Onions are a very good source of vitamins B6 and C, chromium, biotin, and fiber. They are also a good source of folic acid and vitamins B1 and K.
The health benefits provided by onions are mostly due to their content of several organic sulfur compounds. Like garlic, onions also have the enzyme alliinase (released when an onion is cut or crushed). Other constituents that are found in onions include flavonoids (primarily quercetin); phenolic acids (such as ellagic,caffeic, sinapic, and p-coumaric), pectin, sterols, saponins, and volatile oils.
Studies have shown that, like garlic, onions and onion extracts decrease blood lipid levels, prevent clot formation, and lower blood pressure. Studies have also revealed that onions have a significant effect in lowering blood sugar levels. Throughout history, onions have also been used to treat asthma. Their action in asthma is mostly due to their ability to inhibit the production of compounds that cause the bronchial muscle to spasm, which then causes the muscle to relax.