Given their current popularity, you might assume that probiotics—capsules containing a mix of “good” bacteria that are said to rebalance our gut’s bacterial content—would be perfectly harmless.

But a team of gastroenterologists from Augusta University in the US state of Georgia is challenging that assumption.

Their health was fine otherwise, and test results for any known medical conditions that could be causing them came back negative.

Our gut bacteria can sometimes produce too much D-lactic acid through fermentation when they’re overfed with carbohydrates that aren’t properly digested by the small intestine that leads to the colon.

The body can build up so much d-lactic acid that it spills into the blood, where it can damage the body and brain—a condition called D-lactic acidosis.

This rare condition is known to happen in people whose small intestine has been shrunk or damaged by injury, bariatric surgery, or disease.

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