THE CONVERSATION: 7 of the weirdest things coffee does to your body

Coffee, a chemically complicated plant extract containing hundreds of bioactive compounds, can have unusual effects on the body, even when consumed as decaffeinated coffee. While most people associate coffee with its caffeine content and its ability to increase alertness, there are several other intriguing ways coffee affects us.

1. Coffee can stimulate bowel movements: About three out of ten people say they feel the urge to go to the bathroom shortly after drinking coffee. This reaction occurs quickly, regardless of whether the coffee contains caffeine or not. However, it's unclear which specific compounds in coffee cause this effect. Additionally, the time of day when most people drink their first cup of coffee, typically in the morning, may also contribute to this phenomenon.

Certain additives, such as lactose in milk or some sugar-free sweeteners, can also promote bowel movements.

2. Coffee may worsen reflux, eye, and ear conditions: Coffee can exacerbate reflux symptoms for some individuals, leading to respiratory issues like coughing, wheezing, or hoarseness. Caffeine temporarily increases pressure inside the eye in people with glaucoma or ocular hypertension, potentially causing further damage to the optic nerve.

In rare cases, coffee consumption may trigger a condition called patulous Eustachian tube dysfunction, where people hear their own voice or breathing unusually loudly. However, there is little direct evidence that coffee itself causes this condition.

3. Coffee can interact with medications: Coffee can alter the way certain medications are absorbed and metabolized in the body. For instance, it can reduce the absorption of levothyroxine, a medication for treating thyroid disorders, and alendronate, an osteoporosis drug. Caffeine can also slow the metabolism of the antipsychotic clozapine, increasing its concentration in the blood.

In some cases, medications can change how coffee behaves in the body. For example, the antibiotic ciprofloxacin slows the breakdown of caffeine, causing the coffee to remain in the system longer than usual.

4. Coffee can impact cholesterol levels: Coffee contains compounds called diterpenes, including cafestol and kahweol, which can raise total and LDL ("bad") cholesterol. However, the same compounds may lower lipoprotein(a), a substance associated with a lower risk of heart attack or stroke. The effects of coffee on cholesterol are complex, and the overall impact on heart health remains unclear.

5. Coffee may nourish gut microbes: While humans benefit from coffee consumption, some laboratory experiments suggest that our gut microbes also derive benefits. Chlorogenic acids in coffee, which aren't absorbed in the small intestine, reach the colon where gut microbes can ferment them. Coffee also contains complex carbohydrates and melanoidins that can reach the colon, providing a food source for these microorganisms.

Some small human studies indicate that drinking coffee can alter the composition of the gut microbiome, although the evidence is still emerging, and it's too early to conclude that coffee acts as a prebiotic.

6. Coffee can affect iron levels: Consuming coffee with meals can significantly reduce the absorption of non-haem iron, the form found in plant-based foods. This effect isn't primarily due to caffeine but rather the presence of polyphenols in coffee, such as chlorogenic acids, which can bind with iron in the digestive tract, making it less absorbable. This can be particularly relevant for individuals with low iron stores or those relying heavily on plant sources of iron.

7. Coffee can prepare the digestive system: Coffee can stimulate various parts of the digestive system even without the presence of food. It can prompt the pancreas to release trypsin, an enzyme involved in protein digestion. This reaction occurs with both regular and decaffeinated coffee, suggesting that other compounds in the coffee are interacting with the digestive system. However, it remains unclear how this digestive "heads-up" might influence feelings of hunger or satiety.