Onions, bulbing vegetables that grow just above the surface of the dirt, impart a strong flavor and crunch to many dishes. They come in a range of colors, including white, yellow and red, and their flavor can be mild or quite robust. Onions are healthy vegetables that contain vitamins and minerals, few calories and little fat.
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Calories and Fat
A small onion contains 29 calories. It's uncommon to eat an onion on its own, so be sure to account for all the calories contained in recipes that use this vegetable. An onion also adds only 0.1 grams of fat to your daily meal plan totals. This is a very small part of the suggested intake of 44 to 78 grams of fat per day, or 20 to 35 percent of your daily calories.
Carbohydrates and Fiber
One small onion introduces 7.1 grams of carbohydrates into your meal plan. This is a small amount, but it will contribute toward the daily suggested intake of 130 grams. An onion also provides you with 1 gram of fiber, a type of carbohydrate that does not break down for absorption into your body. Your meal plan should ideally contain 25 to 38 grams of fiber per day -- it offers protection against diabetes and some types of cancer.
Eat a small onion, and you'll get 7 percent of the daily recommended amount of vitamin C. Onions are a smart choice when you sustain an injury -- your body needs vitamin C to speed the healing process and bolster your immune system. You'll also take in 5 percent of the vitamin B-6 you need each day per onion. The vitamin B-6, also known as pyridoxine, in onions plays a critical role in helping your body use the protein you eat.
A small onion provides small amounts of several minerals. It contains 3 percent of the potassium you need each day, as well as 2 percent of the daily recommended intake of magnesium, phosphorus and calcium. The minerals present in a small onion influence the efficiency with which your body stores energy, the strength of your bones and teeth and the function of your enzymes.
Incorporating a small amount of onions into your diet may decrease your risk of heart disease and stroke. A 2007 study led by the Institute of Food Research revealed that eating 100 to 200 grams of onions can ease inflammation that impacts arteries; this inflammation can result in heart disease. This benefit is due to a compound known as quercetin, which is also found in tea, apples and red wine.