This was the conclusion that researchers at Augusta University in Georgia came to after studying 30 people with abdominal symptoms such as gas, bloating, and distension.
Of these people, 22 also reported symptoms of brain fog, which is a temporary mental condition that brings on confusion and difficulties with concentration and memory. All 22 were taking probiotics, some more than one brand.
Some reported that their episodes of brain fog — which can last for several hours after a meal — were so bad that they had to give up work.
Although they all had similarly severe abdominal symptoms, those with brain fog were more likely to have two other conditions: an accumulation of bacteria in their small intestine, and higher blood levels of D-lactic acid. In some cases, the acid levels were two to three times the normal.
Lactobacillus bacteria species, one of "the most commonly used probiotics," produces D-lactic acid. The bacteria make the acid when they ferment sugar in the food that is passing through the gut.
The brain fog cleared and, for most patients, the abdominal symptoms "improved significantly" after treatment with antibiotics and stopping use of probiotics.
A paper on the study is now published in the journal Clinical and Translational Gastroenterology.
"What we now know," explains first study author Dr. Satish S. C. Rao, the director of the Digestive Health Clinical Research Center at the Medical College of Georgia at Augusta University, "is that probiotic bacteria have the unique capacity to break down sugar and produce D-lactic acid."
He suggests that taking probiotics might "inadvertently" establish the conditions for brain fog and other symptoms by populating the small intestine with too many bacteria that produce D-lactic acid.
Gut microbes and health
As research tools have improved, scientists have uncovered increasing evidence of the role that bacteria and other microorganisms in the human gut have in health and disease.
The human gut is home to a complex ecosystem of some 300–500 species of bacteria with a total of just under 2 million genes.
These microbe colonies live in partnership with us. They interact with our immune system, help us digest food, and take part in our metabolic processes. In return, we protect them against enemy microbes and provide shelter and nutrients.
The human gut is sterile at birth and soon begins to accumulate microbes from various sources. The variety and composition of the microbe colonies depends on many factors, such as the type of birth, sanitation, method of feeding, physical contacts, and use of antibiotics.
Because of the muscular movement of food along the gut, and because gastric acid, bile, and other digestive juices have an antibiotic effect, the parts of the gut that lie in the stomach and the nearby small intestine are relatively devoid of bacteria in healthy people.
In contrast, the colon — which is found at the other end of the gut near the rectum — contains much denser colonies of bacteria, and their composition is very different.
Here, the dominant strains — including Lactobacilli — are anaerobic, likely because of adaptation to the low-oxygen environment. The bacteria in the parts of the gut nearer the stomach, on the other hand, are predominantly aerobic.
Beware of 'excessive use' of probiotics
For many years, we have heard that taking particular amounts of certain microorganisms — known as probiotics — can benefit human health due to their effect on the gut.
It is now common practice to take probiotics to alleviate gastrointestinal conditions and diseases, and clinicians treating them are also increasingly recommending them.
The most commonly used probiotics are Lactobacillus and Bifidobacterium species.
In the United States, probiotics are classed as dietary supplements and their production is not subject to the same Food and Drug Administration (FDA) regulations as those required of drugs.
There are many commercial probiotics available over the counter. They come in different forms, such as "freeze-dried" pills, sachets that can be mixed with drinks, and yogurts.
While some probiotic manufacturers have tested some of their products in clinical trials, there has been little or no research on whether taking different probiotics together causes the bacteria to work with or against each other.
There are certain scenarios in which probiotic use causes problems, including conditions that affect the movement of food along the gastrointestinal tract. People who take opioids and drugs to reduce stomach acid also experience problems.
Dr. Rao and his colleagues recognize that probiotics can benefit some people in certain situations, such as helping replenish gut bacteria after taking antibiotics. However, they warn against "excessive and indiscriminate use."
First study to show effect in 'intact gut'
All the people who took part in the study underwent extensive gastrointestinal exams to rule out other possible reasons for their symptoms. Also, they completed questionnaires about their symptoms, use of probiotics, consumption of yogurt, and particular food habits.
The team administered metabolic tests that followed what happened when participants consumed carbohydrates. These showed the effect on levels of glucose, insulin, D-lactic acid, and L-lactate acid, which is produced when muscles burn glucose for energy.
The "most severe symptoms" experienced in the 30 patients were "bloating, pain, distension, and gas" in the abdomen. These were similarly intense in the 22 patients with brain fog (who all consumed probiotics) and the 8 without.
The researchers found that the brain fog group was more likely to have a condition called small intestinal bacterial overgrowth (SIBO), in which there are far more bacteria in the small intestine compared with those of healthy individuals.
They also found that three quarters of the brain fog group had higher levels of D-lactic acid in their blood compared with a quarter of those in the group without brain fog.
Other studies have suggested that probiotics may lead to overproduction of D-lactic acid and result in brain fog in people with short bowel syndrome. This is a condition in which the small intestine does not function correctly and results in undigested carbohydrate.
The excess of undigested carbohydrate is what causes SIBO and the resulting higher levels of D-lactic acid.
Dr. Rao says that their study appears to be the first to link probiotic use to brain fog, SIBO, and high levels of D-lactic acid in people with an "intact gut."
"Probiotics should be treated as a drug, not as a food supplement."
Dr. Satish S. C. Rao