BackgroundVitamin C is an essential water-soluble nutrient which cannot be synthesised or stored by humans. It is a potent antioxidant with anti-inflammatory and immune-supportive roles. Previous research has indicated that vitamin C levels are depleted in critically ill patients. In this study we have assessed plasma vitamin C concentrations in critically ill patients relative to infection status (septic shock or non-septic) and level of inflammation (C-reactive protein concentrations). Vitamin C status was also assessed relative to daily enteral and parenteral intakes to determine if standard intensive care unit (ICU) nutritional support is adequate to meet the vitamin C needs of critically ill patients.MethodsForty-four critically ill patients (24 with septic shock, 17 non-septic, 3 uncategorised) were recruited from the Christchurch Hospital Intensive Care Unit. We measured concentrations of plasma vitamin C and a pro-inflammatory biomarker (C-reactive protein) daily over 4 days and calculated patients’ daily vitamin C intake from the enteral or total parenteral nutrition they received. We compared plasma vitamin C and C-reactive protein concentrations between septic shock and non-septic patients over 4 days using a mixed effects statistical model, and we compared the vitamin C status of the critically ill patients with known vitamin C bioavailability data using a four-parameter log-logistic response model.ResultsOverall, the critically ill patients exhibited hypovitaminosis C (i.e., < 23 μmol/L), with a mean plasma vitamin C concentration of 17.8 ± 8.7 μmol/L; of these, one-third had vitamin C deficiency (i.e., < 11 μmol/L). Patients with hypovitaminosis C had elevated inflammation (C-reactive protein levels; P < 0.05). The patients with septic shock had lower vitamin C concentrations and higher C-reactive protein concentrations than the non-septic patients (P < 0.05). Nearly 40% of the septic shock patients were deficient in vitamin C, compared with 25% of the non-septic patients. These low vitamin C levels were apparent despite receiving recommended intakes via enteral and/or parenteral nutritional therapy (mean 125 mg/d).ConclusionsCritically ill patients have low vitamin C concentrations despite receiving standard ICU nutrition. Septic shock patients have significantly depleted vitamin C levels compared with non-septic patients, likely resulting from increased metabolism due to the enhanced inflammatory response observed in septic shock.

Vitamin C (ascorbate) is the major water-soluble antioxidant in plasma and its oxidation to dehydroascorbic acid (DHA) has been proposed as a marker of oxidative stress in vivo. However, controversy exists in the literature around the amount of DHA detected in blood samples collected from various patient cohorts. In this study, we report on DHA concentrations in a selection of different clinical cohorts (diabetes, pneumonia, cancer, and critically ill). All clinical samples were collected into EDTA anticoagulant tubes and processed at 4 °C prior to storage at −80 °C for subsequent analysis by HPLC with electrochemical detection. We also investigated the effects of different handling and processing conditions on short-term and long-term ascorbate and DHA stability in vitro and in whole blood and plasma samples. These conditions included metal chelation, anticoagulants (EDTA and heparin), and processing temperatures (ice, 4 °C and room temperature). Analysis of our clinical cohorts indicated very low to negligible DHA concentrations. Samples exhibiting haemolysis contained significantly higher concentrations of DHA. Metal chelation inhibited oxidation of vitamin C in vitro, confirming the involvement of contaminating metal ions. Although EDTA is an effective metal chelator, complexes with transition metal ions are still redox active, thus its use as an anticoagulant can facilitate metal ion-dependent oxidation of vitamin C in whole blood and plasma. Handling and processing blood samples on ice (or at 4 °C) delayed oxidation of vitamin C by a number of hours. A review of the literature regarding DHA concentrations in clinical cohorts highlighted the fact that studies using colourimetric or fluorometric assays reported significantly higher concentrations of DHA compared to those using HPLC with electrochemical detection. In conclusion, careful handling and processing of samples, combined with appropriate analysis, is crucial for accurate determination of ascorbate and DHA in clinical samples.

Peroxiredoxin 2 (Prx2) is an abundant thiol protein in erythrocytes. It is oxidized readily on exposure to hydrogen peroxide (H2O2) and provides antioxidant protection by cycling between its reduced and disulfide-bonded forms. To test whether Prx2 oxidation could occur in pathological situations where neutrophils are activated, we exposed human erythrocytes to stimulated neutrophils and measured Prx2 oxidation by immunoblotting of nonreducing gels. With phorbol myristate acetate, lipopolysaccharide or Staphylococcus aureus Prx2 dimer increased from <5% to up to 100% depending on neutrophil number and incubation time. Studies with inhibitors and myeloperoxidase-deficient neutrophils showed that H2O2 generated by the neutrophil NADPH oxidase was responsible. Prx2 oxidation was detected at erythrocyte:neutrophil ratios found in blood and reversed over time as the oxidative burst subsided. Acidotic conditions also increased erythrocyte Prx2 oxidation. In a mouse model of endotoxemia induced by lipopolysaccharide, oxidized Prx2 increased transiently from <1 to 15%, then reverted to baseline by 24 h. No increase was seen in mice lacking NADPH oxidase activity. These results indicate that erythrocyte Prx2 scavenges H2O2 produced during inflammation. Oxidized erythrocyte Prx2 could be a sensitive real-time marker of systemic neutrophil activation and an early indicator of inflammation and oxidative stress.

Prx2 remains mostly reduced in a high-pH storage solution with buffering capacity. Addition of DHLA to stored RBCs might be advantageous. Prx2 redox status could be used as a biomarker for the quality of stored RBCs.

The use of a RWD containing surface active agents provided greater subjective satisfaction, reduced lysozyme and total protein deposition, and reduced denatured lysozyme than a RWD containing saline alone.