Leptin as a marker of metabolic disorders in comorbid patients with arterial hypertension

Introduction

Comorbid progression of arterial hypertension (AH) in combination with obesity (OB) and type 2 diabetes mellitus (DM) represents one of the major challenges in modern medicine, contributing to early target organ damage, increased cardiovascular risk, and premature mortality. According to the European Society of Cardiology [1] and the American Diabetes Association [2], this combination of diseases falls under the concept of high cardiometabolic risk and requires an interdisciplinary approach to risk stratification and therapeutic correction. Statistical data from the Centers for Disease Control and Prevention [3] also indicate that patients with AH and T2DM against a background of OB have twice the risk of complications and death compared to those with isolated AH.

A key role in the pathogenesis of this comorbidity is played by adipokine dysfunction — hormones produced by adipose tissue — among which leptin is primary. Leptin is synthesised by adipocytes and regulates energy metabolism, insulin secretion, appetite, sympathoadrenal activity, and participates in inflammation and vascular remodelling. In obesity, hyperleptinaemia is observed without the corresponding biological effect due to the development of leptin resistance — a phenomenon extensively described in the review by A. Engin [4], where the author discusses key mechanisms of impaired leptin signalling, including increased Suppressor of Cytokine Signaling 3, hypothalamic inflammation, and decreased leptin transport across the blood-brain barrier. Similar disruptions are confirmed in the work by J. Liu et al. [5], which describes molecular pathways (JAK2/STAT3, PI3K/Akt) that become ineffective in chronic leptin excess. Leptin overload in the context of leptin resistance is associated with impaired glycaemic control, hyperinsulinaemia, e­levated blood pressure, and myocardial remodelling, as demonstrated in the study by M.S. Poetsch et al. [6], where leptin is considered a factor in cardiovascular dysfunction due to its influence on myocardial hypertrophy and sympathoadrenal activation. Q. Li et al. [7], using a zebrafish model, proved the involvement of the leptin receptor in ventricular regeneration processes, indirectly confirming leptin’s role in cardiac tissue remodelling. Thus, accumulated literature data suggest that leptin is not only a marker of excess fat mass but also an active mediator of metabolic and cardiac maladaptation in the comorbidity of AH, T2DM, and OB.

Ukrainian authors also highlight the importance of leptin as a predictor of metabolic maladaptation. In particular, according to I.V. Cherniavska et al. [8], elevated leptin levels in patients with AH, OB, and T2DM are accompanied by higher body mass index (BMI), increased HbA1c, reduced 25(OH)D levels, and pronounced insulin resistance. This allows leptin to be considered not only a pathogenic factor but also a potential biomarker of the metabolic phenotype. Findings from the study by M.M. Schurko et al. [9] showed that obesity is associated with increased leptin levels, which in turn exacerbate insulin resistance and may act as a trigger for the development of ischaemic heart disease (IHD). Patients with both IHD and metabolic syndrome showed more significant disturbances in carbohydrate and lipid metabolism, indicating more severe disease progression in the presence of metabolic syndrome. In summary, leptin resistance may be a key pathogenic link in the development of insulin resistance, obesity, and, consequently, IHD.

Modern literature pays considerable attention to the relationship between leptin and vitamin D deficiency. The study by M. Obradovic et al. [10] demonstrates a negative correlation between 25(OH)D and leptin levels, explained by the deposition of fat-soluble vitamins in adipose tissue and reduced bioavailability in hyperleptinaemia conditions. Similar results are presented in the work by S.C. Lu, A.O. Akanji [11], who emphasised that high leptin levels are associated with reduced vitamin D levels in patients with obesity and metabolic syndrome. The latest experimental data in the study by A. Birukov et al. [12] confirm that leptin exerts an inhibitory effect on insulin secretion by β-cells and regulates insulin receptor expression in peripheral tissues. This deepens insulin resistance, creating a pathological feedback loop that accelerates T2DM development against the background of OB and AH.

Summarising current scientific evidence, it should be noted that leptin is considered a key component of the cardiometabolic phenotype. Its increase in patients with AH, T2DM, and OB reflects the severity of disorders in carbohydrate, lipid, and endocrine metabolism, which justifies its use as a prognostic marker. At the same time, the significant variability of findings across populations and insufficient standardisation of measurement methods require further clinical-analytical studies. Therefore, it is highly relevant to study leptin levels as an integral predictor of metabolic maladaptation in patients with AH in combination with OB and T2DM. The aim of this study was to determine the characteristics of leptin levels in patients with AH depending on the presen­ce of comorbid conditions — obesity and type 2 DM — and to identify associations between leptin concentration and indicators of carbohydrate metabolism, insulin resistance, 25(OH)D levels, and body mass index.

Materials and methods

A comprehensive study was carried out in accordance with GCP [13] and GLP [14] standards, the ethical and legal principles of the Declaration of Helsinki [15], the Convention on Human Rights and Biomedicine [16], and with the approva­l of the Ethics and Bioethics Committee of Kharkiv National Medical University (Protocol No. 4, dated 7 April 2025). Participation in the study was voluntary, and all patients were informed about potential risks and signed an informed consent form for participation and publication of the study results. Confidentiality, anonymity, and protection from any forms of discrimination or coercion were guaranteed to all participants.

The study was conducted at the outpatient department of the State Institution «L.T. Mala National Institute of Therapy of the National Academy of Medical Sciences of Ukraine» between 2021 and 2024, involving 250 patients with AH with a mean age of 55.26±8 years. The participants were divided into four groups: group 1 included patients with isolated AH (n=49); group 2 — patients with AH and obesity (n=62); group 3 — patients with AH and comorbid type 2 DM (n=77); and group 4 — patients with combined AH, DM, and obesity (n=62). The control group consisted of conditionally healthy individuals without verified metabolic disorders (n=20).

All patients had the body weight and height measured, and BMI was calculated as weight/height² (m²). Body mass index was used to determine obesity (BMI >30 kg/m²), according to WHO criteria [17]. Verification of AH, its grade and stage, was performed according to current European guidelines [18], and the diagnosis of DM was made based on WHO criteria [19]. For each patient, anthropometric parameters, biochemical and hormonal markers (leptin, insulin, HbA1c, and 25(OH)D) were assessed once, in the morning, in a fasting state, by venous blood sampling after 8–12 hours of fasting, which minimised the influence of diurnal variations. Exclusion criteria included: type 1 DM, congenital heart and urinary tract defects, presence of pacemakers, artificial heart valves, heart failure stages IIB and III, acute myocardial infarction, infectious and severe inflammatory processes, and haematological diseases.

Leptin concentration in the patients’ serum was measured using the enzyme-linked immunosorbent assay (ELISA) metho­d on the «Labline-90» analyser (Austria), applying a commercial test kit manufactured by the company «DBC» (Canada), in accordance with the instructions included in the kit. To assess the level of 25(OH)D in serum, the enzyme-linked immunosorbent assay (ELISA) was used on the «Labline-90» analyser (Austria) with a commercial test kit produced by «Monobind Inc» (USA), following the manu­facturer’s instructions provided in the set. Serum insulin level­s were evaluated using the enzyme-linked immunosorbent assay (ELISA) method on the «Labline-90» analyser (Austria), employing a commercial test kit from «Monobind Inc» (USA), according to the instructions supplied with the kit. Glycated haemoglobin (HbA1c) content in the patients’ serum was a­nalysed photometrically based on the thiobarbituric acid reaction, using a commercial test kit developed by the company «Reagent» (Ukraine), in full accordance with the enclosed instructions.

Statistical analysis of the data was performed using Statistica 13.0 and SPSS 26.0 software. The Shapiro — Wilk test was used to assess normality of distribution. Quantitative indicators were presented as median [Q25; Q75]. The Mann–Whitney U test or Kruskal–Wallis test was used for intergroup comparison. Correlation analysis was performed using Spearman’s rank correlation coefficient. Results were considered statistically significant at p<0.05.

Results and discussion

To assess the characteristics of carbohydrate metabolism, hormonal status, and anthropometric parameters in patients with AH, a number of metabolic indicators were analysed depending on the presence of comorbid conditions. A comparative analysis of leptin, insulin, HbA1c, 25(OH)D, and BMI levels was conducted across four clinical groups (Table 1). All biochemical indicators were measured in the morning hours in a fasting state using venous blood samples collected according to a standardised protocol. Enzyme-linked immunosorbent assays (ELISA) with validated commercial test kits were used for analysis. This approach ensured high accuracy of the results and made it possible to identify specific metabolic characteristics for each patient group.

Table 1. Carbohydrate profile indicators in the studied groups

Leptin levels were lowest in patients with isolated AH (23.83±12.52 ng/mL), and significantly higher in groups with obesity (27.68±14.46 ng/mL; p<0.000001) and with AH + obesity + type 2 DM (28.85±12.42 ng/mL; p=0.041). This indicates a close relationship between hyperleptinaemia and obesity, as well as the possible involvement of leptin in the pathogenesis of metabolic disorders associated with comorbidity. Insulin levels were lowest in the AH group (10.96±4.33 μIU/mL) and gradually increased in groups with combined pathology, reaching the highest values in the AH + DM group (25.84±15.71 μIU/mL; p<0.000001). This insulin elevation reflects the presence of insulin resistance, which is significantly exacerbated in the presence of obesity and DM. The HbA1c index was also significantly higher in the groups with DM: 7.57±1.11% in the AH + DM group and 7.34±0.8% in the AH + obesity + DM group (p<0.000001), indicating poorer glycaemic control in these patients. The lowest HbA1c values were recorded in patients with isolated AH — 5.79±0.76%. The 25(OH)D level did not differ significantly between groups (p>0.05), although numerically it was slightly lower in the AH + obesity + DM group (39.31±11.16 ng/mL), which may indirectly indicate a trend towards vitamin D deficiency as comorbidity increases. BMI was lowest in patients with AH (26.81±1.44 kg/m²), while in groups with obesity and combined obesity + DM, it reached the highest values — 34.55±2.85 and 35.45±2.73 kg/m² respectively. Significant differences were observed between all groups, except between the AH + obesity and AH + obesity + DM groups (p>0.05), indicating a similar degree of obesity severity in these patients.

Further analysis confirmed that leptin levels were lowest in the AH group, but significant differences were observed only in comparison with the group combining AH, obesity, and DM. In patients with obesity, leptin levels typically increase due to greater adipose tissue mass, where it is synthesised, and due to increased insulin resistance. Type 2 DM is associated with elevated leptin resistance against a background of marked hyperinsulinaemia, leading to further increases in leptin levels to compensate for this resistance — which explains the differences observed between the patient groups.

The study identified an inverse correlation between leptin and insulin levels (r=-0.418; p=0.036), suggesting a potential antagonistic effect of leptin on insulin secretion, particularly through its anorexigenic effect and its ability to reduce the functional activity of pancreatic β-cells. This mechanism may be one of the pathogenic factors contributing to insulin resistance and metabolic maladaptation in patients with combined forms of comorbidity (Fig. 1).

Figure 1. Correlation between leptin and insulin levels (r=-0.418; p=0.036)

Blue squares — individual values of leptin and insulin levels in serum; red line — regression line.

Developed by the author based on research results.

A direct correlation was established between leptin and glycated haemoglobin levels (r=0.393; p=0.041), confirming leptin’s involvement in the regulation of carbohydrate metabolism. An increase in leptin concentration is associated with higher HbA1c values, which may indicate its role in the develop­ment of chronic hyperglycaemia against the background of leptin resistance. This interaction highlights leptin’s significance as a potential marker of metabolic maladaptation in patients with type 2 DM and related disor­ders (Fig. 2). The graph clearly shows a gradual increase in HbA1c levels as leptin concentration rises. The highest HbA1c values were recorded in patients with the highest leptin levels, consistent with the correlation analysis data.

Figure 2. Correlation between leptin and glycated haemoglobin (r=0.393; p=0.041)

Small squares — individual values of leptin and HbA1c concentrations; red line — regression line.

Developed by the author based on the results of the research.

Considering the identified association between leptin and HbA1c levels, an additional analysis was conducted in the subgroup of patients with type 2 DM, taking into account the degree of glycaemic control. Comparison of the subgroups revealed that in patients with HbA1c levels within 7–8.5%, the median leptin level was 3.1% higher than in those with values above 8.5%: 30.55 [30.1; 31.85] vs 29.61 [29.59; 30.86] ng/mL, respectively (p<0.05) (Fig. 3). The obtained results confirm the complex interaction between leptin, insulin, and glycaemic control indicators, indicating the important role of leptin in the pathogenesis of metabolic disorders in comorbid conditions.

Figure 3. Leptin levels in patients with type 2 DM depending on HbA1c level, p<0.05

Developed by the author based on the results of the research.

A negative correlation was found between leptin and vitamin D levels (r=–0.411, p=0.06), which may be due to the fact that excessive leptin levels can negatively affect the functional effects of 25(OH)D. Thus, with a median leptin level of 31.86 ng/mL, the threshold value of 25(OH)D was below 30 ng/mL; and with a median of 27.8 ng/mL, the threshold value of 25(OH)D was above 30 ng/mL. A positive correlation was observed between leptin levels and BMI (r=0.627, p<0.0001). The obtained results are presented in Table 2.

Table 2. Fasting plasma leptin level in the study group depending on 25(OH)D level and BMI, M [Q25; Q75]

As shown in Table 2, plasma leptin levels were significantly higher in patients with 25(OH)D concentrations below 30 ng/mL compared to those with levels above 30 ng/mL (31.86 vs 27.8 ng/mL; p<0.01). This indicates an inverse relationship between leptin and vitamin D levels, which may suggest mutual pathogenic aggravation. In addition, a significant positive association was found between leptin levels and BMI: with a BMI >30 kg/m², the median leptin level was 31.85 ng/mL vs 27.99 ng/mL with a BMI <30 kg/m² (p<0.01). These results underscore the strong link between leptin, obesity, and vitamin D deficiency as key components of metabolic maladaptation.

Beyond the main biochemical relationships identified in this study, the role of leptin as a systemic mediator of chronic low-grade inflammation should also be considered. According to S.C. Lu, A.O. Akanji [11], leptin is involved in the regulation of pro-inflammatory cytokine production, particularly interleukin-6 and tumour necrosis factor α, which contributes to worsening insulin resistance in chronic hyperglycaemia. This partly explains the observed positive association between leptinaemia and HbA1c levels, indicating not only the metabolic but also the immunoinflammatory activity of leptin.

An important aspect worthy of attention is the presen­ce of sex differences in the regulation of the leptin profile. A. Boucsein et al. [20] emphasised that leptin levels are gene­rally higher in women than in men at the same BMI, which the authors attributed to oestrogen-dependent stimulation of LEP gene expression. This aligns with the observations in the present study, where women exhibited slightly highe­r mean leptin levels regardless of the presence of T2DM or obesity, although this was not the subject of dedicated analysis. M.C. Evans et al. [21] demonstrated that sex hormones affect leptin sensitivity, especially during menopause, which may be important for predicting metabolic risk in the female population. Other researchers — G. Srinivasan et al. [22] — examined leptin’s effect on the contractile activity of uterine tissue and found that it suppresses spontaneous contractions via activation of the NO/cGMP signalling pathway, highlighting its role in reproductive function regulation.

Special attention should also be paid to age-related features. According to Z. Liu et al. [23], older patients tend to show reduced leptin sensitivity due to age-related declines in hypothalamic receptor density. This may partly explain the differences in leptinaemia among subgroups with varying DM or obesity durations. The age-related decrease in leptin’s biological effectiveness may result not only from quantitative changes in leptin itself but also from impaired pre-receptor signalling cascades, as described by V. Oliveira et al. [24]. These researchers demonstrated that leptin resistance in AH is linked to disrupted signalling in the central nervous system, particularly in hypothalamic structures, which control both energy balance and blood pressure regulation. Although the current study did not stratify patients by age, the observed variability in leptin levels among individuals with different metabolic statuses — particularly lower values in patients with higher HbA1c levels — may partly reflect the influence of both the duration of metabolic disorders and potential age-related changes in leptin sensitivity. This is consistent with the literature on the decline in leptin signal efficacy in older adults, and underscores the need for further stratified analysis by age and disease duration.

The role of leptin in regulating energy metabolism and cardiovascular homeostasis, as well as its impact on cognitive functions in metabolic disorders, remains a subject of debate. M. Tanaka et al. [25] showed that elevated levels of the soluble TREM2 receptor are associated with cognitive impairment in patients with type 2 DM, even in the absence of obesity, suggesting complex neuroendocrine links between metabolic status and central nervous system function. In this context, the results of A. Kurylowicz [26] are also of interest, highlighting the influence of oestrogens on adipose tissue physiology and the development of leptin resistance, especially in obesity, underlining the sex- and hormone-specific aspects of metabolic regulation disorders.

It is worth noting that several studies point to potential heterogeneity in leptin resistance depending on adipose tissue location. For example, in the study by K. Tanigaki et al. [27], leptin levels were more strongly correlated with visceral fat than with total fat mass. The authors attribute this to the higher metabolic activity of visceral adipose tissue, which produces more pro-inflammatory mediators. In the present study, BMI was used as a general indicator of obesity; therefore, further analysis of visceral fat content could help clarify the nature of these associations. In turn, S. Pereira et al. [28] expanded the understanding of leptin’s role, demonstrating its tissue-specific effects on glucose and lipid metabolism. The authors noted that leptin’s action largely depends on the type of target tissue, opening new avenues for developing more targeted pharmacological strategies for treating metabolic disorders.

Summarising current concepts of leptin’s biological effects, M. Greco et al. [29] emphasised leptin-activating modu­lators as potential therapeutic agents for correcting metabolic imbalance. In the review, the authors highlighted that such molecules can influence leptin sensitivity, reduce leptin resistance, and consequently improve the regulation of ener­gy homeostasis, appetite, and insulin sensitivity. Expanding research in this direction opens new possibilities for a personalised approach to treating obesity, type 2 DM, and the complications in the context of leptin resistance.

Conclusions

The conducted study focused on determining the dependence of leptin levels in patients with AH on the presence of comorbid conditions — obesity and type 2 DM — as well as identifying associations between leptin concentration and indicators of the metabolic profile, in particular glycated haemoglobin, body mass index, insulin level, and 25-hydroxyvitamin D. The obtained data made it possible to reveal both statistically significant intergroup differences in leptin levels and statistically significant correlations with clinical and biochemical markers of metabolic disorders.

The study involved 250 patients with AH, who were divided into four groups according to the presence of comorbid obesity and type 2 DM. Each participant had the levels of leptin, glycated haemoglobin, insulin, and 25-hydroxyvitamin D measured, and body mass index was calculated. The statistical analysis revealed that patients with a combination of AH, obesity, and type 2 DM had significantly highe­r leptin levels compared to individuals with AH only. Positive correlations were found between leptin concentration and glycated haemoglobin and body mass index, as well as inverse correlations between leptin levels and insulin and 25-hydroxyvitamin D concentrations. These results indicate close associations between leptinaemia and carbohydrate metabolism, body weight, and vitamin status in patients with metabolic comorbidity.

The obtained data provide grounds for considering leptin as an important integral marker of metabolic maladaptation in patients with AH complicated by obesity and type 2 DM. Leptin reflects both excessive fat mass and impaired glucose homeostasis, the development of leptin resistance, and vitamin D deficiency. Such a comprehensive approach to its interpretation allows the use of leptin levels for metabolic risk stratification, differentiated assessment of the severity of the pathological process, and potentially for individualising treatment strategies. Further scientific research should be aimed at studying the dynamics of leptinaemia under the influence of therapeutic interventions, analysing the relationships between leptin and visceral adipose tissue, pro-inflammatory cytokines, and determining its prognostic value in the development of cardiovascular and renal complications in patients with combined metabolic disorders.

References

Надійшла до редакції/Received: 21.07.2025
Прийнято до друку/Accepted: 29.07.2025