In the current study, we found that fasting GLP-1 levels were higher in GDM patients compared to those with normal glucose tolerance. However, this conclusion is based on a single measurement and may not reflect changes throughout pregnancy. Furthermore, GLP-2 and GIP levels showed a significant correlation in the study population investigated.

Following the demonstration for nearly five decades that oral glucose ingestion releases more insulin than intravenous glucose ingestion, efforts to identify incretin peptides began [19]. The discovery of GIP was the first to show that glucose-stimulated insulin release was markedly potentiated. However, as the evidence increased, it was shown that the incretin effect was partially attenuated even after immunoneutralization of GIP, indicating that GIP is not the only intestinal-derived incretin [20, 21].

Incretins have gained importance after the favorable effects of procedures in bariatric surgery on glucose homeostasis and the development of metabolic syndrome [22, 23]. The incretin effect is of great importance for normal glucose tolerance [23]. However, the exact roles and importance of gut-derived peptides under physiological conditions are still controversial. Initial studies focused only on GIP and GLP-1, but nowadays the effects of other derivative peptides are also of interest.

Decreased fasting GIP and GLP-1 levels have been associated with worsening fasting glucose levels in patients with type 2 DM and it has been suggested that basal GIP and GLP-1 may be useful in the early diagnosis of type 2 DM [24]. Although the pathophysiology of decreased GLP-1 release is still controversial, long-term type 2 diabetes and inadequate glycemic control seem to be associated with low GLP-1 responses, while studies evaluating the effects of GIP and GLP-1 in the development of gestational diabetes mellitus present conflicting results [25, 26].

In the literature, some studies indicate that fasting levels of both GLP-1 and GIP are lower in patients with gestational diabetes mellitus (GDM), with low GLP-1 levels being associated with a higher risk of developing GDM [13, 27]. Specifically, research has shown that Asian-Indian women with GDM exhibit high insulin resistance, islet cell dysfunction, and low fasting GLP-1 levels. Incretin axis dysfunction may contribute to this islet cell dysfunction [28]. Early diagnosis, multidisciplinary care, and tailored management with optimal glycemic control are linked to a significant reduction in pregnancy complications as well as long-term consequences for both the mother and the offspring [29].

However, while some studies have reported increased GLP-1 and GIP secretion during oral glucose tolerance tests (OGTT) in women with GDM [15], other research suggests that GLP-1 and GIP levels are not impaired during OGTT in these patients [12]. The discrepancy in results has been attributed to differences in sample sizes used in these studies [12, 13, 30]. In our study, basal GLP-1 levels were found to be elevated in patients with GDM. GLP-1 demonstrated high sensitivity (70%) and specificity (56%) in identifying GDM.

Some human studies on obesity have shown increased levels of GLP-1 and to a lesser degree GIP after diet-induced weight loss, but not all studies showed this effect [23, 31]. GDM and obesity are associated with poor postprandial gut hormone response, impaired satiety, and weight gain [32]; GLP-1 has been implicated in the regulation of blood glucose elevation and insulin resistance in pregnancy, suggesting a role in maternal metabolism and weight gain [27].

In the literature, a study conducted in non-pregnant obese diabetic patients showed impaired insulinotropic effect of GIP and suggested that this impairment may be an important factor in the pathophysiology of GDM [33]. In our study, we think that GLP-1, GLP-2, and GIP are associated with the pathophysiology of GDM.

Based on the available data, changes in incretin levels may play a role in the compensation of insulin resistance in pregnancy. Assessing GLP-1 function in GDM may be an early detection parameter that can be used to plan appropriate follow-up and treatment timing. However, there is only one recent study by Krystynik et al. (2023), no significant differences were found in fasting GLP-1 and GIP concentrations or their AUC between pregnant women with impaired fasting plasma glucose, despite significant variations in BMI values among the groups [34]. Our study showed that basal GLP-1 concentrations were higher in GDM patients especially in obese group. We believe that our study, being the first in its field to differentiate between obese and non-obese pregnant women in terms of GDM presence, is more valuable and will guide future research.

The great interest attracted by GLP-1 and GIP as potent incretins has partially overshadowed efforts to understand the importance of other proglucagon-derived peptides. In contrast to the insulinotropic effects of GLP-1, GLP-2 increases glucagon levels in fasting and postprandial states [35], the main effect of GLP-2 is to reduce intestinal permeability, enhance barrier function, and improve digestion and absorption of nutrients by inhibiting gastrointestinal motility [36].

In the literature, it has been shown that there is a correlation between insulin resistance and GLP-2 secretion in obese individuals [37], while later studies have suggested that it may be the cause of insulin resistance in healthy and diabetic individuals due to its glucagonotropic effect [38, 39]. In addition, animal studies have shown that GLP-2 secretion in the ileum decreases under diabetic conditions [40]. Therefore, it is thought that decreased GLP-2 production may lead to the development of diabetes. The fact that plasma GLP-2 concentrations are higher in obese patients without type 2 diabetes compared to healthy people suggests that GLP-2 functions as a protective factor against glucose metabolism disorder that develops in obesity [21].

The fact that plasma GLP-2 concentrations are higher in obese patients without type 2 diabetes compared to healthy people suggests that GLP-2 functions as a protective factor against glucose metabolism disorder that develops in obesity [21]. In a single and recent study examining the relationship between GLP-2 and GDM, it was suggested that GLP-2 may be associated with weight gain during pregnancy [41], while in our study, GLP-2 was found to be significantly associated with the development of GDM risk. However, the sensitivity and specificity of GLP-2 in the development of GDM were found to be low.

Incretin test results can be influenced by fasting conditions. In a study during the second trimester, postprandial GLP-1 and GIP concentrations were compared between women with gestational diabetes mellitus (GDM) and normal glucose tolerance. Women with GDM had approximately 20% higher postload GLP-1 and GIP concentrations, independent of age, BMI, and gestational age [15]. The increase in GLP-1 was specifically associated with insulin secretion only in women with GDM. Postprandial GLP-1 levels were negatively correlated with birth weight. These findings suggest that women with GDM have elevated postprandial GLP-1 and GIP concentrations, potentially related to insulin secretion. Furthermore, there is a potential role of GLP-1 in fetal growth regulation [15]. When O’Malley et al. compared fasting GLP-1 and GIP measurements in the second trimester according to the presence of obesity, they found that GLP-1 was significantly higher in those with obesity at the first visit, but this difference was not observed in the GDM groups [42]. However, in the study of Bonde et al., which involved participants in the second trimester and postpartum period, the researchers compared postprandial GLP-1 responses between patients with gestational diabetes mellitus (GDM) and their postpartum levels. The results showed that pregnancy was associated with decreased postprandial GLP-1 responses, and patients with GDM had even lower GLP-1 responses compared to their postpartum levels [43]. Another study comparing fasting and postprandial GLP-1 concentrations with plasma insulin and glucose levels found that fasting GLP-1 concentrations were associated with insulin levels, but not with glucose levels. During the oral glucose tolerance test (OGTT), GLP-1 concentrations were associated with insulin levels, but not with glucose levels. No significant associations were found between GIP concentrations and insulin or glucose levels. Additionally, BMI at the time of the OGTT was positively correlated with fasting GLP-1 concentrations. These findings highlight the relationship between GLP-1, insulin, and BMI in glucose metabolism during the second trimester of pregnancy [11].

Overall, the studies provide insights into the concentrations of GLP-1 and GIP in pregnant women with impaired glucose metabolism. While some studies observed significant differences in GLP-1 concentrations between GDM and non-GDM groups, others did not find significant differences. The association between GLP-1 concentrations and insulin or glucose levels varied across studies. Additionally, BMI was found to be associated with GLP-1 concentrations in some studies. However, further research is needed to establish the exact mechanisms and clinical implications of these associations.

An important advantage of our study is that neither the pregnant women nor the research team were aware of GDM status at the time of serum sampling, which minimizes selection bias. Another strength of the study is that very careful sampling was performed to accurately optimize the concentrations of GLP-1,2 and GIP and the results are potentially reproducible.

This study highlights the potential significance of incretin hormones, particularly GLP-1, in the pathophysiology of gestational diabetes mellitus (GDM). Our findings suggest that GLP-1 could be used as a potential biomarker for the diagnosis of GDM. The observed increase in GLP-1 levels in pregnant women with GDM indicate that this hormone may serve as a clinical tool for early diagnosis or management of the condition. Furthermore, investigating GLP-1-targeted therapies in women with GDM may open new avenues for disease management.

This study has some limitations that need to be taken into account. First, the patient population is small, primarily due to the high costs of the tests used. In addition, population matching in terms of age, sex, and BMI is not perfect. Since our study was conducted in the mid-trimester when insulin resistance becomes evident, the lack of evaluation of incretin in the postpartum period limits the power of the study. Another limitation is that there was no measurement of incretin hormones earlier in the pregnancy, which might also affect individual levels as pregnancy progresses. In this study, GLP-1,2 and GIP responses after oral glucose loading were not evaluated when comparing normal glucose tolerance and GDM in pregnant women. The baseline measurement of incretin concentrations may not be the optimal time to assess incretin function, which limits this study. Incretin hormones are also more expensive to use as a screenin/diagnostic tool than the OGTT test, but the test may become cheaper in the future and could be used in patients who do not want or cannot tolerate the test.

Incretins play an important role in glucose homeostasis by reducing insulin resistance and increasing insulin release. New evidence emphasizes the importance of incretins in improving glycemic control and insulin sensitivity, especially in conditions associated with obesity. To date, results from human studies are conflicting.