ONIONS, like garlic, are members of the Allium family and both are rich in sulphur-containing compounds that are responsible for their pungent odours and for many of their health-promoting effects.
A wide variety of allyl sulphides are found in onion, including the four major diallyl sulphides: DMS (diallyl monosulphide), DDS (diallyl disulphide), DTS (diallyl trisulphide), and DTTS (diallyl tetrasulphide).
Also present are a wide variety of sulphoxides, including (+) S-methyl-L-cysteine sulphoxide (MCSO), (+)-S-(1-propenyl)-L-cysteine sulphoxide (PRENCSO), S-methyl-l-cysteine sulphoxide, S-propyl-l-cysteine sulphoxide, and S-propenyl-l-cysteine sulphoxide. Onions are an outstanding source of polyphenols, including the flavonoid polyphenols. Within this flavonoid category, onions are a standout source of quercetin.
In animal studies, there is evidence that onion’s sulphur compounds may work in an anti-clotting capacity and help prevent the unwanted clumping together of blood platelet cells. There is also evidence showing that sulphur compounds in onion can lower blood levels of cholesterol and triglycerides, and also improve cell membrane function in red blood cells.
In human studies, most of the cardiovascular benefits have been demonstrated in the form of overall diet. Multiple studies show onion to be a food that provides protection for the heart and blood vessels when consumed in a diet that is rich in other vegetables and fruits — especially flavonoid-containing vegetables and fruits. The benefits of onion in this overall dietary context extend to prevention of heart attack. In virtually all of these diet-based studies, participants with the greatest intake of vegetables (including onions) gain the most protection.
Support for bone and connective tissue
Human studies have shown that onion can help increase bone density and may be of special benefit to women of menopausal age who are experiencing loss of bone density. In addition, there is evidence that women who have passed the age of menopause may be able to lower their risk of hip fracture through frequent consumption of onions. “Frequent” in this context means onion consumption on a daily basis!
While onion is not as well researched as garlic in terms of specific inflammatory health problems like rheumatoid arthritis or allergic airway inflammation, this allium vegetable has nevertheless been shown to provide important anti-inflammatory benefits. Onionin A — a unique sulphur molecule in onion that is found in the bulb portion of the plant — has been shown to inhibit the activity of macrophages, specialised white blood cells that play a key role in our body’s immune defense system and one of their defense activities involves the triggering of large-scale inflammatory responses. While macrophage activity is typically a good thing, inhibition of their activity can sometimes be critical in getting chronic unwanted inflammation under control.
Onion has repeatedly been shown to lower risk of several cancers, even when we consume it in only moderate amounts. “Moderate” generally means one to two times per week, even though in some studies, it has been used to mean up to five to six times per week. Colorectal cancer, laryngeal cancer, and ovarian cancer are the cancer types for which risk is reduced along with moderate amounts of dietary onion. For other cancer types, however, moderate intake of onion has not been enough to show significant risk reduction. For these cancer types — including esophageal cancer and cancers of the mouth — daily intake of onion is required before research results show significant risk reduction.
Other health benefits
In animal studies, onions have shown potential for improvement of blood sugar balance, though it is not yet clear about the carry over of these benefits for humans who are seeking better blood sugar balance from their diet. Most of the animal studies have been conducted on rats and most have used onion juice or onion extract as the form of onion tested. Future research is needed to clarify onion’s potential for helping lower blood sugar and improving blood sugar control, especially in persons with blood sugar problems.
While not as well researched as garlic in terms of antibacterial benefits, onion has nevertheless been shown to help prevent bacterial infection.
Along with its sulphur-containing compounds, the flavonoid quercetin contained in onion helps provide these antibacterial benefits. We’ve seen studies showing antibacterial activity of onion in relationship to the bacteria Streptococcus mutans and Streptococcus sobrinus. (These bacteria are commonly involved in the production of tooth cavities).
Antibacterial benefits have also been shown in the area of gum (periodontal) disease bacteria, including Porphyromonas gingivalis and Prevotella intermedia. Interestingly, in one study, fresh, chopped, uncooked onion had antibacterial effects on these potentially unwanted gum bacteria, but non-fresh, uncooked onion (raw onion that was chopped and then left to sit for two days at room temperature) did not demonstrate these same antibacterial properties nor did fresh onion that was grated and then steamed for 10 minutes.