Sex-specific association between serum zinc concentration and incidence of abdominal obesity: A nine-year follow-up

Abstract

Background

Zinc is an essential micronutrient involved in metabolic pathways, yet its association with the development of abdominal obesity (AO) is not well established. This study examined the relationship between serum zinc (SZn) concentrations and the incidence of AO, considering potential sex-specific differences.

Methods

We included 1533 adult participants of the fourth phase (2009–2011) of the Tehran Lipid and Glucose Study (mean age of 43.6 ± 13.9 years, 34.3% male) and followed them up to 2018–2022. SZn concentration was measured at baseline by the flame atomic absorption spectrometry. Anthropometric measurements were done at baseline and again at three-year intervals to determine AO incidence. Univariate unrestricted regression spline (UVRS) and sex-stratified Cox proportional hazard models were used.

Results

The mean (SD) of WC was 85.0 ± 6.4 cm, and the mean SZn concentration was 113 ± 46.0 µg/dL at baseline. After a median of 9.3 years follow-up, 30.1% of participants (27% of male and 31.7% of female) developed AO. A non-linear association was observed between SZn and the incidence of AO (non-linearity from the likelihood ratio test P < 0.05). SZn concentrations above 127.5 µg/dL were associated with a reduced risk of AO by 34% in female (HR = 0.66, 95% CIs = 0.47–0.91). No significant association was found in male between SZn and the risk of developing AO over time.

Conclusion

Our study demonstrated that a higher SZn concentration was negatively associated with the risk of developing AO in female. This finding may imply that a higher intake of zinc-rich food sources may protect against developing AO in female.

Introduction

Obesity, caused by excessive fat accumulation, is recognized by the World Health Organization as a major chronic health issue [1, 2]. Abdominal obesity (AO), defined as excess fat around the visceral organs, increases the risk of non-communicable diseases like heart disease, diabetes, and cancer [3, 4]. Obesity-related conditions cause over 5 million deaths annually, with more than half occurring in individuals under 70 years [5]. The prevalence of AO is rising, affecting 41.5% of individuals aged 15 and older globally [6]. In Iran, the rate of abdominal obesity rose from 27.5% in 2004 to 40.4% in 2021 [7].

Obesity is linked to changes in the levels and functions of different minerals in the body [8]. Zinc, the second most abundant trace element in the human body, is an essential micronutrient with catalytic, structural, and regulatory functions in various cellular processes [2]. Impaired zinc homeostasis has been implicated in the pathophysiology of obesity [9]. Zinc plays an essential role in leptin secretion, glucose uptake, free fatty acid metabolism, central nervous system signaling for fat storage, and adipose tissue regulation [10–12]. Alterations in zinc levels have been associated with changes in cortisol, leptin, and insulin resistance in individuals with obesity [13]. Zinc supplementation leads to an increase in the size of visceral adipose tissue in the perirenal area of mice [14]. Clinical investigations have similarly reported associations between zinc and weight, including increased weight and body mass index following zinc supplementation in hemodialysis patients [15], and a correlation between maternal SZn levels and infant birth weight [16].

The current evidence on the relationship between zinc and obesity is primarily derived from case-control, cross-sectional, and clinical trial studies, with inconclusive findings. One study found a negative link between serum zinc levels and obesity [17], while a meta-analysis indicated that obese individuals had lower serum zinc compared to non-obese controls [18]. No prospective cohort study has evaluated the longitudinal association between SZn concentrations and the risk of developing AO. To fill this critical research gap, we conducted a nine-year follow-up study to examine the relationship between baseline SZn concentrations and the future risk of developing AO in adult male and female in the frame.

Materials and methods

Study design and population

This study used a national population-based cohort of the Tehran Lipid and Glucose Study (TLGS) dataset [19]. The TLGS baseline survey was conducted in 1999–2001 among more than 15,000 participants in Tehran. All measurements were taken at baseline and repeated at 3-year intervals using the same standardized approaches [19]. In the current study, 2190 male and female aged 21 years or older who participated in the fourth examination of the TLGS (2009–2011) and had data on SZn and anthropometric measurements were included. After excluding participants with pregnancy (n = 16), breastfeeding (n = 41), cancer (n = 4), diarrhoea (n = 15), AO (n = 498), and those who lost follow-up examinations (n = 83), 1533 subjects remained eligible to be followed over 9-years within 3-consecutive examinations (fifth examination: 2012–2014, sixth examination: 2015–2017, and seventh examination: 2018–2022).

The study protocol was carried out in accordance with the Declaration of Helsinki. The ethics committee of the Research Institute for Endocrine Sciences at Shahid Beheshti University of Medical Sciences reviewed and approved the protocol (Approval number: IR.SBMU.ENDOCRINE.REC.1403.148). All participants in the TLGS provided written informed consent on the day of the examination.

Assessments of covariates

Trained interviewers collected data on demographics, family medical history, and medications during both baseline and follow-up examinations. Information regarding anthropometric variables, including body weight, height, and waist circumference (WC), as well as systolic blood pressure (SBP) and diastolic blood pressure (DBP) has been reported by the TLGS research group in other publications [20]. Physical activity (PA) [21] and biochemical variables, including fasting serum glucose, 2-hour serum glucose, triglycerides, high-density lipoprotein cholesterol (HDL-C), and creatinine, were also documented [22]. SZn concentrations were measured using flame atomic absorption spectrometry with a plate number from Chem Tech Analytical Co. in Kempston, UK. Detailed methods for measuring SZn have been described in other works as well [23]. Mean inter-assay coefficient of variation (CV) was 5.4 ± 0.4% and mean intra-assay CV was 3.7 ± 0.2%.

Definition of terms

AO was defined as WC ≥ 95 cm for both sexes based on the cut-off point of the Iranian population [24]. Type 2 diabetes was defined as fasting blood glucose ≥ 126 mg/dL or 2-hour postprandial blood glucose ≥ 200 mg/dL or using glucose-lowering drugs [25]. Smoking was considered as current smoking and non-smoking (never or past smoking). Menopause was defined, according to the World Health Organization, as the absence of spontaneous menstrual bleeding for more than 12 months without any pathological or physiological cause [26].

Statistical methods

Statistical analyses were performed using SPSS for Windows version 20 (SPSS Inc., Chicago, IL, USA) and STATA version 14 SE (StataCorp, TX, USA). The baseline characteristics of the study participants were compared by sex using independent t-tests for continuous variables and chi-square tests for categorical variables. Data are presented as mean (standard deviation) or percentage. To assess potential non-linear associations and identify the optimal placement of knots in the relationship between SZn concentrations and the incidence of AO, we employed a univariate unrestricted regression spline (UVRS) model. In this model, SZn concentrations served as a continuous predictor variable, while AO was treated as the outcome. The presence of non-linearity in the dose-response relationship was evaluated by examining the fit of the spline model and its significance through the likelihood ratio test. Cox proportional hazard models were used to calculate hazard ratios (HRs) and 95% confidence intervals (CIs) for the development of AO across the best placement of knots, as identified by the UVRS. Two Cox models were developed: Model 1 adjusted for age (years) and baseline body weight (kg), while Model 2 further adjusted for smoking status (yes/no), menopause status (yes/no), type 2 diabetes (yes/no), and physical activity levels (measured in metabolic equivalents (MET) per minute per week). The event date for participants diagnosed with AO was estimated as the midpoint between the follow-up visit when AO was first diagnosed and the preceding visit. Follow-up time was calculated as the difference between this midpoint and the date when the participant entered the study. The Schoenfeld residual test was utilized to assess the Cox proportional hazard assumption. According to the global Schoenfeld residual test, the overall model met the proportional hazard assumption (P > 0.05).

Results

The mean baseline age of the study participant was 43.6 ± 13.9 years and 34% were male. After a median of 9.3 years follow-up, 30.1% of participants (27% of male and 31.7% of female) developed AO. Table 1 shows the basic characteristics of the study participants. The mean baseline SZn concentration was 113 ± 46.0 µg/dL.

Table 1.

Baseline characteristics of the study participants

	Total (n = 1533)	Male (n = 526)	Female (n = 1007)	Pvalue
Age (year)	43.6 ± 13.9*	46.5 ± 15.8	42.1 ± 12.5	0.001
Serum zinc (µg/dL)	113 ± 46.0	114 ± 46.6	113 ± 45.6	0.574
weight (kg)	64.2 ± 8.96*	68.1 ± 8.77	62.1 ± 8.29	0.001*
SBP (mm Hg)	110 ± 15.4*	113 ± 14.2	109 ± 15.4	0.001*
DBP (mm Hg)	73.3 ± 9.61*	75.0 ± 9.37	72.4 ± 9.62	0.001*
BMI (kg/m2)	24.8 ± 3.10*	23.5 ± 2.58	25.5 ± 3.12	0.001*
WC (cm)	85.0 ± 6.48*	86.0 ± 6.33	84.5 ± 6.49	0.001*
Serum Cr (mg/dL)	1.00 ± 0.15*	1.13 ± 0.12	0.94 ± 0.11	0.001*
TG (mg/dL)	125 ± 74.3*	135 ± 78.3	119 ± 71.6	0.001*
FSG (mg/dL)	96.9 ± 26.9	97.1 ± 23.2	96.8 ± 28.7	0.858
2 h-SG (mg/dL)	105 ± 40.4	103 ± 39.5	107 ± 40.8	0.072
HDL-C (mg/dL)	49.8 ± 11.1*	45.1 ± 9.79	52.3 ± 11.0	0.001*
Current smoker (%)	7.6*	18.4	1.9	0.001*
BP-lowering drugs (%)	2.3	1.3	2.7	0.182
Lipid-lowering drugs (%)	2.4	1.7	2.7	0.578
Low-PA levels† (%)	33.7	32.7	34.3	0.569
Education				0.345
Illiterate/primary	24.1	26	23.1
Secondary/diploma	59.1	56.6	60.4
Higher	16.8	17.3	16.5
Job status				0.001*
Employed	37.5	73.7	18
Unemployed	9	16.2	5.2
House wife	-	-	70.5
Marital status				0.001*
Single	14.5	19.6	11.9
Married	79.3	78.3	79.8
Divorced/widowed	4.5	1.3	6.2
Menopause (%)	-	-	23.1	-
T2D (%)	7.6	6.7	8.1	0.313
Data are mean ± SD or percent *P < 0.001; Independent sample t-test for continuous variables and Chi-square test for dichotomous and categorical variables. †Physical activity level less than 600 MET (metabolic equivalent)-min/week. BMI, body mass index; Cr, creatinine; DBP, diastolic blood pressure; FSG, fasting serum glucose; HDL-C, high-density lipoprotein cholesterol; 2 h-SG, 2-hours serum glucose; SBP, systolic blood pressure; SZn, serum zinc; TG, serum triglyceride; WC, waist circumference; T2D, type 2 diabetes

The UVRS analysis, shown in Figs. 1, indicates a statistically significant non-linear association (P < 0.05) between SZn concentrations and the risk of AO, as determined by the likelihood ratio test. Table 2 shows the association between SZn concentration and risk of developing AO. In the crude model, higher SZn concentration (≥ 129.5 µg/dL) was associated with an elevated risk of AO by 58% in male (HR = 1.58, 95% CIs: 1.00-2.51). Nevertheless, after adjustment of confounding Variables, this association was not significant (HR = 1.14, 95% CIs: 0.71–1.84). After adjustment for all potential confounders, higher SZn concentration (≥ 127.5 µg/dL) was associated with a reduced risk of AO by 34% (HR = 0.66, 95% CIs: 0.47–0.91) in female.

Fig. 1.

Non-linear association between serum zinc concentration and the incidence of AO in male (A) and in female (B)

Table 2.

The HR (95% CI) of developing abdominal obesity based on the best knots of SZn concentrations

	Male		Female
SZn (µg/dL)	102.4-129.9	>129.9	102.6-127.5	>127.5
Crude	1.17 (0.71–1.91)	1.58 (1.00-2.51)	0.78 (0.58–1.06)	0.76 (0.56–1.04)
Model 1	0.63 (0.37–1.06)	1.08 (0.67–1.74)	0.79 (0.58–1.08)	0.66 (0.48–0.91)
Model 2	0.65 (0.38–1.09)	1.14 (0.71–1.84)	0.79 (0.58–1.08)	0.66 (0.47–0.91)
Data are HRs and 95% CI Proportional Cox regression was used; SZn < 102.4 and < 102.6 µg/dL, in male and female, was considered as reference, respectively. Model 1, adjusted for age, and baseline body weight Model 2, additionally adjusted smoking, menopause status, diabetes, and physical activity level. SZn; serum zinc

Discussion

In a well-defined group of adult male and female followed for nine years, elevated SZn concentrations were significantly associated with a 34% reduction in the risk of AO among female. This relationship was not found in male. These findings suggest a sex-specific association, indicating that SZn may serve as a potential biomarker for the risk of AO in female. This warrants further investigation into the differences between sexes regarding the role of zinc in obesity.

A cross-sectional study of 1,896 Korean adults in 2016 reported a significant positive association between SZn concentration and AO and total body fat in male while no such association was observed in female [27]. A meta-analysis of 15 observational studies (case-control and cross-sectional) in children and eight case-control studies in adults found that individuals with obesity had lower SZn concentrations compared to control groups ([SMD (95% CI): −1.13 (− 2.03, − 0.23), Z = 2.45, P for Z = 0.014; I² = 97.1%, P for I² < 0.001] and [SMD (95% CI): −0.41 (− 0.68, − 0.15), Z = 3.03, P for Z = 0.002; I² = 62.9%, P for I² = 0.009], respectively) [18]. A case-control study in Bangladesh in 2020 found that SZn concentrations in female with obesity were significantly lower than those in normal-weight female (0.34 ± 0.01 mg/L vs. 0.78 ± 0.08 mg/L) [28]. The National Health and Nutrition Examination Survey (NHANES) in children aged 8–18 showed that a high serum copper/zinc ratio was associated with higher odds of overweight (OR = 1.74), obesity (OR = 5.26), and central obesity (OR = 2.99) [29]. These findings further support exploring the complex relationship between zinc and obesity, especially considering potential sex differences.

Zinc supplementation has shown inconsistent effects in both human and animal studies. In a clinical trial, the administration of zinc supplements (30 mg per day) to obese individuals over a period of 40 weeks led to significant reductions in weight, body mass index, waist circumference, and hip circumference [30]. A 2020 meta-analysis of randomized controlled trials found that zinc supplementation had no overall effect on weight or body composition. However, it did result in a small, non-significant weight loss among overweight or obese individuals [31]. In a 2023 systematic review, researchers concluded that it was not possible to confidently determine the effectiveness of zinc on body composition in overweight or obese individuals [32]. Additionally, a 2024 meta-analysis reported that zinc supplementation led to a significant reduction in body mass index among patients with type 2 diabetes, but it had no effect on overall weight [33]. Currently, due to the limited number of randomized controlled trials, the diverse target populations, and the inconsistent results, it is not possible to confidently assess the impact of zinc supplementation on weight, body composition, and waist circumference. Zinc supplementation may help reduce body weight and abdominal fat from a high-fat diet, according to animal studies [34]. The results of one study showed that short-term (one week) zinc supplementation (30–90 ppm) in mice decreased visceral adipose tissue weight and adipocyte size and stimulated the expression of adipose triglyceride lipase and hormone-sensitive lipase in the visceral adipose tissue but did not decrease body weight [35]. While one study showed that chronic zinc supplementation (30 ppm for 20 weeks) increased visceral adipose tissue weight and adipocyte size, it did not affect weight [14]. In the following, we elucidate the pathways through which zinc helps prevent obesity, and the reciprocal relationship in which obesity causes a decrease in SZn concentrations.

Zinc-α2-glycoprotein (ZAG) is a soluble glycoprotein and anti-inflammatory adipokine with lipid-mobilizing properties contributing to weight loss and obesity reduction [36]. ZAG contains one strong and 15 weak binding sites for zinc, and elevated zinc concentrations can induce ZAG precipitation [37]. ZAG is expressed at higher levels in white and brown adipose tissues (WAT and BAT) and is secreted by adipocytes [38]. ZAG is involved in the regulation of uncoupling proteins (UCP1, UCP2, and UCP3) [39–41] and plays a role in the browning of adipocytes [42, 43]. Zinc appears to be a key regulator of ZAG homeostasis [41]. High zinc concentrations potentially prevent obesity by enhancing ZAG expression [41].

The inflammation associated with obesity increases glucocorticoid concentrations, leading to an upregulation of metallothionein and zinc transporter ZIP14 expression [44, 45]. These proteins facilitate zinc transfer from plasma to adipose tissue and liver [44, 45], contributing to hypozincemia in obesity [44, 45]. ZIP14 expression is upregulated during adipogenesis and the differentiation of preadipocytes into adipocytes, where it plays a critical role in mediating zinc transport from plasma to adipose tissue while also influencing Peroxisome proliferator-activated receptor gamma (PPAR-γ) expression and adipose tissue expansion [46]. As adiposity increases, disruptions in endocrine signaling and alterations in zinc metabolism result in the upregulation of metallothionein and ZIP14, while ZAG expression is concurrently downregulated [41]. Our findings suggest sex differences in the relationship between SZn concentration and AO, which aligns with previous studies that have reported sex-related variations in zinc metabolism in conditions such as metabolic syndrome and hypertension [47, 48]. Our study revealed sex-specific differences in the outcomes, with distinct pattern observed between male and female. However, the limited number of available studies precludes a comprehensive elucidation of the underlying mechanisms driving these sex-based difference. In alignment with our findings, Yang et al. reported that the rs13266634 variants exert differential effects on glucose and lipid metabolism in males and females, with a more pronounced impact observed in males [49]. Further research is warranted to clarify the biological and physiological mechanisms contributing to these sex-specific effects.

Denture adhesives and various industrial processes, such as galvanization, are used to protect metals from corrosion. Additionally, zinc-containing metal alloys are commonly found in cookware. Zinc oxide, a widely used additive, is present in many products, including paints, cosmetics, soaps, deodorants, anti-dandruff shampoos, weaponry, electrical devices, batteries, plastics, inks, pharmaceuticals, textiles, and rubber. Moreover, zinc sulfide is utilized in X-ray screens, luminescent coatings, and fluorescent lighting. These sources may contribute significantly to non-dietary zinc exposure [50].

This study is the first cohort to examine the association between SZn concentration and the risk of AO, emphasizing the necessity of sex-specific analyses in future research. The inclusion of repeated measurements of anthropometric parameters and covariates at 3-year intervals provided a comprehensive assessment of changes over time. While serum zinc levels, independent of dietary intake, can effectively clarify the link between zinc and abdominal obesity, these levels may vary due to their sensitivity to daily intake changes. A thorough history of diet and supplementation could have offered a more complete understanding. Unfortunately, a notable study limitation is the lack of data on dietary and supplement zinc intake. Furthermore, the COVID-19 pandemic during the final follow-up could have impacted the results, as the pandemic likely influenced participants’ lifestyle behaviors, including physical activity, diet, and access to healthcare. These factors could introduce bias or confounding effects, limiting the generalizability of the findings and warranting further investigation in post-pandemic settings.

In conclusion, our study shows that higher SZn concentration is linked to a 34% reduction in the risk of developing AO in females. This suggests that consuming more zinc-rich foods and maintaining elevated serum zinc levels may help protect against the development of AO in women. However, we found no significant association between SZn levels and the risk of AO in males. This indicates that genetic and hormonal differences between males and females might influence zinc metabolism and its outcomes. Given the complex interplay of zinc homeostasis between the sexes, further research is needed to clarify the mechanisms underlying this association.

Author contributions

Z.B., and S.J. designed the study. Z.B., and F.Gh. analyzed the data. Z.B., S.J., and F.Gh. wrote the manuscript. F.A. supervised the study design. All authors read and approved the final manuscript.

Funding

This work was not supported by any funding agency.

Data availability

Data will be presented upon forwarding the request to the corresponding author.

Declarations

Ethics statement

The study protocol was carried out in accordance with the Declaration of Helsinki. The ethics committee of the Research Institute for Endocrine Sciences at Shahid Beheshti University of Medical Sciences reviewed and approved the protocol (Approval number: IR.SBMU.ENDOCRINE.REC.1403.148). All participants in the TLGS provided written informed consent on the day of the examination.

Consent for publication

Not applicable.

Conflict of interest

The authors declare no conflict of interest.