Having a healthy colon is the basis of having a healthy body. Or as 1908 Nobel Laureate, Ilya Mechnikov, bluntly said, "Death begins in the colon".
He also coined the term "probiotic" for friendly bacteria, and it was through his research on bacteria that the correlation was made between longevity and probiotics in the body.
He noted that societies that ate fermented foods in their diets and used kefir bacterial cultures lived healthy, active lives to 100+ years of age.
Having large populations of friendly bacteria in the gut decreases the potential for bad bacteria and fungi to over-run the intestines and compromise immunity and health in general.
Probiotic colonies in the colon also produce highly beneficial enzymes. Enzymes are necessary in the body to provide cells with the nutrients they need in order to function properly. Therefore when there is a drastic inadequacy of probiotic bacteria in the digestive system there is also an inadequacy of enzymes, which leads to poor cellular health and subsequent onset of disease processes.
In a lot of modern-day societies there are so many things that compromise optimal colonisation of beneficial bacteria in the gastrointestinal tract. Couple this with a diet that is lacking in fermented foods and drinks, and the ratio of beneficial microbial residents to detrimental residents in the body becomes too low, opening up the perfect scenario for the detrimental microbes to get a strong-hold and over-run the internal environment, which in turn causes ill-health.