Coffee is usually associated primarily with caffeine and its ability to quickly increase alertness and concentration. However, a cup of coffee contains hundreds of biologically active compounds that can affect the body independently of its caffeine content. A review of these effects has been published in the journal Gut.
One of the best-known unexpected effects of coffee is stimulation of the bowel. About three in 10 people report feeling the urge to have a bowel movement shortly after drinking coffee. This occurs after both regular and decaffeinated coffee, so the effect cannot be explained by caffeine alone. Exactly which compounds are responsible has not yet been established. Coffee additives may also affect bowel function, such as lactose in milk or certain sweeteners.
Coffee can worsen reflux symptoms in some people. Caffeine can also temporarily increase intraocular pressure in people with glaucoma or elevated eye pressure. In addition, people with certain disorders of the Eustachian tube may be advised to reduce their caffeine intake, although there is little direct evidence that coffee itself causes these conditions. The relationship between caffeine and migraine is also complex: caffeine can relieve an attack, but excessive consumption or a sudden reduction in habitual intake may trigger headaches in some people.
Coffee can also interact with medications. For example, it may reduce the absorption of levothyroxine and the osteoporosis drug alendronate. Caffeine, in turn, may be cleared from the body more slowly when certain medications are taken, including the antibiotic ciprofloxacin.
The composition of coffee also affects metabolic parameters. The diterpenes cafestol and kahweol can raise total and “bad” cholesterol levels, although some studies suggest they may have beneficial effects on lipoprotein(a). Other compounds in coffee, including chlorogenic acids, may slightly lower blood pressure and improve blood vessel function. Therefore, it is still difficult to draw a clear conclusion about coffee’s overall effects on cardiovascular health.
Coffee also interacts with microorganisms in the gut. Some chlorogenic acids are not absorbed in the small intestine and reach the colon, where they are metabolized by microbes. Certain carbohydrates and compounds produced during coffee roasting also serve as a food source for gut bacteria. Small human studies suggest that coffee can alter the composition of the gut microbiome, but it is still too early to call coffee a prebiotic.
Another effect is related to iron. Coffee consumed with a meal can significantly reduce the absorption of non-heme iron, particularly the iron found in plant-based foods. Coffee polyphenols contribute to this effect by binding iron in the digestive tract. This may be particularly important for people with low iron stores.
Finally, coffee can activate the digestive system even without food intake. It stimulates the pancreas to release trypsin, an enzyme involved in protein digestion. This effect has also been observed after drinking decaffeinated coffee.
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