Diabetes Mellitus (DM) is one of the fastest growing global chronic diseases of the 21st century [1]. DM is an elevated plasma glucose level due to insufficient insulin secretion and insulin resistance, and is categorized as Type 1 diabetes mellitus, Type 2 diabetes mellitus, gestational diabetes mellitus, and specific types of diabetes mellitus [2]. Gestational Diabetes Mellitus (GDM) is defined by the American Diabetes Association (ADA) as a condition in which diabetes mellitus is diagnosed for the first time in a pregnant woman in the mid- or late-stage of her pregnancy and was not clearly present before the pregnancy. Gestational diabetes mellitus (GDM) is defined by the American Diabetes Association (ADA) as "diabetes that is first diagnosed in a pregnant woman in the middle or late stages of pregnancy and that did not exist before pregnancy [3]. Diabetes Mellitus Type 2 (T2DM) is a progressive metabolic disease characterized by pancreatic beta-cell dysfunction and peripheral insulin resistance, resulting in defective glucose metabolism and chronic low-grade inflammation [4]. Although the etiology and course of GDM and T2GM are not identical, they are closely related and causative of each other. GDM increases the incidence of T2DM in both the pregnant woman herself and the fetus in adulthood. The risk of developing diabetes or prediabetes is at least seven times higher than in unexposed adolescents [5].

Various metals have very important effects in the human body, and their concentration in the human body, either too much or too little, can have a disturbing effect on normal physiological and biochemical functions of the human body. Metals such as Cd, As, and Pb have endocrine disrupting effects that are associated with an increased incidence of T2DM [6]. Exposure to these heavy metals during pregnancy may affect sensitive hormone levels and increase the risk of future progression from GDM to T2DM.

Neuropathy due to GDM or T2DM has become the most prevalent disease in all types of diabetes mellitus. Elevated blood glucose in pregnant women during GDM not only triggers various types of neuropathy but also directly or indirectly affects the neurodevelopment of the fetus or newborn [7]. One of the common clinical complications of T2DM is peripheral neuropathy, dorsal root and sympathetic ganglion lesions in peripheral nervous system and endothelium caused by hyperglycemia [8]. The neurologic changes associated with both GDM and T2DM are closely related to metabolic disorders that result in hyperglycemia, which affects oxidative stress, neurotrophic factor activity, and neurotransmitter transmission [9].

Oxidative stress may be increased in many psychiatric disorders such as depression, anxiety disorders and neurodegenerative changes [10]. The central nervous system is characterized by high oxygen consumption and low antioxidant defense activity, and the body scavenges excess reactive oxygen species from the body under normal conditions to maintain its redox balance. In a high-sugar environment, it affects the elevation or reduction of neurotransmitter metabolites and influences one's mood and mental state [10].

Metabolomics is capable of characterizing normal physiological and pathological states of biological systems, can provide unique insights into how multiple biomolecules interact under specific conditions to aid in the development of biomarkers for complex metabolic disorders (e.g., GDM and T2DM), and can provide insights into disease mechanisms by monitoring differences in metabolite abundance and/or characteristics of patients [11].

In view of the fact that the pathogenesis of both GDM and T2DM is closely related to various metals in the body, and that changes in the concentrations of exposure to these metals can trigger neuropathy via the DNA damage pathway caused by the accumulation of ROS, and ultimately lead to changes in metabolomics in the body, the present study was conducted to detect the relevant indexes for the populations of GDM and T2DM, and the metabolites of the neurotransmitters in the urine of the four selected substances were vanillylmandelic acid (VM-A), 5-Hydroxyindole Acetic Acid (5-HIAA), 3,4-Dihydroxyphenylacetic Acid (DOPAC), Hom vanillic Acid (HVA); heavy metals Cd, Sr, Hg, and Pb, and trace elements Mn, Fe, Mg, Cu, Mo, Se, Sn, Al, and As; and the DNA oxidative damage marker hydroxy-2-deoxyguanosine (8-OH-dG) were used as the detection indicators as well as for metabolomics studies to analyze the associated risk factors and biomarkers for the prediction of hyperglycemia in the hyperglycemic population. The aim is to provide an early diagnostic indicator and construct a predictive strategy for neurological damage in patients with GDM or T2DM.

Participants in the case and control groups were determined according to the diagnostic criteria of the Guidelines for the Management of Gestational Diabetes Mellitus (2021).

(1) Pregnant women with GDM who gave informed consent to participate in this study and pregnant women with normal blood glucose whose prenatal fasting blood glucose and biochemical routine indexes met the diagnosis and treatment criteria of the Guidelines for the Diagnosis and Treatment of Diabetes Mellitus Complicated in Pregnancy (2021). (2) Mothers had complete questionnaires, blood and biochemical routine data. (3) Maternal age is between 24 and 48 years old. (4) No defects, chromosomal disorders, or genetic metabolic diseases at birth

Those with one of the following conditions were excluded from the study. (1) gestational age greater than 42 weeks; (2) development of a disease that interferes with normal growth and metabolism during follow-up; (3) lack of information about the participants; (4) mothers who had polycystic ovary syndrome with glucose intolerance abnormalities or type 1 or type 2 diabetes prior to pregnancy.

(1) Participants with T2DM and normal blood glucose who gave informed consent to participate in this study, and whose fasting blood glucose biochemical routine indexes met the diagnostic and therapeutic criteria of the "Guidelines for the Diagnosis and Treatment of Diabetes Mellitus Comorbid with Pregnancy (2021)". (2) Participants had complete questionnaires, blood and biochemical routine related data. (3) No defects, chromosomal disorders, and genetic metabolic disorders

Persons with one of the following conditions were excluded from this study. (1) the presence of a disease that interferes with normal growth and metabolism during the follow-up period; (2) lack of information about the participants.

Inclusion and exclusion criteria were established for both groups and the questionnaire administrators were trained. The urine collectors and laboratory reagents used were from the same batch. Measuring instruments and conditions were the same, and quality control was done at each sample test to ensure test quality and recall.

Urine samples were collected from pregnant women one week before delivery and from non-pregnant women during hospitalization. Collected urine samples (≥5 ml) were sealed in polyethylene centrifuge tubes (15 ml) and stored at −20°C. All samples were transported to the laboratory refrigerator at −80°C within 12 hours.