You have more than 100 trillion bacteria living in your colon, with some types that are helpful and other types that are harmful. With aging, the ratio of good to bad types of bacteria appears to change, increasing the amounts of harmful bacteria and reducing the amounts of healthful bacteria. Harmful colon bacteria have been shown to increase risk for many diseases by producing chemicals that cause inflammation. A strong new study shows that with aging, there is an increase in the types of harmful colon bacteria that produce amines, specifically TMAO (trimethylamine oxide), that damage arteries to increase risk for heart attacks, strokes and cell damage throughout your body (J Physiol, Feb 4, 2019).
With aging, mice (like humans) suffer progressive changes in their arteries that can lead to heart attacks, strokes and cancers. Older mice, but not younger mice, suffer damage to the inner linings of their arteries and arterial stiffness so their arteries cannot widen to increase blood flow when needed as healthy blood vessels do. Arterial stiffness is associated with increased risk for heart attacks, strokes, and cell damage. Scientists can measure the factors that increase risk for arterial damage with aging.
In this study the researchers looked for the harmful pro-inflammatory, tissue-damaging free radicals. They were also able to measure factors that reduce arterial damage such as healthful antioxidants and nitric oxide. Groups of young and old mice were given poorly-absorbed, broad-spectrum antibiotics to kill large amounts of the bacteria that live in their colons. They used genetic sequencing to determine the types of bacteria in the mice's stools and found that the antibiotics reduced the number of harmful colon bacteria in both young and old mice. After 3-4 weeks, the researchers found no change in the arteries of the young mice, but the old mice had improvements in their arterial stiffness and in the inner linings of their arteries. Both the young mice and the old mice had lower levels of the harmful TMAO, and higher levels of healthful antioxidants.
Note that giving antibiotics to kill colon bacteria was an experimental technique used on mice to mimic changes that occur with aging in humans. The researchers are not suggesting that antibiotics should be given to alter gut bacteria in humans.
Other Studies on Colon Bacteria
Some types of colon bacteria affect risk for many conditions and diseases (JAMA, May 14, 2018), such as:
• Obesity – Colon bacteria break down unabsorbed foods to determine what percentage of the calories from food you absorb (Nature, June 9, 2016;534:213-217). Giving baby mice penicillin, in doses similar to what farmers give livestock, changed their colon bacteria into the types that made them fat and caused them to develop 15 percent more body fat than mice not given antibiotics, and they remained fat into later life (Cell, August 14, 2014;158(4):705-721).
• Type 2 Diabetes – Healthful colon bacteria turn soluble fiber into short chain fatty acids (SCFAs) that reduce inflammation to help prevent and treat diabetes (Front Microbiol, Feb 17, 2016;7:185).
• Heart attacks – Harmful colon bacteria cause inflammation that damages arteries (Am J Clin Nutr, Dec, 2005;82(6):1185-94).
• High Cholesterol – Harmful colon bacteria decrease the rate that your body makes bile to increase blood levels of the harmful LDL cholesterol that increases risk for heart attacks (Cell Metab, 2013;17(2):225-235).
• High Blood Pressure – Healthful colon bacteria increase production of SCFAs that lower high blood pressure (Proc Natl Acad Sci USA, 2013;110(11):4410-4415).
• Inflammatory bowel diseases – SCFAs produced by healthful gut bacteria reduce symptoms of ulcerative colitis and Crohn's disease (Inflam Bowel Dis, Mar 2003;9(2):116-21).
How Colon Bacteria Affect Your Health
Harmful colon bacteria try to enter your colon cells which causes your immune system to produce cells and chemicals that attack and kill bacteria and other invading germs. When your immune system is turned on all the time, these same cells can attack and destroy your own cells to increase risk for arteriosclerosis, diabetes, obesity, heart attacks, cancers, dementia and other diseases. Healthful colon bacteria convert soluble fiber into SCFAs that protect you from disease by decreasing inflammation that can damage every cell in your body, and by lowering high cholesterol and high blood pressure.
Your current diet determines which bacteria live in your gut. Even if your colon is full of harmful bacteria, you can change your colon bacteria by switching to an anti-inflammatory, high-fiber diet that includes a wide variety of vegetables, whole grains, beans, nuts and other seeds. A healthful diet, together with weight control, a regular exercise program, and avoidance of alcohol and smoke, will help to protect you from the many diseases that are linked to the typical Western diet and lifestyle.
Also see Should You Take Probiotics?
Dementia Risk Increased by Harmful Bacteria in Your Colon
How Soluble Fiber Promotes Good Gut Bacteria