Regular tea drinkers who consume at least two mugs a day reduce their risk of developing dementia by around 30 per cent. The risk reduction falls to just 6 per cent among those who drink one or fewer cups of green tea a day. Similar protective effects weren't seen with black tea.
Although it's been suspected for a while that tea helps maintain cognitive abilities, researchers from Tianjin University in China decided to take a closer look, and re-examined 17 studies that had tracked the cognitive abilities of 48,435 people. There was a 'significant reduction' in the risk of cognitive disorders, including dementia, among the tea drinkers.
It's not known exactly known why green tea should have this protective effect, but it does contain flavonoids, which help maintain the health of blood vessels that 'feed' the brain, L-theanine, which is thought to increase levels of calmness, and caffeine, which boosts mental alertness.
In a separate study, researchers have reckoned that the ideal brewing time for black tea is two minutes. That's enough time to release the flavonoids—and brewing for a further two minutes doesn't make much more difference.