Methylxanthines have been reported to increase the severity of fibrocystic breast disease, a benign breast disease, which may increase the risk of breast cancer. Methylxanthines have been reported to increase the severity of fibrocystic breast disease, a benign breast disease, which may increase the risk of breast cancer. Some researchers suggest that if methylxanthines are removed from the diet symptoms will improve. Gastrointestinal Effects Coffee and tea significantly reduce iron absorption by 40 percent and 60 percent respectively. Caffeine itself has a negative effect on calcium balance by reducing both calcium absorption and increasing the calcium lost through the urine.
Coffee also decreases lower esophageal sphincter pressure, possibly contributing to heartburn.
In animals, very high doses of caffeine lead to aggressiveness and psychotic behavior. Caffeine-sensitive subjects report anxiety and severe depression after a dose of only 300 mg of caffeine. Pediatricians are concerned that many children consuming high levels of caffeine display hyperactive behavior. Many regular users are surprised by the difficulty they experience when they try to quit using caffeine. Dependence, as evidenced by withdrawal symptoms, may occur after only 6-15 days of exposure to 600 mg or more daily.