Chronic Systemic Low-Grade Inflammation and Modern Lifestyle: The Dark Role of Gut Microbiota on Related Diseases with a Focus on COVID-19 Pandemic

Inflammation is a physiological, beneficial, and auto-limiting response of the host to alarming stimuli. Conversely, a chronic systemic low-grade inflammation (CSLGI), known as a long-time persisting condition, causes damage to the organs and host tissues, representing a major risk for chronic diseases. Currently, a high global incidence of chronic inflammatory diseases is observed, often linked to the lifestyle-related changes that occurred in the last decade. The main lifestyle-related factors are proinflammatory diet, psychological stress, tobacco smoking, alcohol abuse, physical inactivity, and indoor living and working with its related consequences such as indoor pollution, artificial light exposure, and low vitamin D production. Recent scientific evidence found that gut microbiota (GM) has a main role in shaping the host's health, particularly as CSLGI mediator. Based on the lastest discoveries regarding the remarkable GM activity, in this manuscript we focus on the elements of actual lifestyle that influence the composition and function of the intestinal microbial community in order to elicit the CSLGI and its correlated pathologies. In this scenario, we provide a broad review of the interplay between modern lifestyle, GM, and CSLGI with a special focus on the COVID symptoms and emerging long-COVID syndrome.