A new study, published August 18, 2016, showed that individuals with the highest VO2 max (gold standard measure for cardiorespiratory fitness) also had the most diverse collection of good gut bacteria. We also know that various medications (e.g. antibiotics), artificial sweeteners, and sugar reduce bacterial diversity and that obesity, diabetes, allergies, and inflammatory conditions are associated with low diversity. 
Who cares?! Why does gut bacteria matter?!
1) Gut health influences everything that happens in your body – directly affecting how you feel and perform!
2) It’s your defense system protecting you from any pathogens that enter your digestive tract.
3) They play major roles in metabolism, immune function, regulating inflammation, making key vitamins, detoxification, and producing serotonin and dopamine.