used by the Royal College of Pathologists of Australasia is generally between:

- Plasma: 30-80 μmol/L
- Leucocytes: 1.1-3.0 μmol/10⁹⁴

Vitamin C deficiency – a rarity?

It is generally assumed that vitamin C deficiency is rare, however, research suggests otherwise highlighting a rising incidence in Australia.

A 2023 retrospective study examining serum vitamin C status in 13,000 individuals tested in a public hospital in NSW, observed vitamin C insufficiency in 29.9% and deficiency in 24.5% of individuals.⁵Several studies looking at serum vitamin C levels found sub-optimal levels in the following cohorts:

50% of individuals with diabetic foot ulcers visiting Westmead Hospital, Sydney.⁶
50% of a cohort of patients in a mental health setting in South Australian had vitamin C levels lower than 26 μmol/L.⁷
30% of individuals with periodontal disease consulting a periodontal clinic in Sydney.⁸

The rising incidence of vitamin C insufficiency may be due to several factors including dietary changes (e.g. inadequate consumption of fruit and vegetables), increased chronic illness causing malabsorption, and/or increased requirements.⁹

Modern agricultural practices are considered to be a contributing factor to reduced vitamin C sufficiency, with research suggesting a 15-30% decrease in vitamin C content in a variety fruits and vegetables over the last 50 years due to nutrient depletion as a result of poor soil quality due to over farming.¹⁰

Absorption of vitamin C

Plasma and tissue concentrations of vitamin C are mediated by gastrointestinal absorption. Absorption takes place primarily in the distal ileum,¹⁸followed by renal reabsorption and excretion via the urine.¹⁹Subsequently it is unsurprising that individuals with irritable bowel disease commonly exhibit insufficient serum vitamin C levels.^20,21

Bioavailability

Vitamin C bioavailability declines rapidly as the dose increases.
Small frequent doses appear more efficacious than large doses. Low dose of 200-400 mg results in 100% absorption, however if 500 mg is exceeded, bioavailability reduces, decreasing to approximately 30% when 1000 mg

is consumed orally in one bout. This is due to maximal saturation of the intestinal transporter SVCTI.²²
Because vitamin C is water soluble, almost all the absorbed dose is excreted in urine within 24 hours.²³It is therefore important to be aware of a person’s daily vitamin C intake and ensure they are replenishing levels daily.

PERFECT PAIRINGS

Bioflavonoids

The benefits of vitamin C are enhanced when used in combination with bioflavonoids compared to the use of vitamin C alone.³⁸

Glutathione regeneration: prescribing vitamin C and glutathione together

As powerful antioxidants, both vitamin C and glutathione play an important role in cellular antioxidant defence systems. Both antioxidants appear to augment each other, assisting with the transformation from their reduced forms and the recycling

of one another.³⁹Supplementation with vitamin C has been shown to increase plasma glutathione levels, enhancing antioxidant activity. Deficiency of either vitamin C or glutathione appears to impede the action of the other.³⁹

SAFETY CONSIDERATIONS

Vitamin C is generally well tolerated when used at low doses.⁴⁰
Vitamin C 5–10 g/day orally may produce transient osmotic diarrhea.⁴¹
There is conflicting data regarding increased risk of oxalate containing kidney stones with use of vitamin C, however at low doses this is unlikely to occur.⁴²

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^References

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