Introduction

The World Health Organization (WHO) defines “overweight and obesity” as an abnormal or excessive accumulation of fat that is potentially damaging to health1. Considering the increased obesity rate of the population which could causally contribute to the raised prevalence of obesity-related diseases (e.g. diabetes mellitus, hypertension, dyslipidemia, etc.) and its trend toward youth, the obesity issue has recently become more and more serious worldwide and thus deserves attention2,3,4. The Body Mass Index (BMI) is widely used to assess the degree of obesity in clinical practice. However, this indicator has limitations because it only considers the ratio of weight to height, ignoring the distribution and composition of body fat5. It is therefore necessary to introduce the concept of normal weight obesity (NWO) to facilitate a more comprehensive understanding of the complexity of obesity. NWO refers to those who have a BMI within the normal range (< 25 kg/m2) but have a higher body fat percentage (BF% > 30%). This condition is prevalent among South Asian communities and is linked to various metabolic disorders and potential health hazards6. Studies have shown that NWO is associated with a variety of health problems, including metabolic syndrome, cardiovascular disease, and diabetes7. As a special subtype of obesity, the risk of metabolic syndrome, cardiovascular disease and death in patients with NWO is similar to that of obese patients8, but with a different level in severity and prognosis9. Therefore, investigating NWO independently holds substantial scientific significance and clinical value. Through the identification and examination of NWO, a deeper comprehension of the mechanisms behind obesity-related illnesses can be achieved, leading to the formulation of more efficient preventive and therapeutic approaches10. To guide the clinical practice, it is necessary to differentiate the characteristics between NWO and obese populations in terms of disease phenotypes and regressions. Recently, studies have shown that NWO may increase the risk of cardiovascular and metabolic diseases by mediating the disruption of inflammatory pathways, such as increased levels of C-reactive protein (CRP) and interleukin 6 (IL6)11,12,13,14. Rakhmat et al.15 found that NWO may interfere with normal metabolic processes through insulin resistance, abnormal lipid metabolism and leptin secretion, which leads to abnormal activation of the RAAS system to affect the central nervous system indirectly16,17. The aforementioned mechanisms of metabolic abnormalities and neuroendocrine dysregulation involved in NWO share common pathways with the pathogenesis of psychiatric symptoms such as anxiety and depression. Hence, it is plausible to suggest that NWO could influence mental health conditions via analogous pathological pathways.

The relationship between obesity and mental illness is a complex and multifaceted topic. In recent years, researchers have increasingly focused on the interplay between the two and their underlying mechanisms. First, there is a bidirectional relationship between obesity and depression. Studies have shown that obesity may increase the risk of depression and vice versa18. Second, there is also an association between obesity and symptoms of anxiety and stress. Studies have found that difficulties in emotion regulation and body image problems in obese individuals may exacerbate symptoms of anxiety and stress19. The link between obesity and psychiatric symptoms may involve a variety of biological mechanisms, such as neuroinflammation, dysregulation of the hypothalamic–pituitary–adrenal axis, and dysfunction of the gut-brain axis18. Endocrine disruption frequently occurs alongside obesity, potentially resulting from the influence of hormones and cytokines (such as leptin, insulin, and lipocalin) released by adipose tissue on brain function. These hormones and cytokines can alter mood and behavior by affecting neurotransmitter release and neuronal activity, thereby increasing the risk of psychiatric disorders20. Obesity is strongly associated with metabolic disorders, especially with insulin resistance and metabolic syndrome. Studies have shown that there is a causal relationship between obesity and depression, and that obesity may directly contribute to depression through metabolic pathways21. The gut microbiota also plays an important role in the relationship between obesity and mental illness. Dysregulation of the gut microbiota may affect brain function through the gut-brain axis, contributing to alterations in mood and cognitive function. Studies have shown that obesity-associated dysbiosis of the gut microbiota may affect mental health by influencing the neuroendocrine and immune systems22,23. Chronic inflammation is another important connecting mechanism between obesity and mental illness. Obesity is often accompanied by a low-grade chronic inflammatory state, and this inflammatory state may contribute to the development of mental illness by influencing the inflammatory response of the central nervous system. Inflammatory factors such as C-reactive protein and cytokines can enter the brain through the blood-brain barrier and affect neuronal function and structure, which in turn affects mood and behavior24,25. Although the clear correlation between obesity and mental health has been established, the specific impact of the NWO on mental status remains an ongoing challenge. Therefore, it is necessary to further study the correlation between NWO and mental abnormalities and its influencing factors.

In summary, NWO, characterized by a normal BMI but high body fat percentage, poses significant health risks despite being difficult to detect. Its precise description of body fat percentage makes it an ideal indicator to reflect the real obesity state9. In recent years, although it has gradually attracted attention from the medical community, there is insufficient research on its correlation and potential mechanisms with mental health status. This study aims to understand the potential impact and related relationships of NWO on mental health status and further analyze its main influencing factors, in order to provide more research data support for a deeper understanding of the relationship between the two. This could not only improve clinical guidance and patient outcomes but also drive advancements in related clinical and basic research. The study anticipates uncovering the connections between NWO and mental health, as well as the biological mechanisms involved.

Methods

Research participants

The participants were primarily recruited from those who visited the Health Management (Physical Examination) Center of Peking University Third Hospital for health examinations between January 2019 and December 2022. Requirements for inclusion: (1) Age range: 18 to 70 years old; (2) BMI: 18.5–23.9 kg/m2; (3) Completion of body composition testing; (4) Completion of psychological evaluation. Criteria for exclusion: (1) being diagnosed with any of the following conditions: heart disease, hyperlipidemia, epilepsy, stroke, immune system disorders, metabolism-related diseases, central nervous system diseases, and any other chronic physical illness; (2) being on medications that may alter metabolism, such as immunosuppressants, hormones, special supplements, antipsychotics, etc.; (3) fulfilling the diagnostic requirements for any of the mental disorders related to ICD-10; (4) participating in physical fitness training; (5) having had an irregular life in the previous three months, including excessive drinking, binge drinking, overeating, etc.; and (6) pregnant women. In accordance with the Declaration of Helsinki, protocols involving human participants were reviewed and approved by the Institutional Ethics Committee of Peking University Third Hospital (project: M2023789). After being fully informed goals and methods of the study, all participants signed an informed consent form and consented to the use of the current health survey data, relevant physical exam results, and hematological results. A flowchart demonstrating the participants recruitment process is shown in Fig. 1.

Fig. 1
figure 1

Flow diagram of the study.

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Methods

  1. (1)

    A self-made questionnaire was used to gather general data about the participants, including age, gender, education, occupation, length of disease, prior medical history, and medication history.

  2. (2)

    The Inbody 770 Body Composition Analyzer (Biospace, Korea) was used to conduct the test. The staff input essential details including height, age and gender of the participant. Participants were instructed to wear light clothing, remove accessories, take off socks, stand barefoot on the sterilized metal electrodes of the CaviWipes, They were also directed to firmly grasp the metal handgrip, extend their arms to the sides without touching the armpits, and maintain an upright posture until the test concluded. The device measured key metrics including visceral fat area, body fat percentage, skeletal muscle and other indicators. Body mass index of 18.5 < ~ 24.9 kg/m2 was used as the standard range, and body fat percentage of ≥ 20% for men and ≥ 28% for women was classified as normal weight obesity group, and body fat percentage of < 20% for men and < 28% for women was classified as control group26.

  3. (3)

    Evaluation of mental health: The Symptom Checklist-90 (SCL-90) was used to gauge the patient’s psychosomatic status. The scale consists of 90 items divided into 10 factors: somatization, obsessive-compulsive, interpersonal sensitivity, depression, anxiety, hostility, photic anxiety, paranoid ideation, psychoticism, and additional items (mainly sleeping and eating conditions). A 5-point scale was used, with higher scores being associated with poorer health status. If the total score is ≥ 160 and/or any factor score is ≥ 2, the psychological assessment result of the participant is considered positive. Participants with the score over that of average were classified as psychologically abnormal ones based on the comparison of their SCL-90 total scores with those in the Chinese population’s health norms. Using the Stress Self-Assessment Questionnaire-53 (SSQ-53), the degree of psychological stress was evaluated. The scale contains a total of 53 questions assessing the level of stress experienced by an individual in the last month and is divided into five factors: overall stress, physiology, emotion, cognition and behavior(0-0.08 is classified as grade 1, 0.09–0.19 is classified as grade 2, 0.2–0.28 is classified as grade 3, 0.29–0.42 is classified as grade 4, 0.43–0.62 is classified as grade 5, 0.63–0.91 is classified as grade 6, 0.92–1.3 is classified as grade 7, 1.31–1.72 is classified as grade 8, 1.73-4 is classified as grade 9, and 1.73–4 points and ≥ 2 points for Question 38 or ≥ 3 points for Questions 36 or 51 is a grade 10), and grade 7 and above are considered stressful.

  4. (4)

    Hematological tests: The elbow venous blood of participants was collected and centrifuged for 15 min at 3000 r/min at 4 °C. An analysis of complete blood counts was conducted using SYSMEX XN-2000 Fully Automatic Hematology Analyzer. Blood biochemistry, including blood glucose, blood lipids, liver function, and kidney function, was analyzed using the Beckman Coulter AU5800 Fully Automatic Biochemical Analyzer. Thyroxine, triiodothyronine, free triiodothyronine, thyroid stimulating hormone, and other thyroid-related markers were measured using the Siemens Atellica Analyzer.

  5. (5)

    Statistical processing: All data were analyzed using SPSS 26.0 or R 4.5.1. The two independent samples t-test were used to compare the measurement data between groups that met the normal distribution in the general data, which was expressed as mean ± standard deviation (xˉ±s). The Mann Whitney U test was used to compare the measurement data that did not meet the normal distribution, which was expressed as M (Q1, Q3). The chi-square test was used to compare groups, and count data were reported as n (%). False Discovery Rate (FDR) correction was applied using the Benjamini–Hochberg procedure implemented in R software (version 4.5.1). Pearson’s or Spearman’s correlation analyses were used to examine the associations between somatization factor scores and each hematological parameter. The A-Test validation method was performed to cross-validate the robustness of correlation results, in accordance with the framework proposed by Gharehbaghi27 Specifically, the original time series was phase-randomized 1000 times while preserving the power spectrum, thereby generating a null distribution of correlation coefficients. The p-value was calculated as the proportion of absolute surrogate correlations that exceeded the observed value, which was used to exclude the possibility that the observed correlations were due to random chance .The p-value was calculated as the proportion of absolute surrogate correlations that exceeded the observed value. The relationship between NWO and somatization status was examined using binary logistic regression with adjustments for potential confounding variables. The statistical significance level was set as p-value < 0.05. For multiple comparison correction, the statistical level was set to FDR p-value < 0.10.

Results

Analysis of body composition indicators between the two groups

In this study, body composition indicators from 1181 participants with body mass index (BMI) between 18.5 < BMI ≤ 23.9 were gathered for comparative analysis. Of them, 824 participants (69.8%) fit the description of the NWO population (NWO group) with an average age of 40.82 ± 10.84 years old and a male-to-female ratio of 1: 1.575. The normal population (normal group) consisted of 357 participants (30.2%) with an average age of 37.48 ± 9.90 years old and a male-to-female ratio of 1: 1.767. There were significant differences in various body composition indicators such as body mass index, body fat, and protein between the two groups (Table 1).

Table 1 Body composition analysis of the two groups.
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Analysis of SCL-90 data between the two groups

With a total average score of 1.36 ± 0.35, the SCL-90 score of the NWO group was 122.79 ± 31.88. Of the positive items, 266 (32.3%) of the individuals had psychiatric symptoms. The total SCL-90 score of the normal group was 118.80 ± 28.10, with a mean of 1.32 ± 0.31. Of the positive items, 108 (30.3%) were associated with psychiatric symptoms. Compared with those of the normal group, the total SCL-90 score and the total average score of the NWO group were considerably higher (P < 0.05) (Table 2).

Based on their total scores, the patients were classified as psychologically healthy, mildly psychologically abnormal, moderately psychologically abnormal, and severely psychologically abnormal groups. The total scores of the individual groups were then compared to the normative scores of the Chinese population.

It was determined that there was a statistically significant difference between the NWO group and the normal group based on 558 cases of mental health, 141 cases of mild psychological abnormality, 116 cases of moderate psychological abnormality, and 9 cases of severe psychological abnormality in the NWO group.

Out of ten factors composed in SCL-90, The somatization factor scores in the NWO group were substantially higher than those in the normal group (FDR-adjusted P-value < 0.10). A significant proportion of somatization positivity was observed in 68 (8.3%) participants with positive somatization factor scores (factor score ≥ 2) when compared to those of the normal group (P = 0.033) (Table 2).

Analysis of psychological stress between the two groups

The Stress Self-Assessment Questionnaire-53 (SSQ-53) results showed that the NWO group had greater stress level, with significantly higher scores for both physiological and cognitive stress than those of the normal group (FDR-adjusted P-value < 0.10). It indicated that patients in the NWO group could experience more stress (Table 3).

Table 2 Results of SCL-90 in both groups.
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Table 3 Results of SSQ-53 in both groups.
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Correlation analysis of somatization symptoms and stress levels among NWO participants

Spearman correlation analysis showed correlation coefficients r-values of 0.685 (95% CI 0.642–0.723) and 0.679 (95% CI 0.635–0.718) respectively, with p-value of less than 0.001, indicating that this correlation was significant at the 0.01 level (two-tailed) (Table 4). There was a significant positive correlation between somatization factor scores and overall stress and its grades in participants with NWO. The p-value for the A-Test validation, an additional statistical test, was also < 0.001, cross-validating the conclusion of the main test.

Table 4 Correlation analysis of somatization factor scores and stress levels among NWO participants.
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Analysis of the individuals’ hematological test results between the two groups

Stress-related hematological indices such as the blood cells count, blood pressure level, glycated hemoglobin, total cholesterol, triglycerides, HDL-C, LDL-C, uric acid, ALT, thyroxine, triiodothyronine, and free Triiodothyronine levels of the NWO group were significantly higher than those of the normal group (P < 0.05) (Table 5), indicating that participants in the NWO group may be experiencing an elevated stress response.

Table 5 Analysis of the individuals’ hematological test results between the two groups.
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Correlation analysis of somatization factor scores and various hematology indicators among NWO participants

The results of the correlation analysis suggested that leukocytes, erythrocytes, hemoglobin, and ALT were negatively correlated with the somatization factor scores (all p-values < 0.01), as were AST, free thyroxine, and creatinine (all p-values < 0.05). LDL cholesterol was significantly and positively correlated with somatization factor scores (p-value = 0.032) (Table 6). The p-value from the A-Test validation in an additional statistical test also strongly supports the conclusion of the main test.

Table 6 Correlation analysis of somatization factor scores and various hematology indicators among NWO participants.
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Multiple conditional logistic regression

Based on the results of this correlation analysis, a binary logistic regression analysis was performed with whether or not the somatization factor was positive as the dependent variable, and age group, gender, marital status, the presence of NWO, leukocytes, erythrocytes, hemoglobin, LDL-C, creatinine, AST, ALT, and free thyroxine as the independent variables. The results indicated that the middle-aged and older group (≥ 45 years) was associated with higher odds of somatization symptoms than the younger group (< 45 years) (P-value = 0.047; OR 1.756; 95% CI 1.008–3.059). NWO are also independent factors associated with higher somatization symptoms cross-sectionally (P-value = 0.035; OR 1.882; 95% CI 1.047–3.384) (Table 7).

Table 7 Multiple logistic regression analysis of factors affecting mental health status.
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The Hosmer–Lemeshow test was used to evaluate the goodness-of-fit of the binary logistic regression model. The results showed that P-value was 0.692 (P-value > 0.05), indicating that the model fit was good and there were no significant differences between the predicted values and the actual observed values.

Discussion

It is important to study the mental health status among NWO patients since NWO more accurately represents true obesity status and can be easily neglected. To explore the mental health status of the NWO population and its influencing factors, this study recruited the physical examination population with body mass index in the range of 18.5–23.9 kg/m2 as the research participants. The two groups were categorized into NWO and normal groups as there were significant differences between the two groups in terms of key body composition indicators such as BMI and body fat. It found that the somatization factor scores in the NWO group were significantly higher than those in the control group (FDR-adjusted P-value < 0.10), as well as possessing higher scale total scores (122.79 ± 31.88) and scale total average scores (1.36 ± 0.35) in the Symptom Checklists-90 (SCL-90). It also showed that the NWO group had a greater stress level, with significantly higher scores for both physiological and cognitive stress than those of the normal group (FDR-adjusted P-value < 0.10). This suggests that compared with the normal group, the NWO group had more serious psychological stress abnormalities as well as being closely associated with psychiatric symptoms, especially somatization symptoms. Meanwhile, the results of the correlation analysis between somatization factor scores and stress levels among NWO participants suggested that there might be a correlation between psychiatric symptoms and stress levels in NWO participants. In this study, it was also found that the hematological indexes such as blood routine, blood glucose, blood lipids, liver and kidney functions, and thyroid hormones of the participants in the NWO group were all abnormal and statistically significant differences from those of the control group. In addition, based on the results of the correlation analysis between the somatization factor scores and the hematological indices, a multifactorial binary logistic multiple regression analysis with somatization symptoms as the dependent variable suggested that middle/older age and NWO are independent factors associated with higher somatization symptoms cross-sectionally.

Obesity has been confirmed to negatively affect mental health status through various mechanisms, such as promoting an abnormal inflammatory response, inducing metabolic disorders, or resulting in endocrine dysfunction28,29,30. However, the relationship of NWO and psychiatric symptoms and the underlying mechanisms were barely reported in the literature.

The SCL-90 scale is a self-assessment scale with good consistency and retesting reliability to be used for the evaluation of participants’ psychiatric symptoms, which is widely used in outpatient examinations for mental disorders and psychological illnesses31. The SCL-90 is divided into 10 factors (somatization, obsessive-compulsive, interpersonal sensitivity, depression, anxiety, hostility, photic anxiety, paranoid ideation, psychoticism, additional items (mainly sleeping and eating conditions))32. This study found that the somatization factor score (1.35 ± 0.42) of the NWO group were significantly higher than those of the normal group (FDR-adjusted P-value < 0.10). The proportion of somatization factors in the NWO group (8.3%) was approximately twice that of the control group (4.8%). This suggests that compared with the normal group, the NWO group had more serious psychological stress abnormalities as well as being closely associated with psychiatric symptoms, especially somatization symptoms. It also suggests that somatization symptoms such as headache, palpitations, and digestive discomfort might be typical symptoms of NWO participants.

Somatization is a psychological condition that patients suffer multiple physical symptoms without any identifiable physical pathology detected by the medical examination. It has been regarded as a form of manifestation under the effect of psychosocial stress and can be observed in depression, anxiety, hypochondria and dysthymia and other mental disorders. Characterized primarily by subjective physical complaints, somatization serves as a significant clinical indicator of underlying psychiatric disturbances. Several studies have been conducted in an attempt to unravel the complex relationship between obesity and somatization symptoms, involving multiple inflammatory and neuroendocrine mechanisms. First, visceral adipose tissue (VAT) is strongly associated with chronic systemic low-grade inflammation, and studies have shown that inflammatory markers such as C-reactive protein (CRP) and interleukin-6 (IL-6) play a mediating role in the connection between visceral obesity and cognitive control33. This implies that the inflammatory condition could worsen somatization symptoms by affecting cognitive function. In this study, the leukocyte counts—an indicator reflecting participants’ inflammatory levels—was significantly higher in the NWO group (5.68 ± 1.34) than in the control group. This suggests that NWO may also be associated with somatization symptoms through inflammatory mechanisms similar to those seen in obesity. In addition, obesity is associated with dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis. Previous study has shown that obese individuals exhibit reduced HPA axis adaptability under stress, as demonstrated by the persistence of the cortisol response and heightened IL-6 reactions. This may contribute to higher levels of inflammatory mediators, potentially worsening somatization symptoms34. Chronic elevation of cortisol, a major stress hormone, may contribute to redistribution of fat and increased accumulation of visceral fat, thereby exacerbating occult obesity. In addition, abnormal cortisol levels are associated with various mental health issues, including anxiety and depression, which often manifest as somatization symptoms35. In conclusion, it has been shown that obesity can affect somatization symptoms through multiple mechanisms, including enhanced inflammatory responses, dysfunction of the HPA axis, and abnormal hormonal regulation. The interaction of these mechanisms may provide a multifaceted biological basis for obesity-related somatization symptoms. However, the relationship between NWO, a subtype of obesity, and somatization symptoms remains insufficiently explored. Further research is needed to identify the mechanism of somatization symptoms in the NWO population, and the causal relationship between the NWO and somatization symptoms.

Psychological stress is an organism’s response to a stressor triggered by internal and external factors. The results of this study indicate that the NWO population have higher stress levels, which is similar to the research results reported by Rupal Kumar et al.36 in the obese population. The NWO population could be in a status of stress susceptibility or lack of a reasonable way to cope with stress. Meanwhile, higher stress levels may be associated with NWO status. Further research is needed to clarify the causal relationship between the NWO population and higher stress levels. Our findings indicate that the NWO group also exhibited significantly higher cognitive stress scores compared to those of the normal group. Previous studies have demonstrated an association between obesity and cognitive function, but the type and distribution of adipose tissue may exert distinct effects on brain health and cognition. For instance, the BARICO study revealed that omental white adipose tissue (oWAT) exhibits a stronger relationship with cerebral vascular health compared to subcutaneous white adipose tissue (scWAT), which holds significant implications for understanding the relationship between NWO and cognitive function37. Concurrently, mental reserve may exert a buffering and moderating effect on the obesity-cognitive function relationship. Mental reserve tests primarily assess an individual’s ability to cope with cognitive challenges. Although obesity correlates with reduced verbal ability, processing speed, and cognitive flexibility, this effect may be attenuated in individuals with higher mental reserve38. This suggests that mental reserve may mitigate obesity’s negative impact on cognitive function by providing cognitive protection. Future research should further explore these complex interactions to understand better the potential effects of NWO on cognitive function and its underlying mechanisms. Through correlation analysis, we found significant positive correlations between somatization factor scores and overall stress and its levels in patients with NWO (r-values were 0.685 and 0.679, respectively, FDR-adjusted P-value < 0.10). These results suggest that attention should be paid to the level of overall stress in the health management of patients with normal weight obesity, as this may significantly interact with somatization symptoms.

In this study, it was found that the hematological indexes such as blood routine, blood glucose, blood lipids, liver and kidney functions, and thyroid hormones of the participants in the NWO group were all abnormal and statistically significant differences from those of the control group. It indicated that NWO might be associated with mental health by influencing physiological stress responses. The leukocyte counts of the NWO group (5.68 ± 1.34) were significantly higher than those of the normal group, suggesting a higher level of inflammation in the NWO group. Inflammatory processes were closely related to the development of depression, which has elevated levels of inflammatory markers in the blood of Major Depressive Disorder (MDD) patients39,40. The targeting inflammation thus has been used as an important strategy for depression treatment41. Similarly, as a subtype of obesity, NWO may be associated with psychiatric symptoms through the inflammatory-related mechanism, a relationship that warrants further exploration. Previously, the relationship of obesity, hypertension and psychosomatic health has been evaluated in the research42,43 and obesity is likely to contribute to an increase in diastolic blood pressure in children44. Xu et al.45 reported an association between high blood pressure and an increased risk for the development of depression. Similar to obesity, abnormalities in triglyceride (1.02), High-density lipoprotein cholesterol (1.38 ± 0.31), and Low-density lipoprotein cholesterol (2.93 ± 0.73) levels were also significantly different in the NWO group with those in the control group46,47,48,49. The NWO status thus may also influence mental health by mediating lipid metabolism. Hyperuricemia may cause psychiatric symptoms such as anxiety and depression by activating the RAAS system50, and there is a positive correlation between the elevated level of blood uric acid and the severity of anxiety symptoms51. In this study, the blood uric acid level of the participants in the NWO group (313.67 ± 76.79) was also significantly higher than that of the control group, suggesting that there was a relationship between the blood uric acid level and NWO status. The free triiodothyronine levels were also significantly higher in the NWO group than those in the control group, and the elevated levels of thyroxine and triiodothyronine were statistically significantly different from those in the control group. Thyroid hormones have been previously proved to be a common physiological pathway involved in overweight and obesity to severe anxiety in patients with Major Depressive Disorder (MDD)52,53. It thus suggests that disruptions in the levels of thyroid-related hormones may be associated with unhealthy mental health states in the NWO participants. Psychological stress can contribute to cerebral cortex dysfunction and neurotransmitter imbalance, thus causing a decline in cognitive functions, such as memory and judgement, and changes in hematological indicators, such as blood glucose and thyroid hormone levels54. Meanwhile, the secretion of insulin antagonists (glucagon, catecholamines, glucocorticoids, etc.) is increased to induce the increased blood glucose55 as well as abnormalities of blood lipid levels56. The hyperactivity of the HPA axis is associated with obesity (especially abdominal obesity)57, whereas anxiety and depression are characterized by hyperactivity of the HPA axis and elevated cortisol secretion, which can be improved under the antidepressant treatment58. This study further suggests that NWO status is associated with abnormally elevated inflammatory responses, metabolic and endocrine dysregulation. It is plausible to hypothesize that NWO is associated with unhealthy mental health status, such as anxiety and depression, through mechanisms analogous to those observed in obesity, a connection that warrants further investigation. In summary, the abnormalities of stress-related hematological indicators suggest that participants in the NWO group may be experiencing an elevated stress response, and this low-level stress response may be closely associated with severe mental disorders. Most importantly, the abnormalities of laboratory indicators easily detected at an early stage are expected to be used as the biomarkers for the early identification of the NWO status and active intervention in the clinic. This may achieve advantages for early detection, diagnosis, treatment, and further improvement of the NWO secondary prevention.

As only the somatization factor points were statistically different between the NWO and non-NWO groups when univariate analyses were performed, we performed a correlation analysis between the somatization factor points and various indicators of hematology. The results suggest that decreases in leukocytes, erythrocytes, hemoglobin, AST, ALT, free thyroxine and creatinine, as well as elevations in LDL cholesterol, may be associated with higher somatization factor points. This indicates that abnormalities in these physiological markers should be considered in the assessment and management of somatization symptoms and interventions and treatments should be made accordingly. Based on the results of this correlation analysis, a binary logistic regression analysis was performed with whether the somatization factor was positive as the dependent variable, and statistically significant indicators were selected as the independent variables, the results suggested that middle/older age and NWO are independent factors associated with higher somatization symptoms cross-sectionally.

The interactive relationship between disturbed mental health status and obesity has been widely reported, but those regarding NWO are rarely specified. This study could provide the first line of evidence suggesting a close relationship between NWO and mental health status, evaluate physiological indexes under NWO status, and identify the influencing factors for NWO-related somatization symptoms. However, as a cross-sectional study, the causality of NWO and psychiatric symptoms could not be established yet. Further studies on the relationship between NWO and psychiatric symptoms are needed, which could improve the intervention at an early stage, secondary prevention and prognosis of NWO patients. While cross-sectional designs limit the ability to establish causality, our findings lay critical groundwork for future longitudinal investigations. Prospective studies tracking body composition changes alongside mental health trajectories could elucidate temporal relationships between NWO development and psychiatric symptom onset. This study selected a large comprehensive hospital in the region for research, with the examinee group covering different occupational categories and age groups, making its conclusions reasonably representative. It is worth noting that different geographic distributions and demographic backgrounds may also have potential impacts on the mental health of NWO participants. For example, in certain regions, dietary cultural influences may lead residents to prefer high-fat, high-sugar foods, thereby increasing their body fat content, which could further impact their mental health17. Concurrently, areas with lower socioeconomic status may lack sufficient health education and medical resources, making it difficult for residents to access care59. Conversely, close community ties and strong social support may help alleviate psychological stress and improve mental health outcomes60. Additionally, sociodemographic and lifestyle factors are closely associated with the occurrence of NWO. In a study involving 17,724 participants, researchers found significant associations between NWO and factors such as gender, age, marital status, smoking, and drinking habits61. For instance, males, those who were married or cohabiting, and high alcohol consumption were associated with higher BMI, while higher education levels and infrequent drinking were linked to lower BMI. These findings suggest that the development of NWO is not solely the result of physiological factors but is also influenced by complex social and behavioral factors. Each of these factors may impact the mental health status of individuals with NWO to varying degrees through distinct mechanisms. Therefore, future research should further explore the intricate relationships among these factors to develop more effective and precise mental health intervention strategies for NWO participants. Additionally, other hematological indicators not included in this study warrant further investigation. For instance, growing research in recent years suggests that iron metabolism abnormalities62 and vitamin D deficiency63 may be associated with various psychiatric symptoms. Studies have found that iron metabolism abnormalities may influence neurotransmitter levels, such as serotonin, thereby contributing to depressive symptoms and exacerbating cognitive impairment64,65. Vitamin D deficiency may induce a pro-inflammatory state, which appears to be related to the pathophysiology of depression66,67. Therefore, high-quality studies are still needed to further explore the relationship and specific mechanisms between other blood indicators—particularly serum iron and vitamin D levels—and psychosomatic symptoms like somatization in patients with NWO. Importantly, the observed connection between metabolic dysregulation and psychological distress highlights the necessity for greater integration between internal medicine and preventive psychiatry. Early detection of NWO through routine body composition analysis in clinical settings could enable multidisciplinary interventions targeting metabolic parameters and stress resilience. Such comprehensive strategies may be crucial in disrupting the potential associations between adiposity distribution and mental health deterioration, ultimately informing personalized prevention strategies for the NWO population.

Conclusions

The present study demonstrates that the NWO participants had higher stress levels and were highly associated with psychiatric symptoms, especially somatization. The results of the correlation analysis between somatization factor scores and stress levels among NWO patients suggested that there might be a correlation between psychiatric symptoms and stress levels in NWO participants. In addition, by analyzing the hematological indexes of the participants between the NWO group and the normal group, it shows that the participants in the NWO group had abnormalities in blood cells count, blood lipids, liver and kidney function and thyroid hormone levels. All were statistically significantly different from the control group, and these disturbances in response to inflammatory status, metabolic and endocrine level indicators also suggest that participants in the NWO group may be under the effect of heightened stress response, which could be closely associated with mental health problems. In addition, middle/older age and NWO are independent factors associated with higher somatization symptoms cross-sectionally.