The digestive tract and the brain are crucially linked, according to mounting evidence showing that diet and gut bacteria are able to influence our behavior, thoughts and mood. Now researchers have found evidence of bacterial translocation, or “leaky gut,” among people with depression.

Normally the digestive system is surrounded by an impermeable wall of cells. Certain behaviors and medical conditions can compromise this wall, allowing toxic substances and bacteria to enter the bloodstream. In a study published in the May issue of Acta Psychiatrica Scandinavica, approximately 35 percent of depressed participants showed signs of leaky gut, based on blood tests.

The scientists do not yet know how leaky gut relates to depression, although earlier work offers some hints. Displaced bacteria can activate autoimmune responses and inflammation, which are known to be associated with the onset of depression, lower mood and fatigue. “Leaky gut may maintain increased inflammation in depressed patients,” which could exacerbate the symptoms of depression if not treated, says Michael Maes, a research psychiatrist with affiliations in Australia and Thailand and an author of the paper. Currently leaky gut is treated with a combination of glutamine, N-acetylcysteine and zinc—believed to have anti-inflammatory or antioxidant properties—when behavioral and dietary modifications fail.

Causes of Leaky Gut Regular use of painkillers Regular use of antibiotics Infections (such as HIV) Autoimmune disorders Alcohol abuse Inflammatory bowel disease Gluten hypersensitivity Severe food allergies Radiation therapy Inflammatory disorders Psychological stress Exhaustion

Ulcer Bacteria Linked to Cognitive Decline

One type of harmful bacteria escaping the gut might be Helicobacter pylori, the main cause of stomach ulcers. H. pylori may contribute to cognitive impairment or Alzheimer's disease, according to a study published in the June issue of Psychosomatic Medicine. Compared with uninfected individuals, people who tested positive for H. pylori performed worse on cognitive tests, including ones assessing verbal memory. Some laboratory evidence indicates that H. pylori cells can escape the gut and sneak into the brain. There the cells aggregate with the amyloid proteins characteristic of Alzheimer's and instigate the buildup of plaque, suggests study co-author May Baydoun, a staff scientist at the National Institute on Aging. The National Institutes of Health estimates that about 20 percent of people younger than 40 and half of adults older than 60 are infected with the bacteria, which can be treated with antibiotics.

Bugs That Influence the Brain

Preliminary research suggests that these common gut microbes can also affect our thoughts and feelings.

1. Helicobacter pylori: Children infected with this ulcer-causing bacterium performed worse on IQ tests, suggesting a possible link between H. pylori infection and cognitive development.

2. Lactobacillus helveticus and Bifidobacterium longum: Healthy human volunteers who consumed a probiotic mix of these bacteria exhibited less anxiety and depression.

3. Probiotic bacteria B. animalis subsp. lactis, Streptococcus thermophilus, L. delbrueckii subsp. bulgaricus, L. lactis subsp. lactis: Healthy women who consumed yogurt containing these bugs showed less activity in brain regions that process emotions and physical sensations. Researchers do not yet know whether these effects were beneficial; they also have not discovered the mechanism underlying the observed shift in brain activity.