A “gut feeling,” it turns out, may be more than just an expression.

A new triple-blind study has found that probiotics – the beneficial bacteria found in yogurt and kefir – can lighten your mood as well.

Researchers from Lieden University in the Netherlands found that subjects given a probiotic supplement for four weeks were less apt to be in a negative mood than those who took a placebo.

They divided the 40 participants, all healthy men and women, into two groups of 20. The first was issued a probiotic powder containing three types of beneficial bacteria — Lactobacillus, Lactococcus and Bifidobacterium. The second got an identical-looking powder that contained no bacteria.

Each was also asked to fill out a questionnaire psychologists use to gauge a person’s state of mind, both at the start of the study and when it ended. It showed the probiotic takers were much less apt to suffer from “the blues” – or have aggressive thoughts – than the placebo group.

“These results provide the first evidence that the intake of probiotics may help reduce negative thoughts associated with sad mood,” the researchers wrote. Probiotics, they concluded, might just be “a potential preventive strategy for depression.”

While probiotics are available in supplement form, yogurt is a far more popular way of consuming them. And a previous study at UCLA involving MRI brain scans found that eating yogurt twice daily for a month reduced activity in parts of the brain dealing with emotion and pain and increased it in decision-making areas.

“Our findings indicate that some of the contents of yogurt may actually change the way our brain responds to the environment,” noted Dr. Kirsten Tillisch, that study’s lead researcher.

Experts are also convinced there are strong links between the health of our gut ecosystem and our emotional well-being. One reason may be that levels of tryptophan, which the body uses to produce serotonin, may be increased by having more beneficial bacteria. Serotonin is a substance that affects our moods, and being deficient in it is a known cause of depression.

So it might be accurate to describe yogurt as a genuine “feel-good” food. But be sure the kind you buy has live, active cultures, such as those used in the study. And that it doesn’t contain any fake, toxic sweeteners, such as aspartame or Splenda. Or better yet, buy some plain, whole-milk yogurt (preferably organic) and sweeten it however you like – with fruits, honey or real maple syrup.

If you’ve never tried it, kefir is an even more probiotic-rich product that you can drink, more on the order of a smoothie.