The Glycemic Index of Bananas

People watch their blood sugar for many different reasons, and while the glycemic index can be helpful in this regard, it's not always infallible. Making matters a little more complicated is the fact that the banana glycemic index ranking changes as it ripens.

Don't be so quick as to think this means that you can't count on the glycemic index to help you with your eating habits, or that bananas need to be automatically removed from your diet. But understanding how starch changes to sugar (and what that means for your energy levels) will better equip you to make informed decisions about what you eat.

So, what is the glycemic index? In short, it's a way of measuring how foods affect your blood sugar levels. As Mayo Clinic explains, there are three types of carbohydrates found in foods: starches, sugars and fiber. When the body breaks those carbohydrates down, the sugars and starches are converted into glucose, which your body uses for energy. (The fiber goes through the body undigested.)

The glucose is carried from the blood into cells by a hormone called insulin, which is made in the pancreas. Excess glucose is stored in the liver and released by another hormone called glucagon when availability is getting low. Via this process, the body stays energized.

Read more: 5 Brilliant Banana Recipes You Haven't Tried

But because some foods are richer in carbohydrates than others, and even carbohydrate-rich foods have different ratios of fiber, starches and sugars, different foods will provide different amounts of glucose and at faster speeds. The glycemic index, frequently called GI, measures this effect on a scale of zero to 100. Carbohydrates that break down faster have a higher ranker, with pure sugar being ranked at 100.

The glycemic index can be helpful for people with diabetes, and in some cases, it can be helpful for people who are trying to lose weight. Tufts University notes that low-GI foods tend to be better for weight control than high-GI foods.

Still, Mayo Clinic emphasizes that the glycemic index doesn't give a whole picture of nutrition, and the glycemic index of some foods can change based on what other foods are eaten in combination. If you eat a carbohydrate-rich food with a source of fat, for example, the fat will slow the digestion, thus lowering the GI.

And some people can even benefit from high-GI foods in certain cases. As Harvard Medical School explains, a long-distance runner might prefer high-GI foods because those foods break down quickly and provide practically instant energy.

The other difficult aspect of the glycemic index is that it changes for certain foods, bananas being one of them. Tufts University points out that an unripe banana has a glycemic index that is lower than a ripe banana, just as pasta that's cooked al dente has a lower glycemic index than pasta that's cooked until it's very soft.

Why is this? In short, it has to do with resistant starch, which acts like fiber but isn't exactly the same thing.

According to a review published in June 2019 in Nutrients, green bananas are becoming increasingly popular because they offer such nutritional benefits as fiber, vitamin C, vitamin B6, vitamin A, potassium, phosphorus, magnesium and zinc. They also have resistant starch, which could potentially prevent or treat type 2 diabetes, prevent intestinal disease and reduce blood cholesterol.

Read more: List of Foods Very High in Resistant Starch

Harvard School of Public Health explains that resistant starch is digested slowly and is broken down in the intestines, so less sugar is released into the bloodstream. But as bananas ripen, some of their resistant starch turns into sugar.

The information from Nutrients and Harvard Health recalls what was discussed in a study published all the way back in October 1992 in Diabetic Medicine, which found that diabetics had a more significant response to overripe bananas compared with underripe bananas, though not as much as they did to white bread.

Per numbers provided by Harvard, a banana has a glycemic index of 51, though an underripe banana has a GI of 41. Both would fall under the category of low-GI foods, which have indexes of 55 or less. Doctors recommend people with diabetes try to include more of these foods in their diets, as low-GI foods don't raise blood sugar levels significantly.

How does banana GI stack up to the GI of other fruit? Harvard lists an apple as having a glycemic index of 36 and an orange as having an index of 43. Both fruits would be lower than a ripe banana, but an unripe banana would be marginally lower than an orange. Watermelon, on the other hand, has a higher GI, at 76.

In addition to being low glycemic, bananas offer many other health benefits. Harvard explains that the high levels of potassium in bananas are beneficial in lowering blood pressure and staving off hypertension.

Bananas make a nice fuss-free snack (you can take them on the go, and all you have to do is peel and eat), but they're also easily included in many recipes. Some of them will call for especially ripe bananas — just remember that even though an overripe banana has more sugar and less resistant starch, it's still a low-GI food.

LIVESTRONG.com's Banana Protein Pancakes are made from just three simple ingredients: banana, eggs and olive oil. If you're feeling a little more ambitious, you can enjoy Banana Breakfast Bars, which are made with fat-rich ingredients like flaxseed and coconut oil, which will lower the glycemic index of the finished product.

Because the GI difference between ripe and unripe bananas is so marginal, and both are classified as low glycemic, you should enjoy bananas at the stage of ripeness that you prefer. For many people, a green banana is too hard and sour for their liking; and although some people might like the super sweet taste of an overly ripened banana, the texture might be too soft.

If you're trying to ripen bananas faster, Harvard recommends putting them in a brown paper bag or placing them near ripe fruit. On the other hand, if your bananas have reached the perfect stage of ripeness, you can put them in the fridge to halt their ripening even further. Harvard discourages refrigerating green bananas, as this can be disruptive to their ripening.