The mean HDL-C level found in this study (52.6 ± 0.8 mg/dL) is in the normal range (between 50 and 60 mg/dL) (19) and similar to that reported in a previous Tunisian research on dyslipidemia, conducted among 1484 women aged 35–70 y old, in the same sampling area (25). Compared to data registered elsewhere, the mean HDL-C value in childbearing age Tunisian women is higher than that recorded in Japanese (26), Korean (27), Hispanic and African American women (28) and lower than that reported in Canadian (29), Danish (14) and US women (30). The differences in HDL-C levels between various races and ethnic groups may in part be due to genetic factors but the role of behavioral, environmental and anthropometric covariates seems to be important too (31, 32).
Age appears to be an independent negative risk factor that can affect HDL-C levels in Tunisian women. This is consistent with previous studies reporting a decrease of HDL-C with age in women (27, 33). Many factors could explain this phenomenon such as the frequency of insulin resistance and impaired lipolysis at advanced age that could affect the RCT. Inflammatory processes in aged people as well as hormonal changes are other possible causes of decline in HDL-C with age (34).
Parity and marital status influenced negatively the HDL-C concentration in Tunisian women. After pregnancy, the level of cholesterol bound to HDL particles tends to decrease, which explains the tendency of multiparous women to have lower circulating HDL-C levels than women who have never given birth. These changes in circulating cholesterol levels are likely due to changes in estrogen levels which vary throughout a woman's genital life (35, 36).
Menopause didn’t affect circulating HDL-C levels. This result is in contradiction with those of several authors showing that a worse lipid profile is observed in postmenopausal women in comparison to premenopausal ones due to hormonal changes involving the decrease in estrogen level and increase in luteinizing hormone and follicle stimulating hormone levels (33, 37). In our study, the majority of women (92.3%) are premenopausal, which could explain the absence of relationship between menopause and lipid profile.
In this study, women with high HDL-C levels are more educated and have a lower socioeconomic status than those with mean or low levels of HDL-C. Agongo et al. (2018) (38) found a positive significant association between formal education and socioeconomic status with HDL-C levels in women from rural northern Ghana, while no significant association was found between HDL-C and socioeconomic status of Korean women (27). The mechanisms of association between HDL-C and socioeconomic status are complex due to the influence of lifestyle factors and dietary habits as well as stress variations by social class (39).
Results on the associations between HDL-C levels and lifestyle factors (physical activity, alcohol consumption and smoking) showed that smoking was the only negative risk factor of HDL-C in Tunisian women. Research has shown that physical activity and moderate alcohol consumption are positively correlated with HDL-C contrarily to smoking. According to King et al. (1995), a regular physical activity increases the HDL-C level by 3 to 9 percent in healthy sedentary persons (40). This increase depends on exercise frequency and intensity and is attributed to the stimulation of the production of pre-β HDL-C and RCT (41). The effects of smoking on HDL-C are dose dependent and reversed upon smoking cessation. Nakamura et al. (2020), found that in both men and women, current smokers had significantly (p < 0.001) lower HDL-C than non-smokers (-7.3%, -4.3%) (42). Likewise, Jain and Ducatman (2018) reported lower HDL-C in smokers than in non-smokers (48.8 vs 51.4 mg/dL, p < 0.01) (43). Alcohol consumption in moderation raises the concentration of HDL-C, possibly by increasing cellular cholesterol efflux and plasma cholesterol esterification (44). Brien at al. (2011) reported an increase of HDL-C by 0.1 mmol/L with a quantity of alcohol of about 30 g/day (45). However, the cardioprotective effect of raised HDL-C by alcohol consumption is largely unknown.
While the univariate analysis showed a higher prevalence of chronic diseases in women with normal or low HDL-C levels (overweight, obesity, abdominal obesity and hypertension) than the counterpart group, the multivariate regression analysis revealed that hypertension was the only negative risk factor of HDL-C in Tunisian women. Due to epidemiological and nutritional transition, the prevalence of overall obesity and abdominal obesity in Tunisian women has increased drastically during the last decades (46). In this study, the overall obesity affected the third of Tunisian women and the abdominal obesity concerned almost the half, with a decreasing trend with HDL-C levels. The negative associations between obesity and HDL-C have long been reported and were attributed to the potential role of HDL-C or ApoA-I on adipose tissue content regulation (47, 48). Hypertension is a well-established risk factor for CVD and is strongly associated with dyslipidemia, a group of metabolic derangements including low HDL-C levels. This association occurs at the vascular endothelial level leading to an increase of oxidative stress and endothelial dysfunction (49). The inverse association between HDL-C and hypertension was reported elsewhere (50). Halperin et al. (2006) found that men in the highest quintile of HDL-C had a 32% decreased risk of developing hypertension compared with those in the lowest quintile (51). Likewise, Tohidi et al. (2012) found that women with HDL-C level between 1.0 and 1.5 mmol L− 1 had 33% lower risk of hypertension compared with those having HDL-C levels < 1 mmol L− 1 (52).
Family history of chronic diseases (CVD, hypertension, diabetes, obesity) was not correlated with HDL-C levels in this study, except the family history of cancer. In addition, the intake of lipid-lowering drugs is evenly divided between participants. According to Steyn et al. (1989), women with high levels of HDL-C were less likely to have a history of hypertension or diabetes (53) than those with low HDL-C concentrations. Opoku et al. (2019) reported negative significant associations of the history of coronary heart disease and the history of stroke with HDL-C in Chinese women (50). In the Bogalusa Heart Study, children with fathers’ history of myocardial infraction had low ApoA-I levels and a high ApoB/ApoA-I ratio, whereas their HDL-C levels were not outside normal limits (54). In this study, family history of cancer revealed an inverse association with HDL-C levels between the univariate and the multivariate analysis. After adjustment for all sociodemographic, metabolic and lifestyle factors, the family history of cancer was a strong positive predictor of HDL-C in Tunisian women. Similar findings were observed in a cohort study on US veterans, which reported a slight increase in cancer mortality among participants with high HDL-C levels (> 50 mg/dL). However, other epidemiological studies reported that a low HDL-C level may be a risk of cancer deaths or a prognostic factor of many types of cancer in obese subjects (29). These controversial results need further investigations on the relationship between HDL-C and cancer disease.
Significant differences were noticed between biological characteristics in women with high HDL-C levels and the counterpart group. Triglyceridemia, ApoB, TC/HDL-C and ApoB/ApoA-I ratios, SBP and DBP were lower in women with high HDL-C concentrations contrary to TC and ApoA-I levels. Increased plasma triglyceride levels have been associated with an increased risk of CVD even when HDL-C levels were adjusted for (55). ApoA-I is the major structural and functional HDL protein which accounts for approximately 70% of total HDL protein and is significantly associated with HDL particles (56). However, more than 90% of all ApoB in blood is found in LDL (57). Clinical studies have reported that elevated ApoB levels, an increased apoB/apoA-I ratio and low levels of apoA-I were better predictors of cardiovascular events than LDL-C, TC and triglyceride levels even in patients receiving statins (57). SBP and DBP were lower in women with high HDL-C levels. This result confirms the protective role of HDL-C against risk factors of CVD such as raised blood pressure or hypertension. Despite the significant differences in biological characteristics between women with high HDL-C and those with normal or low HDL-C, all mean concentrations were within the normal range for both groups in our study.