The Benefits of Coffee and Tea for the Brain

Polyphenols are plant-based compounds famous for their positive impact on human health. Imagine my surprise, when, in the largest prospective study of plant-based eaters to date, I saw the average dietary polyphenol intake of nonvegetarians was higher than the vegetarians and vegans. Why? Mainly because they drank more coffee, which is by far the leading source of polyphenols in the United States. Since dietary polyphenols may help protect against Alzheimer’s, is coffee consumption good for the brain?

On one hand, caffeine has been shown to reduce amyloid plaques and reverse cognitive impairment in Alzheimer’s disease mouse models. On the other hand, caffeine is a well-known vasoconstrictor, meaning it squeezes down on arterial blood flow, and indeed a single dose of 250mg of caffeine (about 2½ cups of coffee worth) temporarily reduced blood flow in the brain by as much as 30%, though for doses smaller than 175mg of caffeine, researchers only find a significant constriction of blood flow in caffeine-naïve or low-caffeine consumers. The body may be upregulating the arterial adenosine receptors blocked by caffeine in habitual caffeine users to preserve cerebral blood flow. How does this all shake out in terms of dementia risk?

A meta-analysis compiling the results of eight studies that followed hundreds of thousands of individuals for between 5 and 25 years found no overall difference in Alzheimer’s disease rates or dementia in general between coffee drinkers and non-coffee drinkers. This is consistent with mendelian randomization studies that found no significant relationship between genetic determinants of habitual coffee consumption and Alzheimer’s disease (yes, your coffee habit may be due in part to your genes), or memory, or global cognition. It’s possible, though, that there is a non-linear relationship between coffee and cognition that can’t be captured in a binary yes or no.

A more granular meta-analysis found that the risk of cognitive deficits and dementia in those over age 60 may be less in “low” coffee consumers (defined as less than about 2½ cups a day) than in those who don’t drink coffee at all. This is consistent with a PET scan study showing less amyloid pathology in the brains of non-demented adults who drank 2 or more cups of coffee a day (compared to less than two cups). But compared to those who daily drink one to two cups of coffee, those who drink more than six cups a day have smaller total brain volumes and about 50% higher odds of dementia. So it’s possible the apparent lack of association between coffee and dementia when comparing total coffee drinkers versus noncoffee drinkers may be obscured by deleterious effects of high coffee consumption potentially balancing protective effects of low coffee consumption. However the green tea data appears to have a linear dose-response, meaning the any green tea consumption is better than none, when it comes to risk of cognitive deficits, and the more the better.

The potential cognitive benefits of tea may extend late into life. In a study of more than 7,000 of the oldest old in China (average age 91, but up to 115), regular green tea consumption was associated with significantly better cognitive function. What about black tea? A meta-analysis found that while higher green tea was associated with a significant reduction in the risk of cognitive disorders, black or oolong tea consumption was not. Interventional studies have found black tea can acutely improve attention and alertness but population studies did not find it related to the risk of dementia or cognitive decline.

One of the compounds that is lost when green tea is oxidized to black is EGCG. Compared to those randomized to placebo, about two cups of green tea’s worth of EGCG acutely improves self-reported calmness and stress reduction but not cognitive performance. There are more than a thousand compounds in tea, though. Why not test the whole food? Researchers in Japan did just that, randomized cognitively healthy elderly men and women to three grams a day (a level teaspoon) of matcha, powdered green tea. That’s the polyphenol equivalent of drinking three to six cups a day of regular green tea (where you throw the leaves away). Compared to those randomized to a placebo powder for 12 weeks, the women getting the matcha experienced some cognitive improvement, but the men did not.

What about matcha for those already suffering from cognitive dysfunction? A before-and-after pilot study of a dozen individuals with dementia found 2 grams of matcha a day for three months appeared to significantly improve their cognition. With no control group, no definitive conclusions can be made, but it did inspire a year-long randomized double-blind placebo-controlled trial. Nursing home residents with dementia were randomized to 2 grams of matcha a day for 12 months and unfortunately no significant cognitive effect was found compared to the placebo powder group. Once dementia sets in it may be too late for green tea to make a difference.