Researchers followed more than 3500 Japanese adults, 40-64 years of age, for 20 years and found that those who ate lots of dietary fiber were at reduced risk for developing dementia (Nutr Neurosci, Feb 6, 2022;1-8:). Fiber is sub-classified as:
insoluble fiber that passes unabsorbed through your body
soluble fiber that can be broken down by bacteria in your colon into short chain fatty acids (SCFAs) that are absorbed into your bloodstream, where they help to lower high blood sugar, high cholesterol, high blood pressure and inflammation
The study found that those who ate the most soluble fiber had the lowest incidence of dementia.

How a High-Fiber Diet May Help to Prevent Dementia
Normal brain cells live a certain amount of time and then die. To keep the breakdown products of these dead cells from accumulating in your brain and harming you, you have specialized cells called microglial cells that eat the dead nerve cells and their cellular debris to remove them from nervous tissue in your body. However, aging can damage these microglial cells so they are less able to do their job, so dead cells and their cellular debris begin to accumulate in your brain to increase risk for dementia.

A study in mice showed that just four weeks on a high-fiber diet helped to prevent dementia in aging mice by protecting their microglial cells from being damaged (Front Immunol, August 14, 2018). Researchers can measure the amount of brain damage that occurs with aging by measuring the pro-inflammatory immune response to the cell damage. The high fiber diet reduced levels of inflammatory interleukin-1 beta, which has been linked to dementia in humans, including Alzheimer’s disease.

Fiber Increases Short Chain Fatty Acids
The high-fiber diet also promoted growth of healthful bacteria that produce short chain fatty acids (SCFAs) that decrease inflammation, and blood levels of SCFAs in the mice increased markedly. Dietary fiber cannot be absorbed in your upper intestinal tract because you lack the enzymes necessary to break fiber down into its smaller components that can be absorbed. When the non-absorbed fiber reaches your colon, some of the healthful bacteria ferment it and break it down into SCFAs that can be absorbed into the bloodstream. Many studies show that SCFAs specifically dampen down inflammation (Nutrients, Oct 3, 2011;(10): 858–876), which is associated with damage to brain cells and other tissues that commonly occurs with aging. SCFAs also help to lower high cholesterol and high blood pressure.

Healthful and Unhealthful Colon Bacteria
You have more than 100 trillion bacteria in your colon. Some are healthful, while others are harmful. Eating lots of fiber helps to grow the healthful bacteria that are happy to stay in your colon. Eating a diet low in fiber and high in fat and protein encourages the growth of harmful bacteria that try to penetrate the cells lining your colon. Your immune system responds to these attacking bacteria by producing white blood cells and chemicals called cytokines to try to kill these bacteria. This constant attack by bacteria trying to invade your colon cells causes your immune system to stay active all the time. This is called inflammation, where your immune system starts to attack you and uses the same mechanisms that it uses to kill germs to damage your cells.

Dementia, Aging and Changes in Colon Bacteria
Aging is associated with increased risk for loss of brain function, and more than 30 percent of adults in North America over the age of 85 suffer from dementia. This loss of brain function with aging may be linked with the way aging changes the composition of bacteria in your colon (Nutr Healthy Aging, 2016;4:3-16). As humans age, the numbers of different colon bacteria markedly and progressively decrease. The loss of bacterial variety is associated with increased risk for brain damage, such as loss of memory, Alzheimer’s disease and Parkinson’s disease (J Lifestyle Med, Jan 2018; 8(1):1-7). With aging, there is also a marked increase of certain colon bacteria that secrete chemicals that turn on your immunity to cause inflammation that can damage your brain (Mutat Res, 2010;690:50-6). Other studies show that these harmful colon bacteria increase risk for certain cancers as well as causing loss of brain function in the hippocampus, a part of the brain that governs both memory and anxiety (Neural Plast, 2014;2014:563160).

Scientists have identified more than 1,000 bacterial species in the human colon, and each person has an average of about 160 of these different species of colon bacteria (Nature, 2010 Mar 4; 464(7285):59-65). Greater variety and larger total numbers of healthful bacteria in your colon are linked to lower risk for a host of diseases, including dementia, in later life. A low-fiber Western diet markedly reduces the number of different types of bacteria in your colon (Cell Metab, 2014 Nov 4;20(5):779-786), but the good news is that animal studies show that a high-fiber diet can rapidly increase the diversity and number of healthful colon bacteria at any age, even if a low-fiber diet has been followed for a long time (Nature, Jan 14, 2016; 529(7585):212-215).

We do not have any good studies on colon bacteria and life span in humans, but we do have studies that show that certain colon bacteria have extended the lives of a worm called C. Elegans that normally lives only two weeks. These colon bacteria were found to produce a chemical called colanic acid that activates mitochondria to turn more nutrients into energy (Cell, 2017 Jun 15; 169(7): 1249-1262.e13).

My Recommendations
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 a high-fiber diet that includes a wide variety of vegetables, whole grains, beans, nuts and other seeds. At this time, there is no good evidence to support taking probiotics (pills or supplements containing strains of good bacteria), and bacteria you take as a pill Will not continue to grow in your colon after you stop taking them. You need to make the diet changes to support a colony of healthful bacteria, and once you do that, there will be no need to take bacteria in pills. See Should You Take Probiotics? A healthful diet, together with weight control, a regular exercise program, and avoidance of alcohol and smoke, will help to protect your brain from dementia and the many other diseases that are linked to the typical Western diet and lifestyle.

Checked 1/15/23