In 2019 alone, over 2,600 articles on “coffee” or “caffeine” have been published, according to MEDLINE, the most comprehensive database on life sciences and biomedical research. That means on every single day, we can find 5-10 published research articles on coffee or caffeine. Often these studies reported results that contradict with each other. More than 150 million of Americans drink coffee on a daily basis. The truth is, no matter what we read, many of us can’t live without coffee and we won’t stop anytime soon. We may as well understand it better and adjust our coffee routines according to the evidence.
Caffeine is present in 60 plants. Coffee and tea are the most common source of caffeine, but there are many more 1. The table below summarized the amount of caffeine in some common drinks and food. The caffeine content in coffee varies depending on source of beans, roast method and brewing method.
Table I. Caffeine content in some common drinks and food 1.
Espresso is my favorite coffee drink. Its dark appearance and strong taste make people think it is stronger than a regular coffee. However, the high-pressure steam during brewing process extracts more aromas and less caffeine. Thus one serving of espresso has much less caffeine than a cup of drip coffee, given that they are made from same amount of coffee beans.
The popular K-cup yields 75-150 mg caffeine in an 8-oz cup, according to the official Keurig website).
Studies have shown that the effects of caffeine on our body are dose-dependent. Many unwanted effects are related to consumption of large amount of coffee, say, more than 6 cups a day.
Daily consumption of 400 mg of caffeine is considered safe, as this amount is less likely to be associated with adverse effects 2. That is about 3 cups of home brewed coffee in an 8 oz cup, or 1 Tall cup of coffee at Starbucks, or 10 shots of espresso. Today an average American adult drinks two cups of coffee each day.
Our body processes caffeine differently. Caffeine is primarily metabolized in liver by the enzyme cytochrome P450 1A2 (CYP1A2). DNA variation in this enzyme contributes different rate of metabolism of caffeine and a wide range of half-life of caffeine in our body, which varies around 2-12 hours 3.
In the short term, coffee improves our alertness, cognitive function and learning ability. Moreover, long-term drinking of coffee leads to protective effect against several diseases and better and longer living, as have been well supported by research.
Type 2 diabetes People who drink coffee daily has reduced risk for diabetes. Interestingly, decaffeinated coffee has similar protective effect against diabetes 4,5.
Parkinson Disease Parkinson is a progressive neurodegenerative disease that affects 0.3% of population of 40 years or older 6. Multiple large-cohort studies showed that coffee or caffeine intake is associated with lower risk of Parkinson disease 7,8.
Gout Elevated serum uric acid leads to gout. Increasing intake of either regular or decaffeinated coffee is associated with lower serum uric acid level and lower risk of gout 9,10.
Headache Caffeine is used in combination with aspirin and tylenol for treating tension headache and migraine, and the combined form is more effective than using aspirin and tylenol alone 11,12.
However, it is important to keep in mind that long-term use of large amount of coffee induces headache 13. Headache is also the most common symptoms of coffee withdrawal, occurring to 50% of affected individuals 14.
Longevity Studies have shown that long-term coffee drinking is associated with lower mortality 15,16. It does not necessarily mean that drinking coffee makes us live longer. Some argue that healthier people may simply drink more coffee than less healthy people. On the other hand, it has also been shown that having more than 4 cups of coffee daily is associated with higher mortality, suggesting moderate consumption of coffee is key to its health benefits 17.
Athletic Performance I love this one. Each time when I am training for a marathon, I make sure to drink coffee before my training run. In this case, I believe the aromas and taste of coffee affect my mental fitness, as much as what caffeine does to my physical fitness. However, there are increasing number of studies in recent years supporting caffeine not only improve endurance of runners, but also other aspects of performance in a wide range of sports 18,19.
Several adverse effects of coffee have been well studied. These effects appear to be dose dependent, being more prominent in people who drink more than 5 cups of coffee each day.
High blood pressure Coffee drinking can increase blood pressure by up to 10 mmHg in people who do not normally drink coffee, but have a much smaller effect on people who regularly drink coffee 2,20.
Anxiety – People who have history of anxiety or insomnia may experience more severe symptoms after drinking coffee. However, this is more likely due to temporary effects of coffee, and there is no evidence supporting long-term use of coffee induces psychiatric disorders 21.
Reduced bone density and osteoporosis – Coffee consumption is associated with reduced bone density and higher risk for osteoporosis, particularly in people with lower calcium intake 22,23.
Interference with other medicine Coffee has been shown to inhibit absorbance of certain medicine if taken together. Some examples include iron supplement for treating anemia, Alendronate for treating osteoporosis and Levothyroxine for treating hypothyroidism 24–26.
Many studies on coffee and caffeine reported mixed effects on health, both benefits and harms. Many factors may contribute to the complexity of this topic. For instance, genetic variation in metabolism and response to caffeine may cause inconsistent outcomes in different studies. Future studies are needed to address these remaining questions.
Cancers – Some studies have shown that coffee or caffeine is associated with reduced risk for certain cancers. However, the evidence has been inconclusive and vary among different cancer types.
Cardiovascular diseases – Caffeine can possibly affect function of heart and blood vessels through several different mechanisms. However, studies have shown that caffeine from moderate coffee consumption does not add extra burden to cardiovascular system. Moreover, some studies suggest that moderate coffee drinking is associated with lower risk of heart attack and stroke, and lower mortality 27,28.
Some patients with arrhythmias (irregular heart rhythm), may feel heart racing after drinking coffee due to the stimulant effect of caffeine. No evidence suggests that moderate coffee drinking triggers irregular heart rhythm 29.
Alzheimer Disease – Alzheimer disease affects older adults and is the most common cause of dementia. Although some researchers found protective effects of coffee against Alzheimer disease in both human and animal model, some other studies do not support this notion 28,30,31.
GERD – Many patients and their physicians empirically associate coffee drinking with heartburn or gastroesophageal reflux disease (GERD). However, evidence for such connection has been inconsistent 32,33.
For those of us who drink coffee daily, there is no scientific evidence suggesting that we should stop now. Instead, coffee brings multiple health benefits and protects us from several diseases. Moderation seems to be the key for striking the balance between benefits and adverse effects. If we adhere to moderate coffee assumption (i.e., no more than 2 cups a day), we may be on our way to better fitness and a longer life.
Over 500 chemicals have been characterized from roasted coffee and their concentration in brewed coffee varies drastically. Many studies have shown how these non-caffeine chemicals, such as antioxidants, benefit our health 34,35. Hopefully future studies will offer us more reasons to enjoy our favorite coffee drinks.
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I received my MD from PUMC in Beijing China and my Ph.D. in Biochemistry from Stony Brook University on Long Island. Over the years, I have worked in the fields of genetic research and clinical medicine in different parts of the US, including PA, MO, CT, FL, NY and MI. My research has been published in multiple scientific journals. Currently I live in Ann Arbor, MI with my husband and our children and Mango the orange tabby. I love hiking, running, baking, cooking and biking.