Dietary sugar is supposed to be absorbed in the upper intestinal tract, but new research from Yale suggests that taking in excessive amounts of sugar can cause some of the sugar to pass through the intestines unabsorbed. This sugar arrives in your colon where it can harm you by keeping healthful bacteria from growing in your colon and encouraging the overgrowth of harmful bacteria (PNAS, Dec 17, 2018). Specifically, unabsorbed sugar in your colon can prevent the good bacteria from producing a key protein called "Roc" (regulator of colonization), which is required for growth of the healthful species Bacteroides thetaiotaomicron (B. theta) in your colon. This research was done in mice, but it should also apply to humans.
How Too Much Sugar Harms You
We have known for many years that eating too much sugar can harm you by causing high rises in blood sugar, which can damage cells throughout your body and can also lead to diabetes. The new theory proposed by this study is that some excess sugar also passes unabsorbed all the way to your colon where it can harm you by changing the bacteria that live in your colon.
The Differences between Healthful and Harmful Colon Bacteria
More than 100 trillion bacteria live in your colon and these bacteria help to govern your immune system. The healthful bacteria are happy eating the food that reaches them in your colon, while the harmful bacteria are not happy with the food that you eat and instead try to invade the cells lining your colon. Your immune system tries to defend you by producing huge amounts of white blood cells and chemicals that work to destroy the invading bacteria by punching holes in their outer membranes and trying to kill and eat them. This constant invasion of your colon cells by harmful bacteria can cause your immune system to stay overactive all the time, which is called inflammation.
Findings of This New Study
The good and bad bacteria compete for space in your colon. Healthful bacteria called B. theta produce a chemical called Roc that enhances their growth. Normally the sugars glucose and fructose do not reach your colon, but eating and drinking excess sources of sugar can cause these sugars to overload your intestines so that they are not all absorbed and reach your colon where they prevent the B. theta from producing Roc. Without Roc, the B. theta do not thrive and multiply, which allows harmful bacteria to replace them in your colon. The researchers at Yale demonstrated this by engineering a strain of B. theta in which sugar did not suppress Roc, and showed that these engineered bacteria were able to thrive in the colons of mice fed a high-sugar diet. If these results can be duplicated in humans, they will show how eating large amounts of sugar can affect the numbers of good and bad bacteria that grow in your colon.
I will watch for more research on the effects of excess sugar on colon bacteria, but meanwhile, this study gives one more important reason to limit or avoid foods with added sugars and drinks with sugars in them, including fruit juices. Anything that hinders the growth of healthful bacteria and fosters the growth of harmful bacteria can increase your risk for obesity, heart attacks, strokes and some types of cancers. For a more complete discussion on how harmful gut bacteria can cause disease and shorten lives, see What You Eat, Not Your Genes, Determines Your Microbiome