We initially observed that the multivariate technique of interdependence is a good way to evaluate the inflammatory pattern in the elderly and that the association between IL-6, TNF-α, and zinc are responsive markers for the diagnosis of inflamm-aging, for which, until now, there are no normative diagnostic values in the literature 37. In addition, there were no differences between the clusters when comparing the prevalence of sarcopenic individuals. The elderly residents in not-for-profit LSIEs presented the best results regarding the inflamm-aging condition more frequently in the female sex. Concentrations of LDL, cholesterol, and triglycerides were higher in the inflamm-aging group. In the analysis of the net effect of the independent variables, a greater influence of age, HDL, LDL, and triglycerides was observed in the characterization of the inflamm-aging group.
The process of low-intensity chronic inflammation (inflamm-aging) results in a continuous increase in the production of pro-inflammatory cytokines, including IL-6 and TNFα 3–5. For example, Xia et al. 37 propose that pro-inflammatory cytokines play a key role in the mechanism of inflamm-aging, with serum concentrations of IL-6 and TNFα markers representative of inflamm-aging 37. When we observed the characteristics of cluster 1 in relation to the proinflammatory cytokines, we found higher values of IL-6 (8.15 pg/ml) and TNFα (12.6 pg/ml) than those observed in another study performed with young and elderly adults, both in IL-6 (2.57 pg/ml) and TNFα (4.94 pg/ml) 38. In addition, the IL-6 values found in cluster 1 were lower when compared to severe inflammatory conditions such as sepsis (IL-6 = 64.1 pg/ml) and systemic inflammatory response syndrome (IL-6 = 41.1 pg/ml (43), a fact that classifies the inflamm-aging cluster with low intensity inflammation. These results are very important to define the risk group to COVID-19 among institutionalized elderly, recognizing that patients who do not survive to this disease present higher values of IL-16 compared to patients who do not die 30.
In addition to the increase in the synthesis of proinflammatory cytokines triggered in inflamm-aging 1, the increase in bioavailability of zinc can be considered an important marker to characterize this condition due to the relationship presented with pro-inflammatory cytokines 39 of zinc in reducing the transcription of proinflammatory cytokines while favoring the transcription of anti-inflammatory cytokines 7,39. This information is consistent with the results of zinc and its relationship with IL-6 and TNFα observed in the clusters. The plasma zinc values (76.27 μg/ml) observed in the inflamm-aging cluster were lower than those in the non-inflamm-aging cluster (98.11 ug/ml). In addition, zinc (F = 116,83) was the variable that best described the clusters, followed by IL6 (F = 101,09) and TNFα (F = 72,98), demonstrating the importance of inserting this marker in future studies aimed at identifying inflamm-aging, corroborating studies that point to this variable as an important modulator of inflammatory activity 39.
This analysis is very important because there is research that has already demonstrated that zinc has antiviral effects; improving immune responses and suppressing viral replication provide a protective role against the COVID-19 pandemic, likely by improving the host's resistance against viral infection 40.
In contrast to the inflamm-aging cluster, the configuration of the proinflammatory and zinc cytokines in the cluster without inflamm-aging presents a framework with a lack of inflammation due to the reduced values of TNFα and IL-6 and high values of zinc. Mocchegiani et al. 39 defend the ability of pro-inflammatory cytokines to regulate the bioavailability of zinc observed in the clusters, justifying this antagonistic relationship between these two markers. The cluster with a transient inflammatory state presents characteristics of both clusters, in which the concentration of IL-6 was similar to that presented by the cluster without inflamm-aging and the zinc index was similar to that presented by the cluster with inflamm-aging, whereas the TNFα value presented an intermediate pattern between the other two clusters, being therefore characterized as a "transient inflammatory state".
The deficiency of zinc, common in the elderly 41, associated with increased secretion of inflammatory cytokines, also plays an important role in oxidative stress and endothelial cell apoptosis 6. Cardiovascular diseases are associated with IL-6 and TNF-α, which in turn act in distinct but complementary ways 42. IL-6 is considered an independent risk factor for the prothrombotic state, whereas TNF-α induces endothelium-dependent relaxation, interfering with the binding and transmigration of leukocytes through the endothelium 42. The concentrations of IL-6 and TNF-α are associated with dyslipidemia, a factor that compromises endothelial function 5,7.
Systemic immune system dysregulation may be able to cause acute respiratory distress syndrome (ARDS) but multiple organ failure and finally lead to death in severe cases of SARS-CoV-2 infection. Therefore, inflammatory biomarker dosage is also part of preliminary guidelines for the diagnosis and treatment of coronavirus 2 (SARS-CoV-2) infection 43.
In addition, zinc deficiency alters key transcription factors and adhesion molecules at LDL receptors, increasing the risk of hypertriglyceridemia, which reinforces the deleterious context for cardiovascular health 6,39. These findings may explain the greater net effect of LDL and triglycerides on inflamm-aging compared to other biochemical variables and reinforce the theory of endothelial injury as a theoretical model to elucidate chronic inflammation with a low intensity characteristic of inflamm-aging 44,45. Regarding HDL, although contradictory, the increase in its plasma concentration was also associated with inflamm-aging in the ordinal logistic regression model. This is because all groups presented reduced HDL values, with a slightly reduced mean in the cluster without inflamm-aging, which was already expected due to their overall reduction in cholesterol and LDL concentrations.
This framework of association with the inflamm-aging process is evidenced in the results of the present study, which demonstrated that individuals with inflamm-aging present higher amounts of LDL, triglycerides, and total cholesterol, resulting in an increased risk of endothelial injury 42. This probably happens considering that the small increases in the concentrations of LDL, triglycerides, and total cholesterol are able to promote greater infiltration of lipoproteins in the arterial walls, generating accumulation and, consequently, endothelial injury 46. In cluster 2, the proinflammatory cytokines presented lower values compared to cluster 1, a fact that did not occur in the variables related to the lipid profile. This indicates that the concentrations of LDL and triglycerides in cluster 2 resulted in better regulation of TNF-α and IL-6 by the immune system due to the action of anti-inflammatory cytokines (e.g., IL-10) or in a shorter time of exposure to an unresolved inflammatory state in relation to the inflamm-aging cluster, since zinc was similar between these clusters.
This association of blood fat levels with inflammaging becomes even more serious when we consider that the presence of cardiovascular disease is associated with a worse prognosis and mortality for COVID-19 47. In practical terms, a study by 48 reported an almost two-fold increase in mortality in patients with cardiovascular disease. With regard to the relationship between inflamm-aging and COVID-19, we can highlight that understanding this inflammatory condition is fundamental for the identification of elderly people with a higher risk of mortality if they are exposed to the virus. The scientific community 47,49,50 has proposed that the course of treatment and prognosis of COVID-19 should be stratified according to the presence or absence of comorbidities prior to SARS-COV-2 infection. Thus, those patients who have acute respiratory failure syndrome (SIRA), a characteristic symptom of COVID-19 but do not have any comorbidities, are stratified into TYPE A patients. Those who, in addition to SIRA, also have some comorbidities are classified as TYPE B. Finally, those classified as TYPE C are those who additionally have multiple organ injuries 49. Thus, the higher the classification, the worse the prognosis for the disease. Based on this, it is possible to assume that the elderly with inflammatory characteristics already have a worse prognosis.
Regarding the sociodemographic variables, we observed that cluster 3 presented predominantly male elderly (56.1%) and elderly with low socioeconomic status (53.1%). The greater presence of men in this cluster could be explained by the fact that women present a higher chronic inflammatory state in relation to men, justified by the reduction in the production of estrogen, a hormone with an anti-inflammatory protective effect, caused by menopause, which therefore increases the possibility of low intensity inflammatory conditions 51,52. With respect to socioeconomic conditions, it is expected that elderly people residing in for-profit LSIEs will present a more debilitated global health state 53. For the clusters inflamm-aging and transient inflammatory state, no great differences were observed that could characterize these clusters regarding their demographic condition. This corroborates the findings obtained for the biochemical and anthropometric measures, in which no differences were observed for these groups. Furthermore, because residency in the not-for-profit LSIE groups several socioeconomic classes in the same group, it is pertinent to seek new strategies for future studies, which use more precise data to analyze this variable, to differentiate individuals using the indicator that best stratifies socioeconomic condition.
Moreover, we can observe that long-lived elderly people (≥ 80 years old) had a greater influence on the characterization of inflamm-aging. This was expected, since it is an age group that presents greater factors of health aggravation. Long-lived elderly individuals have presented a higher prevalence of arterial hypertension 54, associated with higher cardiovascular risk and lower functional capacity 55. In a study conducted with long-lived elderly Brazilians of both sexes, when analyzing 9 chronic noncommunicable diseases, a prevalence of 67.0% of multimorbidity was observed in long-lived elderly compared to those aged less than 80 years 56.
In addition, we can infer that long-lived elderly individuals (>80 years) who present a greater amount of fat in the blood may present a higher relative risk of death from COVID-19. Therefore, it is possible to suggest that mechanisms associated with endothelial injury are associated with inflamm-aging, serving as the basis for future studies on this condition in institutionalized elderly and prevention factors to reduce COVID-19 mortality. In this way, public health strategies should direct their efforts to the fight against the control of diseases associated with mechanisms promoting endothelial injuries, considering this as the main factor associated with inflamm-aging. In addition, studies should be performed that seek to establish other diagnostic values for the inflammatory condition of the elderly, especially those that chronically verify the effect of this condition on the health problems of the elderly. We suggest the analysis of biochemical markers in place of inflammatory markers as a way to lower the future costs of a preventive evaluation to groups at higher risk.