Drinking coffee can provide a much-needed mental jolt, but new research suggests that you should stop at five or six cups per day; drinking more than that could harm your brain. Researchers looked at about 400,000 people who regularly drank coffee (either caffeinated or decaffeinated). Daily intake was divided into several categories, from one to two cups to more than six cups. Approximately 18,000 people had their brain volume measured with MRI. At the 11-year follow-up, the investigators found that compared with light coffee drinkers (one to two daily cups), heavy coffee drinkers (more than six cups per day) tended to have smaller amounts of total brain volume, especially in the hippocampus, the region responsible for short- and long-term memory.

Heavy drinkers also had a 53% higher risk of being diagnosed with dementia compared with light drinkers. The risk was approximately the same whether people drank caffeinated or decaffeinated coffee.

It’s unknown how coffee may affect the brain in this way. The researchers pointed out that high amounts of cafestol (a molecule in coffee that can increase cholesterol) could be a contributor.

The findings don’t suggest that coffee lovers give up their daily java, but people may want to be mindful of their number of refills.