Active principles and properties of onions
Several prospective and epidemiological studies have shown that high consumption of vegetables and fruit reduces the risk of cardiovascular disease, certain cancers and other chronic diseases1,2. More specifically, studies indicate that the consumption of vegetables from the alliaceae family (onion, garlic, shallot, chives, green onions, leeks) could have a protective effect against stomach and intestinal cancers3,4. To date, there is insufficient evidence to link with other types of cancers (such as prostate, breast, esophageal and lung cancers) 4.
Cancer. Some epidemiological studies point to the link between onion consumption and the decrease in the incidence of different types of cancer5. First, a synthesis of case-control studies carried out in Italy and Switzerland reveals that eating one to seven servings of onions per week reduces the risk of cancer of the colon, larynx and ovaries6. Added to this is a lower risk of cancer of the esophagus, oral cavity and pharynx with an intake of seven servings and more onion per week6. The same trends are observed for cancers of the brain7, stomach and esophagus8 (studies in China). Researchers in the Netherlands report an inverse relationship between onion consumption and the incidence of stomach cancer9. Finally, mortality from prostate cancer would be reduced thanks to a high intake of onions10. The results of these observational studies should be interpreted with caution as they do not take into account several important factors, such as the variety and method of cooking onions, as well as the precise amounts consumed. In addition, some studies have failed to demonstrate significantly such protective effects against cancer11-13.
The onion may act at different stages of cancer development14. Studies show that onion extracts can inhibit mutation processes that trigger cancer15. They also decrease the proliferation of cancer cells15-17. These results come from in vitro and animal studies. The compounds involved and the precise mechanisms of action are becoming better known and research is continuing.
Cardiovascular illnesses. The onion contains compounds that act on various cardiovascular risk factors18. The majority of studies on the subject have been conducted in vitro or in animals, apart from a few preliminary studies in humans. The onion is recognized for its ability to reduce platelet aggregation in vitro, which is, however, 13 times lower than that of garlic19. Note that aggregation of platelets in the blood increases the risk of thrombosis and, therefore, cardiovascular disease. A recent study found that the addition of raw onions to pig feed for six weeks did not alter platelet aggregation, but significantly decreased blood triglyceride levels20. It should be noted that the amounts used in these studies are equivalent to a daily consumption of one half to one and a half onions in humans.
A preliminary human study indicated that the consumption of approximately three medium onions (500 g) in a soup decreased platelet aggregation ex vivo (test using blood from subjects) 21. In humans, the daily consumption of about 220 g of cooked onion for two weeks has not shown positive effects on platelet aggregation22. Some studies have shown that onions had lower antiplatelet activity after cooking19,23. The antiplatelet activity is believed to be partly attributable to the sulfur compounds and flavonoids (quercetin) of the onion. These two compounds could act synergistically22,24. The precise modes of action have yet to be determined25.
Antioxidants. Antioxidants are compounds that protect the body’s cells from damage caused by free radicals. These are highly reactive molecules that would be involved in the development of cardiovascular diseases, certain cancers and other diseases related to aging26. The main classes of onion antioxidants are anthocyanins and flavonols (more specifically quercetin) 5. Anthocyanins give red color to certain onion varieties, and flavonols color yellow onions5. It should be noted that these antioxidant compounds are mainly lodged in the outer layers of onions27. White onions contain few antioxidants compared to yellow and red ones16. In addition, red onion varieties generally have an antioxidant content5,15 and an activity with