IS COFFEE HARMFUL OR HEALTHFUL?
Victoria J. Drake, Ph.D.
Coffee is an infusion of ground, roasted coffee beans and is one of the most widely consumed beverages in the world. The drink contains more than a thousand different components, including carbohydrates, lipids, vitamins, minerals, alkaloids, phenols, and nitrogenous compounds. Overall, there is little evidence of health risks and some evidence of health benefits for adults consuming moderate amounts of coffee (3-4 cups/day, providing 300-400 mg/day of caffeine). Beneficial effects on mental state, along with coffee's aroma and flavor, are likely reasons why coffee is such a popular beverage. Because of the many variations in the manner in which coffee is prepared and consumed, it is difficult to interpret epidemiological studies.
Caffeine (1,3,7-trimethylxanthine) is an alkaloid that occurs naturally in coffee beans. Caffeine consumption has been linked to several positive effects on mental state, including increased alertness, elevated mood, decreased fatigue, and improved work performance. Moreover, two prospective epidemiological studies have reported that moderate coffee drinking significantly decreases the risk of suicide. While the specific mechanisms for effects on health and behavior are not established, it is known that caffeine functions by blocking two adenosine receptors (A1 and A2A) in the brain and consequently has an overall stimulatory effect on the central nervous system.
Boiled coffee beans also contain significant amounts of the two diterpenes (a class of compounds with four isoprene units), cafestol and kahweol. Some studies have associated these compounds with increased levels of LDL (the "bad" cholesterol) and total cholesterol. Filtered coffee, on the other hand, contains very low levels of cafestol and kahweol because they are mostly removed by the paper filter during brewing. Indeed, filtered coffee consumption does not appear to elevate serum cholesterol concentrations.
Coffee naturally contains polyphenolic compounds called chlorogenic acids, which are esters of quinic and cinnamic acids. The most prevalent individual chlorogenic acid is 5-O-caffeoylquinic acid but often simply called chlorogenic acid. Chlorogenic acid has been shown to exhibit antioxidant activity in vitro; however, the extent of antioxidant activity in vivo is unclear.
Coffee contains a number of other compounds that could potentially affect human health, such as various micronutrients like magnesium, potassium, niacin, and vitamin E. Additionally, coffee is a dietary source of several lignans (phytoestrogens) that may alter glucose metabolism as well as trigonelline (N-methylnicotinic acid), a phytochemical that has been shown to have a hypoglycemic effect in diabetic rats.
Prevention of type 2 diabetes mellitus
Several prospective epidemiological studies have consistently reported that habitual intake of caffeinated coffee lowers the risk of type 2 diabetes mellitus (DM). Interestingly, decaffeinated coffee was recently reported to decrease type 2 DM risk and also to positively alter glucose metabolism, suggesting that non-caffeine components in coffee may offer protection against the disease. More prospective research is needed to determine whether consumption of decaffeinated coffee also lowers type 2 DM risk. Additionally, mechanistic studies investigating the effects of individual constituents of coffee will provide insight into the observed anti-diabetic effect.
Prevention of Parkinson's disease
To date, results of most case-control and prospective cohort studies suggest that routine coffee and caffeine consumption decreases the risk of Parkinson's disease in men but not in women. However, one study in 77,713 women participating in the Nurses' Health Study found that coffee consumption decreased Parkinson's disease risk in women who had never used postmenopausal estrogen, and high coffee intakes actually increased risk of Parkinson's disease in women using estrogen replacement therapy. More prospective research is necessary to determine whether coffee or caffeine intake alters Parkinson's disease risk and whether postmenopausal use of estrogen modifies the association.
Prevention of liver diseases
Some studies have found that coffee intake decreases risk of liver diseases, including cirrhosis and hepatocellular carcinoma. Caffeine and chlorogenic acid have been reported to inhibit hepatic carcinogenesis in animals, yet more mechanistic studies are necessary to determine the exact relationship.
Prevention of colorectal cancer
Case-control studies have generally found that coffee drinking lowers risk of colorectal cancer; however, prospective cohort studies have largely reported no effect. It is generally recognized that prospective studies in which nutrient intake is measured in healthy people and disease outcome is determined later have a stronger experimental design than case-control studies, which rely on comparing subjects diagnosed with disease to other subjects without the disease. Thus, more prospective research on coffee consumption and colorectal cancer is warranted to make any definitive conclusions about the association.
Effects on mental state
Coffee intake has been shown to improve overall mood and mental performance; however, these effects are highly dependent on caffeine dose and habituation. Negative effects, such as increased anxiety and sleep disturbances, have also been reported with coffee intake, especially in non-habitual consumers.
Case-control and prospective epidemiological studies on the effects of coffee intake on cardiovascular disease report conflicting results. Therefore, it is not clear whether coffee drinking increases risk for various cardiovascular diseases, including myocardial infarction, stroke, and coronary heart disease. However, studies have associated consumption of caffeinated coffee with increases in blood pressure and plasma homocysteine levels, two independent risk factors for cardiovascular diseases.
It is well-established that acute caffeine administration increases blood pressure, but recent studies suggest that chronic consumption of caffeinated coffee may not raise blood pressure, presumably because tolerance to caffeine develops.
Most studies to date report that moderate coffee drinking does not elevate risk for pancreatic, lung, bladder, or renal cancer.
Osteoporosis and bone fracture
Studies investigating the effect of coffee or caffeine consumption on osteoporosis and bone fracture risk have reported mixed results. While large-scale, prospective studies are needed to determine whether coffee or caffeine intake alters osteoporosis or fracture risk, currently available data suggest that limiting coffee consumption to three cups daily (300 mg/d of caffeine) and ensuring adequate calcium and vitamin D nutrition may help prevent any adverse effects on bone status.
Caffeine and pregnancy complications
Some studies have reported that high intakes of caffeine during pregnancy are associated with adverse effects, including spontaneous abortion, retarded fetal growth, and low birth weight. Studies suggest that caffeine does not influence preterm delivery or birth defects. Currently, it is recommended that pregnant women limit daily caffeine intake to 300 mg (about three cups of coffee).
Special risk groups
In addition to pregnant women, older adults may be more vulnerable to any adverse effects of caffeine, including acute increases in blood pressure and any negative effects on bone status. Caffeine distributes primarily to lean body mass instead of adipose tissue, and older adults have lower lean mass to adipose ratios than younger adults; thus, the caffeine dose may be more concentrated in older individuals. Another group that may be susceptible to the negative effects of caffeine is children, but studies in this age group are limited, and additionally, most studies have focused on behavioral endpoints. High caffeine intakes in children have been linked to increased anxiety and sleep disturbances.
Please see the Linus Pauling Institute's Micronutrient Information Center for more information on coffee and health.
Last updated December 2007