"Is this paper reliable?"

Association of serum vitamin C with all-cause and cause-specific death: Data from National Health and Nutrition Examination Survey (NHANES 2003-2006)

Highlights
•
Association of circulating vitamin C with mortality remains controversial.

•
Serum vitamin C presented a U-shaped relationship with all-cause or cardiovascular disease mortality.

•
Serum vitamin C exceeding a threshold was positively associated with all-cause and cardiovascular disease mortality.


Abstract
Objectives
The association between levels of circulating vitamin C and mortality remains controversial. The aim of this study was to explore the non-linear association between serum vitamin C levels and all-cause or cause-specific mortality.

Methods
We included 9902 US adults with their serum vitamin C levels from the National Health and Nutrition Examination Survey (NHANES 2003-2006). Their survival information was retrieved from baseline until 2015 using the national death index. Multivariable Cox proportional hazards models were used to show the risk for all-cause or cause-specific death according to baseline serum vitamin C levels. Smooth curve fitting and threshold effect analyses were used to clarify potential nonlinearity.

Results
During a median follow-up of 10.6 y, there were 1558 all-cause deaths, including 320 from cancer, 374 from cardiovascular disease (CVD), and 120 from respiratory diseases. Serum vitamin C levels had a U-shaped relationship with all-cause or CVD-associated mortality. Interestingly, serum vitamin C levels lower than the threshold value (1.06 mg/dL) were negatively associated with all-cause (fully adjusted hazard ratio [HR], 0.71; 95% confidence interval [CI], 0.59–0.86) and CVD (fully adjusted HR, 0.70; 95% CI, 0.47–1.03) mortality. In contrast, serum vitamin C levels higher than the threshold value (1.06 mg/dL) were positively associated with all-cause (fully adjusted HR, 1.33; 95% CI, 1.15–1.54) and CVD (fully adjusted HR, 1.60, 95% CI, 1.23–2.10) mortality, respectively.

Conclusion
Serum vitamin C levels showed a U-shaped relationship with all-cause and CVD-associated deaths among US adults using the NHANES data.

Introduction
Vitamin C, also known as ascorbic acid, plays a key role in the physiologic synthesis of l-carnitine, collagen, and some neurotransmitters. Vitamin C needs to be acquired exogenously from the diet or supplemented as it cannot be physiologically synthesized in humans [1,2]. In recent years, the antioxidant effects of vitamin C have been found to reduce the risk for several types of cancer and cardiovascular diseases (CVDs), and their disease-related outcomes [1], [2], [3], [4], [5]. Some observational studies have reported dietary vitamin C intake to be inversely correlated with mortality in humans [1,3,4]; however, some studies have indicated that dietary vitamin C presented a U-shaped curve with all-cause death [6,7]. These inconsistent findings might be due to challenges associated with the accurate evaluation of vitamin C intake from foods due to different storage, processing, and cooking methods [2,7].

As an integration of consumption, absorption, and metabolism of vitamin C from the diet, serum vitamin C levels might more accurately reflect vitamin C status in the human body [2,7]. Although one clinical trial [8] and some prospective cohorts have reported a lower mortality risk among individuals with higher levels of serum vitamin C [2,5,[9], [10], [11], [12]]， the second National Health and Nutrition Examination Survey (NHANES II) did not find this relationship among women [13]. Additionally, although a dose–response meta-analysis presented an inverse relationship between serum vitamin C levels and all-cause mortality [7], a British study did not find a linear relationship [14]. However, previous studies have rarely focused on the possibility of a nonlinear relationship between vitamin C levels and all-cause mortality. Although vitamin C might have protective effects on various disease processes due to its antioxidation properties, it can be potentially toxic when exceeding a certain threshold level [15], [16], [17]. The currently accepted reference range for the serum vitamin C level is between 0.4 and 0.99 mg/dL, with 0.2 to 0.39 mg/dL classified as insufficient and <0.2 mg/dL as deficient [18,19]. However, few studies have systematically assessed the cutoff point for excessive serum vitamin C levels. Thus, to date, the exact association between serum vitamin C levels and mortality has not been largely explored, and it is also not known whether there is any threshold effect between serum vitamin C level and risk for death.

Therefore, this cohort study using randomly selected US adults from the NHANES (2003–2006) was conducted to evaluate the curvilinear relationship between serum vitamin C and all-cause or cause-specific death (including cancer, CVD, and respiratory disease).

Section snippets
Participants and design
NHANES has been conducted to collect nationally representative data on health and nutrition among residents of the United States. A multistage, complex, probability sampling design was used to generalize the NHANES findings broadly across the US population [9,13,20]. Questionnaires, physical examinations, and laboratory tests have been collected periodically since 1999. The public use of NHANES data was approved by the National Center for Health Statistics Ethics Review Board, and each

Baseline characteristics of study participants
Among the 9902 adults from NHANES who were included in the final analysis, the mean age was 45.6 ± 17.4 y, and women accounted for 51.6% of the participants. Table 1 shows the baseline sociodemographic and clinical characteristics of the selected individuals according to serum vitamin C quintiles. Significant differences were observed between serum vitamin C quintiles for all included characteristics, except for a history of CAD. For the higher serum vitamin C quintiles, participants were more

Discussion
Among a large sample of US adults aged ≥18 y from NHANES with a median of 10.6 y of follow-up, both a lower and higher concentration of serum vitamin C were correlated with a higher all-cause or CVD mortality. Serum vitamin C levels showed an independently significant U-shaped relationship with all-cause or CVD-associated death. The threshold effect analysis further indicated that overall, serum vitamin C at 1.06 mg/dL could be a possible optimal level with respect to all-cause or CVD mortality.

Conclusions
The serum level of vitamin C presented a U-shaped relationship with all-cause and CVD mortality among 9902 US adults from NHANES. Beyond a certain threshold (1.06 mg/dL), an increased risk was observed in all-cause and CVD deaths among adults with a higher serum vitamin C level. The present findings indicated that excessive circulating serum vitamin C levels could lead to potential health risks.

Acknowledgments
We thank Jie Liu, Department of Vascular and Endovascular Surgery, Chinese PLA General Hospital) for their helpful review and comments regarding the manuscript.