2.1Data collection and study population
The NHANES is an ongoing, multistage, stratified survey of the noninstitutionalized population of the United States. The NHANES database contains a wealth of information on a variety of health and nutrition-related topics, including demographics, dietary intake, physical activity, health conditions and environmental exposures. Data are collected through a combination of interviews, physical examinations, and laboratory tests.
This cross-sectional study was conducted across 2 cycles of the NHANES (2007–2010). Since 2007, the NHANES database has been completely harmonized and improved on questionnaires for sexual minorities. The restrictions on available publicly available data vary according to the survey year, but always include complete personal information about those between the ages of 20 and 59. Consequently, respondents in this age group were selected for the current study. Prior to 2010, the NHANES database contained measurements of the complete nine biomarkers that could be used to calculate PhenoAgeAccel. Therefore, we used data from the NHANES database from 2007 to 2010 From 2007 to 2010, a total of 20,686 individuals were included in the NHANES survey. Our study included 7,926 participants aged 20 to 59 years old. Participants who were missing data on the nine biomarkers (albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase, white blood cell count) that comprise PhenoAgeAccel (N = 4450), or sexual orientation identity (N = 415) were excluded. Ultimately, the study comprised a total of 3061 individuals.
2.2 Variable definitions for sexual orientation identity
The NHANES measures sexual orientation with a question about sexual identity. The participants of the study identified their sexual orientation by selecting from a set of predetermined options, which included heterosexual (meaning sexually attracted solely to the opposite sex), homosexual (meaning sexually attracted solely to the same sex), bisexual (meaning sexually attracted to both men and women), an alternative option, or uncertainty. After excluding those who answered “no,” “don’t know,” or “I don’t know the answer,” we categorized the remaining participants as gay, straight, bisexual, or other based on their responses. Other category included participants who did not identify as gay, heterosexual, or bisexual. The responses homosexual or gay/lesbian, bisexual, and other (LGBT+) were combined to create a sexual minority category.
2.3 Criteria for determining Phenotypic age acceleration6
The calculation of phenotypic age involves nine clinical chemical biomarkers (albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase, white blood cell count) and chronological age. Phenotypic age is a commonly used metric in medical research to evaluate the expected age of a population based on an individual's estimated risk of mortality. This indicator is frequently employed to identify risk factors for morbidity and mortality, evaluate the effectiveness of interventions, and provide insights into the mechanisms of aging. The formula used to calculate phenotypic age is as follows:
Phenotypic Age = 141.50 +\(\frac{{ln}\left\{-0.00553\times {ln}\left(1-mortality risk\right)\right\}}{0.090165}\)
where Mortality risk = 1 − exp (\(\frac{-1.51714 \times e\text{x}p \left(\text{x}b\right)}{0.0076927 }\)) and xb = − 19.907 − 0.0336 × albumin + 0.0095 × creatinine + 0.1953 × glucose + 0.0954 × ln (C − reactive protein) − 0.0120 × lymphocyte percentage + 0.0268 × mean cell volume + 0.3306 ×red blood cell distribution width + 0.00188 × alkaline phosphatase + 0.0554 × white blood cell count + 0.0804 × chronological age
In our study, we calculated PhenoAgeAccel as an indicator of physiological aging, which adjusts for chronological age. This metric quantifies the dissimilarity between an individual’s Phenotypic Age and chronological Age, serving as a tool to ascertain whether said individual exhibits an advanced (positive value) or delayed (negative value) aging phenotype relative to their physiological attributes. The formula used to calculate PhenoAgeAccel is as follows: PhenoAgeAccel = Phenotypic Age - chronological Age. This approach enables us to evaluate the rate of aging and identify factors that influence the aging process in medical research.
2.4 Covariates
The covariates in this study came from the population background section of NHANES (2007–2010) including sex (male, female), age, body mass index (BMI), race/ethnicity (other Hispanic, non-Hispanic black, non-Hispanic white, other), education level (high school and below, college or above), family income to poverty ratio (PIR) (≤ 1.3, 1.3 to 3.5, > 3.5), smoking status (current smoker, former smoker, never smoked), drinking status (yes, no), obesity (yes, no), depression (yes, no) and moderate-to-vigorous physical activity (MVPA). To determine an individual's BMI, their weight in kilograms was divided by the square of their height in meters. Those with a BMI equal to or greater than 30 were considered to have obesity21. The analysis focused on identifying clinically significant symptoms of depression, as defined by the standard cut point of The 9-item Patient Health Questionnaire (PHQ-9) scores of 10, in accordance with medical research guidelines.22 The calculation of MVPA was performed by means of the following formula: [the product of moderate leisure-time activity minutes and moderate leisure-time days] added to [the product of vigorous leisure-time activity minutes and vigorous leisure-time days] resulted in the total number of MVPA minutes per week23.
2.5 Statistical Analyses
Data analysis was performed in accordance with the analysis guidelines and used recommended survey weights for NHANES data. For continuous variables, mean and standard deviation are commonly presented. Analysis of variance (ANOVA) is used for between-group comparison of continuous variables. For categorical variables, frequencies and percentages were presented. The chi-square test was used for analyzing differences among groups of categorical variables. The sex-stratified linear regression model was used to examine the differences of PhenoAgeAccel and nine biomarkers that composed phenotypic age between sexual minorities and heterosexuality. For all regression analyses, model 1 was basic model (unadjusted) and model 2 was adjusted for ethnicity/race, education, PIR, smoking, drinking, obesity, depression and MVPA. All regressions were conducted using heterosexuals as the reference group. In addition, we performed stratified analyses to explore difference in PhenoAgeAccel between sexual minorities and heterosexual adults among different populations based all covariates. The results are presented in the form of β-values, accompanied by their respective 95% confidence intervals (CIs). The data analysis was performed using Empower (R) (www.empowerstats.com, X&Ysolutions, inc. Boston MA) and R (http://www.R-project.org). The plots were all created using R version 4.3.0. Statistical significance was defined as a P value of 0.05.