The Connection Between Physical Exercise and Gut Microbiota: Implications for Competitive Sports Athletes

Abstract

Gut microbiota refers to those microorganisms in the human digestive tract that display activities fundamental in human life. With at least 4 million different bacterial types, the gut microbiota is composed of bacteria that are present at levels sixfold greater than the total number of cells in the entire human body. Among its multiple functions, the microbiota helps promote the bioavailability of some nutrients and the metabolization of food, and protects the intestinal mucosa from the aggression of pathogenic microorganisms. Moreover, by stimulating the production of intestinal mediators able to reach the central nervous system (gut/brain axis), the gut microbiota participates in the modulation of human moods and behaviors. Several endogenous and exogenous factors can cause dysbiosis with important consequences on the composition and functions of the microbiota. Recent research underlines the importance of appropriate physical activity (such as sports), nutrition, and a healthy lifestyle to ensure the presence of a functional physiological microbiota working to maintain the health of the whole human organism. Indeed, in addition to bowel disturbances, variations in the qualitative and quantitative microbial composition of the gastrointestinal tract might have systemic negative effects. Here, we review recent studies on the effects of physical activity on gut microbiota with the aim of identifying potential mechanisms by which exercise could affect gut microbiota composition and function. Whether physical exercise of variable work intensity might reflect changes in intestinal health is analyzed.

Fig. 1

The main factors that influence the composition of the gut microbiota

Fig. 2

Main biomolecular interactions during regular physical exercise and training. The tight interplay between the gut microbiota and the gut/brain axis, HPA axis, or muscle/gut axis may help to explain the renowned beneficial effects of exercise on several organs and functions. Depending on the nature and intensity of physical training, the composition and activities of intestinal bacteria may vary. This process subsequently contributes to modulating immune function (by improving the sensitivity of Toll-like receptors that recognize bacterial DNA and through the production of butyrate), reducing intestinal inflammation (mediated by various myokines, such as IL-6, TNF-α, and IL-10) to lower salivary cortisol via the gut/brain axis, and improving the psychophysiological conditions of patients with inflammatory bowel disease or suffering from anxiety, stress-induced depression, obesity, mobility, musculoskeletal disorders and respiratory diseases. HPA hypothalamic pituitary adrenal axis, BDNF brain-derived neurotrophic factor, GABA γ-aminobutyric acid, SCFAs short-chain fatty acids, LPS lipopolysaccharides, FXR farnesoid X receptor, TLR4 toll-like receptor 4, AMPK AMP-activated protein kinase. Credits: Original figure by I. A. Charitos

Fig. 3

Each intense and prolonged training leads to physiological stress and transient but significant changes in immune defense, enhancing the release of stress hormones, pro- and anti-inflammatory cytokines and reactive oxygen species. Changes may affect a the activity of natural killer cells, b the number and the correct function of T and B cells, c the function of upper airway neutrophils, d the salivary concentration of IgA, and e the oxidative activities of granulocytes. MHC expression is suppressed for several hours during recovery from prolonged endurance exercise. Therefore, endocrinological alterations (such as an increase in cortisol secretion), repetitive muscle microtrauma, and a lack of energy can lead to both irregular immunomodulatory effects and intestinal dysbiosis [106, 107]. According to this hypothesis, altered function in two independently regulated pathways (the first concerning the influence of the immune system on the intestinal mucosa, the second related to the relationship between intestinal mucosa and several tissues) may contribute to creating a unifying vicious cycle responsible for both unhealthy status and poor performance. Credits: Original figure by I. A. Charitos