Tea is rich in something called "flavonoids" which are antioxidants and fight inflammation. Tea drinkers have a significantly lower risk of stroke and heart disease, and tea is known to boost brain health.
One study, for example, found that compared with older adults who drank less than three cups a week, those who drank more than two cups of green tea a day had a significantly lower risk of age related declines in memory. Regular tea drinkers also have higher bone density levels and slower rates of bone loss.
One cup or eight ounces of black tea contains 14-70 mg of caffeine, and green tea 24-45 mg, compared to 95-200 mg in the same sized portion of coffee.
The main disadvantage of drinking tea includes the aesthetic of stained teeth.
Another is the potential impact on your iron levels due to tanins, another type of antioxidant that interferes with the absorption of non-heme, or plant-based iron from foods like greens and beans (more of an issue if you choose to follow a vegetarian diet).
Coffee is also rich in antioxidants called Polyphenols and a Harvard study found that drinking three to five cups of coffee a day may be less likely to die prematurely from type 2 diabetes, cardiovascular disease, Parkinson’s disease and certain cancers than those who drink less or no coffee.
Natural substances in unfiltered coffee (meaning boiled or espresso) have been shown to raise cholesterol levels slightly. Coffee is also more acidic than tea, so if you have stomach or digestive issues you may tolerate tea better.
Coffee has long had a reputation for bone issues, but it remains unclear how significant the effects are. One study found that a high intake of coffee (>4cups) reduced bone density by 2-4%, but the effect didn’t translate to an increased risk of fracture.
The caffeine content of coffee is higher than tea (see above). Caffeine is a stimulant, so if you’re sensitive to it coffee may leave you feeling overly stimulated, jittery, and anxious. If you have high blood pressure, you should limit your caffeine intake because caffeine can cause a short, but dramatic spike in blood pressure. Also two or more cups of coffee a day can uncrease the risk of heart disease in people with a fairly common genetic mutation, which slows the breakdown of caffeine in the body.
Unfortunately the testing for this is limited, and not typically offered by doctor's offices.
Finally, coffee is a known diuretic, meaning it flushes water from the body. Research shows that your body can adjust to a regular habit, but if you only have it once in a while, or you have more than usual, it could leave you dehydrated.