Onions are a commonly eaten vegetable all over the world and contain many bioactive components. Onion extracts have shown potent anti-atherogenic effects related to a variety of bioactivity. Onion (Allium cepa) is an important source of food phytochemicals with proven antioxidant properties, such as organo-sulfur compounds, phenolic acids, flavonoids and anthocyanins. 30 volatile compounds have been identified from the dark onion, and a total of 49 types of volatile compounds have been identified from the regular onion. Mainly these are sulfur compounds, but with a significant difference in type and their relative content between fresh and dried onion. Other active components of the onion were found to be quercetin and the glucosides of quercetin, isorhamnetin and kaempferol. Their concentrations in the Tropea variety were 150 times higher than that of Montoro, but the total anthocyanins in the latter are 30 times higher than those of Tropea.
There are no scientifically recognized studies on the health properties of onion for humans. There is dozens of evidence on experimental animals and popular traditions handed down through the generations. Let’s start with the first ones.
- Diet of diabetic rats supplemented with onion or with single components (alliin, allithride and S-methylcysteine oxide) leads to a reduction in plasma glucose concentrations and body weight (Jain and Vyas, 1974).
- In one study, onion extracts or the bioactive components quercetin and catechin were observed to potentiate anti-free radical (ROS) activity, which in turn prevented LDL oxidation and lipid peroxidation in male Wistar rats undergoing to oxidative stress caused by mercury.
- In another study, it was found that the onion extract attenuates atherosclerotic lesions, decreases the adrenomedulin content in the plasma and the aortic calcitonin receptor. In addition, onion extract increased plasma glutathione level, SOD activity and vasodilator nitric oxide (*NO) content, while the inflammatory response was reduced.
- Some experimental studies have suggested that onions have anti-thrombotic effects through the platelet inhibitory response. Therefore, onion intake could have a thrombophilic-based cardiovascular disease prevention capacity. In one study, the results showed that onion could inhibit platelet-induced thrombosis in dogs. This effect appears to be mediated by the sulfur-rich compounds of the onion, as the researchers did not find a significant association between the quercetin concentration and anti-thrombotic activity.
- Interestingly, the anti-thrombotic effects of quercetin-rich onion peel (OPE) extracts on arteries in rats were confirmed in another study. OPE drastically reduced TG and blood glucose without affecting blood cholesterol levels. Furthermore, in vivo arterial thrombosis was significantly abolished in OPE-fed 2mg and 10mg groups. Furthermore, the expression of the tissue factor induced by thrombin, a coagulation initiator, in human umbilical vein endothelial cells was significantly reduced by OPE.
- Onions also found anti-hypertensive effects in other experiments. For example, onion as a food reduced oxidation markers in plasma in rats made hypertensive by NAME (nitric oxide antagonist) and in SHR rats subject to stroke. In addition, onions improved the nitrate / nitrite ratio (NO products) excreted in the urine and NOS activities in the kidneys in stroke-prone SHRs, but not in L-NAME-induced hypertensive rats. These findings could partly explain the mechanisms by which onion lowered blood pressure on these hypertensive rats (see bibliography).
Among the most abundant sulfur compounds in onion, the most important is allylpropyl disulfide. This substance, together with the flavonoid quercetin, gives the onion a potential antitumor and chemopreventive activity, especially with regards to gastric, colon and prostate cancer. This food also contains flavonoids with a diuretic action, and glucokinin, a saponin with antidiabetic activity. A few years ago is the definitive characterization of a gluco-kampferol (polyphenol) as a hypoglycemic active ingredient in onion extract and how it acts at the insulin level (Gautam M et al., 2015; Ikechukwu and Ifeanyi, 2016) . An example of the number of poly-sulphides contained in the onion is shown in the following figure. Not all of them have the same biological properties, and among them it has not yet been possible to identify those with hypotensive or hypoglycemic properties.
The complete mixture (oil) is known to possess bactericidal, antifungal, anticancer and hypotensive activity. It is proved by popular experiences that the onion eaten raw helps digestion and has internal antibiotic activity. Cooked, on the other hand, shows strong diuretic properties, which has led scholars to suggest that the organo-sulfur derivatives oxidized by cooking may be responsible for this effect. More recent is the confirmation that the whole bulb extract has a preventive action on obesity (Yoshinari O et al., 2012).
The first report that dealt with the hypoglycemic effect of onions dates back to more than 50 years ago (Galal and Mawad, 1965), confirmed almost a decade later in laboratory animals. Since then, many experiments have confirmed the blood glucose, lipid and thrombotic risk lowering activities of onion extracts in experimental animals (see the history of the bibliography). The first systematic study correlating the intake of onions in diet and the incidence of type 2 diabetes was published about 10 years ago (McNaughton SA et al., 2008). An excellent review on the management of type 2 diabetes and dietary lifestyle was published a few years ago (Haque N et al., 2011), while the use of onion extract as a supplement was excellently reviewed by Akash M et al. (2012).
But it was only a few years ago that a study was published on the effects of onion consumption in subjects predisposed to cardiovascular disease, renal failure and diabetes (Bahadoran Z et al., 2017). Individuals who had previously participated in a study on blood glucose control and lipidemia (Tehran Lipid and Glucose Study; 2006-2008 to 2012-2014) were evaluated at the end of the study. Among those who consumed more vegetables of the Allium genus (including onion), the incidence of cardiovascular disease was 64% lower, that for CRI by 29% and for diabetes mellitus by 37%.
The above may suffice to make it clear how in the diet of the diabetic patient, and / or with heart disease, the onion must be consumed regularly. Since it is also a food with antioxidant and chemopreventive action, it serves to keep the risk of other diseases low due to a lifestyle not really suited to the general concept of health.
- edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Bahadoran Z et al. J Hypertens. 2017; 35(9):1909-16.
Gautam S et al. Planta Med. 2015 Feb; 81(3):208-14.
Akash MS et al. Nutrition. 2014 Oct; 30(10):1128-37.
Yoshinari O et al. Nutrients. 2012 Oct 22; 4(10):1518-26.
Haque N et al. Pak J Biol Sci. 2011; 14(1):13-24. Review.
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Bang MA, Kim HA et al. Nutr Res Pract. 2009; 3(3):242-46.
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Mathew PT et al. Indian J Phys Pharmacol 1975; 19(4):213.
Galal EE et al. J Egypt Med Assoc 1965; 48:Suppl:14-45.