Gestational diabetes mellitus (GDM), defined as glucose intolerance first detected during pregnancy, is one of the most common pregnancy complications and has been associated with adverse health outcomes for both mothers and their offspring [1]. GDM affects approximately 5% to 17% of all pregnancies worldwide, and the prevalence has increased over the past 20 years and this upward trend is expected to continue due to a rising number of overweight and obese women of childbearing age [2]. GDM is a significant public health concern, therefore, early identification of modifiable factors to prevent its development is crucial [1].

Dietary components are associated with GDM development, making these modifiable factors ideal for informing GDM prevention strategies. Dietary intervention trials with probiotics or myo-inositol supplements were efficient in reducing GDM risk [3-5]. Data from observational studies reveal that distinct dietary components including energy, nutrients (total fat, cholesterol, and heme iron), and selected food items (red/processed meats and eggs) are associated with GDM risk [1,6]. Because of potential interactions among these nutrients and food items, it is difficult to single out each item’s specific effect [6].

Dietary pattern analyses or dietary indexes like the Healthy Eating Index (HEI), takes into consideration interactive and cumulative effects of nutrients or foods. To date, most of the studies examining maternal dietary patterns and GDM have been conducted in Western populations [6]. Findings from the Nurses’ Health Study II suggest that women who adhered to the low consumption of the prudent dietary pattern, high consumption of the Western dietary pattern, or a low-carbohydrate dietary pattern [7] were associated with an elevated risk for developing GDM, while Mediterranean diet (MD) and high HEI adherers demonstrated lower GDM risk [8]. Studies conducted in Iran [9], the Mediterranean basin [10], and Australia [11] also verified lower GDM risk among MD adherers. Furthermore, adherence to a prudent dietary pattern was associated with lower risk of GDM among Iceland women [12]. However, a study from Singapore representing multi-ethnic Asian cohort identified different dietary patterns compared to results of similar studies conducted in Western populations, and it found that the seafood-noodle-based-diet was associated with a lower risk of GDM [13].

To the best of our knowledge, only one study including 3,063 pregnant women conducted in Guangzhou, China examined dietary patterns in relation to GDM [14]. In this study, four dietary patterns vegetable, protein-rich, prudent, and sweets/ seafood patterns were identified. The highest tertile of vegetable score and sweets/seafood score were associated with reduced (odds ratio [OR], 0.79; 95% confidence interval [CI], 0.64 to 0.97) and increased (OR, 1.23; 95% CI, 1.02 to 1.49) risk of GDM respectively. There are several limitations to this study. First of all, the study was conducted in Guangzhou, China. Given the vast differences in dietary habits between Northern and Southern China, the results of this study are not generalizable. Second of all, this study only used frequency of food intake to analyze dietary consumption and did not collect information on portion sizes, thus preventing the adjustment of total energy intake. Therefore, the effect of diet pattern and risk of GDM still needs to be verified in China. To examine the dietary pattern in Chinese population and evaluate its relationship with GDM risk, we conducted a case control study based on a birth cohort in Taiyuan, China.

The study participants were recruited from the First Affiliated Hospital of Shanxi Medical University in Taiyuan, China when they came to the hospital for delivery between March 1, 2012 and December 30, 2016. Women aged 18 years or older with gestational age of 20 weeks or more and without mental illness were eligible for the study. Although 10,320 pregnant women were enrolled in the study, 91 pregnant women with previous diabetes and 94 pregnant women whose gestational age less than 20 weeks were excluded, resulting in a total sample size of 10,137 pregnant women.

All study procedures were approved by the Institutional Review Board (IRB) at the Shanxi Medical University with the approval IRB number of 2011143. Written consents were obtained from each participant. Information on demographic factors, reproductive and medical history, smoking, and alcohol were collected using standardized questionnaires [15] administered by trained interviewers. Information on birth outcomes and pregnancy complications were acquired from medical records.

Before propensity score matching, there were difference between the case and control groups in several variables including maternal age, education, BMI, gestational week, parity, gestational hypertension, weight gain, preterm, and family history of diabetes. With the use of propensity score matching, 1,464 GDM cases were matched with 2,928 control subjects. After matching, the standardized differences were less than 0.1 for all variables, suggesting only small difference between the two groups (Table 1).

The number of factors were determined according to the Kaiser criterion (eigenvalues over 1.0), the factor analysis identified three major factors vegetables, cereals, and meats that could explain 52.67%, 52.48%, and 50.47% of the variance of the original information respectively. The factor loading of the three patterns during these periods was shown in Table 2. The three dietary patterns identified in these three periods were similar. The first factor loaded heavily with the following food or food groups: green leafy vegetables, other vegetables, and bean products. The second factor loaded heavily with cereals, pickles, alga, fruits and nuts. The third factor loaded heavily with meats, dairy, eggs, nuts, and fruits. We labeled these three factors as “vegetables,” “cereals,” and “meats,” respectively.

As shown in Table 3, the vegetable dietary pattern was associated with lower risk of GDM before and after propensity score matching. The lowered GDM risk that associated with vegetable pattern was consistent across the three time periods with a slightly stronger effect during the second trimester (P for trend=0.025, 0.018, and 0.001, respectively). When the analysis stratified by exercise status, the OR was 0.93 (95% CI, 0.87 to 0.99), 0.92 (95% CI, 0.87 to 0.98), 0.90 (95% CI, 0.84 to 0.95) during the three periods in exercise subjects; the OR was 1.07 (95% CI, 0.66 to 1.75), 1.01 (95% CI, 0.64 to 1.61), 1.01 (95% CI, 0.63 to 1.62) in non-exercise subjects. The cereals and meats dietary patterns were not statistically significantly associated with GDM risk.

The contents of the main components in the “vegetables” dietary patterns were shown in Table 4. The vegetables and bean products consumed by the case and control subjects were similar throughout the three periods. The beans and vegetables consumed for the four quartiles were approximately 130, 185, 240, and 335 g/day in cases, and 135, 185, 240, and 330 g/day in controls, respectively.

Among subjects who younger than 35 years old, control subjects consumed more vegetables (223.9±85.7, 223.9±87.0, 226.6±86.3 g/day) than the case subjects consumed (215.9±89.7, 217.3±89.8, 218.5±91.1 g/day) significantly during 1 year before conception (P=0.011), the first trimester (P=0.038), and the second trimester (P=0.010). Among subjects whose BMI less than 24, control subjects consumed more vegetables (221.8±84.0, 222.1±85.6, 224.5±84.7 g/day) than the case subjects consumed (214.4±86.4, 215.7±87.0, 216.4±87.0 g/day) significantly during 1 year before conception (P=0.024), the first trimester (P=0.055), and the second trimester (P=0.016). Among subjects older than or equal to 35 years old or subjects whose BMI greater than or equal to 24, there was no significant difference in vegetable intake between cases and controls during the periods.

As shown in Tables 5 and 6, analysis stratified by BMI and maternal age suggested that statistically significant associations between the vegetable pattern diet and risk of GDM were only found in women with BMIs less than 24 and who were younger than 35 years old. The lowered GDM risk was also consistent across the three time periods for the women mentioned above. As shown in Supplementary Table 1, stratified analysis by parity was also conducted, and, significant results were only found in nulliparous women during the second trimester of pregnancy.

In our study, women who adhered to the vegetable dietary pattern had lower GDM risk during the year before pregnancy as well as the first and second trimesters of pregnancy. The GDM risk lowered 6% to 9% for every quartile increase in the vegetable pattern score. Our results were consistent with a previous study conducted in China, which also found lowered GDM risk for women who adhered to a vegetable dietary pattern (root vegetables, beans, mushrooms, melon vegetables, seaweed, other legumes, fruits, leafy and cruciferous vegetables, processed vegetables, nuts, and cooking oil) [14]. The vegetable pattern in our study was slightly different from that in the previous study [14], as fruits, nuts, and cooking oil were excluded from our vegetable pattern. Although Chinese women are advised to follow a set of dietary customs after conception [18], the dietary pattern identified a year prior to pregnancy was the same as that identified in the first and second trimesters of pregnancy. The vegetable pattern in these three periods was all statistically significantly associated with lowered GDM risk. In a Multi-Ethnic Asian Cohort: the GUSTO study, consumption of the vegetable-fruit-rice diet was associated with lower risk of GDM in Chinese participants [13]. Unlike the GUSTO study, our vegetable based dietary pattern excluded rice and fruits.

Although dietary patterns identified in Western populations were different from that in Chinese populations [6,10,11,19], healthy diets rich in vegetables, whole grains, nuts and fish, low in red and processed meats and snacks were generally associated with lowered GDM risk. Vegetables are known to be rich in vitamins and dietary fiber. GDM risk was reported to be associated with vitamin D [20-22], vitamin C [23,24], and dietary fiber [25,26], and these nutrients were the main ingredient of vegetables. Thus, maybe fiber and ameliorated diet quality in general is associated with the lowered GDM risk. However, the lack of meat association with GDM risk is also an issue, as the increased vegetables pattern is usually associated with a reduced meats intake.

The role of vegetable dietary pattern may be related with the reduced concentrations of C-reactive protein and other inflammatory markers [27] that involved in GDM development [28], the role of dietary fiber that reducing adiposity and improve insulin sensitivity [26,29] and improving lipid homeostasis [25,30]. However, the precise pathway remains unclear, and further studies are needed to explore the mechanism behind the relationship between vegetables pattern and GDM risk.

The vegetable dietary pattern was found to be associated with lowered risk of GDM in our study and previous study [14]; however, it still needs verified by further well-designed trials before it is recommended to public. Due to the GDM risk factors include high BMI, advanced maternal age [1], so the stratified analysis was conducted, and the protective effect of vegetable dietary pattern diet was found in women who had BMIs less than 24, and were younger than 35 years old. It may due to subjects in these groups consumed more vegetables, and they focus more on life quality and exercise more during periconceptional periods. It was also supported by our results that the protective effect of vegetable pattern diet was only found in exercise subjects. The protective effect of vegetable pattern diet was not shown in older or obese/overweight women, and it is possible that GDM risk is much higher in these population [31]; hence, the protective effect was not shown. Vegetable dietary patterns suggest a possible way to prevent GDM with diet, but it still needs to be verified in future studies. It is imperative to find appropriate methods, frequencies, and portion sizes of vegetable dietary pattern interventions in order to effectively prevent GDM.

The association between vegetable pattern diet and GDM risk was analyzed during 1 year before conception, the first trimester and the second trimester. Although the OR of vegetable pattern diet in relation to GDM risk observed in the second trimester was slightly smaller than that observed in the first trimester and 1 year before conception; however, we thought that there is no essential difference between the effect among these periods.

There are several strengths and limitations of our study. In terms of strengths, diagnosis of GDM in our study was obtained by investigating medical records that were based upon national guidelines of GDM diagnosis. This was likely to minimize potential disease misclassification. Another strength is that information on potential confounders was collected using a standardized questionnaire, thus allowing us to control these potential confounders. A limitation may be that self-reported dietary intake could have led to measurement errors and the resulting misclassification of dietary intake may have weakened the detection of an association of specific dietary patterns with GDM. Another limitation is that the subjects were enrolled from a hospital setting, potentially limiting the generalizability of the result. The food intake data during 1 year before pregnancy and during the first and second trimesters of pregnancy were collected in our study, the recall bias maybe existed, and women may couldn’t correctly refer differences in food intake between first and second trimester, and healthier women may underlines the amount of vegetables in these periods, and this may result in the over estimation of the protective effect of vegetables dietary pattern for the risk of GDM. Additionally, some characteristics of the cases and controls were not equilibrium distributed; however, they were adjusted when estimated the association between dietary patterns and risk of GDM.

In conclusion, the vegetable diet pattern characteristically abundant in green leafy vegetables, other vegetables, and bean products was associated with lowered GDM risk. Future studies, preferably consisting of appropriately designed trials, are necessary to verify the results and provide strong evidence to inform GDM prevention strategies.