Our findings were similar to those of previous studies that reported a correlation between a poor SRH and a high hs-CRP level ([7–9, 26]. In a study of 4,049 respondent older adults without significant cognitive deficit by Szybalska et al. , a worse SRH was associated with increased interleukin-6 (IL-6) and CRP levels. Leshem-Rubinow et al.  analyzed the correlations between SRH and the inflammation-sensitive biomarkers hs-CRP and fibrinogen in 13,773 healthy individuals and observed higher biomarker levels in the group with the lowest SRH level; hs-CRP showed a correlation in both men and women, but fibrinogen only showed a correlation in men. Shanahan et al.  studied 13,236 young adults and reported that on adjusting for acute/chronic diseases, medication history, and health behaviors, a lower SRH level was associated with a higher hs-CRP level; however, on adjusting for BMI, the correlation in female participants was weakened, whereas the correlation in male participants remained significant. However, a study of 16,256 Japanese individuals reported a significant correlation between a poor SRH and a high hs-CRP level only in female participants . Thus, while correlations between a poor SRH and a high hs-CRP level have been reported, the above studies show limitations such as restricted age of participants, including only older adults  or only young adults , and lack of consideration for diseases that could affect the relationship between SRH and CRP . Moreover, there have been few studies on the relationship between SRH and CRP [7, 28] so far; there have been no such studies in Koreans.
Several studies have reported correlations between a poor SRH and pro-inflammatory cytokines, including IL-6 [17, 29]. CRP is produced by hepatocytes under the regulatory control of IL-6 and other inflammatory cytokines , and these pro-inflammatory cytokines cause sickness behaviors, such as weakness, depression, exaggerated pain (hyperalgesia or allodynia), and lack of appetite . In other words, the relationship between a poor SRH and a high hs-CRP level can be explained by differences caused by pro-inflammatory cytokines, and this, in turn, can explain our results.
Inflammatory indices in women are known to be altered by the menstrual cycle, menopause, and hormone therapy , and menopause and estrogen replacement therapy have been reported to affect obesity and inflammation in women . CRP levels can be presumed to change depending on the hormonal environment, and this could act as a confounding factor in the relationship between SRH and hs-CRP . In our study, we were unable to investigate whether participants were taking female hormones or their stage in the menstrual cycle. As we only accounted for female menopause, we may not have observed significant results for the association between SRH and hs-CRP levels in women. Moreover, biological sex is known to affect CRP-related genetic variation , and according to a study by Kettunen et al. , allelic variants in the CRP gene are associated with CRP levels, and men and women show differences depending on the CRP genotype. Hence, the differences in genetic variation between men and women could have affected our results.
Sex differences have been reported in the relationship between CRP levels and mortality [37–40], but it is unclear why a high CRP level is only associated with an increased mortality risk in men. Dong et al analyzed the middle-aged Chinese population; hs-CRP was associated with increased risk of developing CVD . In addition, Lee JH et al.  studied 23,233 rural Koreans and reported that a high CRP level was more strongly associated with higher mortality in men than in women. There have been several studies reporting a stronger correlation between SRH and mortality in men than in women [40, 43]. Specifically, men with a poor SRH have been reported to show a higher risk for conditions related to mortality, such as CVD and cancer. When assessing SRH, the subject rates their current overall health; it has been reported that men rate their own health in comparison to that of other men, and male SRH tends to mostly reflect serious and life-threatening disease, whereas female SRH tends to reflect other factors unrelated to mortality and chronic, non-life-threatening disease, resulting in a weaker correlation between SRH and mortality for women [43–45]. Moreover, in a study of Korean adults by Shin et al. , women tended to rate their own health more poorly than men, and in a study by Lee SY et al. , traditional Korean gender roles had a negative effect on women, and the risk of a poor SRH was higher among Korean women than among women from the US. Similarly, in our study, we only observed a correlation between SRH and hs-CRP among male participants. The discrepancy between men and women could be related to the fact that CRP is more likely to reflect CVD and mortality in men than in women and that SRH is more likely to directly reflect health and mortality in men than in women.
In our study of Korean adults aged ≥19 years, when we analyzed all participants, the poor SRH group was more likely to have high hs-CRP levels (>1.0 mg/L) than the very good SRH group. Especially in male participants, as SRH went from very good to very poor, there was a corresponding increase in the risk of high hs-CRP level (>1.0 mg/L). These results can be explained by the fact that immune-related activity is associated with vague symptoms of malaise and interoceptive perception . Such findings are consistent with those of a previous study of healthy adults, in which a poorer SRH was associated with increased serum inflammatory marker levels (IL-6 and CRP) .
Even after correcting for all sociodemographic characteristics, health-related factors, and chronic diseases known to be associated with low-level inflammation, among male participants, the very poor SRH group showed 1.74 times higher risk of high hs-CRP level than the very good SRH group, but there was no significant relationship among female participants. This finding could be because SRH is a dynamic evaluation for judging the trajectory of health, which reflects both clinical stage and preclinical stage disease . Therefore, even after correcting for chronic diseases associated with hs-CRP, we still observed a correlation in male participants.
Our study has several limitations. First, because this was a cross-sectional study, it was not possible to infer causal relationships, and we could only investigate the correlation between SRH and hs-CRP. Nevertheless, the value of this study is that we used data from the KNHANES, which is representative of the Korean population, and that it was a large-scale study of Korean adults. Second, SRH assessment was performed at specific times. Future monitoring studies are necessary to ascertain the long-term relationships between SRH and hs-CRP. Third, we only used hs-CRP as an inflammatory marker; further studies will need to investigate the correlations of SRH with other indicators (e.g., IL-6, tumor necrosis factor-alpha). Finally, because the study was based on data from a survey of South Koreans, the results could have been affected by the racial characteristics of Koreans and may thus be difficult to apply to people of other races. Despite these limitations, our study showed a strong correlation between a poor SRH and a high hs-CRP level in male Korean adults and is valuable as the first study to examine the relationship between SRH and hs-CRP in Korean adults.