Gestational diabetes mellitus (GDM), which is the most common metabolic disorder during pregnancy, is defined as glucose intolerance that is first recognized in pregnancy and affects around 5% of all pregnancies [1, 2, 3]. Although there is no consensus on the national guidance of GDM, pregnant women with indicative results of DM on the basic of diagnostic criteria of World Health Organization (WHO) (fasting glucose $\ge$7 mmol/L, random glucose $\ge$11.1 mmol/L, 2 h-75 g oral glucose tolerance test (OGTT), or HbA1c $\ge$48 mmol/mol) should be considered to be “diabetes in pregnancy” or appropriate management initiated [4]. It was estimated that almost one in every seven live births were affected by GDM all over the world in 2017, and approximately 85% of total 21.3 million live births overwide were affected by diabetes during the whole pregnancy [5]. GDM is associated with increased risk of a wide range of serious maternal, fetal, and neonatal adverse outcomes (e.g., miscarriage, macrosomia, shoulder dystocia, and stillbirth) [6, 7]. Therefore, screening and diagnosing GDM in early pregnancy could be effective to reverses hyperglycemia and reduce the risk of associated adverse pregnancy outcomes, and could be of great importance to optimize the health of women and their children.

Insulin resistance, which has been known as one of the key pathophysiological features observed in type 2 diabetes as well as GDM [8], was found to present prior to conception and persisted across pregnancy [9], and is suggested to play a vital role in the mechanism of GDM. The insulin sensitivity is defined by the reduced sensitivity by approximately 50–60% in late pregnancy as opposed to pre-pregnancy among pregnant woman with/without normal glucose tolerance [10]. Neutrophil gelatinase-associated lipocalin (NGAL), also named lipocalin-2, first identified as a matrix protein of specific granules of human neutrophils [11], was found to exert an effect on the regulation of insulin sensitivity and was associated with insulin resistance [12]. To further investigate the potential role of NGAL in the GDM development, a broad range of studies [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30] was conducted to evaluated the differences of circulating NGAL levels between GDM cases and healthy pregnant women. But large variations in the results were observed across studies, a little difference was obtained from studies by Sweeting AN et al. [28] and Wang YY et al. [29] compared to the relatively significant differences obtained from most studies [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. There is a need for more convincing pooled estimate, and further explorations for the source of underlying between-study heterogeneity that influences the concentrations of circulating NGAL.

Up-to-date, the most clinically and cost-effective methods of GDM screening remain controversial, and the ability to diagnose GDM in the first trimester compares to the second or third trimester of pregnancy keep inconsistent [31]. Thus, this meta-analysis was conducted to verify the circulating NGAL levels in those with GDM compared to healthy pregnant women, and potential influencing factors, which might be useful in providing more evidence to detect GDM in early pregnancy.

In this meta-analysis, we firstly pool the current evidence on the role of circulating NGAL in detection of GDM. Result from comparation between 1080 GDM cases and 1736 controls, our finding shows that the circulating NGAL levels in GDM patients are higher than that of healthy pregnant women, although significant heterogeneity of results was detected. Subgroup analyses further indicated that these differences were more significant in women with younger maternal age (30 years), as well as in studies comparing to not BMI-matched controls, but not in studies conducted among Caucasian population.

Over the last decades, the need and interest in the identification of molecules for the detection of disease onset and progression has wildly emerged. Since first identified in 1994, NGAL has become increasingly relevant as a biomarker in several diseases (e.g., acute kidney injury, Alzheimer’s disease, multiple sclerosis, cardiovascular diseases, depression, etc.) [12, 39]. By pooling the current evidence, our results were in chord with findings from previous studies mainly conducted in Chinese population [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30], showing that circulating NGAL levels were higher in GDM cases compared to healthy controls. For instance, in a cohort study involving 41 women with a singleton pregnancy, who developed GDM in the past 12 months, and healthy group of 82 normal pregnancies, the levels of circulating NGAL were significantly higher in those with GDM than that of control group [51.3 ng/mL (39.8–66.1) vs. 17.8 ng/mL (15.5–20.9); p 0.001] in the first trimester [13]. Furthermore, another study including 49 GDM subjects and 39 age-matched women with heathy pregnancies not only reported significantly higher level of serum NGAL in GDM women compared with the control in maternal blood (4.80 $\pm$ 1.99 vs. 3.66 $\pm$ 1.13, p = 0.001) and cord blood (4.70 $\pm$ 2.08 vs. 3.85 $\pm$ 1.44, p = 0.027), but also found positive correlation between NGAL levels and different clinical markers of insulin resistance (Pearson’s correlation coefficient or Spearman’s correlation coefficient ranged from 0.253 to 0.624, all p 0.05) [30]. Consistently, a Chinese study involving 260 pregnant women showed that, plasma NGAL concentrations tended to be statistically higher in women with GDM and normal pre-pregnancy BMI (group 2) and in those with GDM and over-weighted BMI (group 3) compared with healthy pregnant women with normal BMI (group 1) (p 0.001 for both group comparisons) [22]. All these evidence consistently suggested that plasma NGAL might play a significant role in the development of GDM.

Although the broad functions of NGAL have been described, including innate immune response (e.g., chelator of bacterial siderophores and anti-plasmodial regulator) [40], iron homeostasis (e.g., inductor of cell proliferation/cell differentiation and inductor of cell death ) [41], modulation of the inflammatory response (e.g., anti-inflammatory modulator and pro-inflammatory modulator) [11], the exact mechanism by which NGAL has potential role in the occurrence and development of GDM were still unclear. Agent-mediated decreases in NGAL concentrations significantly correlated with increases in insulin sensitivity [12]. Moreover, NGAL was found to promote insulin resistance in cultured adipocytes and hepatocytes [42]. According to the current evidence, NGAL has not only been proposed to be an iron delivery protein [41], but also been probably involved in the regulation of insulin sensitivity through the influence on iron homeostasis [39, 42]. During an oral glucose tolerance test, the GDM Serum ferritin levels might be independent predictors of 2-h glucose [43], and iron intake in healthy women has been found to be positively associated with higher risk of type 2 diabetes [44]. Consistent with this surmise that iron is required for the effect of NGAL on insulin action, the iron-free NGAL might be ineffective in causing insulin resistance in cultured hepatocytes [42]. Consequently, NGAL may play a role in the development of GDM through insulin resistance; still, whether the increase circulating NGAL levels contributes to GDM progression is not yet known.

Subgroup analyses indicated that the variation of circulating NGAL levels between GDM cases and healthy pregnant women increased when studies were performed in women with maternal age 30 years. As a modulator of the inflammatory response, NGAL involved in progression of inflammatory/anti-inflammatory states [11]. Relatively stronger immune function in young pregnant women could result in higher increased production of NGAL when exposed diverse pro-inflammatory stimuli. However, the inconsistency between studies might be partly ascribed to the difference in diagnostic tools and methods used in the detection of NGAL [45]. Besides, more significant difference in the levels of circulating NGAL was observed in BMI not matched studies, which could interpret by the interaction between NGAL and BMI. Circulating NGAL was a marker for obesity and its associated pathologies by providing both clinical and experimental evidence [12]. In addition, nutritional status among pregnant women might exert an effect on the result because Asian population tend to consume more carbohydrates (rice) in their diet than other populations and this might affect their insulin resistance [46]. Of note, heterogeneity of definitions for GDM [47], parity [48], gestational age at GDM diagnosis [49], and concurrent diseases (e.g., hypertension or hyperglycemia) [50] might exert great effects on the results. Hence, findings of our subgroup analyses still need to be further confirmed.

At present, GDM presents a particular public health challenge given its rapidly increasing prevalence in the context of the global obesity epidemic [51]. Screening and diagnosing GDM in early pregnancy are beneficial to reduce the burden of disease caused by GDM. Our study suggested that NGAL may be helpful for early identification of GDM, but this potential role needed more evidence to prove, further prospective studies are needed.

This meta-analysis had several limitations. Firstly, between-study heterogeneity was significantly assessed for overall estimate (I${}^{\mathrm{2}}$: 98.4%), but subgroup analyses showed that the main source of heterogeneity was not well described with high I${}^{\mathrm{2}}$(ranged from 87.2% to 99.1%) observed in all stratums. However, the significant heterogeneity in meta-analysis might be susceptible to the influence of bias, confounding, potential measurement error. Secondly, most included studies were conducted in Asian population, while only two studies measured circulating NGAL in Caucasian participants [13, 28], and no relevant studies were found for Africans. Moreover, subgroup analyses for Caucasians did not obtain any difference, probably due to the different measurement and small sample size. Thus, this finding should be interpreted with more care. Finally, all included studies were case-control studies with variations in matching method, and the bias was unavoidable.

Our findings imply that the expression levels of NGAL in patients with GDM were higher than in healthy pregnant women, indicating that NGAL might be a useful detecting index for the judgment of the occurrence of GDM. This suggests that NGAL should be valued at the GDM screening in order to improve the maternal and fetus health. However, more evidence from larger prospective studies are needed to demonstrate the exact role of circulating NGAL during the early pregnancy.

Study concept and design—LX and HH. Data extraction—ZC, and SYW. Data analysis—JCH and SYW. Manuscript drafting—ZC and HH. HH, JCH and LX contributed to the article revise. All authors gave final approval of the version to be published.

Not applicable.

We would like to express my gratitude to all those who helped me during the writing of this manuscript. Thanks to all the peer reviewers for their opinions and suggestions.

This research was supported by Ningbo Clinical Research Center for Digestive System Tumors (GrantNo.2019A21003).

The authors declare no conflict of interest.