17: Vitamin C (Chapater 19)
- Page ID
- 117100
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Vitamin C (ascorbic acid) is involved in a variety of biological processes, acting primarily as an electron donating enzyme cofactor and reducing agent (antioxidant). Clinical deficiency of vitamin C results in scurvy, a relatively rare disease in high-income countries. Nevertheless, moderate vitamin C deficiency (hypovitaminosis C) has been described in selected vulnerable groups such as the institutionalized elderly, with gingival inflammation and fatigue appearing to be the most sensitive clinical markers for moderate vitamin C deficiency. Excess intakes of vitamin C may produce osmotic diarrhea in some people, and although oxaluria has been reported, this measurement is prone to errors in the preparative or investigative procedure. There are currently no reliable functional indices of vitamin C nutriture. Static biochemical indices such as plasma or leukocyte ascorbic acid concentrations are most frequently used. The former reflect recent dietary intake within a limited range (30–80mg/d). Plasma ascorbic acid concentrations increase linearly with dietary intake but only up to a plateau of about 70µmol/L, and thus are unsuitable for detecting excessive intakes of vitamin C. A number of non-nutritional factors, including sex, body weight, smoking, and severe infection, affect plasma ascorbic acid concentrations. Plasma concentrations are a more reliable marker of vitamin C status than dietary intake assessments. Urinary excretion of vitamin C occurs when the renal reabsorption threshold has been reached and plasma “saturation” occurs (between about 60 and 70µmol/L). Urinary excretion is influenced by recent dietary intake. Moreover, the sensitivity and specificity of this test is low, and 24h urine samples are generally required. Leukocyte ascorbic acid concentrations are considered a relatively reliable index of tissue ascorbic acid stores in healthy people; they are less responsive than plasma to short-term fluctuations in vitamin C intakes, but their assay is difficult and requires a larger blood sample. Furthermore, leukocyte ascorbic acid concentrations may not be a reliable marker of vitamin C status in some conditions, such as severe infection. The body pool size of vitamin C has been estimated using isotope dilution techniques. More research is needed to identify useful functional tests of vitamin C status.
- 17.1: Vitamin C (19.1)
- This page discusses the historical role of citrus fruit in preventing scurvy and the isolation of vitamin C in 1932 as the first chemically synthesized vitamin. It explains that due to genetic mutations, humans cannot synthesize vitamin C from glucose. Both L-ascorbic acid and its oxidized form, L-dehydroascorbic acid, share similar activity, with ascorbic acid primarily occurring as the mono-anion, ascorbate, in physiological conditions.
- 17.2: Functions of Vitamin C (19.2)
- This page discusses the role of ascorbic acid as a vital cofactor in hydroxylation reactions, including collagen synthesis and neurotransmitter production. A deficiency can lead to scurvy, causing various health issues. Ascorbic acid also functions as an antioxidant and improves iron absorption. While it may offer protective benefits against cardiovascular disease and cancer, more research is necessary to confirm these associations.
- 17.3: Absorption and metabolism of vitamin C (19.3)
- This page discusses vitamin C absorption in the small intestine via the SVCT-1 transporter, noting a 70-90% absorption rate at moderate doses (30-180 mg) that drops to ≤ 50% at over 1g. High doses can cause osmotic diarrhea from unabsorbed vitamin C. The body maintains vitamin C levels through absorption and renal functions, with the highest concentrations found in neuronal and glandular tissues.
- 17.4: Deficiency of vitamin C in humans (19.4)
- This page discusses scurvy's manifestations, noting their rarity in high-income countries due to adequate vitamin C intake, though it still affects certain groups, such as the elderly and those with unhealthy lifestyles. Infantile scurvy is uncommon due to sufficient vitamin C in breast milk and formulas.
- 17.5: Food sources and dietary intakes of vitamin C (19.5)
- This page discusses the synthesis and variability of vitamin C in plants, influenced by factors like growing conditions and location. High-income countries primarily source vitamin C from fresh fruits and vegetables, with citrus and kiwifruit being significant, while in low-income countries, intake varies seasonally. Staple foods and animal products are low in vitamin C. Additionally, the bioavailability of vitamin C from food is comparable to that from supplements.
- 17.6: Nutrient reference values for vitamin C (19.6)
- This page discusses varying Average Requirements (ARs) and Recommended Intakes (RIs) for vitamin C established by expert groups like IOM and EFSA, ranging from 40mg/d to 110mg/d. U.S. recommendations specify 75mg/d for women and 90mg/d for men, with higher needs for pregnant, lactating women, and smokers. Although upper intake limits suggest low toxicity risk, some countries set ULs at 1–2g/d, cautioning against excessive use due to potential gastrointestinal issues and health risks.
- 17.7: Indices of vitamin C status (19.7)
- This page discusses the lack of reliable functional tests for vitamin C status, emphasizing the reliance on static biochemical tests such as measuring plasma or leukocyte ascorbic acid levels for assessment. Detailed insights into these testing methods are also included.
- 17.8: Plasma ascorbic acid (19.8)
- This page discusses the transport and measurement of ascorbic acid (vitamin C) in plasma, emphasizing its role as a key indicator of vitamin C status influenced by various factors. It defines deficiency thresholds, noting that low-middle-income populations often exhibit inadequate levels. The European Food Safety Authority (EFSA) recommends plasma levels ≥ 50µmol/L for adequacy.
- 17.9: Ascorbic acid in leukocytes and specific cell subsets (19.9)
- This page discusses leukocytes, which include lymphocytes, monocytes, and granulocytes, highlighting their varying ascorbic acid levels and responses to vitamin C. Leukocyte ascorbic acid is a stable indicator of tissue vitamin C status, influenced by factors like infection and smoking. Research shows neutrophils increase ascorbic acid with supplementation but plateau at higher doses.
- 17.10: Ascorbic acid in erythrocytes and whole blood (19.10)
- This page discusses the limitations of using erythrocyte ascorbic acid concentrations to assess vitamin C status, noting that they are less sensitive to dietary intake compared to plasma concentrations. While useful for non-fasting individuals, variability and measurement challenges are present. Whole blood levels are also not commonly used for deficiency evaluation, with concentrations below 17 µmol/L indicating deficiency and above 28 µmol/L being acceptable.
- 17.11: Urinary excretion of ascorbic acid and metabolites (19.11)
- This page discusses the excretion of ascorbic acid primarily through urine, highlighting that high intakes can increase oxalate levels. Urinary ascorbic acid reflects recent dietary intake but is not a reliable indicator of vitamin C status. A renal threshold of about 60µmol/L requires intakes of at least 100mg/d for saturation. Careful measurement is necessary due to instability, with HPLC recommended as the preferred analysis method.
- 17.12: Ascorbic acid in other cells and fluids (19.12)
- This page discusses the varying concentrations of ascorbic acid (vitamin C) in body fluids, revealing low levels in saliva and alveolar lining fluid that do not reflect plasma or dietary intake. Notably, cerebrospinal fluid contains higher levels, indicating its importance in the central nervous system. Seminal fluid may have elevated levels, with lower concentrations in infertile men associated with oxidative stress.
- 17.13: Body pool size (19.13)
- This page discusses isotope dilution techniques utilizing 14C or 13C-labeled ascorbic acid to measure vitamin C body pool size through blood or urine. In healthy males, the average pool size is about 1500mg, with a range from <300mg to 3000mg influenced by daily intake. Pool sizes below 600mg are linked to psychological issues, and concentrations under 300mg can result in scurvy symptoms.
- 17.14: Functional tests of vitamin C status (19.14)
- This page discusses the lack of reliable functional tests for assessing vitamin C status, pointing out that capillary fragility is not effective, while erythrocyte fragility is nonspecific. Proposed markers include collagen cross-linking and vitamin C-dependent epigenetic marks in leukocyte DNA. Preliminary studies indicate potential correlations between vitamin C status and epigenetic modifications, highlighting the need for further research.