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While moderate coffee consumption has been linked to a reduced risk of several chronic diseases, high caffeine intake may have poor health outcomes depending on the consumer, a review article indicated.

Drinking two to five cups of coffee daily may lower the risk of chronic illnesses such as type 2 diabetes, cardiovascular disease, and endometrial and liver cancers, reported Rob van Dam, PhD, of the Saw Swee Hock School of Public Health at the University of Singapore, and colleagues.

But too much caffeine may generate negative health outcomes, including anxiety, insomnia or psychomotor agitation. Limiting caffeine consumption to 400 mg a day for the general population, and 200 mg for pregnant and lactating people is recommended, they wrote.

The group also noted that higher caffeine intake may be especially dangerous during pregnancy, as side effects like poor fetal growth and miscarriage have been observed.

And the evidence is largely from epidemiological studies; it's far from certain that the beneficial associations are genuinely causal.

While current guidelines recommend no more than four to five cups of coffee a day for the general population, van Dam told 乌鸦传媒 that caffeine sensitivity differs based on the person consuming it.

"There is large variation in how fast individuals metabolize caffeine due to several genetic, lifestyle, and physiological factors," van Dam said in an email.

Even at recommended levels, caffeine drinkers may experience unpleasant side effects on nervousness or ability to sleep. "Consumers should be aware of these symptoms and reduce their intake, or only use sources of caffeine earlier in the day to avoid side effects," van Dam said.

Wendy Setiawan, PhD, of the Keck School of Medicine at the University of Southern California in Los Angeles, said that this review provides a comprehensive summary of the current research on coffee, caffeine, and health.

"The key message is that you cannot recommend drinking coffee or caffeine to prevent disease... or even to prolong life," Setiawan, who was not involved with this research, told 乌鸦传媒. "But in general, for those people who are not pregnant or do not have underlying health conditions, I think moderate consumption of coffee or tea can be a part of a healthy diet."

Caffeine is the most widely consumed psychoactive agent, researchers wrote. In the U.S., around 85% of adults consume coffee everyday, and standard daily intake is around 1.5 cups. While the majority of adults drink coffee as their primary source of caffeine, adolescents rely on soft drinks and tea.

Recent epidemiological studies suggest that coffee and caffeine consumption are linked to a number of health benefits, van Dam and colleagues noted. But coffee contains hundreds of other biologically reactive phytochemicals, including polyphenols, the alkaloid trigonelline, melanoidins, magnesium, potassium, and vitamin B. These compounds may lower oxidative stress, and improve gut health and metabolism.

Acknowledging the health benefits of coffee compounds is crucial to research on its effects on health, especially when identifying whether it is coffee, or the caffeine itself, that improves outcomes, Setiawan said.

"[The researchers] focus a lot on caffeine, but as we know coffee contains tons of other biologically reactive compounds that could have been related to all of these health benefits," she stated. She said that understanding which compounds have the most nutritional benefits would be an interesting next step for research.

Van Dam and colleagues found that coffee had no association with increased risk of high blood pressure or hypertension. Some concerns have been raised about levels of cholesterol in unfiltered coffee, such as French press, Turkish, or Scandinavian boiled coffee. But moderate consumers should not be concerned about their risk of chronic disease, van Dam said.

Other potential benefits of caffeine consumption include improved energy and metabolism, and researchers also identified studies that showed an inverse relationship between caffeinated coffee consumption and Parkinson's disease.

While this review identified several health benefits associated with coffee consumption, van Dam said "it should be emphasized that these are epidemiological studies and that we can not fully exclude the possibility that there are other characteristics of coffee consumers that explain their lower risk of these diseases."

The researchers noted that studies on caffeine intake and health included in this review may have been subject to other limitations, including measurement error and some exposure misclassification, as most studies do not account for brew strength, cup size, or milk or sugar added to coffee.

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