Association Between Self-Reported Tobacco Smoke Exposure and Serum Non-High-Density Lipoprotein Among Adults, Beijing


 Background: Cigarette smoking, including secondhand exposure, is a leading cause of several atherosclerotic diseases (AS). Non-high-density lipoprotein cholesterol (non-HDL-C) is considered as a valuable predictor for dyslipidemia and AS. There is a need to identify the association of tobacco smoke exposure (TSE) and the marker of sub-clinical AS.Objective: To examine the association of TSE, including exposed status, cigarette smoking intensity and burden with serum non-HDL-C level.Methods: A cross-sectional study was carried out on 18-79 years old residents in 2017 in Beijing. All participants were interviewed on their tobacco exposed status (unexposed, passively exposed, actively exposed), smoking intensity (number of cigarettes smoked per week) and smoking burden (pack-years of smoking) among current smokers. Non-HDL-C was calculated by subtracting high density lipoprotein cholesterol (HDL-C) from total cholesterol (TC). Covariates were age, education status, alcohol drinking status within 30 days, hypertension, type 2 diabetes, physical inactivity and BMI. We plot graphs to show the tendency between the number of cigarettes smoked per week, passively exposed days per week and non-HDL-C levels, respectively. Multiple regression models were used to assess the association between non-HDL-C level and TSE after the adjustment for covariates. Results: Of the 12,798 participants, mean age was 44.9±15.4 years, 46.8% were male. The prevalence of actively exposed, passively exposed and unexposed to tobacco smoke was 25.7%, 35.7% and 38.6%, respectively. Of the tobacco passively exposed subjects, 21.7% had to exposed to second-hand smoke every day. Among current smokers, 41.2% were smoking more than 120 cigarettes per week, and 56.6% were smoking more over 20 pack-years. With the increasing of the number of cigarettes smoked per week, the serum level of non-HDL-C in male increased gradually (Male: F=2.83, P=0.04; Female: F=1.23, P=0.32; Total: F=3.29, P=0.02). After multivariable adjustment, smokers had a higher non-HDL-C level (OR=1.34, 95%CI: 1.20,1.59, P=0.001) compared with tobacco unexposed subjects. However, passively smokers were not associated with non-HDL-C levels regardless of gender (OR=0.98, 95%CI: 0.78,1.23, P=0.87). Compared with lighter smokers, male smokers who smoked more than 140 cigarettes per week had significantly higher non-HDL-C levels (OR=1.30, 95%CI: 1.00,1.70, P=0.04). Male higher burden smokers (>20 pack-years) had more risk of having higher non-HDL-C level (OR=1.88, 95%CI: 1.46,2.43, P<0.001) compared with other smokers. However, there were no significant differences between non-HDL-C and smoking intensity or burden among female smokers.Conclusions: This study showed that strong associations between TSE and non-HDL-C levels, especially in male smokers. Findings of this work emphasize the importance of encouragement to focus on blood lipid levels among smokers.

Height, weight, waist circumference, and blood pressure were measured. Height was measured in the standing position to the nearest 0.1cm without shoes. Weight was measured to the nearest 0.1kg while the subjects were minimally clothed without shoes. Body mass index (BMI) was calculated as weight (kg) divided by square of height (m 2 ). Overweight and obesity were de ned based on BMI of 24 and 28 Kg/m 2 as cutoff points recommended by the Working Group on Obesity in China 13 . Resting blood pressure (BP) and heart rate were measured 3 times with 1-minute intervals in the sitting position after 15 min of rest on the left arm by standard methods using electronic sphygmomanometer (HBP-1300, OMRON). Hypertension was de ned as a systolic blood pressure (SBP) ≥ 140 mmHg, and/or diastolic BP (DBP) ≥ 90 mmHg, or previous medical diagnosis and the use of antihypertensive medications in the last two weeks 14 . Diabetes mellitus (DM) was de ned as either previous medical diagnosis of DM, or ful llment of the diagnostic criteria for diagnosis based on fasting ≥ 126 mg/dl (7.0 mmol/l) in this survey 15 . Fasting blood samples were obtained from participants after a 10 hours' overnight fast. TC and HDL-C were measured by an enzymatic method using Hitachi auto-analyzer (Tokyo, Japan). We calculated non-HDL-C by subtracting HDL-C from TC.

Ascertainment Of Tse
The question on tobacco use included frequency, amount of smoking currently and in the past, and smoking cessation. Smoker was de ned as a documented lifetime use of more than 100 cigarettes 16,17 . The primary TSE variables were tobacco exposure status (unexposed, passively exposed, actively exposed), smoking intensity (number of cigarettes smoked per week among current smokers) and smoking burden (pack-years of smoking).
Based on the self-reported smoking patterns, tobacco exposure status was divided into three subgroups: tobacco unexposed (non-smoking and no exposure to secondhand smoke), passively exposed (non-current smokers but exposed to secondhand smoke), actively exposed (current smokers).
Smoking intensity and burden were two measures of tobacco exposure levels among current smokers, who were de ned as smoking within the previous 30 days. Smoking intensity could re ect short-term exposure levels and it was divided into three categories: current light smoker (< 70 cigarettes per week), moderate smoker (≥ 70 and < 140 cigarettes per week) and heavy smoker (≥ 140 cigarettes per week) 18 . Smoking burden which was a comprehensive indicator of smoking amount and duration, was divided into two categories according to the median level: lower burden (< 20 pack-years) and higher burden (≥ 20 pack-years) 19 . Pack-years of smoking was de ned as the product of the average number of packs of cigarette smoked per day multiplied by the average days of smoking in one year.

Covariates
We selected covariates into the analysis,which were age, gender, education status, alcohol drinking within 30 days, hypertension, type 2 diabetes, physical inactivity and overweight and obesity. Education status was self-reported as primary school and below, secondary school, or university degree and above. According to whether they had consumed alcohol within 30 days, participants were divided into two groups as current drinkers or not. Physical inactivity was de ned as less than 150 minutes of moderate intensity activity or equivalent quantity per week according to WHO classi cation criteria 20 .

Statistical analysis
Statistical analyses were performed in complex sampling module which considering sampling weight and post-weight using SPSS software (ver. 21.0 for Windows; SPSS, Chicago, IL, USA) for all analyses. All tests were two-sided, and P < 0.05 was considered statistically signi cant.
Moreover, subgroup analysis was performed according to sex to explore the distribution of population characteristics among different genders. Distribution of tobacco exposure status among male and female were showed Table 2. Secondhand smokers (tobacco passively exposed) were divided into 4 subgroups (0, 1 ~ 3, 4 ~ 6 and 7) according to days per week passively exposed to tobacco. Current smokers (tobacco actively exposed) were divided into 5 subgroups according to numbers of cigarettes smoked per week (< 40, 40~, 80~, 120~, and ≥ 160). Moreover, smoking intensity (light, moderate, heavy) and smoking burden (lower, higher) among current smokers were showed in Table 2. Logistic regression models were used to explore the different distribution in the same gender groups. Average non-HDL-C levels of subjects in each subgroup was calculated and plotted. The ANOVA analysis was conducted to compare the differences between groups (Fig A-B).
Participants were divided into two groups according to the mean value of serum non-HDL-C level with the cut-off value as 3.35mmol/l. Multivariate logistic regression analysis controlling for confounding factors including age, education status, alcohol drinking within 30 days, hypertension, type 2 diabetes and BMI was also performed to examine the association between non-HDL-C level and tobacco exposed status, smoking intensity or burden. Data are presented as odds ratio (OR) with 95% con dence interval (CI) ( Table 3-4). Table 3 Logistic regression results to estimate of association between higher non-HDL-C levels (cut-off value: 3.35mmol/l) and tobacco exposed status Participants

Demographics and clinical characteristics of the study population
The demographics and clinical characteristics of the study population were shown in Table 1. Among the 13,240 participants, 442 subjects were excluded because of the important information lost, such as smoking status and laboratory examinations. The remaining 12,798 participants were divided into three categories according to tobacco exposure status, actively exposed (n = 3286), passively exposed (n = 4573) and unexposed (n = 4939), respectively.
Of all participants, the mean age of tobacco actively exposed, passively exposed and unexposed groups were 46.3 ± 0.5, 45.5 ± 0.6 and 51.2 ± 0.9 years old, respectively. Of speci c interest, tobacco actively exposed, passively exposed subjects were younger than unexposed subjects (P < 0.01). While a majority of men were current smokers (51.2%), and 50.4% of women were tobacco unexposed subjects. There were differences in age and education status among three subgroups after adjusting for age and sex.

Characteristics Of Tobacco Exposure Status Among Exposed Participants
Of the tobacco passively exposed subjects, 21.7% had to exposed to second-hand smoke every day and the proportion among female was up to 27.0%.
Among current smokers, 41.2% were smoking more than 120 cigarettes per week, and 56.6% were heavy burden smokers who smoked more over 20 packyears (Table 2).

Mean Non-hdl-c Levels According To Tobacco Exposed Status
The tendency for average non-HDL-C levels according to numbers of cigarettes smoked per week in smokers, and days per week passively exposed among non-smokers are respectively showed in Fig A-B. With the increasing of the number of cigarettes smoked per week, the serum level of non-HDL-C increased gradually among male smokers (F = 2.83, P = 0.04) (Fig A). However, there was no difference therein among passively exposed subjects (Fig B).

Tse With Elevated Non-hdl-c Levels
The results of strati cation analysis and multi-factor logistic regressions between non-HDL-C and tobacco exposed status are presented in Table 3 Table 4 showed the logistic regressions results between non-HDL-C status and smoking intensity or burden among current smokers. Compared with lighter smokers, heavy smokers had signi cantly higher non-HDL-C levels (OR = 1.30, 95%CI: 1.02, 1.67, P = 0.03) after adjusting for age, sex, education status, alcohol drinking within 30 days, hypertension, type 2 diabetes, physical inactivity and BMI. Higher burden smokers had more risk of higher non-HDL-C levels (OR = 1.85, 95%CI: 1.46, 2.35, P < 0.001) compared with other current smokers. Subgroup analysis of different sex explored these relationships among male smokers remain signi cant. However, of the female smokers, the association between smoking intensity or burden and non-HDL-C levels were not found.

Discussion
Cigarette smoking has been linked to progression of AS, which may necessitate different measures of exposures and bio-markers 21 . Given the long latency period between tobacco exposure and AS, the identi cation of sensitive bio-markers of dyslipidemia is important for assessment of potential AS 22 . Several studies have recommended the use of non-HDL-C as a valuable predictor for dyslipidemia and cardiovascular risks 23,24 . In the current study, we demonstrated that tobacco actively exposed (current smoking), smoking intensity and burden were all positively associated with higher levels of the AS bio-marker non-HDL-C in the general adult population in Beijing, even after adjusting for several parameters that might affect the result. These data suggested that non-HDL-C may be useful bio-markers of smoking-induced AS, which might predict the risk and process of AS diseases and carry out preventive measures among smokers.
The relations between non-HDL-C and TSE in this study were consistent to some previous studies. Cigarette smoking was observed to increase non-HDL-C levels in middle-age male Japanese subjects 25 . Smoking was associated with high non-HDL-C in non-drinkers at middle and elderly ages (40s or older) and the authors suggested that elderly smokers had better stop or reduce smoking from the viewpoint of prevention of dyslipidemia 26 . Moreover, Yang et al.
discovered an increased smoking amount was signi cantly associated with a decreased HDL-C level 27 . However, there were some inconsistent results from other studies 12,28 . According to Srinivasan et al.'s study, non-HDL-C levels varied in terms of sex, age and ethnic group 12 . The possible reason of these mixed ndings was the signi cant difference on the de nition of tobacco exposure status.
Smoking intensity was positively associated with hsCRP independent of the duration of exposure, which suggested that acute tobacco exposure was associated with subclinical vascular damage. Smoking burden as an indicator of chronic tobacco exposure was related to abnormalities in all markers of in ammation and subclinical atherosclerosis in both former and current smokers 29 . Effects of smoking have been attributed to the mediational in uence of in ammation, plate aggregation and/or endothelial dysfunction 30 . In this study, we found male smokers who smoked more than 140 cigarettes per week and more than 20 pack-years, had signi cantly higher non-HDL-C levels. Further studies and objective assessment of TSE are needed to elucidate the underlying mechanisms and the causal effects of smoking on dyslipidemia.
In the present study, no signi cant associations were found between second-hand smokers and elevated non-HDL-C. Several studies performed in Asian populations also did not nd any consistent relationship between secondhand smoke (SHS) and lipid levels among adults, but the effects of secondhand smoke exposure on lipid index among adolescents had been con rmed by several studies [31][32][33] . Chen et al. made meta-analyses and reported secondhand smoke (SHS) was negatively associated with HDL-C in the lower age group (7-18 years), but no effect in the upper age group (27-74 years) 34 . Children and teenagers are at a critical stage of growth and development, and their blood lipids are susceptible to living environmental factors, such as smoking, pesticides 11,35 . This may explain why SHS is more likely to lead to signi cant dyslipidemia in younger people than in adults. Further longitudinal assessments of exposure to tobacco in adults are required to validate the ndings.
Higher burden male smokers (> 15 pack-years) had more risk of higher non-HDL-C levels compared with other smokers. Smokers should pay attention on their blood lipid levels in order to prevent atherosclerosis.

Declarations
Ethics approval and consent to participate The study protocol was approved by the Ethical Review Board of BJCDC (The number is no. 5 of 2017). All participants signed written informed consent.

Consent for publication
All authors have approved this manuscript for submission.

Competing interests
There was no any potential competing interests.