Gestational diabetes mellitus (GDM) is the most prevalent medical illness in pregnancy and is defined as glucose intolerance of varying degrees, with onset or first detection during pregnancy (1). The average prevalence of GDM ranges from 9 to 30%, and up to 31.5% in some areas (2, 3). The prevalence of GDM has been progressively increasing due to changes in lifestyle and dietary structure (4). GDM carries significant short- and long-term health concerns for both mothers and their children. Mothers are at an increased risk of adverse pregnancy outcomes, such as premature rupture of membranes, infection, preterm labor, gestational hypertension, pre-eclampsia, excess amniotic fluid, and cesarean section; in severe cases, they may suffer ketoacidosis and have a lifetime risk of type 2 diabetes mellitus (T2DM), which is up to 20 times higher than that in normal pregnant women (5, 6). The offspring may have a significantly increased risk of hypoglycemia, macrosomia, neonatal epigenetic alterations, neonatal respiratory distress syndrome, and in severe cases, they may suffer high risk of fetal death (5, 6). In addition, offspring will carry a lifetime risk of obesity, and T2DM and metabolic syndrome are more common for them (7, 8). As a result, it is crucial for health care providers to work with pregnant women to prevent the development of GDM.

GDM has many influencing factors, including race or ethnicity, family history of diabetes mellitus, age at delivery, obesity, overweight, and lack of exercise (9); dietary factors also play an important role in its development (10). Diet quality is defined as the degree of adherence to dietary patterns recommended in dietary guidelines or indicators of a varied diet (11). In contrast to single food or nutrient intake, diet quality has been demonstrated to be a reasonable and important measure of total nutritional intake in several studies (12–14), and is a promising tool for examining the relationship between overall diet and diseases (15). Therefore, high diet quality reflects the achievement of more optimal nutrient intake profiles and a lower risk of diet-related non-communicable diseases (including T2DM) (16). A higher-quality diet is an important protective factor for diabetes (17) and is negatively associated with fasting glucose and glycated hemoglobin in adults with T2DM (18). A high-quality diet during pregnancy can help to decrease the risk of pathoglycemia, hypertension, pre-eclampsia, and excessive weight gain (19, 20); poor diet quality increases the risk of preterm birth, neonatal intensive care unit admissions, small for gestational age babies, low birth weight, and congenital heart defects (21, 22). However, the role of diet quality in the risk of GDM development has not been systematically evaluated. In addition, studies have found that the quality of a woman's diet does not change significantly before or during pregnancy (23). Schwingshackl et al. (24) encourages all women of childbearing age to adopt healthier eating behaviors, even before they become pregnant. Therefore, this study aimed to systematically review the available evidence regarding the relationship between diet quality and GDM before or during pregnancy.

The study protocol was registered in PROSPERO (no.: CRD42022372488; https://www.crd.york.ac.uk/PROSPERO/).

This study provides a systematic review and summary of the existing literature on the relationship between diet quality and GDM risk. The results, which included 19 studies (108,084 participants) with a total of eight diets, demonstrated that higher quality diet (MD, DASH diet, AHEI, PDI, or adherence to national dietary guidelines) before or during pregnancy reduced the risk of GDM, whereas poorer diet quality (higher DII or LCD diet) was associated with a high risk of GDM.

Although the exact molecular mechanism remains to be elucidated, the results of this study are biologically plausible. As shown in Table 5, high diet quality was mainly characterized by a higher intake of fruits, vegetables, legumes, and whole grains and a lower intake of red meat, processed meat, and trans fats. Vegetables and fruits are rich in antioxidants, fiber, polyunsaturated fatty acids, and micronutrients that can reduce glucose absorption, increase insulin secretion, and improve insulin sensitivity to assist glucose metabolism (49). Whole grain foods provide more nutrients, fiber, and phytochemicals, which serve to increase satiety, prolong the time for food to go through the digestive system, promote gut health, and reduce the glycemic response (50, 51). Vegetables, fruits, and whole grains can be directly or indirectly involved in the management of intestinal inflammation by altering intestinal flora and reducing the systemic inflammatory response (52). In contrast, red and processed meats are rich in saturated fat, hemoglobin, iron, nitrosamines, and other compounds associated with β-cell destruction, oxidative stress, insulin resistance, and GDM (53). In addition, such foods promote inflammation, alter cellular metabolic processes in the adipose tissue, liver, and pancreas, and increase the inflammatory response in GDM (54, 55).

Studies have shown that dietary patterns (i.e., increased intake of higher-quality foods and reduced intake of poor-quality foods) are associated with a lower risk of GDM before and/or during pregnancy (56). Healthier eating patterns like the MD, DASH and AHEI diets can lower the risk of GDM by 15–38% (57). In contrast, a diet that is high in added sugars and organ meats, and low in fruit, vegetables, and seafood (51), a low-carbohydrate pre-pregnancy diet (58), and non-compliance with national dietary guidelines (19) were associated with a higher risk of GDM. Our findings are similar. Research on food for the prevention of GDM has received a lot of attention and has shown some promise. However, studies have shown that pre-pregnant and pregnant women may not meet the minimum dietary recommendations (59); the adherence to all food types during pregnancy even decreased (60). In conclusion, although women of childbearing age and pregnant women were given varied nutritional or eating advice, there were still grounds for concern regarding the actual quality of their diet. Yu et al. (61) recommended that health practitioners or policymakers should tailor strategies to the quality level of women's diets.

In contrast to dietary patterns, dietary quality assessment can combine large amounts of dietary data into a practical dietary indicator, thereby increasing the feasibility of translating food intake into daily food consumption and providing visualization of the intake of different food groups (62). Studies have shown that stress in women of childbearing age is inversely associated with poor diet quality (63). Borge et al. (64) found a positive association between better maternal diet quality during pregnancy and functioning of the child. A high dietary quality is a strong predictor of chronic diseases (all-cause mortality, cardiovascular disease, and T2DM) (65, 66). However, only a few studies have examined the relationship between diet quality and GDM. This systematic research discovered that high quality diet reduced the risk of developing GDM. As a result, assessment of diet quality can have the potential to be a quick and easy way to screen for dietary habits associated with GDM before or during early pregnancy.

Donazar-Ezcurra et al. (67) found that, compared to pre-pregnancy, healthy dietary measures adopted during pregnancy seem to be ineffective because they require more time to properly curb the development of GDM. However, similar to the results of a previous systematic review (19, 68), we found that high quality diet before and during pregnancy was beneficial for preventing GDM. At present, the well-documented risk factors for GDM include advanced maternal age, family history of diabetes, having a macrosomic baby, non-Caucasian race/ethnicity, being overweight or obese, and cigarette smoking (69). This study found that the impacts of adjustment variables such as family history of diabetes, socioeconomic status, physical activity, and gravidity on the risk of developing GDM should be considered when systematically evaluating outcome analysis.

Some limitations of this meta-analysis should be considered. First, studies were observational, making causal inferences difficult. Although the studies adjusted for some confounders, the possibility of residual confounding cannot be ruled out completely. Most studies accounted for maternal age, BMI, and family history of diabetes; however, most of the included studies did not adjust for GWG, socioeconomic or alcohol status, previous macrosomia, or polycystic ovary syndrome, which may be important risk factors for GDM (70) (Supplementary Figure 1). Secondly, the most studied countries (61%) were the United States and China, where populations have different dietary habits. Although many studies assessed dietary intake with validated measurement tools (e.g., FFQ), these dietary data were self-reported. Additionally, the timing of the dietary data assessment was heterogeneous. We did not know how much time elapsed between the assessment of diet quality and the diagnosis of GDM; in some studies, diet was assessed years before pregnancy, and in others, it was assessed during pregnancy. Finally, no evidence of publication bias based on Egger's test was found in this meta-analysis.

We suggest the following recommendations for future studies. During the sampling and survey phases, studies should be conducted in diverse populations with varying racial/ethnic backgrounds, socioeconomic status, BMI ranges, and diet culture. Research should improve the collection methods of food intake, which can combine contemporary Internet technology such as applets and real-time recording, to capture the complexity of dietary habits more accurately. During the design and analysis phase, appropriate analytical methods should be used, and adjustments for covariates should be demonstrated through causal considerations and graphs, particularly for physical activity, gravidity, and socioeconomic status. Comparability studies should be improved by increasing the uniformity of the timing of dietary assessments and the outcomes measured. When possible, adequately powered randomized controlled trials should be conducted to support better causal inferences. In addition, one should investigate what are the micronutrient levels in women of childbearing age or in pregnant women with high or poor dietary quality.

In conclusion, positive association between better maternal diet quality during pregnancy and functioning of the child. However, only a few studies have examined the relationship between diet quality and GDM. This study found that higher diet quality (MD, DASH diet, AHEI, PDI, or adherence to national dietary guidelines) before or during pregnancy reduced the prevalence of GDM; while poorer diet quality (higher DII or LCD) increased the risk of developing GDM. And then, the assessment of diet quality can have the potential to be a quick and easy way to screen for dietary habits associated with GDM before or during early pregnancy. Further studies are necessary to ascertain the relationship between food quality and GDM.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

XG, QZ, and XJ conceived and designed the experiments. XG and QZ performed the experiments and wrote the paper. XG, YP, and YL analyzed the data. XC contributed materials and analysis tools. All authors read and approved the final manuscript prior to submission.