The banana is one of nature's most nutritious foods. It contains many simple carbohydrates, which your body can break down rapidly for energy, and it's also rich in vitamins and minerals like potassium and vitamin B-6. Eating too much of any type of food has its downsides, however, even if it's a food as healthy as the banana.
Eating too many calories, no matter where they come from, leads to weight gain. When part of a healthy balanced diet, bananas alone shouldn't cause you to gain weight.
Bananas and Weight Gain
Bananas are unlikely to "cause" weight gain because putting on weight is associated with eating more calories than you burn and not necessarily with specific foods. So if you maintain an energy balance, you won't gain weight even if you eat a lot of bananas. However, as fruits go, bananas have a relatively high calorie density — eating many of them on a regular basis can raise your calorie intake more than other fruits. A large banana has 120 calories, but a large orange has just 85, and 1 cup of sliced strawberries has just 55. Dr. Melina Jampolis, CNN.com's physician nutrition specialist, urges all of her patients to avoid thinking of fruit as a "free food" and to recognize that eating too much of it can add several hundred or more extra calories to their diets every day.
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High Sugar Content
According to Harvard University, bananas are classified as a "medium-level" glycemic food, meaning they have a glycemic index level that is high enough to cause some fluctuations in blood sugar levels. If you eat a lot of foods that have relatively high GI levels, you may develop an elevated risk of Type 2 diabetes or heart disease, or have a harder time with managing your weight. Since bananas do contain fiber, they are more slowly digested than other foods with similarly high sugar contents, but very ripe bananas may have a higher GI and be more likely to contribute to health issues if you eat too many of them.
Eating too many bananas could also be harmful because it may prevent you from achieving a healthy balance in your diet. Although bananas contain plenty of beneficial nutrients, they don't contain all of the same nutrients as other fruits, vegetables, whole grains and protein foods, all of which are important in a daily eating plan. If the bulk of your diet is made up of bananas, you could develop nutrient deficiencies over time.
When to Stop
There's no surefire way to tell how many bananas is "too many" to eat. Harvard Medical School recommends that adults eat 2 servings of fruit per day. Examples of one serving include 1 whole banana, apple orange or pear or 1/2 cup grapes, blueberries or strawberries. You can also eat dried or canned fruit to meet the daily goal. Eating more than the recommended amount isn't likely to do you any physical harm beyond adding extra calories to your diet, but it's healthiest to enjoy bananas in moderation along with a variety of other nutritious foods.