Daily dose of probiotics -- live bacteria and yeasts -- well known to be good for the digestive system, may however, be less effective when taken alongside a balanced diet and impair certain aspects of memory, suggests a research.

"If you're eating poorly, then probiotics might be helpful. But if you're already eating healthy, they may not be that beneficial," said Margaret Morris, Professor at University of New South Wales in Australia. For the study, published in the journal Molecular Psychiatry, the team examined the impact of a commonly used probiotics 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.

The results showed that in fat rats with "grossly dysregulated" gut health, being fed on junk food, probiotics positively changed the bacterial make-up in their digestive tract and benefited brain function, preventing spatial memory loss.

But for rats on a healthy diet, the probiotics had little impact on microbial diversity and actually impaired recognition memory. "We were surprised to find that after feeding a healthy diet to the rats, the probiotics actually resulted in some memory impairment with regards to object recognition," Morris said.

Although the study is looking at rats, people need to exercise caution while taking probiotics. "It's very hard for us to say that they are definitively good or bad. 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," Morris added.