**7. Covid-19 discovers our Achilles heel**

Covid-19 probably interferes with the tryptophan uptake pathway and therefore T cell and Interferon responses via a chaperone mechanism with the Angiotensinconverting enzyme (ACE2) receptor through which it enters cells. This amino-acid uptake mechanism also malfunctions with mutations that lead to Hartnup disease that includes a pellagra-like syndrome [55]. Some clinical manifestations of Covid, such as on gut, skin and cognition, or "long Covid" (whose multi-organ symptoms as happened to pellagrins were often doubted), could be "formes fruste" or new versions of pellagra as is supported by documented abnormal tryptophan and nicotinamide metabolism [56–59]. At risk groups such as the elderly or those in poor countries on poor diets may be at risk as they start off from an NAD depleted state. It remains to be seen if supplementation would help before, during or after this or other acute infections.
