A first type of cardiovascular benefit from bananas is related to their potassium content. Bananas are a good source of potassium, an essential mineral for maintaining normal blood pressure and heart function. Since one medium-sized banana contains a whopping 400-plus mg of potassium, the inclusion of bananas in your routine meal plan may help to prevent high blood pressure and protect against atherosclerosis.
The effectiveness of potassium-rich foods such as bananas in lowering blood pressure has been demonstrated by a number of studies. For example, researchers tracked over 40,000 American male health professionals over four years to determine the effects of diet on blood pressure. Men who ate diets higher in potassium-rich foods, as well as foods high in magnesium and cereal fiber, had a substantially reduced risk of stroke. We've also seen numerous prospective clinical research trials showing substantial reductions of blood pressure in individuals eating the potassium-rich DASH Diet.
A second type of cardiovascular benefit from bananas involves their sterol content. While bananas are a very low-fat food (less than 4% of their calories come from fat), one type of fat that they do contain in small amounts are sterols like sitosterol, campesterol, and stigmasterol. As these sterols look structurally similar to cholesterol, they can block the absorption of dietary cholesterol. By blocking absorption, they help us keep our blood cholesterol levels in check.
A third type of cardiovascular benefit from bananas involves their fiber content. At about 3 grams per medium banana, we rank bananas as a good source of fiber. Approximately one-third of the fiber in bananas is water-soluble fiber. For one medium-sized banana, this amount translates into 1 gram of soluble fiber per banana. Soluble fiber in food is a type of fiber especially associated with decreased risk of heart disease, making regular intake of bananas a potentially helpful approach to lowering your heart disease risk.
Bananas are a fascinating fruit in terms of their carbohydrate and sugar content. Even though bananas are a fruit that tastes quite sweet when ripe—containing 14-15 grams of total sugar—bananas receive a rating of low in their glycemic index (GI) value. GI measures the impact of a food on our blood sugar. This low GI value for bananas is most likely related to two of their carbohydrate-related qualities.
First, as mentioned previously, a medium-size banana contains about 3 grams of total fiber. Fiber is a nutrient that helps regulate the speed of digestion, and by keeping digestion well-regulated, conversion of carbohydrates to simple sugars and release of simple sugars from digesting foods also stays well-regulated.
Within their total fiber content, bananas also contain pectins. Pectins are unique and complicated types of fiber. Some of the components in pectins are water-soluble, and others are not. As bananas ripen, their water-soluble pectins increase, and this increase is one of the key reasons why bananas become softer in texture as they ripen. As their water-soluble pectins increase, so does their relative concentration of fructose in comparison to other sugars. This increase in water-soluble pectins and higher proportional fructose content helps normalize the rate of carbohydrate digestion and moderates the impact of banana consumption on our blood sugar. The bottom line here are some surprisingly digestion-friendly consequences for a fruit that might be casually dismissed as being too high in sugar to be digestion-friendly.
Similar to the importance of their water-soluble pectins is the digestive importance of fructooligosaccharides (FOS) in bananas. FOS are unique fructose-containing carbohydrates that are typically not broken down by enzymes in our digestive tract. Instead, they move along through the digestive tract until they reach our lower intestine and get metabolized by bacteria. This process helps maintain the balance of "friendly" bacteria (for example, Bifidobacteria) in our lower intestine, and as a consequence, it also supports our overall digestive health.
In one study involving female participants, eating two bananas each day for two months led to significant increases in Bifidobacteria. Along with these increased levels of Bifidobacteria, participants also experienced fewer gastrointestinal problems and more regular bowel function when compared to other women in the study who drank a banana-flavored beverage that did not contain any actual banana.
The unique mix of vitamins, minerals, and low glycemic carbohydrates in bananas has made them a favorite fruit among endurance athletes. Their easy portability, low expense, and great taste also help support their popularity in this exclusive group.
A 2012 study of distance cyclists found that eating the equivalent of about one half a banana every 15 minutes of a three-hour race was just as good at keeping energy levels steady as drinking an equivalent amount of carbohydrate and minerals from a processed sports beverage. Bananas have long been valued by athletes for prevention of muscle cramps. Since bananas are a good source of potassium, and since low potassium levels are known to contribute to risk of muscle cramps, it is logical to think about the potassium content of bananas as being the reason for fewer muscle cramps after consumption of bananas. There is actually some recent research in support of this reasoning. In a recent study, consumption of one or two bananas prior to an hour of exercise was shown to keep blood potassium levels higher after the training. But there are still some big unanswered questions here, since researchers are not convinced that low potassium levels are the most frequent cause of muscle cramps with training.