One of the main findings of our study was Covid-19 milder severity in the young population. Our second finding was that the GI symptom frequency and liver test disorder were less common in the young population.
Covid-19 is a disease whose severity and mortality increase with age, and it is mostly mild in young people. Studies have shown that patients over the age of 60 are approximately 4 times more mortality than those under the age of 60 (5.2%, 1.4% respectively.) (7). In the young population, the mildness of the disease than the elderly seems likely to be related to a better immune system. Age-related defects in the function of T and B lymphocytes may lead to a deficiency in the control of SARS-CoV-2 replication and longer pro-inflammatory responses, potentially leading to a worse outcome (15). Also, the fact that the respiratory defense mechanisms and secretion clearance is better in young people and the absence of chronic diseases may be related to this situation.
In addition to respiratory symptoms, GI symptoms frequently occur in coronavirus infections. In two recent coronavirus outbreaks (SARS 2002, MERS 2012), respiratory tract symptoms are often accompanied by GI symptoms (16–18). Although the mechanism of SARS-CoV-2 causing GI symptoms is unclear, it is angiotensin-converting enzyme-2 (ACE-2) binding receptor for SARS-CoV-2, and ACE-2 is also present in large numbers in the digestive system. It is thought to bind to the ACE-2 receptor in the digestive system and cause symptoms such as nausea, vomiting and diarrhea (19, 20).
In our study, GI symptoms were present in 6.8% of patients at the time of admission. In a study in which 206 patients were evaluated in the literature, diarrhea was observed in 32.5% of the patients and the average age of the patients was 62.5 (21).
In another study, 204 patients with an average age of 52.9 years, 38 (18.6%) patients had GI symptoms such as diarrhea, vomiting, and abdominal pain (10). In the study in which 651 patients were evaluated, 11.4% of the patients had GI symptoms and the mean age of those with GI symptoms was 46.1 (22). As seen in these studies, the frequency of GI symptoms decreases as the age decreases. The cause of fewer GI symptoms in young people is not fully known. The better condition of both the immune system and the respiratory barrier systems in young people may decrease the load of SARS-CoV-2 in the GI tract and prevent GI symptoms. However, larger studies are needed to compare young people with older individuals, examining the SARS-CoV-2 genome burden in feces and its relationship to symptoms.
Similar to lung tissue, there is ACE-2 receptor expression in liver tissue. Especially in cholangiocytes, ACE-2 expression is evident and is 20 times higher than hepatocytes (59.7%, 2.6% respectively.) It has also been found that ACE-2 in cholangiocytes is expressed at levels comparable to that in pulmonary cells. Although cholangiocytes have many important functions in the liver, such as regeneration and immune response, it is not known whether the disorder in liver tests is due to direct liver inflammation or systemic inflammation by SARS-CoV-2 that binds to the ACE-2 receptor (23, 24).
In our study, in the first biochemical study performed to patients, 13.7% of the patients had impairment in liver tests. In a study by Chen et al., 43.4% of patients had liver test disorder. Liver tests were significantly impaired in one patient and were significantly higher with levels which are ALT 7590 U/L, AST 1445 U/L (5).
In another study, 37% of patients had impaired liver tests (6). The mean ages of these three studies were 55, 59.7, and 49 years, respectively. In another study, the average age of which was lower (41 years) compared to these studies, a lower rate (%16) of liver test disorders was detected(14). In addition, in a meta-analysis of 3024 diseases conducted in China where 21 studies were analyzed, there was a significant relationship between disease severity and liver enzymes disruption (12).
According to the literature, as the age decreases in Covid-19, fewer liver test disorders are observed. As the age decreases, the severity of the disease decreases relatively, and there is a need for large-scale experimental studies to investigate whether the defect in liver tests is due to inflammation or systemic inflammation caused by SARS-CoV-2 that binds to ACE-2.
Limitations
The first limitation of our study was that it was retrospective due to the risk of transmission. In addition, due to the low number of patients, the comparative statistical analysis could not be made between those with and without GI symptoms or liver test disorders.