Raw onions are very low in calories, with only 40calories per 100 grams (3.5 oz). By fresh weight, they are 89% water, 9% carbohydrates and 1.7% fiber, with tiny amounts of protein and fat.
The table below contains information on all the main nutrients in 100g onions.
Carbohydrates content is about 9-10% of both raw and cooked onions.They consist mostly of simple sugars, such as glucose, fructose and sucrose, as well as fibers.
A 100 gram (3.5 oz) portion of onions contains 9.3 grams of carbs, and 1.7 grams of fiber, so the total digestible carbohydrate content is 7.6 grams.
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. Fructans are so-called prebiotic fibers. In other words, the beneficial bacteria in the gut can use them for fuel.
This leads to the formation of short-chain fatty acids, such as butyrate, which may improve colon health, reduce inflammation, and cut the risk of colon cancer.
However, fructans are also known FODMAPs (fermentable oligo-, di-, monosaccharides and polyols), which some people cannot digest. FODMAPs may cause unpleasant digestive symptoms in sensitive individuals, such as those who suffer from irritable bowel syndrome (IBS).
Brief speaking, Onions consist mostly of water, carbs and fiber. The main fibers in them, fructans, can feed the friendly bacteria in the gut, but they can also cause digestive problems in some people.
3. Vitamins and Minerals
Onions contain decent amounts of several vitamins and minerals, including vitamin C, folate (B9), vitamin B6 and potassium.
Vitamin C is an antioxidant vitamin that is needed for immune function and maintenance of skin and hair. Folate (B9) is a water-soluble B-vitamin that is essential for cell growth and metabolism, and is especially important for pregnant women .Vitamin B6 was found in most foods, this vitamin is involved in the formation of red blood cells. Potassium is the essential mineral can have blood pressure-lowering effects, and is important for heart health.
4. Other Plant Compounds
The health benefits of onions are attributed to their antioxidants and sulfur-containing compounds.
Onions are also among the main dietary sources of flavonoids in many countries, specifically a beneficial compound called quercetin.
The most abundant plant compounds in onions are listed below:
1) Anthocyanins: Only found in red or purple onions, anthocyanins are powerful antioxidants and pigments that give onions their reddish color.
2) Quercetin: An antioxidant flavonoid that may lower blood pressure and improve heart health.
3) Sulfur-compounds: Mainly sulfides and polysulfides, which may have protective effects against cancer.
4) Thiosulfinates: Sulfur-containing compounds that may inhibit the growth of harmful microorganisms and prevent formation of blood clots.
Red and yellow onions are richer in antioxidants than other types. In fact, yellow onions may contain almost 11 times more antioxidants than white onions.Cooking onions can significantly decrease some of the antioxidants.
Onions are rich in plant compounds and antioxidants, especially quercetin and sulfur-containing comppunds. Colored varieties(yellow onion and red onion or purple onion) contain more antioxidants than white ones.
Same as with many other foods, onions have both "good" and "bad" aspects.
They are high in antioxidants and sulfur-containing compounds, some of which may have a number of beneficial health effects. Although more research is needed, onions have been linked to improved bone health, lower blood sugar levels and reduced risk of cancer.
On the other hand, they may cause problems in some people, especially digestive problems in those who are intolerant to FODMAPs.
All of that being said, onions are usually consumed in small quantities anyway. Their main purpose is providing flavor, not nutrition. For individuals who tolerate (and enjoy) onions, they can be a valuable component of a healthy diet.