Background
Vitamin C is an essential water-soluble nutrient which cannot be synthesised by humans, owing to loss of the terminal enzyme in the biosynthetic pathway [
1]. The vitamin has numerous physiological roles in the body through acting as an electron donor. It is a potent antioxidant, protecting important biomolecules (proteins, lipids and DNA) from oxidation, thus preserving essential tissue structure and function [
2]. Vitamin C also acts as an enzyme cofactor for a number of biosynthetic enzymes involved in hormone synthesis and generation of metabolic energy [
3], as well as an ever-expanding family of gene-regulatory enzymes [
4,
5]. Recent research has indicated important roles for vitamin C in the regulation of gene transcription via transcription factors and epigenetic modifying enzymes [
4,
5]. Its function as a cofactor for the metalloenzymes which synthesise the vasopressors noradrenaline and vasopressin suggests an important role in critically ill patients, particularly those with sepsis and septic shock [
6]. Several early studies have indicated that critically ill patients, including those with sepsis, exhibit very low circulating vitamin C levels compared with healthy control individuals [
7,
8], and deficient levels correlated with multiple organ failure [
9]. Owing to the pleiotropic functions of vitamin C in the body, depleted vitamin C status in critically ill patients may compromise their recovery.
Patients in the intensive care unit (ICU) routinely receive enteral nutrition and/or parenteral nutrition for those who cannot receive enteral nutrition or cannot receive it in sufficient amounts [
10]. The route of feeding—enteral or parenteral—is determined by the presence or absence of a functioning intestine and the haemodynamic status of the patient [
11]. Nutritional requirements are often tailored in the setting of fluid restriction and organ failure [
11]. Nutrition therapy provides essential macronutrients and micronutrients, such as vitamins A, C and E, which are thought to help prevent oxidative stress and modulate immune responses [
12]. Although total parenteral nutrition (TPN) typically comprises 100 mg/day vitamin C, one study has indicated that restoration of normal circulating vitamin C levels in critically ill patients requires a 30-fold higher intake [
13]. This is likely due to the enhanced metabolic demands for vitamin C in these patients owing to inflammatory and/or infectious processes.
However, whether critically ill patients are receiving adequate vitamin C intakes via standard ICU nutritional therapy to meet their enhanced needs remains largely underexplored. We therefore assessed the plasma vitamin C status of critically ill patients in intensive care and related this to infectious status (septic shock or non-septic), inflammation (C-reactive protein concentrations) and daily enteral and/or parenteral intakes.
Methods
We carried out an observational study with 44 critically ill patients recruited in the ICU of Christchurch Hospital, Christchurch, New Zealand (December 2015–August 2016). All procedures involving human participants were approved by the Southern Health and Disability Ethics Committee (15/STH/36).
All patients aged > 18 years admitted to the ICU were eligible, except those not expected to survive 72 h or when consent could not be obtained. Proxy consent was obtained from the treating physician in consultation with family members when patient consent was not possible. Consent was sought from patients as soon as they had sufficiently recovered. Only one patient withdrew consent; that patient’s data was discarded. The final cohort comprised 44 patients: a septic shock group (n = 24), a non-septic group (n = 17) and three patients who could not be categorised on the basis of the clinical notes and therefore were not included in subgroup analysis. Inclusion in the septic shock group required meeting the following criteria: receiving ≥ 5 μg/minute noradrenaline or adrenaline, receiving intravenous antimicrobial therapy specifically for infection, evidence of organ dysfunction (i.e., Sequential Organ Failure Assessment [SOFA] score ≥ 2 for at least one of respiratory function [ratio of partial pressure of arterial oxygen and fraction of inspired oxygen < 300], liver function [bilirubin > 33 μmol/L], coagulation [platelets < 100 × 103/μL] and renal function [creatinine > 171 μmol/L or creatinine increase > 50% of baseline if chronic renal failure]). One patient entered the study with unusually high plasma vitamin C concentrations (>100 μmol/L) and was classified as an outlier. This patient was excluded from all analyses involving vitamin C and has been presented separately as a case study.
Several ICU scoring systems were used at enrolment: the Simplified Acute Physiology Score II (SAPS II) [
14] and the Acute Physiology and Chronic Health Evaluation (APACHE) II and III scores [
15]. Organ failure was assessed using the SOFA score [
16] daily for the duration of the study.
Vitamin C and C-reactive protein analysis
A blood sample (4-ml ethylenediaminetetraacetic acid tube) was collected from each patient through an already established arterial line at enrolment (baseline = 0 h) and at 12, 24, 48, 72 and 96 h. The blood was immediately placed on ice and processed for vitamin C analysis within 2 h of collection by centrifugation at 4 °C to separate plasma, and precipitation of proteins and stabilisation of vitamin C was carried out with perchloric acid and the metal chelator diethylenetriaminepentaacetic acid as described previously [
17]. The acidified supernatants were stored at −80 °C for batch analysis. Urine samples were collected from a subset of patients through an indwelling urinary catheter at times 0, 12, 24, 48, 72 and 96 h. Plasma and urine vitamin C concentrations were determined using reversed-phase high-performance liquid chromatography with electrochemical detection as described previously [
17]. Half of each acidified sample was treated with the reducing agent tris(2-carboxyethyl)phosphine (TCEP) to determine the amount of dehydroascorbic acid present [
18]. Treatment of the samples with the reducing agent TCEP showed no significant gain in vitamin C concentrations, other than two of the samples which exhibited haemolysis. Plasma C-reactive protein concentration was assessed by endpoint nephelometry, and urinary creatinine concentrations were determined by the classical Jaffe reaction at Canterbury Health Laboratories, an International Accreditation New Zealand laboratory.
Patient vitamin C intake assessment
The daily vitamin C intake of the patients was determined from the volume of enteral nutrition or TPN administered to the patients. The enteral nutrition (Jevity, Glucerna, and Osmolite [Abbott Nutrition, Lake Bluff, IL, USA], Fresubin® 1000 complete and Fresubin® Energy Fibre [Fresenius Kabi, Bad Homburg, Germany]) contained 100–110 mg vitamin C/L, and the TPN (Olimel® N9; Baxter, Old Toongabbie, Australia) contained 62.5 mg vitamin C/L plus an additional 150 mg/L sodium ascorbate, which was included as a pH stabiliser (Baxter Healthcare, Christchurch, NZ), providing a total of 196 mg/L ascorbic acid (vitamin C).
Statistical analysis
Data are represented as mean and SD or box plots showing median values, 25th and 75th percentiles as boundaries, and whiskers as the range. To compare plasma and urinary vitamin C, as well as C-reactive protein concentrations, in septic shock and non-septic patients over the study period, we performed linear mixed effects models (LMEMs) using the lme4 [
19] and lmerTest [
20] packages in R version 3.2.4 [
21]. Patient identity was included in all models as a random variable to account for repeated measures of the same patient over the 4 days. The Satterthwaite method was used to calculate
P values from
t tests. We included an interaction effect in both models of day sampled × patient group (septic shock or non-septic). Post hoc analysis was conducted for each model using the multcomp package in R [
22]. Model assumptions were verified. All effects were statistically significant at α = 0.05.
Using the pharmacokinetic data from the study by Levine et al. [
23], whereby the steady-state plasma vitamin C concentrations for vitamin C-deficient individuals were determined following vitamin C of doses of 30–1000 mg/day, we constructed a dose-response model. Using this model we estimated plasma vitamin C concentrations for the critically ill patients in this study on the basis of the known daily enteral and/or parenteral nutritional intake they received. To do this we constructed a four-parameter log-logistic response model using the drc package in R [
24] and the following function: ((f(x) = c + \frac{d-c}{1 + \exp(b(\log(x)-\log(e)))}). All model assumptions were met.
Discussion
Our study revealed that nearly 70% of critically ill patients had hypovitaminosis C, including a high percentage with vitamin C deficiency (32%), despite receiving standard ICU nutritional support. Patients with septic shock have further depletion of plasma vitamin C, with nearly 90% having hypovitaminosis C and 40% having vitamin C deficiency, likely owing to enhanced activation of inflammatory pathways in response to infection, as reflected by higher C-reactive protein levels in these patients. This level of deficiency is significantly greater than we have observed in a middle-aged community-dwelling cohort (
n = 369), which exhibited < 13% hypovitaminosis C and only 2.4% deficiency [
26]. On average, the plasma vitamin C values of those receiving enteral and parenteral nutrition were approximately one-third the values predicted from intake, suggesting that recommended ICU intakes are not sufficient to meet the needs of critically ill patients. It should be noted that the pharmacokinetic data used to calculate the predicted vitamin C concentrations in our participants provide a conservative estimate, because all of the individuals in the pharmacokinetic study had vitamin C deficiency prior to supplementation [
23], so the difference between measured and predicted vitamin C concentrations in our participants may be even greater.
There are a number of possible explanations for the discrepancy in predicted and measured vitamin C concentrations. It is possible that enteral and parenteral preparations contain less vitamin C than stated, owing to the instability of the vitamin in the presence of oxygen, transition metal ions, ultraviolet light and higher temperatures [
27,
28]. Furthermore, critically ill patients are at significant risk for gastrointestinal dysfunction and consequently poor absorption for those patients who receive enteral feeding [
29]. Critically ill patients often exhibit gastrointestinal dysmotility, as well as feeding intolerance such as vomiting, aspiration of gastric contents, or high gastric residual volumes. Patients who are receiving sedatives and vasopressors are also at risk of feeding intolerance, and gastroparesis can be caused by pharmacologic agents that are frequently used in the ICU, such as opioids [
30]. Additionally, gastric hypoperfusion in the context of shock or vasopressor use will contribute to poor nutritional uptake of essential vitamins [
31]. Although early parenteral nutrition is discouraged in critically ill patients owing to increased risk of infection [
31], parenteral feeding bypasses any issues with gastrointestinal dysfunction. Parenteral nutrition also bypasses the saturable intestinal uptake of orally administered vitamin C via sodium-dependent vitamin C transporter 1 (SVCT1) [
23]. However, even the participants in our study receiving TPN, which provided an average of approximately 200 mg/day vitamin C, exhibited hypovitaminosis C.
Fluid resuscitation may haemodilute the plasma concentration of vitamin C. When we estimated haemodilution by comparing the patients’ haematocrit with gender-specific norms, this accounted for only one-fourth to one-third of the difference between the measured and predicted vitamin C status. However, this correction is likely an overestimate because critically ill patients also have lower haemoglobin levels owing to inflammation-induced suppression of synthesis and blood sampling or loss. Vitamin C removal from the plasma via dialysis can also occur, and at least 50% may be lost during continuous renal replacement therapy [
32‐
34]. Our study cohort comprised only five patients who were on continuous venovenous haemodiafiltration for acute renal dysfunction whilst in the ICU. One of these patients was presented as a case study, highlighting a rapid and > 50% drop in vitamin C over the first 24 h in the ICU. Depleted plasma vitamin C status might also be due to glomerular hyperfiltration and/or dysfunctional tubular reabsorption of the vitamin via SVCT1 [
35]. However, assessment of a subset of patients indicated a lower urinary concentration of vitamin C than we have previously observed in healthy individuals [
25]. Furthermore, estimation of total vitamin C excretion based on urinary output provided values that were comparable to those of healthy individuals, who excrete 25% and 50% of a 100-mg dose (for males and females, respectively) [
23,
36]. Thus, these estimates suggest that our subset of critically ill patients with low plasma vitamin C concentrations were not excreting more vitamin C than healthy individuals.
Vitamin C acts as a potent antioxidant in vivo, scavenging a wide range of reactive oxygen and nitrogen species, and in the process becoming oxidised to dehydroascorbic acid [
2]. The levels of dehydroascorbic acid detected in plasma are normally very low [
7,
37]. Although some studies have shown higher levels of dehydroascorbic acid in samples collected from patients assumed to be under enhanced oxidative stress [
38‐
40], we did not observe a significant increase in total vitamin C when our samples were treated with a reducing agent. This indicates that dehydroascorbic acid is not present in the circulation in significant amounts, even in critically ill patients when blood samples are collected and processed in a manner which minimises ex vivo oxidation of vitamin C.
Several studies have shown subnormal vitamin C status in critically ill patients, and this correlated with the severity of the illness and with multiple organ failure [
7,
9,
13]. It is likely that the low plasma vitamin C levels in these patients are a marker for correspondingly low tissue status, because we and others have shown that tissue vitamin C status correlates closely with plasma vitamin C levels, both in humans and in animal models [
41,
42]. In the present study, we were able to compare patients with septic shock and non-septic patients and found that a greater proportion of patients with septic shock had hypovitaminosis C and vitamin C deficiency. This is likely due to enhanced metabolic turnover as a result of the elevated inflammatory response observed in septic shock. Indeed, the patients with septic shock in our cohort had higher C-reactive protein levels than the non-septic patients for the first 48 h of the study. C-reactive protein is an acute-phase protein synthesised by the liver during acute inflammation in response to infection, trauma, ischaemia and burns along with other inflammatory conditions, and it is often used as a biomarker of sepsis with high levels linked to higher incidences of organ failure and mortality [
43]. In our study, significantly higher C-reactive protein levels were observed in patients with lower vitamin C status, which has been observed previously in hospitalised patients [
44]. Furthermore, administration of vitamin C to septic patients has been shown to lower C-reactive protein and other pro-inflammatory biomarkers, as well as to decrease organ failure [
45,
46].
The American Medical Association’s Nutrition Advisory Board recommends an intake of 100 mg/day vitamin C via liquid nutrition [
47]. However, hypovitaminosis C was not prevented in our patients with the use of standard enteral and/or parenteral nutrition, despite our TPN containing additional vitamin C (i.e., a total of nearly 200 mg/L). These results agree with data reported by others who have shown that parenteral intakes of ~ 300 mg/day did not change their patients’ plasma vitamin C concentrations [
13,
27]. In comparison, intakes of 150–200 mg/day will saturate the plasma of healthy individuals [
23], although we and others have observed decreased response to supplementation in individuals with hypovitaminosis C [
48,
49]. Uncomplicated surgical patients require > 500 mg/day of vitamin C, with much higher doses required in surgical ICU patients [
50]. Parentaeral administration of ≥ 1000 mg/day vitamin C to critically ill patients was barely sufficient to raise plasma vitamin C concentrations above hypovitaminosis C levels [
13,
27]. Recent pharmacokinetic data in critically ill patients indicate that ≥ 2000 mg/day is required to normalise plasma vitamin C levels [
51], and 3000 mg/day has been shown to result in saturating levels (i.e., 68 μmol/L), as evidenced by enhanced urinary excretion [
13].
Vitamin C can be metabolised via dehydroascorbic acid to oxalate, which has the potential to form calcium oxalate crystals in individuals predisposed to renal stone formation. A few cases of acute oxalate nephropathy have been reported over the last 30 years in patients with impaired renal function following administration of high-gram doses of intravenous vitamin C [
52‐
54]. Administration of gram doses of vitamin C to individuals with normal renal function, however, results in < 0.2% oxalate excretion when samples are collected and stored in a manner which avoids artefactual oxidation [
55]. Furthermore, no cases of oxalate nephropathy have been reported for low-gram doses of parenteral vitamin C.