Vitamin C, also known as ascorbic acid, is a water-soluble vitamin naturally present in some foods, particularly fruits and vegetables. It is used as a food additive and is available as a dietary supplement [1
]. Because humans are unable to synthesize it, vitamin C is considered to be an essential dietary micronutrient [1
]. The Chinese Dietary Reference Intakes recommend an estimated average requirement (EAR) for vitamin C for adults of 85 mg/day [2
]. A regular and adequate intake is required to prevent vitamin C insufficiency.
Along with the rapid economic growth and urbanization observed in China over the past few decades, the Chinese population has experienced a nutritional transition characterized by dramatic changes in the dietary patterns, eating, and cooking behaviors [3
]. Between 1991 and 2011, the consumption of animal source foods increased; over the same time frame, the consumption of coarse grains, legumes, and other healthy foods, e.g., vegetables, declined, and fruit intake remained low [5
]. Consistent with these findings, the China Health and Nutrition Survey (CHNS, 1989–2009) showed a decrease in dark vegetable consumption in adults, accompanied by a significantly decreased dietary vitamin C intake, from 117.2 and 111.0 mg/day in 1989 to 85.2 and 83.1 mg/day in 2009 on average for males and females, respectively [6
]. Results of the subsequent China National Nutrition and Health Survey (2010–2012) showed that the average dietary vitamin C intake in the Chinese population was 80.1 mg/day, and the mean probability of adequacy of vitamin C was 38.9%, suggesting that dietary vitamin C intake continued to be inadequate in most Chinese [7
]. Compared with a reported range from 80 to 230 mg/day of mean vitamin C intake among 27 centers in 10 European countries [8
], vitamin C intake in Chinese appeared low [8
]. Thus, we must estimate vitamin C intake in the setting of the recent nutritional transition in China before we can recommend interventions to improve dietary vitamin C intake in the Chinese population.
Although fruits and vegetables remain the food groups with the highest contribution to total vitamin C intake [7
], significant disparities in food availability, food choice, and consumption pattern among different ethnic groups worldwide could influence the primary food sources of vitamin C intake from one country to another [10
]. In addition, the increasingly westernized lifestyle of China, characterized by the pronounced growth in the consumption of processed foods and beverages and a propensity to eat away from home [5
], may lead to changes in the typical pattern of food sources of vitamin C in this country. Besides fruit and vegetable consumption, the role of sociodemographic factors, including income, educational level, and nutritional status [11
], and the independent influence of smoking [12
] in vitamin C status have been evaluated. In China, previous studies estimated the vitamin C intake stratified by gender, age, residence region, and household income level. However, independent roles of the sociodemographic factors and dietary habits have not been investigated yet, despite evidence that individuals from rural villages or households with low income and members of the aging population have a lower vitamin C intake [6
Therefore, the present study aimed to estimate dietary vitamin C intake and vitamin C food sources and evaluate the prevalence of risk of inadequate vitamin C intake in the adult population using newly collected data from the China Nutritional Transition Cohort Study carried out in 2015. To identify independent determinants of the vitamin C intake status, this study further explored potential influencing factors. These findings would be valuable for developing and implementing public health strategies to meet the recommended vitamin C intake and provide guidance for population-specific nutrition education and intervention programs.
On the basis of the antioxidant property of vitamin C, increasing evidence indicates that vitamin C intake is essential for human health by reducing the risk of progression to cardiovascular disease, type 2 diabetes mellitus, or other diseases in which oxidative stress plays a pivotal role [1
]. Understanding dietary vitamin C intake and the extent to which the intake does not meet the recommendations and the underlying influencing factors will be valuable for targeted interventions to assure adequate vitamin C intake. The present study, which used three-day 24 h dietary recalls, found that the average vitamin C intake was 78.1 mg/day in adults aged 18–65 years, and the prevalence of risk of insufficient dietary vitamin C intake in the study population was 65.1%. The top four food sources of vitamin C were light and dark vegetables, fruits, and tubers, contributing a combined 97.3% of the total vitamin C. After fully adjusting for covariates, the subjects living in towns or rural villages resulted in potentially vulnerability to the risk of insufficient vitamin C intake, while the subjects having middle school education, those living in suburban regions or southern regions, those with a high vegetable consumption frequency, and those with a moderate and high total daily energy intake were protected against the risk of insufficient dietary vitamin C intake. The independent influences of educational level, residence area, geographic location, vegetable consumption frequency, and total daily energy intake were relatively strong in the areas of lower intake of vitamin C intake distribution, which is of great concern. Like all dietary methods, 24 h recalls are subject to random errors that lower the precision and to systematic errors that can reduce the accuracy at each stage of the measurement protocol [20
]. Here, although our study was well designed to reduce random errors and systematic errors by incorporating standardized quality control procedures and collecting 24 h recalls per person across three consecutive days, including two weekdays and one weekend day, dietary vitamin C underreporting and some degree of misclassification might have been possible, which should be taken into account when considering the findings in the present study.
China National Nutrition and Health Survey in 1982, 1992, 2002, and 2010–2012 reported that the average dietary vitamin C intake in the Chinese population was 129.4, 100.2, 88.4, and 80.1 mg/day, suggesting a progressive decline in vitamin C intake over the past three decades [7
]. The mean vitamin C intake (78.1 mg/day) in adults in 2015 in the present study seemed to be slightly lower than the above-mentioned findings from 2010–2012 nationwide. To evaluate the changes in vitamin C intake among the same population between different survey periods, we further focused on vitamin C intake in adults aged 18–49 years from nine provinces in the present study, which were 87.4 mg/day in males and 81.0 mg/day in females on average, and thus considerably lower than that in 1989 (117.2 mg/day for males and 111.0 mg/day for females), and showing a slightly increasing trend in males and a decreasing trend in females relative to vitamin C intake in 2009 (85.2 mg/day for males and 83.1 mg/day for females) [6
]. Thus, it is evident that the dietary vitamin C intake in Chinese adults over the periods studied significantly decreased at the same time of the nutritional transition in China, and the intake in recent years has remained low and needs to be improved. Vitamin C intake from foods varied from about 80 to over 230 mg/day in adults in 10 European countries in a survey during 1995–2000 [8
]; that of the US adult population in 2001–2002 ranged from 101.8 to 116.2 mg/day [21
], and the most recent estimate for the Spanish population in 2013 was 84.4 mg/day [9
]. The intake amount in the Chinese population appears to be lower than that of western countries. These differences may result from ethnic disparities in dietary patterns and nutrient intake from meals, largely related to portion size, recipe, and cooking method [22
Around 38.9% of Chinese population had adequate vitamin C intake in the China National Nutrition and Health Survey 2010–2012, indicating more than 60% of the population was at risk of vitamin C inadequacy [7
]. The present study showed that the prevalence of risk of insufficient dietary vitamin C intake in adults was 65.1% by using EAR cut-off point, partially consistent with the aforementioned estimation of the previous study [7
]. In our study, the proportion of subjects with a lower likelihood of inadequate vitamin C intake using RNI was 25.9%. The specific recommendations for vitamin C intake vary in different countries. The reported percentage of the population not meeting the recommended vitamin C intake was 36% in the Spanish population [9
], and 31% in the US population [21
], which is significantly lower than what we found in our study, and likely due in part to disparities in fruit and vegetable consumption. There were increased trends in daily fruit and vegetable consumption in European and North American countries from 2002 to 2010 [23
]; conversely, an unbalanced diet was adopted by the Chinese population, especially by middle-aged residents, characterized as high in energy-dense, nutrient-poor foods and low in fruit and vegetables [24
]. Here, based on comparisons of dietary intake to EAR and RNI for vitamin C, we found that the dietary vitamin C intake in Chinese adults is a potential problem and needs to be addressed.
Indeed, fruits and vegetables are by far main contributors to dietary vitamin C. These two groups provided about 87% to the total vitamin C intake in the total population assessed in the present study, an estimate that tends to be higher than that reported in Spanish (70%) [9
], Polish (75%) [26
], and other European populations (80%) [8
]. Additionally, the most important source of vitamin C was fruits in the US [10
]. Conversely, vegetables ranked first in Chinese (68.2%) and European (19.9–42.9%) populations, and in China it was characterized by a higher contribution of light greens (47.2%) as the consumption of dark greens gradually decreased [6
]. Fruit contribution in China (18.5%) was less than in the US (around 50.0%) and in European countries (23.3–59.3%) [8
]. Therefore, we found that vegetables are the most important food source of vitamin C and fruit consumption is lower than vegetable consumption in the Chinese population. Consistent with other countries [8
], tubers were also an important contributor to total vitamin C and ranked fourth in the Chinese adult population in the present study. Previous studies have found beverages ranked third or fourth among contributors of total vitamin C intake in subjects from the US [10
] and European countries [8
]; however, in the present study, drinks afforded a negligible proportion (data not shown), and the largest contribution observed was only 1.2% in subjects living in northern parts of China. Taken together, different patterns of food source contributions to total vitamin C were observed in China and western countries. Interestingly, the present study also observed a differentiated percent contribution of food sources in relation to sociodemographic factors, regarding especially fruit contributions.
The 15 provinces surveyed in the present study (Figure S1
) vary substantially in geography, economic development, public resources, and health indicators. The nutrition and health behaviors and outcomes of each participant are influenced by changes in community organizations and programs and by changes in types of household and individual economic, demographic, and social factors. All these factors largely contributes to the skewed distribution of dietary vitamin C in the study population. Previous studies in China found that vitamin C intake was lower in populations from rural villages or households with lower income, as well as in the aging population [6
]. This is the first study exploring the potential influencing factors behind these observations. Both the distribution of vitamin C intake and the prevalence of risk of insufficient dietary vitamin C intake in adults significantly differed by gender, educational level, residence region, geographic location of residence, smoking, alcohol intake, and vegetable consumption frequency. Educational level, residence region, geographic location, vegetable consumption, and total energy daily intake were further identified as independent indicators for the risk of vitamin C inadequacy in Chinese adults. The availability of fruit and vegetables was linked to favorable vitamin C intake in adults [27
]. We investigated the impacts of fruit and vegetable consumption on vitamin C intake and found that a high frequency of vegetable consumption independently protected against the risk of dietary vitamin C inadequacy. No significance was observed for fruit consumption, which may be due to the lack of fruit consumption on a daily basis among more than 66% of the population on the basis of the findings of fruit consumption frequency in the present study. Similar to the findings in European countries [8
], a clear geographic gradient of vitamin C intake was observed in our study, with higher intakes and negative association with the risk of vitamin C inadequacy in the southern regions as compared to the northern regions. In contrast to residency in suburban areas, residency in both town and rural villages increased the risk of insufficient dietary vitamin C. All these findings indicate that the vitamin C intake status may depend on parameters reflecting economic level, dietary habit, and food availability in the population from various regions. The association between gender and vitamin C intake varied [8
]. Here, gender was not found to associate with the risk of insufficient dietary vitamin C intake after considering total energy intake in the model. Some evidence indicated the inverse association between smoking and vitamin C intake, and smoking was negatively associated with the preference for foods rich in vitamin C [8
]. Conversely, only fruit contribution in current smokers in the present study was lower than in either never or former smokers, according to percent contributions of food sources to vitamin C intake, but this was not sufficient to match the higher vitamin C intake from vegetables and tubers. The vitamin C intake distribution significantly differed depending on the smoking status, in that smokers tended to have higher vitamin C intake; however, an independent influence of smoking on vitamin C was not observed in our study. The association between vitamin C intake and alcohol intake was similar to that observed for smoking. It was suspected that the different vitamin C intake distributions associated with smoking and alcohol intake in the present study were linked to a significant higher proportion of males in the smoker and drinker groups relative to their counterparts, and to the observed vitamin C intake variation in relation to gender found in our study. Regarding the impacts of smoking and alcohol intake on vitamin C-enriched foods and vitamin C intakes, future study is still required.
This is a representative and large study evaluating the intake and food sources of vitamin C across sociodemographic factors and exploring independent determinants for vitamin C intake, using data of adults from 15 provinces in China. However, the findings cannot be extrapolated to the general population. Another limitation of this study is that the 24 h dietary recalls for three consecutive days were generally collected in summer, and seasonal variations in food consumption rich in vitamin C could not be taken into account. Moreover, there are always recall biases and a potential underestimation in dietary recalls being self-reported measurements. In addition, this study considers only intakes of vitamin C from dietary sources. Dietary supplements are likely to be considerable sources of vitamin C in some particular populations. It should be borne in mind that the users of supplements may differ from non-users with regard to demographic characteristics [28