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洋蔥 (學名：Allium cepa)，現在北京人稱蔥頭、甘肅人稱洋蒜、新疆人稱皮牙孜、是一種常見的蔥科蔥屬植物。
二年生或多年生草本。根弦線狀，濃綠色圓筒形中空葉子，表面有蠟質；葉鞘肥厚呈鱗片狀，密集於短縮莖的周圍，形成鱗莖 (俗稱蔥頭)；傘狀花序，白色小花；蒴果。根莖外邊包著一層薄薄的皮 (白、黃或紅色)，裡面是一層一層的肉，一般是白色或淡黃色。
洋蔥的成長可由種子培育、或是更常見的、由球莖Sets來培育。洋蔥球莖通常是一年播很密集的種子來培育的；讓大部份的種子最後都發育不良，只結很小的球莖。再將這些小球莖分散來種，就能夠很容易的在隔年時，成長為成熟的洋蔥。不過這樣長出來的洋蔥也常常會有雙球莖的問題。無論那一種培育的方法，都能用在培育大蔥Spring Onion或綠蔥上。這些蔥是洋蔥發芽之後，還沒完全長成的時候收成的，綠蔥的英文名字除了Green Onion之外，也常被叫做Welsh Onion、或是Allium fistulosum (青蔥的學名；不會結乾球莖的一種蔥類)。
洋蔥是500種蒜屬植物中的一種，其他較常見的成員還包括了蒜頭、蝦夷、蔥及韭菜。數千年來，人類把洋 蔥 拿來食用或是治療各種疾病，上至感冒、下至香港腳。
美國國家癌症研究所正在努力探查它潛在的抗癌特性。根據國家癌症研究所雜誌在1989年發表 一篇在中國所作的研究，指出食用越多蒜屬蔬菜的人，胃癌的罹患率越低。 而另有多篇研究顯示，在動物及試管實驗中，洋蔥所含的多種化學物質可以抑制癌細胞的生長。
洋蔥的化學性質非常複雜。 它裏面含有 100 種以上硫 磺類的化合物，很多都是極怪異且複雜的結構。它也含有大量的類黃酮，包括槲黃素在內 ，這種化合物根據顯示可消除好些強力致癌物及腫瘤刺激物的作用，因此受到廣泛的研究 。
紐約州立大學珂爾巴尼分校的 Eric Block 博士發現洋蔥 裏 面的某種硫磺成份在試管實驗中，可預 防引起氣喘和發炎的生化連鎖反應。而有趣的是，洋蔥原本就是治療呼吸道疾病的傳統偏方。研究指出，每天吃一顆洋蔥的人，好膽固醇的含量會增加。此外，洋蔥也可能幫助降低血壓並預防血凝塊。
The onion (Allium cepa) also known as the bulb onion or common onion, is used as a vegetable and is the most widely cultivated species of the genus Allium. This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), the Egyptian onion (A. ×proliferum), and the Canada onion (A. canadense). The name "wild onion" is applied to a number of Allium species but A. cepa is exclusively known from cultivation and its wild original form is not known. The onion is most frequently a biennial or a perennial plant, but is usually treated as an annual and harvested in its first growing season.
The onion plant has a fan of hollow, bluish-green leaves and the bulb at the base of the plant begins to swell when a certain day-length is reached. In the autumn the foliage dies down and the outer layers of the bulb become dry and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly the onion fly, the onion eelworm and various fungi that cause rotting. Some varieties of A. cepa such as shallots and potato onions produce multiple bulbs.
Onions are cultivated and used around the world. As a foodstuff they are usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make pickles or chutneys. They are pungent when chopped and contain certain chemical substances which irritate the eyes. Onions contain phenolics and flavonoids that have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties.
Nutrition and Health
Most onion cultivars are about 89% water, 4% sugar, 1% protein, 2% fibre and 0.1% fat. They contain vitamin C, vitamin B6, folic acid and numerous other nutrients in small amounts. They are low in fats and in sodium, and with an energy value of 166kJ (40 kcal) per 100 g (3.5 oz) serving, they can contribute their flavour to savoury dishes without raising caloric content appreciably.
Onions contain chemical compounds such as phenolics and flavonoids that basic research shows to have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties. These include quercetin and its glycosides quercetin 3,4'-diglucoside and quercetin-4'-glucoside. There are considerable differences between different varieties in potential antioxidant content. Shallots have the highest level, six times the amount found in Vidalia onions, the variety with the smallest amount.
Some people suffer from allergic reactions after handling onions. Symptoms can include contact dermatitis, intense itching, rhinoconjunctivitis, blurred vision, bronchial asthma, sweating and anaphylaxis. There may be no allergic reaction in these individuals to the consumption of onions, perhaps because of the denaturing of the proteins involved during the cooking process.
While onions and other members of the genus Allium are commonly consumed by humans, they can be deadly for dogs, cats, guinea pigs, monkeys and other animals. The toxicity is caused by the sulfoxides present in raw and cooked onions which many animals are unable to digest. Ingestion results in anaemia caused by the distortion and rupture of red blood cells. Sick pets are sometimes fed with tinned baby foods and any that contain onion should be avoided. Nor is it good for pets to be fed onion-containing leftovers such as pizza, canned spaghetti, Chinese dishes and onion rings. The typical toxic doses are 5 g (0.2 oz) per kg (2.2 lb) bodyweight for cats and 15 to 30 g (0.5 to 1.1 oz) per kg for dogs.