Coming across less than stellar news about probiotics is rare, like reading about Mother Teresa’s doubts on her faith.

However accustomed as we are to the rushing train of probiotic and prebiotic benefits rolling through our labs, microbial science is still a track of discovery where conflicting or negative results deserve ample attention.

Researchers at University of New South Wales in Australia set up an experiment to see if a probiotic could prevent diet-induced memory deficits. These detrimental effects on cognition are linked to the high density Western diet.

Jessica E Beilharz and colleagues described their results recently in Molecular Psychiatry journal in an article titled Cafeteria diet and probiotic therapy: cross talk among memory, neuroplasticity, serotonin receptors and gut microbiota in the rat.

The Study

• Rats were pre-exposed to vehicle, low or high doses of VSL#3 (8 probiotic strains) for 2 weeks.
• Half of the rats were switched from chow to a cafeteria diet for 25 days and VSL#3 treatment continued until death.

Results

• High-dose VSL#3 prevented the diet-induced memory deficits on the hippocampal-dependent place task.
• High-dose VSL#3 caused deficits on the perirhinal-dependent object task irrespective of diet or dose.
• Gut microbial diversity was as expected reduced by the cafeteria diet.
•  VSL#3 was able to increase the abundance of some taxa contained in the probiotic such as Streptococcus and Lactobacillus and also other taxa including Butyrivibrio, which were decreased by the cafeteria diet.

Metabolic effects

The cafeteria diet increased the expression of many genes related to neuroplasticity and serotonin receptor (5HT) 1A in the hippocampus.

Object memory performance was correlated with perirhinal 5HT2C expression and microbiota PC3.

The authors concluded:

“These results show that probiotics can be beneficial in situations of gut dysbiosis where memory deficits are evident but may be detrimental in healthy subjects.”

Clearly, more research is needed before we tell healthy people to abstain from probiotics. But the findings are troubling and deserve to be repeated.