A medium banana (about 118 grams) contains 105 calories, 28 grams of carbohydrates, 450 mg of potassium, 3 grams of fiber, and essentially zero fat. That simple nutritional profile makes it one of the most convenient whole foods available, but there’s more going on inside a banana than most people realize.
Carbohydrates and Natural Sugars
Carbohydrates make up the bulk of what’s in a banana. A medium one has about 28 grams of carbs, with 15 grams coming from naturally occurring sugars (a mix of glucose, fructose, and sucrose). The remaining carbohydrates come from fiber and starch, and the ratio between sugar and starch shifts dramatically as a banana ripens.
A green banana contains 8 to 12 grams of resistant starch, a type of carbohydrate your body can’t fully digest. Resistant starch passes through your small intestine intact, functioning more like fiber. It feeds beneficial gut bacteria and causes a slower, smaller rise in blood sugar compared to regular starch. As the banana turns yellow, that resistant starch converts into simple sugars. A ripe yellow banana has only 1 to 3 grams of resistant starch left, and an overripe brown banana has less than 1 gram. This is why green bananas taste starchy and chalky while brown-spotted ones taste noticeably sweeter, even though the total calorie count barely changes.
Potassium
Bananas are famous for potassium, and they deliver. One medium banana provides about 450 mg, roughly 10% of what most adults need daily. Potassium is an electrolyte that helps regulate fluid balance, muscle contractions, and nerve signals. Most people in Western diets fall short of the recommended intake (around 2,600 to 3,400 mg per day depending on age and sex), so a banana makes a meaningful dent. That said, bananas aren’t the richest source. A medium baked potato or a cup of cooked spinach contains more potassium per serving, but bananas win on convenience since they come in their own wrapper and need no preparation.
Vitamins
The standout vitamin in a banana is B6. A medium banana contains roughly 0.4 mg, which covers about 25% of the FDA’s daily value of 1.7 mg. Vitamin B6 is involved in over 100 enzyme reactions in the body, most related to protein metabolism, immune function, and the production of brain chemicals like serotonin and dopamine. It’s unusual to find this much B6 in a fruit; most rich sources are animal proteins like poultry and fish.
Bananas also contain smaller amounts of vitamin C (about 10% of daily needs) and traces of vitamins A and E. These aren’t standout levels compared to citrus fruits or berries, but they add to the overall nutritional package.
Other Minerals
Beyond potassium, a medium banana contains about 32 mg of magnesium (roughly 8% of daily needs), 0.32 mg of manganese (about 14% of daily needs), and a small amount of copper. Magnesium supports muscle and nerve function and plays a role in blood sugar regulation. Manganese is involved in bone formation and the metabolism of carbohydrates and amino acids. These aren’t headline-grabbing amounts, but they contribute to your overall mineral intake, especially if bananas are part of your regular diet.
Fiber
A medium banana has about 3 grams of dietary fiber, a mix of soluble and insoluble types. Soluble fiber dissolves in water and forms a gel-like substance that slows digestion, which helps moderate blood sugar spikes after eating. Insoluble fiber adds bulk and helps move things through your digestive tract. Three grams isn’t exceptional (a cup of raspberries has 8 grams, and a cup of lentils has about 15), but for a grab-and-go snack with no prep, it’s a solid contribution toward the 25 to 38 grams most adults should aim for daily.
Green bananas offer an additional digestive benefit through their resistant starch, which functions similarly to soluble fiber. If you’re specifically looking for gut health benefits, less-ripe bananas have the edge.
Antioxidants and Plant Compounds
Bananas contain several antioxidant compounds, though they’re not as rich in these as deeply colored fruits like blueberries or pomegranates. The flesh contains about 232 mg of total phenolic compounds per 100 grams of dry weight. These phenolics help neutralize free radicals, unstable molecules that can damage cells over time. One specific antioxidant identified in bananas is gallocatechin, a compound in the same family as the catechins found in green tea, though at lower concentrations (about 30 mg per 100 grams of dry weight in the flesh).
Interestingly, the banana peel contains far higher concentrations of these compounds. Total phenolics in the peel measure roughly four times higher than in the flesh, and gallocatechin is about five times more concentrated. Most people don’t eat the peel, of course, but this has driven interest in using banana peels in food processing and supplements.
Bananas also contain dopamine, but before you get excited: the dopamine in a banana doesn’t cross the blood-brain barrier. It won’t affect your mood or brain chemistry. It does, however, act as an antioxidant in the body.
Protein and Fat
Bananas are not a meaningful source of protein or fat. A medium banana has about 1 gram of protein and essentially zero fat. This makes them a poor choice as a standalone meal since they won’t keep you full for long. Pairing a banana with a protein or fat source, like peanut butter, yogurt, or a handful of nuts, creates a more balanced snack that sustains energy longer.
How Ripeness Changes What You Get
The banana sitting on your counter is chemically different today than it will be in three days. As bananas ripen, enzymes break down resistant starch into simple sugars. A green banana is higher in resistant starch and lower in sugar, making it a better option if you’re watching blood sugar responses. A spotted, overripe banana is almost entirely simple sugar with less than 1 gram of resistant starch remaining, which makes it sweeter and easier to digest but more likely to cause a quick blood sugar spike.
Texture, flavor, and digestibility all shift with ripeness too. Green bananas can cause bloating in some people because resistant starch ferments in the gut. Ripe bananas are gentler on sensitive stomachs. Neither is “better” across the board; it depends on what you’re looking for.