Bananas are an excellent natural fast food, providing 'moderate-releasing' sugars that give an instant, sustained energy boost, without the side effects of a junk food binge. They also contain beneficial nutrients that can help prevent or overcome a number of illnesses.
Bananas contain three natural sugars - sucrose, fructose and glucose. They also contain the soluble fiber pectin, which improves the elimination of toxins. Bananas are an excellent source of potassium, which can help reduce the risk of high blood pressure and stroke, and are a good source of Vitamin C, B6, zinc, iron, folic acid and calcium.
Bananas are an excellent snack for anyone engaged in strenuous exercise, because of their combination of instant and slow-release energy as well as potassium, which can help prevent cramping.
The starch in bananas is not easily digestible, which is why they should only be eaten ripe, when most of the starch has turned to sugar. This happens when the skin starts to become speckled brown. Fully ripened fruit also contains the most antioxidants, as demonstrated by research conducted in Austria, which suggests that as fruits fully ripen, almost to the point of spoilage, their antioxidant levels actually increase .
The plantain is a close relative of the banana, but it should only be eaten cooked, as it is virtually indigestible raw. However, it is a major source of energy and can be used much like potatoes, eaten boiled, fried or baked.
As the fiber found in ripe bananas is so easily digestible, they are beneficial in the treatment of both constipation and diarrhea. They are also an exceptionally rich source of fructooligosaccharide, a compound called a prebiotic because it nourishes probiotic (friendly) bacteria in the colon. These beneficial bacteria produce vitamins and digestive enzymes that improve our ability to absorb nutrients, plus compounds that protect us against unfriendly microorganisms.
According to a recent survey undertaken by M.I.N.D. (a leading mental health charity in the UK), many people suffering from depression felt much better after eating a banana. This is because bananas contain tryptophan, a type of protein that the body converts into serotonin, a chemical known improve your mood, help you relax, and generally make you feel happier.
Research has also found that bananas are beneficial for stomach ulcers and forms of cancer, and the scholar Ibn al-Qayyim mentioned that bananas are 'best when ripe and sweet.' See article Ibn al-Qayyim: Bananas are Beneficial for the Throat, Lungs and Bowel
So a banana really is a natural remedy for many ills, and because it has a thick outer skin, it contains less pesticides that many other fruit, and makes it a convenient fruit to eat on the go.
-  Müller T, Ulrich M, Ongania KH, Kräutler B. Colorless tetrapyrrolic chlorophyll catabolites found in ripening fruit are effective antioxidants. Angew Chem Int Ed Engl. 2007;46(45):8699-702.