Coffee and tea are among the plants that are highest in caffeine. Their use as beverages makes caffeine the most consumed psychoactive agent in the world. Coffee is commonly used to increase alertness and work productivity. Synthetic caffeine is added to soft drinks, energy drinks, and products intended to reduce fatigue or promote weight loss.
The caffeine content varies with the type of drink: It is high in coffee, energy drinks, and caffeine tablets; intermediate in tea; and low in soft drinks. Coffee is the predominant source of the caffeine ingested by adults. The evidence for caffeine’s effects on people is ambiguous, and some risks and benefits deserve special attention because of the impact they may have on our health.
Characteristics of Caffeine
The half-life of caffeine varies according to age. In adults, it is 2.5-4.5 hours; in newborns, 80 hours; in children older than 6 months, it remains stable over time with respect to weight. Smoking accelerates caffeine metabolism by reducing the half-life by 50%. Oral contraceptives, however, double caffeine’s half-life. Caffeine metabolism is reduced during pregnancy (it is greater in the first trimester), with a half-life of more than 15 hours. Caffeine clearance can be slowed by several classes of drugs (eg, quinolones, cardiovascular drugs, bronchodilators, and antidepressants) that increase its half-life because they are metabolized by the same liver enzymes.
Caffeine passes the blood-brain barrier and, having an adenosine-like structure, inhibits adenosine’s effects by binding to adenosine receptors. In the brain, caffeine reduces fatigue, increases alertness, reduces reaction times, may reduce the risk for depression, and increases the effectiveness of nonsteroidal anti-inflammatory drugs in treating headaches and other types of pain.
Caffeine and Chronic Diseases
The evidence available on the relationship between caffeine and health has several methodological limitations. Observations of the acute effects of caffeine may not reflect long-term effects because tolerance to caffeine’s effects may develop over time. Smoking and unhealthy lifestyles are confounding factors in epidemiological studies of caffeine intake. In addition, the estimate of the amount and frequency of caffeine intake is often inaccurate because it is mainly based on self-assessment systems. Finally, prospective studies of caffeine consumption are mainly based on coffee and tea consumption, but it is unclear how much the observed outcomes can be translated to intake of other beverages such as energy drinks.
Considering the very high prevalence of arterial hypertension worldwide (31.1% of adults), many questions have been raised about the influence of coffee consumption on blood pressure (BP) and the risk for arterial hypertension. Administration of 200-300 mg caffeine is shown to induce a mean increase of 8.1 mm Hg systolic BP and 5.7 mm Hg diastolic BP. The increase is observed in the first hour after caffeine intake and lasts no longer than 3 hours.
Yet, the moderate and usual consumption of coffee does not increase, but may even reduce, the risk of developing high BP. In contrast, occasional coffee consumption can have hypertensive effects, and moderate and usual consumption in patients with high BP does not appear to increase the risk for uncontrolled BP and can reduce the risk for death from any cause. The inverse association between coffee consumption and hypertension risk was confirmed in a review and meta-analysis of cross-sectional and cohort studies.
With respect to lipid metabolism, cholesterol levels may increase after caffeine consumption because of cafestol. Concentrations of cafestol are high in unfiltered coffee, intermediate in espresso and moka pot coffee, and negligible in instant or filtered coffee. Studies on the impact of coffee on lipid levels have led to inconsistent results, however. Data have shown that people who drink more coffee have higher triglycerides, total cholesterol, and low-density lipoprotein cholesterol (LDL-C) levels. Other data have shown that caffeine promotes LDL receptor expression and clearance of LDL cholesterol.
Experimental and cohort studies have not shown an association between coffee consumption and atrial fibrillation (AF). In fact, evidence suggests that coffee consumption tends to reduce the risk for AF in a dose-response relationship. Similarly, coffee consumption is not associated with increased risk for cardiovascular events in the general population or among patients with a history of hypertension, diabetes, or cardiovascular disease.
The Coffee and Real-Time Atrial and Ventricular Ectopy study evaluated the acute effects of coffee consumption on cardiac ectopy using wearable sensors with continuous recording. It did not demonstrate any increase in daily premature atrial contractions with coffee consumption, compared with abstaining from caffeine.
In patients with type 2 diabetes, a study performed in Japan showed that coffee consumption was associated with reduced all-cause mortality. The results suggested a dose-response relationship, and drinking coffee and green tea appeared to reduce mortality risk further. The results were not generalizable, however, because of the study population’s ethnic homogeneity.