Abstract
Background
The relationship between metabolic syndrome (MetS) and osteoarthritis (OA) remains debated, necessitating further exploration to clarify potential causal links.
Methods
This study included 271,019 participants from the UK Biobank and NHANES, and conducted Mendelian randomization (MR) analyses to determine the relationship between BMI, MetS, and OA. We used generalized linear modeling and restricted cubic spline plots to identify non-linear associations, as well as a mediation analysis of the possible mediating effect of MetS between BMI and OA. MR analyses were used to assess genetic causality, with sensitivity and subgroup analyses ensuring robustness.
Results
MetS was present in 27.3% of UK Biobank and 32.4% of NHANES participants, with OA prevalence at 17.2% and 12.6%. BMI showed a significant positive association with both MetS and OA, with risk increasing notably at a BMI of 27. MetS mediated a small proportion of the BMI effect on OA but was not an independent risk factor after BMI adjustment [1.03 (P < 0.05) in UK Biobank and 1.18 (P > 0.05) in NHANES]. Waist circumference was the primary MetS component influencing OA risk. Subgroup analyses indicated MetS increased OA risk in younger participants and showed a “U” shaped association across BMI levels. MR results consistent with observational studies.
Conclusions
There’s a complex interplay between BMI, MetS, and OA. Although MetS mediated a small proportion of the BMI effect, it was not an independent risk factor for OA in the overall population. The impact of MetS on the risk of OA varied by age and BMI levels.
Supplementary Information
The online version contains supplementary material available at 10.1186/s13098-025-02059-y.
Keywords: Osteoarthritis, Metabolic syndrome, UK biobank, NHANES, Cross-sectional study, Mendelian randomization
Background
Osteoarthritis (OA), the most prevalent degenerative joint disease, is the leading cause of pain and joint dysfunction globally, affecting over 500 million individuals [1]. Its rising prevalence, driven by an aging population and increasing obesity rates, imposes a significant economic burden on the whole world [2–4]. OA typically affects the hip and knee joints, and cartilage damage, subchondral bone remodeling and synovitis are the main features of it3. Besides aging, genetic predisposition, trauma, and metabolic abnormalities also contribute to OA development [5]. Currently, except for total joint arthroplasty, effective treatments for severe OA are limited. This emphasizes the critical necessity to identify and intervene in potential risk factors to prevent disease onset and progression [1].
Metabolic syndrome (MetS) refers to a cluster of metabolic abnormalities, including centripetal obesity, insulin resistance, dyslipidemia, and hypertension [6]. MetS and its components are linked to a wide range of diseases, including cancer, cardiovascular conditions, and musculoskeletal disorders [7–9]. Obesity is a well-established risk factor for OA, and its frequent coexistence with MetS suggests a plausible connection between MetS and OA [10]. However, the connection between MetS and OA remains unclear and contentious. Some scholars reported that MetS is linked to the onset and progression of OA, particularly in women, and poses an independent risk factor for OA even after controlling for BMI [11, 12]. Conversely, a 32-year longitudinal study involving 6,274 participants found no significant link between MetS components and knee osteoarthritis (KOA) [13]. These conflicting findings, arising from differing research designs and methodologies, urgently require further large-scale investigations to clarify the relationship.
The UK Biobank and NHANES databases, comprising over 500,000 and 100,000 participants respectively, offer robust foundations for extensive population-based research. Mendelian randomization (MR) is an epidemiological method that investigates the causal relationship between exposures and outcomes, utilizing single nucleotide polymorphisms (SNPs) as instrumental variables (IVs). Alleles are allocated randomly during meiosis in accordance with Mendelian principles, forming natural exposure and control groups akin to randomized controlled trials (RCTs). This genetic randomization occurs independently of environmental influences, thereby mitigating confounding factors and reverse causation [14].
In this work, we first performed a cross-sectional analysis to systematically examine the associations between BMI, MetS, and OA using data from the UK Biobank and NHANES. Concurrently, we conducted MR analyses utilizing genome-wide association study (GWAS) data to investigate the potential genetic causality between various exposures and OA. By integrating observational data from large populations with genetic evidence from MR studies, we aim to achieve a more reliable and comprehensive understanding of the connection between BMI, MetS, and OA.
Methods and materials
Study population
The UK Biobank is a comprehensive population-based prospective cohort study that has collected phenotypic and genetic data from over 500,000 participants. NHANES is an ongoing biennial survey collecting nutrition and health data from the U.S. population. For this study, we included 264,626 white participants from the UK Biobank and 6,393 non-Hispanic white participants from NHANES after excluding those with missing data, and all participants providing written informed consent.
Ascertainment of exposure and outcomes
BMI data were obtained from the measurements recorded in both the UK Biobank and NHANES databases. The diagnosis of metabolic syndrome was made according to the definition of the International Diabetes Federation, and detailed criteria are displayed in the Supplementary material [15]:
In the UK Biobank, disease diagnoses were coded using the International Classification of Diseases, 10th edition (ICD-10). Diagnoses of hip osteoarthritis (ICD-10: M16) and knee osteoarthritis (ICD-10: M17) were obtained from self-reports, primary care records, and hospital inpatient data. In the NHANES database, diagnoses of OA were obtained from the “Medical Conditions” questionnaire, where participants were asked whether they had received a diagnosis of arthritis from a medical professional and, if so, to specify the type of arthritis. Participants were considered to have OA if they answered “Osteoarthritis” or “Osteoarthritis or degenerative arthritis”.
Covariates
To control for potential confounders, we included the following covariates from both databases: age (years), gender (male or female), income level (average total household income before tax in UK Biobank: < £18,000, £18,000–51,999, > £52,000; family income poverty ratio level in NHANES: < 1.3, 1.3–3.5, > 3.5), education level (below high school, high school or above), drinking habits (no drinking, < 2 times/week, ≥ 2 times/week), smoking status (no smoking, used to smoke, currently smoking), physical activity (MET min/week), uric acid (mg/dL), and serum calcium (mmol/L). Physical activity was quantified using metabolic equivalents (MET), calculated as time × frequency × intensity for all types of activities [16].
Statistical analysis
Observational studies
In the observational component of the study, we first analyzed the relationship between BMI and the risk of MetS and OA using generalized linear models. Four different models were used to assess the effects of various potential confounders by incrementally adding covariates. We then utilized restricted cubic spline (RCS) plots with 4 knots to construct non-linear associations between BMI and the risk of MetS and OA, stratified by gender. After ensuring that the relationship met the criteria for mediation (i.e., a significant relationship between exposure to outcome, exposure to mediator, and mediator to outcome), we included all covariates for mediation analyses to explore the mediating role of MetS in the relationship between BMI and OA. The mediation analysis was conducted under the standard causal assumptions of no unmeasured confounding between exposure, mediator, and outcome variables. To investigate whether MetS was an independent risk factor of OA after adjusting for BMI, we included BMI as a covariate in analysis after adjusting for all other covariates. Meanwhile, four models were used to analyze the impact of the MetS’s different components on OA.
Mendelian randomization studies
The GWAS for MetS was derived from a study of 461,920 individuals of European ancestry by van Walree et al. [17]. To accurately reflect the underlying genetic structure, this study’s results were not corrected for BMI. Data were obtained from the CNCR website.
The GWAS for BMI was sourced from a study by Yengo et al. [18] involving 681,275 individuals of European ancestry, which significantly increased the sample size through meta-analysis. Data were accessed via the GIANT website.
The GWAS data for WC, TG, HDL-C, and FG were obtained from the IEU OpenGWAS project website (https://gwas.mrcieu.ac.uk/) [19, 20]. The GWAS summary data for hypertension were obtained from the FinnGen Database, including 412,113 individuals of European ancestry.
GWAS for OA were sourced from a study by Tachmazidou et al., which performed a genome-wide meta-analysis of approximately 17.5 million single nucleotide variants in up to 455,221 individuals using UK Biobank and arcOGEN resources [21]. The design framework of this study is shown in Fig. 1.
Fig. 1.
Design framework of this research. BMI: Body mass index; GVIF: Generalized variance inflation factor; GWAS: Genome-wide association study; IVW: Inverse variance weighted; MetS: Metabolic syndrome; MR: Mendelian randomization; OA: Osteoarthritis; PSM: Propensity score matching
Mendelian randomization utilized SNPs as IVs, which were chosen to satisfy the following three assumptions: (1) significant association with the exposure, (2) association with the outcome only through the exposure, and (3) no association with confounders affecting the exposure-outcome relationship [22]. In order to find IVs with sufficient validity and satisfying the three basic assumptions of MR, a rigorous process of screening was employed and details are shown in the supplementary material. (1) screening for SNPs significantly associated with exposure at the genome-wide level (P < 5 × 10− 8); (2) the effect of linkage disequilibrium was removed (r2 < 0.001, window size = 10,000 kb): (3) SNPs significantly associated with the outcome (P < 5 × 10− 8) were excluded to avoid pleiotropy of IVs (4) palindromic variants that could not be oriented (i.e., C/G or A/T) were also removed. (5) we calculated the F-values of the IVs, and SNPs with F < 10 were removed to avoid weak instrumental variable bias [23].
We utilized a two-sample univariate Mendelian randomization (UVMR) approach to explore the causal relationship between BMI, MetS, and OA, followed by multivariate Mendelian randomization (MVMR) to explore the genetic causality of BMI and MetS on OA after adjusting for each other, as well as the relationship of MetS components with OA. For UVMR, the inverse-variance weighted (IVW) method was used as the main analysis, with additional analyses conducted using the weighted median, MR-Egger, and weighted mode methods.
Sensitivity analysis and subgroup analysis
In the observational study, we assessed multicollinearity among variables by generalized variance inflation factor (GVIF) analysis, with GVIF < 2 indicating the absence of multicollinearity. After the primary analysis, we performed a 1:1 propensity score matching with BMI at the RCS turning point including all covariates, considering the presence or absence of MetS. We used the nearest neighbor matching method with a 0.2 caliper for PSM, evaluating matching balance through hist and jitter plots [24]. Five models were used to analyze the relationship between BMI, MetS, and OA risk after PSM. Identified key factors influencing the MetS-OA relationship including age, gender, and BMI were then guided subsequent subgroup analyses.
For MR study, we calculated the Cochran’s Q statistic and I² index to assess heterogeneity in the IVW method, and evaluated potential horizontal pleiotropy between IVs by the MR-Egger intercept [25]. MR-PRESSO provides corrected causal effects by removing outliers and was used to detect and correct for horizontal pleiotropy [26]. The symmetry of the funnel plot was also used to assess potential pleiotropy, and the scatterplot assessed the reliability of the results.
Foreign package, rms package, mediation package and mendelR package in R software (version 4.3.1) was used for statistical analyses, and all tests were two-tailed, with p < 0.05 considered to be statistically significant.
Results
Descriptive statistics
This study included a total of 264,626 white adult participants from the UK Biobank and 6,393 from NHANES. The overall study design is shown in Fig. 1, and the baseline characteristics of the participants are provided in Table 1 and Supplementary Table 1. The mean ages of participants were 56.06 ± 8.02 years for the UK Biobank and 50.41 ± 17.82 years for NHANES. The median BMIs were 26.64 [24.09, 29.79] kg/m² and 27.14 [23.85, 31.20] kg/m², respectively. MetS was identified in 27.3% of the UK Biobank participants and 32.4% of NHANES participants. OA was diagnosed in 45,431 UK Biobank participants and 804 NHANES participants, with prevalence rates of 17.2% and 12.6%, respectively. Notably, the mean age and median BMI were higher in participants with OA, and the proportion of women and participants with MetS was also higher compared to non-OA participants.
Table 1.
Baseline characteristics of participants from UK biobank
| level | Overall (n = 264,626) | Non-OA (n = 219,195) | OA (n = 45,431) | P |
|---|---|---|---|---|
| Age (mean (SD)) | 56.06 (8.02) | 55.28 (8.03) | 59.81 (6.81) | < 0.001 |
| Gender (%) | ||||
| Male | 126,219 (47.7) | 105,644 (48.2) | 20,575 (45.3) | < 0.001 |
| Female | 138,407 (52.3) | 113,551 (51.8) | 24,856 (54.7) | |
| Education (%) | ||||
| Below high school | 125,187 (47.3) | 99,351 (45.3) | 25,836 (56.9) | < 0.001 |
| High School or above | 139,439 (52.7) | 119,844 (54.7) | 19,595 (43.1) | |
| Income_level (%) | ||||
| Low income | 46,536 (17.6) | 34,246 (15.6) | 12,290 (27.1) | < 0.001 |
| Middle income | 142,332 (53.8) | 116,987 (53.4) | 25,345 (55.8) | |
| High income | 75,758 (28.6) | 67,962 (31.0) | 7,796 (17.2) | |
| Alcohol (%) | ||||
| No drinking | 14,279 (5.4) | 11,422 (5.2) | 2,857 (6.3) | < 0.001 |
| Light to moderate drinking | 121,353 (45.9) | 99,710 (45.5) | 21,643 (47.6) | |
| Moderate to heavy drinking | 128,994 (48.7) | 108,063 (49.3) | 20,931 (46.1) | |
| Smoking (%) | ||||
| No smoking | 147,445 (55.7) | 124,333 (56.7) | 23,112 (50.9) | < 0.001 |
| Used to smoke | 93,434 (35.3) | 74,577 (34.0) | 18,857 (41.5) | |
| Now smoking | 23,747 (9.0) | 20,285 (9.3) | 3,462 (7.6) | |
| PA (median [IQR]) | 1,668.00 [782.00, 3,360.00] | 1,659.00 [777.00, 3,312.00] | 1,695.00 [817.00, 3,612.00] | < 0.001 |
| BMI (median [IQR]) | 26.64 [24.09, 29.79] | 26.28 [23.83, 29.23] | 28.74 [25.65, 32.60] | < 0.001 |
| MetS (%) | ||||
| No MetS | 192,421 (72.7) | 164,740 (75.2) | 27,681 (60.9) | < 0.001 |
| MetS | 72,205 (27.3) | 54,455 (24.8) | 17,750 (39.1) | |
| WC (median [IQR]) | 90.00 [80.00, 99.00] | 89.00 [80.00, 98.00] | 95.00 [85.00, 105.00] | < 0.001 |
| HDL-C (median [IQR]) | 54.37 [45.55, 65.08] | 54.68 [45.82, 65.39] | 53.06 [44.63, 63.53] | < 0.001 |
| TG (mean (SD)) | 153.93 (90.14) | 151.51 (89.79) | 165.65 (90.89) | < 0.001 |
| FG (mean (SD)) | 91.72 (20.74) | 91.41 (20.71) | 93.21 (20.83) | < 0.001 |
| SBP (mean (SD)) | 139.03 (19.31) | 138.48 (19.31) | 141.64 (19.14) | < 0.001 |
| DBP (mean (SD)) | 82.21 (10.61) | 82.03 (10.69) | 83.07 (10.20) | < 0.001 |
| Calcium (mean (SD)) | 2.38 (0.09) | 2.38 (0.09) | 2.38 (0.10) | < 0.001 |
| Uric acid (median [IQR]) | 5.11 [4.22, 6.08] | 5.06 [4.17, 6.03] | 5.32 [4.47, 6.32] | < 0.001 |
OA: Osteoarthritis; PA: Physical activity; BMI: Body mass index; MetS: Metabolic syndrome; WC: Waist circumference; HDL-C: High density lipoprotein; TG: Triglyceride; FG: Fasting glucose; SBP: Systolic pressure; DBP: Diastolic pressure;
Relationship of BMI to the risk of MetS and OA
We employed four different models to explore the impact of different BMI categories on the risk of MetS and OA, with the results presented in Fig. 2. The analysis demonstrated that BMI was significantly and positively associated with both MetS and OA across all models. Compared to the lowest BMI quartile (Q1), the odds ratios (ORs) increased with higher BMI levels, though the ORs gradually decreased with the inclusion of more covariates. After adjusting for all covariates, the ORs for MetS in the highest BMI quartile (Q4) were 27.78 (26.62–28.99) in the UK Biobank and 21.93 (17.28–27.83) in NHANES. For OA, the ORs in Q4 were 3.71 (3.58–3.84) in the UK Biobank and 1.91 (1.49–2.44) in NHANES.
Fig. 2.
Relationship of BMI to the risk of MetS and OA. (A1): Effect of BMI on MetS risk in UK Biobank; (A2): Effect of BMI on MetS risk in NHANES; (B1): Effect of BMI on OA risk in UK Biobank; (B2): Effect of BMI on OA risk in NHANES; (C1): RCS plot of BMI to MetS in UK Biobank; (C2): RCS plot of BMI to MetS in UK Biobank after sex stratification; (D1): RCS plot of BMI to OA in UK Biobank; (D2): RCS plot of BMI to OA in UK Biobank after sex stratification; (E1): RCS plot of BMI to MetS in NHANES; (E2): RCS plot of BMI to MetS in NHANES after sex stratification; (F1): RCS plot of BMI to OA in NHANES; (F2): RCS plot of BMI to OA in NHANES after sex stratification. BMI: Body mass index; MetS: Metabolic syndrome; OA: Osteoarthritis; model 1: No adjustment; model 2: Adjusted for age and gender; model 3: model 2 + education + income + personal behavior (Smoking, alcohol, physical activity); model 4: model 3 + serum calcium + uric acid; * P < 0.05, ** P < 0.01, and *** P < 0.001
Additionally, restricted cubic spline (RCS) plots revealed non-linear relationships (non-linear P < 0.05) between BMI and outcome events (Fig. 2). The effect of BMI on MetS and OA was minimal at lower BMI levels but increased rapidly beyond a certain threshold. Notably, in the RCS, the turning point for both MetS and OA was around a BMI of 27, where the OR exceeded 1, indicating an elevated risk of these conditions. Age stratification revealed no gender differences in the elevated risk of MetS with increasing BMI, but women exhibited a higher risk of developing OA compared to men at the same BMI level.
Effects of MetS on OA and mediating effects between BMI and OA
We used four different models to explore the association between MetS and the development of OA, with results presented in Fig. 3A. MetS was significantly associated with an increased risk of OA in all analyses from both databases. This correlation was strongest in unadjusted analyses, with ORs of 1.94 (1.90–1.98) in the UK Biobank and 1.72 (1.48–2.00.48.00) in NHANES. After multi-model adjustment, the ORs decreased to 1.55 (1.51–1.59) and 1.36 (1.15–1.60), respectively. Adjusting for age and gender had the strongest impact on this relationship.
Fig. 3.

Analysis of the role of MetS and its components on OA. (A): Results of multi-model analysis of MetS on OA; (B): Mediation analysis of MetS in the relationship between BMI and OA; (C): Multivariate analyses results after mutual adjustment of BMI and MetS; (D1)-(D4): Effect of MetS components on OA risk in UK Biobank; (E1)-(E4): Effect of MetS components on OA risk in NHANES; ACME, Average causal mediation effects (indirect effect); ADE, Average direct effects; BMI: Body mass index; MetS: Metabolic syndrome; OA: Osteoarthritis; model 1: No adjustment; model 2: Adjusted for age and gender; model 3: model 2 + education + income + personal behavior (Smoking, alcohol, physical activity); model 4: model 3 + serum calcium + uric acid; * P < 0.05, ** P < 0.01, and *** P < 0.001.* P < 0.05, ** P < 0.01, and *** P < 0.001
The results of the mediation analysis are displayed in Fig. 3B, which demonstrates that MetS significantly mediates the association between BMI and OA. However, the proportion mediated by MetS was relatively small: 8.6% (7.0%−9.0%) in the UK Biobank and 28.5% (4.1%−75.0%) in NHANES.
Relationship between metabolic abnormalities and OA after adjusting for BMI
To avoid interaction effects between BMI and MetS, they were adjusted for each other as covariates based on the previous model 4, with results shown in Fig. 3C. After adjustment, the effect of BMI to OA remained significant, with ORs of 3.65 (3.51–3.79) in the UK Biobank and 1.75 (1.34–2.28) in NHANES for the Q4 level. However, the OR for MetS decreased substantially, from 1.55 (1.51–1.59) to 1.03 (1.01–1.06) in the UK Biobank, and was no longer statistically significant (OR: 1.18, p > 0.05) in NHANES.
Analysis with four models for five variables of waist circumference (WC), triglyceride (TG), high density lipoprotein cholesterol (HDL-C), diastolic blood pressure (DBP), and fasting glucose (FG) was conducted to explore the connection between individual MetS components and OA after testing for multicollinearity, with results displayed in Fig. 3. Given that SBP exhibits a greater GVIF, we used DBP to represent blood pressure and did not additionally adjust for BMI. WC was the only component that maintained a significant effect on OA after multi-model adjustment, with ORs of 3.54 (3.40–3.69) in the UK Biobank and 1.84 (1.39–2.43) in NHANES. Other metabolic factors, such as DBP and FG, had little effect on OA. In the UK Biobank sample, TG and HDL-C showed a mild protective effect on OA after adjusting for other metabolic factors and covariates, whereas this relationship was not observed in the NHANES sample.
Mendelian randomization analysis
We first performed two-sample UVMR analyses between BMI, MetS, and OA, followed by multivariate Mendelian randomization (MVMR) analyses, with results presented in Fig. 4A. A total of 471, 325, 183, and 450 SNPs were used in UVMR and MVMR, respectively. In UVMR, the OR for BMI on OA was 1.87 (1.76–1.99); the OR for BMI on MetS was 1.88 (1.84–1.92); and the OR for MetS on OA was 1.72 (1.55–1.91). In MVMR, the genetic causal effect of BMI on OA was still significant after mutual adjustment, with an OR of 1.82 (1.61–2.06). However, the effect of MetS on OA was substantially attenuated and no longer significant, with an OR of 1.03 (0.87–1.21).
Fig. 4.
Results of the analysis of univariate MR and multivariate MR. (A): Effects between BMI and MetS and OA in MR analysis; (B): Effects between components of MetS and OA in MR analysis; (C): Results of subgroup analysis of MetS for OA; BMI: Body mass index; CI: Confidence interval; DBP: Diastolic pressure; FG: Fasting glucose; HDL-C: High density lipoprotein; IVW: Inverse variance weighted; MetS: Metabolic syndrome; MVMR: Multivariate Mendelian randomization; OA: Osteoarthritis; OR: Odds ratio; SNP: Single nucleotide polymorphism; TG: Triglyceride; UVMR: Univariate Mendelian randomization; WC: Waist circumference
The results of mediated MR analyses, shown in the Supplementary Fig. 5, indicated that MetS mediated part of the genetic effect of BMI on OA, with a mediator proportion of 34.7% (25.2%−44.2%). The results of these MR analyses are consistent with our observational findings.
To analyze the genetic causal effects of individual metabolic factors on OA, we performed MVMR on different components of MetS, A total of 209 SNPs were used, with results displayed in Fig. 4B. After mutual adjustment, only WC demonstrated a significantly elevated genetic risk for OA, with an OR of 2.26 (1.99–2.58). No significant effects of TG and HDL-C on OA were observed, while hypertension and fasting glucose exhibited weak protective genetic effects on OA.
Subgroup analysis and sensitivity analysis
Subgroup analyses based on age, gender, and BMI revealed that MetS was associated with an elevated risk of OA at younger ages (age < 50), but this association was not significant at older ages (70 ≤ age) (Fig. 4C). Gender-specific analyses showed that the increased risk of OA due to MetS was significant in males (OR: 1.04, 1.00–1.08.00.08) but not in females in the UK Biobank sample. In the NHANES sample, the elevated risk of OA due to MetS was more pronounced in females (OR: 1.29, 1.00–1.65.00.65). Interestingly, MetS exhibited a “U” shaped relationship with OA across different BMI levels from the UK Biobank, with `MetS linked to an increased risk of OA at the lowest (Q1) and highest (Q4) BMI levels, but not at intermediate levels (Q2). Similar trends were also observed in the NHANES sample.
We performed propensity score matching (PSM) on samples from the UK Biobank and NHANES using BMI at the RCS turning point, with or without MetS. Matched baseline information is presented in the Supplementary Tables 8–11. Multi-model analyses, shown in Table 2, indicated that the effect of BMI on OA remained significant across all models, while the effect of MetS on OA diminished and was no longer significant after adjustment, consistent with our previous analyses.
Table 2.
Results of multi-model analysis of BMI and MetS on OA after PSM
| UK Biobank | model 1 | model 2 | model 3 | model 4 | model 5 | |
|---|---|---|---|---|---|---|
| BMI | BMI ≤ 27 | |||||
| BMI > 27 | 2.04 (1.98–2.10) *** | 2.09 (2.03–2.15) *** | 2.09 (2.03–2.15) *** | 2.08 (2.02–2.14) *** | 2.08 (2.02–2.14) *** | |
| MetS | Non-MetS | |||||
| MetS | 2.04 (1.98–2.10) *** | 1.02 (0.99–1.05) | 1.02 (0.99–1.05) | 1.01 (0.98–1.04) | 0.99 (0.96–1.02) |
| NHANES | model 1 | model 2 | model 3 | model 4 | model 5 | |
|---|---|---|---|---|---|---|
| BMI | BMI ≤ 27 | |||||
| BMI > 27 | 1.26 (1.04–1.53) * | 1.24 (1.01–1.52) * | 1.25 (1.02–1.53) * | 1.25 (1.02–1.53) * | 1.24 (1.01–1.53) * | |
| MetS | Non-MetS | |||||
| MetS | 1.26 (1.04–1.52) * | 1.18 (0.98–1.44) | 1.19 (0.98–1.45) | 1.20 (0.99–1.46) | 1.20 (0.98–1.45) |
BMI: body mass index; MetS: metabolic syndrome; model 1: No adjustment; model 2: Adjusted for age and gender; model 3: model 2 + education + income + personal behavior (Smoking, alcohol, physical activity); model 4: model 3 + serum calcium + uric acid; model 5: model 4 + * P < 0.05, ** P < 0.01, and *** P < 0.001
For MR analyses, information on instrumental variables used and sensitivity analyses are provided in the Supplementary Table 12. All instrumental variables satisfied F > 10, and no evidence of pleiotropy was found (P > 0.05) in UVMR analyses, with no outliers detected in MR-PRESSO.
Discussion
In this large-scale observational and MR study, we identified non-linear relationships between BMI and the risk of MetS and OA. In both analyses, the risk curves initially flattened and then increased significantly, with a notable turning point around a BMI of 27, aligning with recommendations for anti-obesity treatments [27]. Although MetS mediated a small proportion of the effect of BMI on OA, it was not an independent risk factor for OA after adjusting for BMI. Genetic effects of metabolic syndrome may be realised exclusively through BMI rather than an independent way. Among the components of MetS, elevated WC was the primary contributor to OA risk, while other components had limited effects. Subgroup analyses revealed intriguing results: MetS was associated with an increased risk of OA in younger participants even after adjusting for BMI, and a “U” shaped association between MetS and OA risk was observed across different BMI levels. Through the use of multiple methods, our study provides a deeper understanding of the relationship between MetS and OA risk, contributing evidence to the epidemiology, prevention, and management of OA.
Obesity is a well-established risk factor for OA, influencing the condition through increased mechanical loading on joints and causing structural changes that impair joint function, and an increase in waist circumference is typically associated with higher body weight and BMI [28]. There may also be reverse causation between obesity and OA, i.e., patients with OA are less active due to pain and dysfunction, which may exacerbate obesity and thus create a vicious cycle. Controlling and reducing weight is beneficial in preventing and slowing OA progression [29]. The mechanical loading theory posits that OA is primarily driven by the increased weight burden on joints. Furthermore, obesity often leads to lipid abnormalities and other metabolic disruptions, contributing to the rising prevalence of MetS alongside obesity [30]. The increase in body weight is a significant factor linking MetS to the heightened risk of OA. Numerous studies have demonstrated a significant association between MetS and OA prior to adjusting for BMI, but this relationship often disappears after such adjustments, which is consistent with our study [13, 31]. These results suggest that it is most likely the mechanical changes following obesity confounding the association between Mets and OA.
In addition to mechanical loading, inflammation is another crucial factor linking obesity, MetS, and OA. OA is characterized by chronic, low-grade inflammation, encompassing both local and systemic inflammatory processes [32]. Macrophages and cytokines, including IL-1β and TNF, play vital roles in local inflammation, and patients with obesity or MetS often exhibit systemic low-grade inflammation [33]. Previous studies revealed higher levels of pro-inflammatory cytokines were observed in overweight individuals, with adipose tissue macrophages shifting from an anti-inflammatory M2 phenotype to a pro-inflammatory M1 phenotype as weight increases [10]. MetS also influences multiple pathways, such as the AMPK pathway, leading to macrophage polarization towards the pro-inflammatory M1 phenotype [33]. Moreover, hyperlipidemia can also elevate levels of reactive oxygen species and cytokines like IL-6 and IL-8, exacerbating inflammatory responses [34].
Except for abdominal obesity, the connection between other MetS components and OA is complex and controversial. In this study, TG and HDL-C showed a negative correlation to OA in the UK Biobank data and the opposite in NHANES. Various cross-sectional and cohort studies have found associations between hyperlipidemia and increased OA risk [35, 36]. However, Schwager et al. found no significant relationship between dyslipidemia and OA, which is supported by evidence from MR studies [37]. Hypertension may contribute to OA through multiple mechanisms, including angiogenesis, shared molecular pathways, and other factors leading to disease onset and progression [38]. However, a study of 2,234 participants from South Korea found a negative correlation between hypertension and OA after adjusting for multiple covariates [39]. Evidence from MR studies on this relationship is also inconsistent [40, 41]. The association between elevated fasting glucose and OA similarly varies, with different studies yielding conflicting results [42, 43]. These inconsistencies suggest that MetS components other than WC may not play an important role in OA development.
A growing number of studies have shown that classifying obesity solely on the basis of BMI masks heterogeneity in fat distribution and metabolic status. So-called “metabolically healthy obese (MHO)” individuals are at lower risk for traditional cardiovascular outcomes, but are not completely harmless [44]. In contrast, “metabolically unhealthy non-obese (MUNO)” individuals, despite being of normal weight, are at substantial risk for a wide range of chronic diseases, including musculoskeletal disorders [45], due to abdominal adiposity and metabolic abnormalities. Our study showed that the association between metabolic syndrome and osteoarthritis was significantly attenuated and no longer statistically significant after adjusting for BMI, suggesting that metabolic abnormalities alone (without overweight) may have a small effect on osteoarthritis risk in the general population. However, subgroup analyses showed that the effect of metabolic abnormalities was more pronounced in the younger age group and in the lowest BMI quartile, a pattern consistent with the MUNO phenotypic profile.
One key finding from our study is the differential effect of MetS on OA across various age groups. As a degenerative disease, age is a major determinant of OA risk. Järvholm et al.‘s study identified a non-linear relationship between age and KOA, with a sharp increase in OA incidence among men aged 50–70 [46]. This implies that metabolic changes and systemic inflammation due to MetS may be more impactful in younger individuals, whereas joint wear and degeneration become predominant factors as age increases.
The “U” shaped association between MetS and OA risk across different BMI levels is particularly intriguing. Abdominal obesity, a component of MetS, signifies primary metabolic disturbances in patients with low BMI, indicating that metabolic disorders rather than mechanical loading are primary OA risk factors in these individuals. In participants with mid-range BMI, the risk of OA is minimized, possibly due to secondary metabolic abnormalities induced by overweight, where mechanical loading remains a significant joint health risk [47]. At higher BMI levels, MetS represents severe obesity and more extensive metabolic dysfunctions, contributing to the increased OA risk through multiple systemic impacts, including non-MetS-related organ anomalies [48].
Our study has several strengths. Firstly, utilizing large datasets from the UK Biobank and NHANES enhances the study’s credibility. Secondly, we employed multiple analytical methods and models to comprehensively examine the relationships between BMI, MetS, and OA, combining MR with observational studies for mutual validation. Thirdly, extensive sensitivity analyses further ensured the robustness of our findings. However, there still have some limitations. Firstly, both observational and MR studies’ sample populations were primarily white individuals of European ancestry, and the results could not be directly applied to other ancestry populations. Secondly, as a cross-sectional study, causal inference strength is inherently limited, however, we supplemented the MR analysis to enhance the credibility of causality. Thirdly, given that MR reveals the cumulative effect of genetic variants over an individual’s entire lifetime, some of the results may differ from clinical observations. In addition, the two databases have slightly different diagnoses for patients with OA, which may lead to limited comparability between them. Although sensitivity analyses were conducted, the possibility of unknown horizontal pleiotropy cannot be completely excluded, and sample overlap may affect the precision of the estimates. Our findings highlight the distinct associations between metabolic factors and OA across different age groups. Future follow-up and prospective studies may further clarify these changing patterns.
Conclusion
This large observational and MR study demonstrated a non-linear effect of BMI on MetS and OA. MetS mediated part of the BMI effect on OA, but its proportion was low, and metabolic factors were less influential than mechanical factors, age, and sex in OA development. However, metabolic abnormalities increased OA risk in younger individuals and those with lower or very high BMI. This study provides comprehensive epidemiological evidence elucidating the MetS-OA relationship and identifies varying MetS impacts across different populations, offering new insights for future research and management strategies.
Supplementary Information
Acknowledgements
Thanks to the UK Biobank and NHANES and providers and researchers of all the publicly available datasets used in the study.
Abbreviations
- BMI
Body mass index
- DBP
Diastolic blood pressure
- FG
Fasting glucose
- GVIF
Generalized variance inflation factor
- GWAS
genome-wide association study
- HDL-C
High density lipoprotein cholesterol
- ICD-10
International Classification of Diseases, 10th edition
- IV
Instrumental variable
- IVW
Inverse-variance weighted
- MetS
Metabolic syndrome
- MR
Mendelian randomization
- MVMR
Multivariate Mendelian randomization
- OA
Osteoarthritis
- OR
Odds ratio
- PSM
Propensity score matching
- RCS
Restricted cubic spline
- RCT
Randomized controlled trial
- SNP
Single nucleotide polymorphism
- TG
Triglyceride
- UVMR
Univariate Mendelian randomization
- WC
Waist circumference
Author contributions
All authors made significant contributions to the design and implementation of this study. J.X.W., L.P., M.Y. and J.G. conceived the study. J.X.W. and L.P. performed the data analysis, J.X.W. wrote the first draft of the manuscript. X.X., P.X., and F.G supervised all the analyses and are responsible for the interpretation of the data. Z.Y., J.C.W. and K.X. performed the validation analyses. H.Y., M.Y. and P.X. performed data organization. All authors reviewed the draft manuscript.
Funding
This research was funded by Scientific Research and Innovation Platform for Intelligent and Precise Treatment of Bone and Joint Diseases in Shaanxi Province (No. 2024PT-13), the Science and Technology Program of Xi’an, Shaanxi Province (No. 24YXYJ0085), the 2024 Xi’an Honghui Hospital New Technology and New Business Project, and Postdoctoral Fund of Shaanxi Province (No. 2023BSHGZZHQYXMZZ02).
Data availability
The data supporting the findings from this study are available within the manuscript. This study was conducted using the UK Biobank Resource (Application 46478). Researchers can apply to use the UK Biobank resource for health-related research that is in the public interest ([https://www.ukbiobank.ac.uk/register-apply/]).
Declarations
Ethics approval and consent to participate
This study was based on publicly available databases, and all participants had provided written informed consent in the original studies; therefore, no additional ethical approval was required.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Junxiang Wang, Leixuan Peng and Peng Xu are co-first authors of the article and contributed equally to this work.
Contributor Information
Junfei Guo, Email: drjfguo@163.com.
Mingyi Yang, Email: ymy25808@163.com.
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Associated Data
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Supplementary Materials
Data Availability Statement
The data supporting the findings from this study are available within the manuscript. This study was conducted using the UK Biobank Resource (Application 46478). Researchers can apply to use the UK Biobank resource for health-related research that is in the public interest ([https://www.ukbiobank.ac.uk/register-apply/]).



