Cancer
Some evidence suggests that moderate consumption of bananas may be protective against kidney cancer. A 2005 Swedish study found that women who ate more than 75 servings of fruits and vegetables cut their risk of kidney cancer by 40 percent, and that bananas were especially effective. Women eating four to six bananas a week halved their risk of developing kidney cancer.
Bananas may be helpful in preventing kidney cancer because of their high levels of antioxidant phenolic compounds.
Health risks
Eaten in moderation, there are no significant side effects associated with eating bananas. However, eating the fruits in excess may trigger headaches and sleepiness, Flores said. She said that such headaches are caused by "the amino acids in bananas that dilate blood vessels." Overripe bananas contain more of these amino acids than other bananas. "Bananas can also contribute to sleepiness when eaten in excess due to the high amount of tryptophan found in them," she said. Magnesium also relaxes the muscles — another sometimes-benefit, sometimes-risk.
Bananas are a sugary fruit, so eating too many and not maintaining proper dental hygiene practices can lead to tooth decay. They also do not contain enough fat or protein to be a healthy meal on their own, or an effective post-workout snack.
Eating bananas becomes significantly risky only if you eat too many. The USDA recommends that adults eat about two cups of fruit a day, or about two bananas. If you eat dozens of bananas every day, there may be a risk of excessively high vitamin and mineral levels.
The University of Maryland Medical Center reported that potassium overconsumption can lead to hyperkalemia, which is characterized by muscle weakness, temporary paralysis and an irregular heartbeat. It can have serious consequences, but you would have to eat about 43 bananas in a short time for any symptoms of hyperkalemia to occur.
According to the NIH, consuming more than 500 milligrams of vitamin B6 daily can possibly lead to nerve damage in the arms and legs. You would have to eat thousands of bananas to reach that level of vitamin B6.