To our knowledge, this is the first study focusing on the associations of the whole blood metalloids with the severity and outcome of COVID-19. We performed a retrospective study of 306 patients with COVID-19 for their whole blood levels of essential minerals including magnesium, calcium, chromium, manganese, iron, copper, zinc, and toxic metal(loid)s including arsenic, cadmium, mercury, thallium and lead according to the disease severity (severe or non-severe) and outcomes (recovered or deceased), respectively. Our data revealed the whole blood magnesium, cadmium, chromium, manganese, iron, copper, zinc, arsenic and thallium displayed significant differences between severe and non-severe group (p < 0.05), and the whole blood chromium, cadmium and arsenic displayed differences between the deceased group and recovered group (p < 0.05). These data suggested the whole blood levels of essential minerals and toxic metal(loid)s may be associated with the disease progression and outcome, and could be potential risk factors correlated with the severe illness and mortality of COVID-19.
For essential minerals, as the world awaits an effective vaccine, nutrition may play an important and safe role in helping mitigate patient morbidity and mortality. Optimal nutritional status of relevant nutrients is important for a well-functioning immune system during the COVID-19 crisis [8, 9]. Among the many essential nutrients, magnesium is the second most abundant intracellular cation after potassium, and involved in > 600 enzymatic reactions in the body, including those contributing to the exaggerated immune and inflammatory responses exhibited by COVID-19 patients [19]. Our data suggested the significant lower levels of magnesium in severe patients with COVID-19 than in the non-severe patients, which is consistent with the hypothesis by Stefano Iotti that a low Mg status might foment the transition from mild to severe clinical manifestations of the COVID-19 [20]. Several aspects of the COVID-19 mimic the metabolic events shown to occur during latent subclinical magnesium deficiency, such as a drop of T cells, increased plasma concentration of inflammatory cytokines, and endothelial dysfunction [19, 20]. Constant monitoring of magnesium status was thought as an possible strategy to influence disease contraction and progression [19].
Calcium was reported as a key role in viral fusion for many enveloped viruses such as SARS-CoV, MERS-CoV and Ebola virus [21]. Low levels of serum total and ionized calcium were reported in COVID-19 patients [22]. Hypocalcemia had already shown to be common in patients with SARS and in patients with Ebola virus disease, and may occur also in COVID-19 [21]. However, our data suggested a mild increase of whole blood total calcium (63.55 to 68.20 mg/L) in severe COVID-19 patients than non-severe cases. We suspected that these amount of increased calcium may be from the protein-binding forms or due to the variations of the blood cells, which need to be confirmed in future studies.
Chromium is an essential micronutrient involved in carbohydrate, lipid and protein metabolism primarily by increasing insulin efficiency [23, 24]. Previous study has confirmed the drastic increase of urinary chromium in patients with diabetes [23]. Chromium is also a naturally occurring heavy metal found commonly in the environment, which could affect various components of the immune system and may result in immunostimulation or immunosuppression [25]. Higher levels of chromium in severe and deceased patients with COVID-19, suggested by our study, may be related to the comorbidity of diabetes and the immune dysfunction in COVID-19.
Transition metals such as manganese, iron, copper, and zinc are essential for all forms of life, as 30% of enzymes require a metal cofactor [26]. Clinical deficiency of manganese, iron or zinc in the host increases the incidence of infectious disease and mortality [27], which is consistent, to a certain extent, with our findings that the whole blood manganese, iron or zinc decreased in the severe COVID-19 patients than non-severe patients. Among then, manganese has been reported of playing a important role in innate immune activation and host anti-viral defense, as it released from organelles into the cytosol upon virus infection and facilitates the activation of cGAS and STING signaling [27]. For iron, systemic dysregulation resulted from COVID-19 hyperinflammation has been recently reviewed [28]. Serum iron level was reported as a potential predictor of COVID-19 severity and mortality [29]. Decreased serum iron level could predict the transition of COVID-19 from mild to severe and critical illness [30, 29], which was in agreement with our findings. Zinc is one of the micronutrients that could be consumed to reduce the intensity of SARS-CoV-2 infection and perhaps lessen the respiratory tract infection through the antiviral actions [7]. Zinc deficiency can contribute to defective cell-mediated immunity and to increased susceptibility to various infections, including pneumonia [31]. Several lines of evidence suggest a link between zinc and COVID-19, including the observation that chloroquine, a drug being repurposed for COVID-19, is a known zinc ionophore [32]. SARS-CoV and SARS-CoV-2 use the host zinc metalloenzyme, ACE2, as an entry point to cells [33]. Consistent with these overall findings, our data suggested the lower level of whole blood zinc, and also the transition metal manganese and iron, were associated with the severe illness of COVID-19. Our finding that these essential minerals including manganese, iron, zinc and magnesium positively correlated with each other with the highest coefficients further implied a possible synergistic effect of the metals on the disease.
In contrast to the transition metals above, copper showed a different changes, as it increased in the severe patients compared with the non-severe cases. This difference may be associated with the two sides of copper, as it is an essential micronutrient for both pathogens and the animal hosts they infect, and also be toxic in cells due to its redox properties and ability to disrupt active sites of metalloproteins [34]. Animal host can thwart pathogen growth by limiting their copper nutrients, similar to the well-documented nutritional immunity effects for starving microbes of essential zinc, manganese and iron micronutrients [34]. Meanwhile, a common hallmark of infection irrespective of the agent (viral, bacterial, fungal) is a marked and progressive rise in serum copper, including the lung infections [35–37]. In addition, the increased serum copper to zinc ratio has been reported to be associated with the inflammation conditions [38], as similar with the findings in the present study.
Besides the micronutrients discussed above, environmental pollutants, including the heavy metals or metalloids, are believed to be associated with the viral epidemic/pandemic events and prevalence [14]. In the present study, the heavy metals in whole blood were investigated, among which, cadmium was found to be higher in the deceased cases. Cadmium has cumulative toxicity to many organs due to its long biological half-life. It was reported that exposure to cadmium and lead could cause adverse effect on human health on the respiratory system with lung function impairment [39].
However, the arsenic, lead and thallium showed lower levels in whole blood of the severe cases than the non-severe cases, and the arsenic also showed a lower level in deceased cases than the recovered cases. These results were unexpected, as exposure to heavy metals was commonly reported to induces respiratory dysfunction and positively correlated with the occurrence of respiratory diseases [40]. Considering that the heavy metals in whole blood were found to be at relatively low levels in our study, which were all within the baseline range or mildly lower than the lower limit in all the patients with COVID-19, we suspected that the decrease of the circulatory heavy metals in severe patients with COVID-19 may be attributed to the abnormalities of peripheral blood system [41]. The positive correlations among arsenic, thallium and lead suggested by our results implied a similar mechanism existed for these elements. The mechanisms of lower levels of whole blood arsenic, thallium and lead in severe or deceased COVID-19 patients need to be verified and clarified in the future studies.
There are some limitations should be noted. This study was based on only 306 severe or non-severe patients with COVID-19 in Wuhan of China. Specifically for the severe group, deceased cases accounts for 14.4% (N = 15). Thus future multi-center studies on a larger cohort were needed to verified the findings. In addition, the present study mainly focused on the associations of disease severity and mortality with whole blood metal(loid)s, but did not excluded the effect of comorbidities, which should also be noticed in future studies.
In conclusion, we provided a comprehensive analysis of the abnormalities of 12 metal(loid)s for the COVID-19 disease. Whole blood calcium, chromium and copper were higher, while magnesium, manganese, iron, zinc, arsenic, thallium and lead were lower in the severe patients. Among the severe patients, chromium and cadmium were higher, while arsenic were lower in the deceased group. Our study determined the iron and arsenic, and age and sex were independent factors associated with the disease severity, while chromium, cadmium and the co-morbidities of cardiovascular disease were independent factors associated with the mortality. These results suggest variations of whole blood metal(loid)s as associated factors correlated with the disease progression and fatal outcome, which could be persistently monitored, and would be helpful in the evaluation of the dynamic changes in patients with COVID-19.