Altered tryptophan absorption and metabolism could underlie long-term symptoms in survivors of coronavirus disease 2019 (COVID-19)

•Coronavirus disease 2019 (COVID-19) causes long-term symptoms including fatigue, depression, sleep disturbances, and muscle weakness.•COVID-19 causes altered metabolism of tryptophan in favor of the kynurenine pathway and disturbed tryptophan absorption due to altered expression of intestinal angiotensin-converting enzyme 2.•Both an increased kynurenine pathway and decreased absorption of tryptophan could be main contributors to long-term symptoms of COVID-19. The global pandemic of COVID-19 has been lasting for more than one year and there is little known about the long-term health effects of the disease. Long-COVID is a new term that is used to describe the enduring symptoms of COVID-19 survivors. Huang et al. reported that fatigue, muscle weakness, sleep disturbances, anxiety, and depression were the most common complaints in COVID-19 survivors after 6 months of the infection. A recent meta-analysis showed that 80% of COVID-19 survivors have developed at least one long-term symptom and the most common five were fatigue, headache, attention deficit disorder, hair loss, and dyspnea. In this paper, we discuss the hypothesis that altered tryptophan absorption and metabolism could be the main contributor to the long-term symptoms in COVID-19 survivors.