Consuming probiotics may be much less effective than thought when taken alongside a balanced diet and could impair certain aspects of memory, a new research has warned.
Researchers from University of New South Wales (UNSW) in Australia studied the impact of a commonly used probiotic on the gut health and cognitive function of rats, which were fed either a healthy diet or a "cafeteria diet" high in saturated fat and sugar.
In fat rats with "grossly dysregulated" gut health, thanks to being fed junk food, probiotics positively changed the bacterial make-up in their digestive tract and benefitted brain function, preventing spatial memory loss.
However, for rats on a healthy diet, the probiotics had little impact on microbial diversity and actually impaired recognition memory.
"If you're eating really badly then probiotics might be helpful. But if you're already eating healthily, they may not be that beneficial," said Margaret Morris, professor at UNSW.
"We were surprised to find that, in the rats we were feeding a healthy diet, the probiotics actually resulted in some memory impairment with regards to object recognition," said Morris.
"Although this study is looking at rats, I think the main takeaway message is that we need to exercise caution when we recommend that people take probiotics," she said.
"Probiotics may offer a great opportunity to improve health so long as they are replacing the correct bacteria - the challenge is accurately determining which beneficial microbes are absent," she said.
Western-style diets high in saturated fat and sugar have been consistently shown to have detrimental effects on the brain and cognitive function, and can "rapidly alter the composition and metabolic activity" of microbes in the gut, researchers said.
There is an emerging body of research that suggests gut bacteria can impact brain function.
The researchers pre-exposed groups of rats to either a low or high dose of the probiotic medicine for two weeks before their diet was changed from healthy "chow" to cafeteria-style food (including cookies, cakes and meat pies). This diet change lasted for 25 days.
The cafeteria diet "dramatically altered the microbiota" resulting in rats with much less microbial diversity in their gut, researchers said. However, these rats also saw the greatest health benefits from the probiotics.
They increased the abundance of certain bacteria-types contained in the probiotic such as Streptococcus and Lactobacillus and other bacteria-types such as Butyrivibrio, which were decreased by the cafeteria diet. Furthermore, memory impairment was prevented.
The research appears in the journal Molecular Psychiatry.
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