Coffee may cut the risk of dementia by blocking the damage cholesterol can inflict on the body, research suggests.
The drink has already been linked to a lower risk of Alzheimer's Disease, and a study by a US team for the Journal of Neuroinflammation may explain why.
A vital barrier between the brain and the main blood supply of rabbits fed a fat-rich diet was protected in those given a caffeine supplement.
UK experts said it was the "best evidence yet" of coffee's benefits.
Caffeine is a safe and readily available drug and its ability to stabilise the blood brain barrier means it could have an important part to play in therapies against neurological disorders
Dr Jonathan Geiger
University of North Dakota
The "blood brain barrier" is a filter which protects the central nervous system from potentially harmful chemicals carried around in the rest of the bloodstream.
Other studies have shown that high levels of cholesterol in the blood can make this barrier "leaky".
Alzheimer's researchers suggest this makes the brain vulnerable to damage which can trigger or contribute to the condition.
The University of North Dakota study used the equivalent to just one daily cup of coffee in their experiments on rabbits.
After 12 weeks of a high-cholesterol diet, the blood brain barrier in those given caffeine was far more intact than in those given no caffeine.
"Caffeine appears to block several of the disruptive effects of cholesterol that make the blood-brain barrier leaky," said Dr Jonathan Geiger, who led the study.
"High levels of cholesterol are a risk factor for Alzheimer's disease, perhaps by compromising the protective nature of the blood brain barrier.
"Caffeine is a safe and readily available drug and its ability to stabilise the blood brain barrier means it could have an important part to play in therapies against neurological disorders."
A spokesman for the Alzheimer's Disease Society said that the study shed "important light" on why previous research had showed benefits for drinking coffee.
"This is the best evidence yet that caffeine equivalent to one cup of coffee a day can help protect the brain against cholesterol.
"In addition to its effect on the vascular system, elevated cholesterol levels also cause problems with the blood brain barrier.
"This barrier, which protects the brain from toxins and infections, is less efficient prior to brain damage caused by Alzheimer's disease or strokes."She called for more research into whether the same effect could be seen in humans.