It may not come as a surprise to most people that coffee can have a positive effect on mood. Many of us rely on this benefit before hitting the road in the morning and throughout the day at work.
But coffee may have other mood benefits that simply elevating mood shortly after its intake. Numerous smaller studies have indicated that coffee can help prevent mild to moderate depression—a disorder that affects 15 percent of people in high-income nations.
More recently researchers at Qingdao University Medical College in China have carried out a meta-analysis of the results of 15 of the previously conducted studies looking at the correlation between coffee consumption and depression.
The results of the meta-analysis, which was published in the March 2016 issue of the Australian and New Zealand Journal of Psychiatry, suggest that there is an eight percent reduced risk of developing depression per average cup of caffeinated coffee consumed per day.
We don't yet know how coffee help alleviate depressive disorder. But the researchers suggests two potential mechanisms.
Although depression is often taken to be a result of low serotonin-levels in the brain, a new hypothesis is emerging. According to this hypothesis, depression is a result of a bad immune reaction that causes an inflammation to the brain.
The Chinese researchers speculate that coffee may help alleviate this kind of inflammation, owing to its special combination of anxioxidants—which include chemicals such as chlorogenic acid, nicotinic acid, trigonelline, quinolinic acid, tannic acid, pyrogallic acid.
Another hypothesis is that the anti-depressive effects of caffeinated coffee consumption is a direct result of the caffeine. Caffeine is a psychostimulant that can increase wakefulness and motivation.
When we are awake and alert after a good night's sleep, there is very little adenosine in the central nervous system. During several hours without sleep, however, adenosine slowly accumulates. Adenosine activates adenosine receptors, which causes drowsiness and a lack of motivation and energy.
Like adenosine, caffeine binds to adenosine receptors in the central nervous system but unlike adenosine, caffeine does not activate the receptors. Instead, it blocks them, which prevents them from causing the normal cellular response that leads to a lack of energy and motivation.
This response explains why coffee consumption can help us stay awake and alert in the short run. However, it does not by itself explain how coffee can help alleviate depression in the long run.
One mechanism suggested by the Chinese researchers is that coffee helps make the brain's main motivational chemical dopamine more effective. By binding to adenosine receptors, caffeine increases the amount of dopamine that binds to dopamine receptors in the striatum--a part of the forebrain that coordinates decision-making, motivation and reward perception, among many other things.
Two typical characteristics of depression are anhedonia (the inability to feel pleasure) and a lack of motivation. If coffee helps increase motivation and the ability to feel pleasure by binding to adenosine receptors, this might explain its long-term effect on depressive disorder.
Wang L, Shen X, Wu Y, Zhang D. Coffee and caffeine consumption and depression: A meta-analysis of observational studies. Aust N Z J Psychiatry. 2016; 50(3):228-42.