28/12/2020
Nutrition facts
Raw onions are very low in calories, with only 40 calories per 3.5 ounces (100 grams).
By fresh weight, they are 89% water, 9% carbs, and 1.7% fiber, with tiny amounts of protein and fat.
The main nutrients in 3.5 ounces (100 grams) of raw onions are (1Trusted Source):
Calories: 40
Water: 89%
Protein: 1.1 grams
Carbs: 9.3 grams
Sugar: 4.2 grams
Fiber: 1.7 grams
Fat: 0.1 grams
Carbs
Carbohydrates make up about 9–10% of both raw and cooked onions.
They consist mostly of simple sugars, such as glucose, fructose, and sucrose, as well as fiber.
A 3.5-ounce (100-gram) portion contains 9.3 grams of carbs and 1.7 grams of fiber, so the total digestible carb content is 7.6 grams.
Fibers
Onions are a decent source of fiber, which accounts for 0.9–2.6% of the fresh weight, depending on the type of onion.
They are very rich in healthy soluble fibers called fructans. In fact, onions are among the main dietary sources of fructans (2Trusted Source, 3).
Fructans are so-called prebiotic fibers, which feed the beneficial bacteria in your gut.
This leads to the formation of short-chain fatty acids (SCFAs), such as butyrate, which may improve colon health, reduce inflammation, and cut your risk of colon cancer (4, 5Trusted Source, 6Trusted Source).
However, fructans are considered FODMAPs, which may cause unpleasant digestive symptoms in sensitive individuals, such as those with irritable bowel syndrome (IBS) (7Trusted Source, 8Trusted Source, 9Trusted Source).
SUMMARY
Onions consist mostly of water, carbs, and fiber. Their main fibers, fructans, can feed the friendly bacteria in your gut, though they may cause digestive problems in some people.