The gut microbiome is influential on long-term health, and its quality declines with age. Microbial populations that produce beneficial metabolites such as butyrate or propionate decline in number, replaced by microbial populations that invade tissue and cause chronic inflammation. Physical exercise influences health and the gut microbiome, but as noted here, the evidence for exercise to beneficially regulate these microbial populations largely results from animal studies. Data in humans is still comparatively lacking, even though epidemiological studies strongly suggest a relationship between exercise and a better gut microbiome.

Although the general characteristics of the gut microbiome in healthy people are not yet completely defined, the gut microbiomes of people with disease (e.g. metabolic syndrome, physical frailty, cognitive dysfunction, etc.) show a gradual change toward an imbalanced composition compared to those in healthy people. These imbalanced microbiome characteristics may contribute to disease onset and may play a role in a vicious cycle.

Age-related changes in the composition and diversity of the gut microbiome aggravate the immune system to regulate inflammatory responses. Collapse of the immune system causes age-related diseases. The gut microbiome is related to the immune system in that both vary in composition with age. Although the gut microbiota of humans is determined to some extent at birth, the composition continually changes throughout life according to the external environment. This age-dependent gut microbiome is closely correlated with host inflammation and pathophysiology as the host ages. The gut physiology induced by this altered gut microbiome can cause host sensitivity to microbiota, leading to chronic and severe inflammatory responses.

Exercise can significantly alter the composition of the gut microbiome, although the mechanism by which this occurs remains unclear. Some studies have assessed the effects of exercise as a treatment on metabolic disorders in mice with diabetes. When db/db (type 2 diabetes) and db/+ (control) mice were made to exercise at a low intensity, the proportion of Bifidobacterium spp. increased in the db/+ mice that exercised. In another study, wild-type mice were subjected to voluntary wheel running for 12 weeks. After the exercise intervention, the Bacteroidetes:Firmicutes ratio increased, preventing diet-induced obesity. In addition, 4-week-old C57BL/6J mice that were made to exercise on a treadmill had an increased relative abundance of Butyricimonas and Akkermansia.

It is difficult to elucidate the long-term effects of exercise in humans because the gut microbiota is influenced by several genetic and environmental factors. For this reason, previous studies have primarily sought to demonstrate the correlation between the gut microbiome and physical function. These studies have shown that the gut microbiome has distinct characteristics. This led to the hypothesis that improvements in physical function through exercise training could also be associated with the gut microbiome. Therefore, it is important to study alterations in the gut microbiome according to the type and intensity of exercise. However, to our knowledge, no studies have determined which exercise types (e.g., resistance or aerobic exercise) are more effective in influencing the gut microbiome.