More than 100 trillion bacteria live in your colon. Some are healthful, while others can be harmful. Your healthful bacteria eat the same foods that you do and stay in your colon. They convert soluble fiber in all foods made from plants into short chain fatty acids (SCFAs) that lower high cholesterol and high blood pressure, and reduce inflammation. SCFAs soften your stool to prevent constipation, help to prevent obesity, treat diarrhea and irritable bowel syndrome, lower high blood sugar, cholesterol and blood pressure, reduce the pain and swelling of arthritis or psoriasis, and possibly even help to prevent and treat diabetes, arteriosclerosis and certain cancers. These “good” bacteria help to protect you from the “bad” bacteria by causing your intestinal cells to produce a layer of mucus that coats the cells lining your intestines, to help prevent the harmful bacteria from invading your intestinal cells and passing into your bloodstream.

The harmful bacteria are not happy to stay in your colon and eat what you eat, so they attack you and try to penetrate your colon cells. Your immune system responds to these harmful bacteria by producing white blood cells and chemicals that attack invading bacteria and kill them by punching holes in their cell membranes and even eating them and removing them from your body. This is good as long as your immune system can stop after it kills the invading germs. However, if the germs keep on penetrating your colon cells, your immune system stays active all the time to cause inflammation and to attack your own tissues. For example, it can punch holes in your arteries to form plaques that break off to cause heart attacks, and damage the DNA in normal cells to turn them into cancer cells.

Building a Colony of Healthful Bacteria
If you wish to encourage the growth of healthful bacteria in your colon, you have two choices: probiotics or prebiotics. Probiotics are living microorganisms, varieties of the good bacteria, that can live and multiply in your colon. Prebiotics are nondigestible food components that cannot be absorbed in the upper intestinal tract and travel to the colon where they serve as a medium where good bacteria can grow and multiply.

Probiotic Foods and Supplements
Many fermented or cultured foods are called probiotics because they contain healthful bacteria that pass to your colon when you eat them. For example, live-culture yogurt is a readily available source of good bacteria. However, the lactobacilli in most live cultures of yogurt will not colonize in your intestines, so they disappear if you stop eating yogurt regularly. Other probiotic foods include kefir, buttermilk, kombucha (fermented tea), tempeh, miso and natto (fermented soybean products), kimchi, sauerkraut, some pickles, and some fermented cheeses. With any of these foods, check the label to see if they contain live cultures, or culture your own. If the food is pasteurized or otherwise processed, the bacteria will have been killed.

Probiotic bacteria are also offered in a multitude of over-the-counter supplements, but these products should be treated with skepticism since you have no way to know what is actually in them or what effect (if any) they will have in your body. See my report on Should You Take Probiotics?

Prebiotic Foods and Supplements
All plants contain fiber, so when you eat a plant-based diet, you will consume a wide variety of prebiotic foods. Fiber is not absorbed in your upper intestinal tract, so it passes to your colon where it serves as the medium to encourage growth of the good bacteria. Soluble fiber is the most important component of your diet to foster a large colony of healthful bacteria. Prebiotic supplements are available, but it is easier and cheaper just to eat plenty of the foods that provide this benefit. Good sources of soluble fiber include whole grains, beans, seeds, vegetables and nuts.

Psyllium husks are an example of a prebiotic fiber supplement that is widely sold under brand names such as Metamucil, primarily to help prevent constipation and with various other health claims. The massive Harvard Women’s Health Initiative study followed more than 160,000 postmenopausal women, 3.7 percent of which took psyllium supplements. After an average follow-up of 15.4 years, 3,032 of the women in the study had developed colorectal cancer, and the researchers showed that taking the psyllium products offered no protection from colorectal cancer (Cancer Epidemiology, Biomarkers & Prevention, October 10, 2019).

Probiotic and Prebiotic Supplements Won’t Correct an Unhealthful Lifestyle
A study from Washington University showed how quickly an unhealthful diet can cause changes in your gut bacteria (Cell Host and Microbe, March 26, 2019). Since healthful bacteria in your colon eat the same food that you do, the best way to grow healthful bacteria is to eat a diet with lots of the foods that encourage the growth of healthful bacteria. Soluble fiber in plants appears to be the most effective way to grow healthful bacteria in your colon, prevent disease and prolong your life. See The More Vegetables, The Better.

The number of harmful colon bacteria increases with certain harmful lifestyle factors such as:
• eating a lot of pro-inflammatory foods (such as meats, foods with added sugars, sugared drinks and other refined carbohydrates)
• not eating lots of vegetables and fruits
• drinking alcohol
• smoking
• not exercising
• being overweight, particularly storing fat in your belly

My Recommendations
Since we have no way to know what is actually in probiotic or prebiotic supplements, I recommend that you concentrate on the foods that are known to help you grow a colony of healthful colon bacteria – fruits, vegetables, whole grains, beans, nuts and other seeds – and work to change any of the harmful lifestyle factors listed above that apply to you. You may benefit from eating a probiotic food such as live-culture yogurt any time you need to take a prescription for antibiotics to treat an infection.