In the present study, a prevalence of diabetes of 15.9% was reported, suggesting that it was prevalent in Chinese adults. This study also investigated the relationship between the intake of each vitamin with diabetes and found that dietary folate and vitamin E intakes were significantly higher in diabetic patients than in controls, while dietary thiamine, riboflavin, niacin and vitamin C intakes were significantly lower. After adjusting for potential confounders, subjects with diabetes had a greater odds ratio for dietary folate intake and lower OR for niacin intake in comparison to those without diabetes. In addition, the plots of restricted cubic splines presented an inverted U-shaped association between dietary folate intake and the risk of diabetes. According to gender, there were significant differences in the folate intake for women, and niacin intake for men between the diabetes and normoglycemia groups.
B group vitamins are essential water-soluble nutrients. Folate (vitamin B9) can be found in a wide variety of food, such as vegetables, fruits and nuts [25]. Dietary intake of folate was inversely associated with incident diabetes in Korean or Japanese women reported by previous two prospective cohort studies [10–11]. Diabetes is an oxidative stress disease, folate deficiency has been linked to oxidative stress in diabetic patients [8]. Deficiency of folate has also been reported to severely hamper biosynthesis and secretion of insulin in pancreatic β-cells [26]. However, our study showed that people with diabetes had higher dietary folate intakes than the controls. The anti-diabetes drug metformin may cause folate deficiency. In an randomized controlled trial (RCT), after eight weeks of folate supplementation, diabetic men taking metformin showed improved serum total antioxidant capacity [27]. Thus, there is a possibility that higher folate intake in diabetics may be due to the reverse causality. The diabetics might have been advised to change their dietary habits during routine examinations.
Niacin (vitamin B3) can be found in meat (especially liver and heart), fish, nuts, and some fruits and vegetables and coffee [28]. Niacin is known to lower triglycerides and low-density lipoprotein cholesterol levels while significantly increasing high-density lipoprotein cholesterol levels [29]. Few studies have assessed the relationship between dietary niacin intake and diabetes. Inadequate intake of niacin from food may have not been recognized as a problem. The present study indicated that individuals with low niacin intake had a high likelihood of having diabetes. However, a Japanese study concluded that dietary intake of niacin was not associated with a reduced risk of diabetes [11]. An RCT study [30] showed that the use of niacin for 3 years in subjects with normal baseline glucose levels was associated with an increase in blood glucose levels and the risk of developing impaired fasting glucose. While Sazonov et al. [14] reported that niacin has clinically insignificant negative impact on glycaemia. However, further research about the relationship between dietary niacin intake and the risk of diabetes is needed.
Thiamine, also known as vitamin B1, plays a role in various cell functions, such as energy metabolism and the breakdown of sugars and carbon skeletons [31]. Riboflavin, also known as vitamin B2, plays a role in tryptophan metabolism, iron absorption, gastrointestinal tract, brain function, mitochondrial function, and other vitamins’ metabolism [32]. In this study, the dietary intakes of thiamine and riboflavin in diabetic patients were significantly lower than that in the control group. Moreover, riboflavin intake was independently associated with the risk of diabetes in women. Insufficient thiamine level causes inadequate glucose metabolism in mitochondria [31]. Taking thiamine for a month has been shown to decrease glucose in diabetics when compared to controls [33]. Meanwhile, Md Maroof Alam et al. [34] suggested that supplementation with dietary riboflavin might help in the reduction of diabetic complications.
Vitamins C as an antioxidant was found decreased in diabetic patients, possibly due to an increased need to control the excessive oxidative stress caused by abnormal glucose metabolism [8]. Plasma vitamin C concentrations have been significantly and inversely associated with glycosylated hemoglobin and fasting and postprandial blood glucose and oxidative stress [35–36]. Vitamin E has a very important antioxidant role in the organism and has been found decreased in patients with diabetes [37]. However, other studies have shown no association with risk of diabetes or no effect on insulin sensitivity [38–39]. Our study showed lower dietary vitamin C and higher vitamin E intakes in diabetic patients. The effect of vitamins C and vitamin E on the risk of diabetes is likely due to its role as an antioxidant.
Several potential limitations warrant mention. First, this study is a cross-sectional design, which does not allow to establish the temporality of cause-effect relationship with certainty. Second, the information of using glucose-lowering drugs or vitamin supplements was not investigated, which may have affected the results. Third, vitamin intake data were obtained through a three-day dietary questionnaire review, which could not accurately reflect an individual’s dietary intake due to faulty memory and underreporting. Finally, the study sample was recruited in Bengbu city and was not necessarily representative of all the general populations, which may limit the generalizability of our findings. In spite of this, we have adjusted for some known and proposed potential confounders for reliability in binary logistic regression models and restricted cubic splines analyses, and our findings added to the limited data available on the association of dietary vitamin intakes with the risk of diabetes in Chinese populations.