Onion health benefits
Onion (Allium cepa L.) and green onion (Allium ascalonicum) are vegetables originally native to central Asia and are among the oldest cultivated plants in the world 1). Onions are normally available in three color varieties. Yellow or brown onions (called red in some European countries), are full-flavored and are the onions of choice for everyday use, with many cultivars bred specifically to demonstrate this sweetness. Yellow onions turn a rich, dark brown when caramelised and give French onion soup its sweet flavor. The red onion (called purple in some European countries) is a good choice for fresh use when its color livens up the dish; it is also used in grilling. White onions are the traditional onions used in classic Mexican cuisine; they have a golden color when cooked and a particularly sweet flavor when sautéed. While the large, mature onion bulb is most often eaten, onions can be eaten at immature stages. Young plants may be harvested before bulbing occurs and used whole as spring onions or scallions. When an onion is harvested after bulbing has begun, but the onion is not yet mature, the plants are sometimes referred to as “summer” onions. Additionally, onions may be bred and grown to mature at smaller sizes. Depending on the mature size and the purpose for which the onion is used, these may be referred to as pearl, boiler, or pickler onions, but differ from true pearl onions which are a different species. Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient and pickler onions are often preserved in vinegar as a long-lasting relish. Onions are available in fresh, frozen, canned, caramelised, pickled, and chopped forms. The dehydrated product is available as kibbled, sliced, ring, minced, chopped, granulated, and powder forms.
Onion powder is a seasoning widely used when the fresh ingredient is not available. It is made from finely ground, dehydrated onions, mainly the pungent varieties of bulb onions, and has a strong odour. Being dehydrated, it has a long shelf life and is available in several varieties: yellow, red, and white. The bulbs and leaves of onions have a wide variety of flavors and textures, and therefore many culinary uses. Green onions, which are closely related to onions, are characterized by their less pungent onion flavor and are commonly used in cooked dishes or are pickled. All of these vegetables have been valued in many cultures for their pungent flavors and culinary uses and for their health benefits for over 4000 years 2). Consumption patterns vary widely, but onions are typically consumed in larger quantities than garlic, chives, shallots, or leeks 3). Data from the Food and Agriculture Organization of the United Nations show that global per capita production of garlic and onions has been increasing steadily since 1980 4). The most pungent yellow and white onions have typically been used worldwide for cooking; however, consumption of sweeter onions, chives, and green onions (shallots) for uncooked use in the U.S. is increasing 5).
Onions are a commonly consumed vegetable all over the world, and contain bioactive components like phytochemicals. Onions are rich in two chemical groups that have perceived benefits to human health. These are the flavonoids and the alk(en)yl cysteine sulphoxides. Two flavonoid subgroups are found in onion, the anthocyanins, which impart a red/purple color to red onions and flavanols such as quercetin and its derivatives responsible for the yellow and brown skins of many other onion varieties 6). The alk(en)yl cysteine sulphoxides are the flavor precursors, which, when cleaved by the enzyme alliinase, generate the characteristic odor and taste of onion. The downstream products are a complex mixture of compounds which include thiosulphinates, thiosulphonates, mono‐, di‐ and tri‐sulphides. Compounds from onion have been reported to have a range of health benefits which include anticarcinogenic properties, antiplatelet activity, antithrombotic activity, antiasthmatic and antibiotic effects.
Cancer prevention properties
While epidemiological studies support some association between increased intake of onions and/or garlic and decreased risk of certain cancers, the data are limited and sometimes conflicting 7). Additionally, Allium vegetables (garlic, onions, shallots, leeks, and chives) are frequently grouped together for epidemiological analysis, which prohibits separation of effects. The strongest epidemiological evidence points to protective effects of garlic and/or onions against cancers of the digestive tract. A recent meta-analysis of 19 case-control and 2 cohort studies showed that consumption of large amounts of total Allium vegetables reduced risk of gastric cancer when comparing the highest and lowest consumption groups 8). Results were similar for individual Allium vegetables, including garlic, onion, leeks, Chinese chives, scallions, and garlic stalks, but not onion leaves. The summary odds ratio (OR) for decreased risk of gastric cancer with an increment of 20 g/day of total Allium vegetables, (the average weight of one garlic bulb), was 0.91 9). An odds ratio (OR) is a measure of association between an exposure and an outcome. The odds ratio represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure. An odds ratio (OR) 0.91 means consuming garlic, onion, leeks, Chinese chives, scallions, and garlic stalks are associated with lower odds of gastric cancer. On the other hand, investigators with the European Prospective Investigation into Cancer and Nutrition 10) study were unable to corroborate the findings of the above meta-analyses. After 6.5 years of follow-up and 330 cases of gastric cancer, Allium vegetables (garlic, young garlic, onion, and shallot) collectively were associated with a weak, non-significant inverse association with gastric adenocarcinoma 11). However, after 11 years of follow-up and 683 cases, even this weak association was no longer evident for total gastric adenocarcinoma. Allium vegetables were also not associated with decreased risk of cardia, noncardia, intestinal, and diffuse subtypes of gastric cancer 12). The European Prospective Investigation into Cancer and Nutrition cohort, which included men and women aged 35–70 recruited from 1992–1998, had relatively high fruit and vegetable consumption even in the lowest quintile of intake, thus making it difficult to compare high to low or no intake.
The association of onion and garlic intake on colorectal adenoma was also assessed using 562 cases and 5932 controls from subjects in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial 13). Investigators assessed onion and garlic intake collectively for the 12 months prior to screening using a food frequency questionnaire. After adjustment for multiple non-dietary colorectal cancer risk factors, the fifth quintile of garlic/onion intake was associated with an odds ratio of colorectal adenoma of 0.87 compared to the lowest quintile. However, a meta-analysis of 8 different prospective cohort studies on total Allium vegetable intake and colorectal cancer risk showed no decrease in risk with increased Allium consumption 14). And the World Cancer Research Fund/American Institute for Cancer Research 15) expert panel found no statistically significantly decreased risk of colorectal cancer with highest garlic intake in two prospective cohorts, while three case-control studies showed significantly decreased risk of colorectal cancer for those with the highest garlic intake and three other case-control studies did not.
On esophageal cancer, Chen et al 16) found that consumption of raw garlic/onion at least one time per week was associated with an adjusted odds ratio of esophageal cancer of 0.2 in Taiwanese men. Cooked garlic or onions were not addressed in this study. The Italian and Swiss case-control study 17) also reported that consumption of ≥7 portions of onions per week was protective against esophageal cancer odds ratio of 0.12 and that increasing garlic intake was protective with the highest intake associated with an odds ratio of 0.74. The World Cancer Research Fund/American Institute for Cancer Research report reviewed one cohort and eight case-control studies that assessed the effect of garlic, onions, or Allium vegetables collectively on risk of esophageal cancer 18). Only one of the case-control studies showed a statistically significant decrease in risk.
In a population-based case-control study in Shanghai, Hsing et. al. 19) found that individuals in the highest of 3 intake categories of total Allium vegetables had a 53% decreased odds ratio of prostate cancer compared to those with the lowest intake (odds ratio = 0.47). Both garlic (odds ratio= 0.47) and scallion (odds ratio = 0.30) alone were also associated with decreased odds ratio, while Chinese chives, leeks, and onions were not.
The Italian and Swiss case-control networks study showed that those with the highest intake of both onions and garlic did not have significantly decreased odds ratio of prostate cancer compared to those with the lowest intake (odds ratio onions = 0.29; odds ratio garlic = 0.81), although there was evidence of a trend of decreased odds with increasing intake. A third study 20), using data from the Vitamins And Lifestyle cohort, analyzed the association of garlic supplements on prostate cancer risk and found that garlic supplement use was not associated with a decreased risk of prostate cancer among men in western Washington state (hazard ratio = 1.00; means the chance fo dying from prostate cancer is the same between treatment and control group) 21).
In addition to the above in colorectal, esophageal, and prostate cancer, the investigators of the Italian and Swiss case-control studies investigated other possible associations between onions or garlic and cancers of the oral cavity/pharynx, larynx, renal cells, breast, ovary, and endometrium 22). Statistically significant inverse associations were observed for those who ate onions ≥7 times/week for oral cavity/pharyngeal cancers (odds ratio = 0.16), laryngeal cancer (odds ratio = 0.17), and ovarian cancer (odds ratio = 0.27) compared to those who did not eat onions. Additionally, 1–7 servings of onions/week was associated with significantly decreased odds of laryngeal (odds ratio = 0.44) and ovarian cancers (odds ratio = 0.57) compared to nonusers, and >2 compared to 0 servings/week was associated with decreased risk of endometrial cancer (odds ratio = 0.40). No significant associations were observed for onions or garlic and breast cancer or for onions and renal cell carcinoma.
A study of cases from the Hawaii Tumor Registry and controls randomly selected from a list of Oahu residents found that onions (odds ratio = 0.5), but not garlic (odds ratio = 0.7), were associated with decreased risk of lung cancer in those in the highest quartile of intake compared to the lowest 23). When lung cancer subtypes were considered, onions were more strongly associated with decreased risk of squamous cell carcinoma (odds ratio = 0.1) than with adenocarcinoma (odds ratio = 0.6). The association with squamous cell carcinoma was modified by CYP1A1 genotype, as onions were more protective in those with the homozygous wild-type *1/*1 genotype compared to those with *1/*2 or *2/*2 genotypes.
Further studies on the VITamins and Lifestyle (VITAL) cohort 24) revealed that high use of garlic supplements over 10 years prior to baseline was associated with 45% decreased odds of hematological malignancies compared no or low use. Additional analyses of data from the European Prospective Investigation into Cancer and Nutrition cohort found that after 9 years of follow-up, garlic and onions combined had no effect on cervical carcinoma in situ or on invasive squamous cervical cancer 25).
Onion extracts exhibited potent anti-atherogenic effects (protection against atherosclerosis) that were related to a variety of bioactivities 26). In a study, onion (Allium cepa L.) extracts as well as the bioactive components quercetin and catechin were observed to enhance paraoxonase 1 activity and radical scavenging activity, which in turn prevented LDL “bad” cholesterol oxidation and lipid peroxidation in male Wistar rats subjected to oxidative stress caused by mercuric chloride 27). In another study 28), onion extract was found to lessen atherosclerotic lesions, increase endogenous aortic hydrogen sulfide (H2S) production, and decrease plasma adrenomedulin content, aortic adrenomedulin content, aortic calcitonin receptor-like receptor, and receptor activity-modifying protein 1/2 mRNAs. Additionally, plasma GPx level, SOD activity, plasma endothelial NOS activity, and NO content were increased, while MDA and inflammatory response were reduced by onion extract. In another study, the relationship between onion intake and acute myocardial infarction (heart attack) in Italy was analyzed 29). Compared to the control group, the risks of acute myocardial infarction (heart attack) for the group consuming less than one portion of onion per week and more than one portion per week significantly decreased. All of these effects made onions a potential candidate for anti-atherogenic therapy.
Some experimental studies have suggested that onions have anti-thrombotic (anti blood clots) effects via platelet inhibitory response and inhibiting mitogen-activated protein kinase activation 30). Therefore, onion intake might have a capacity for preventing platelet-mediated cardiovascular diseases 31). In a study, results showed that onion could inhibit thrombosis induced by platelets in dogs 32). It was demonstrated that periodic platelet-mediated thrombus formation followed by embolization caused a reduction in cyclic flow. However, in five dogs, 0.09 ± 0.01 mL/kg onion juice administered intravenously attenuated cyclic flow reductions within 20 min, followed by a 60 ± 14% reduction in collagen-induced ex vivo whole blood platelet aggregation. In addition, in six dogs given 2.0 g/kg onion homogenate intragastrically, cyclic flow reductions were lessened within 2.5–3 hours in five of the dogs, accompanied by a 44 ± 24% reduction in ex vivo aggregation. Moreover, as measured by thrombosis/thrombolysis models in rodents in another study, a variety of onion cultivars exhibited natural anti-thrombotic effects 33). Furthermore, researchers found no significant association between quercetin concentration and anti-thrombotic activity. Interestingly, the anti-thrombotic effects of quercetin-rich onion peel extracts on arteries in rats were stated in another study 34). The onion peel extract markedly reduced blood triglyceride and glucose without affecting blood cholesterol levels. In addition, in vivo arterial thrombosis was significantly abolished in groups fed with 2 mg and 10 mg onion peel extract. Additionally, thrombin-induced expression of tissue factor in human umbilical vein endothelial cells, a coagulation initiator, was greatly diminished by the onion peel extract. Furthermore, extracellular signal-regulated kinase and c-Jun N-terminal kinase signaling pathways activated by thrombin treatment were blocked by pre-treatment with onion peel extract.
Onions were also found to have anti-hypertensive effects (anti high blood pressure) in some other experiments 35). For instance, dietary onion decreased the TBARS in plasma in N(G)-nitro-l-arginine methyl ester —induced-hypertensive rats and stroke-prone spontaneously hypertensive rats 36). In addition, onions improved the nitrate/nitrite (products of nitric oxide) excreted in urine and the NOS activities in the kidneys in stroke-prone spontaneously hypertensive rats, but not in nitro-l-arginine methyl ester- induced-hypertensive rats. These results might in part explain the mechanisms by which onion exerted an anti-hypertensive effect on these hypertensive rats. In addition, the anti-hypertensive effects of onion were observed with different mechanisms. Onion peel extract was demonstrated to concentration-dependently reduce the aorta contractions induced by potassium chloride or phenylephrine. Moreover, the onion peel extract activity could not be attenuated by removing aorta endothelium, or the inhibition of nitric oxide, cGMP and prostaglandin synthesis induced by nitro-l-arginine methyl ester (100 μM), methylene blue (10 μM) and indomethacin (10 μM), respectively. In addition, the relaxation in phenylephrine-precontracted aorta mediated by onion peel extract was not abolished by atropine, which blocked the acetylcholine-induced relaxation. Furthermore, after three weeks’ intervention with onion peel extract, a reduction of blood pressure was observed in the hypertensive rats fed with fructose.
On the downside, onions can be a potent and long-lasting refluxogenic agent in heartburn patients 37). Patients with gastroesophageal reflux (GERD) often describe heartburn after “spicy meals.” One ingredient common to most such meals is onion. Scientists investigated the effects of onion on acid reflux and reflux symptoms in 16 normal subjects and 16 heartburn subjects. Subjects were studied with an esophageal pH probe for 2 hours after the ingestion of a plain hamburger and a glass of ice water. The identical meal, with the addition of a slice of onion, was ingested on another day. Variables measured were number of reflux episodes, percentage of the time pH was less than four, heartburn episodes, and belches. Ingestion of onions did not increase any of the reflux variables measured in normals. However, onions significantly increased all measures in heartburn subjects, compared with the no-onion condition, and compared with normals under the onion condition. Onions can be a potent and long-lasting refluxogenic agent in heartburn patients, therefore if you suffer from gastroesophageal reflux or heartburn its best to avoid onion in your meals.
Much of onion’s health benefits and the majority of studies on onions focus on their sulfur-containing components 38). The characteristic flavors and odors of Allium vegetables (garlic, onions, shallots, leeks, and chives) arise from their sulfur-containing compounds. In fact, sulfur comprises up to 0.5% of the dry weight of onions 39). Sulfur-containing compounds in garlic and onions are largely derived from the precursors γ-glutamyl-S-alk(en)yl-L-cysteines and S-alk(en)yl-L-cysteine sulfoxides (ASCOs) 40). Isoalliin (trans-(+)-S-(propen-1-yl)-L-cysteine sulfoxide) is the predominant ASCO in onions (5, 13) (Figure 1). Propiin 41). The intermediates are highly reactive and rapidly produce thiosulfinate compounds via condensation reactions. In onions, cleavage of isoalliin and other precursor compounds and the subsequent condensation of the sulfenic acid intermediates results in the formation of lachrymatory factor (thiopropanal S-oxide), and in thiosulfonates, bis-sulfines, sulfides including DAS, DADS, and DATS; zweiebelanes, and cepaenes, all of which contribute to the flavor of onions 42).
Temperature, pH, time, processing, and the food matrix can influence the activity of alliinase and the stability of the bioactive compounds. Heating results in denaturation of alliinase, which leads to decreased allicin metabolites in garlic and onions. This decrease in sulfur compounds is associated with a decrease in garlic odor and in a reduction of garlic’s anticancer and antimicrobial potential 43). While multiple sulfur compounds may have potential preventive activities, the compounds must first be generated from the parent compounds in the vegetables. In a study of the effect of processing or cooking on onion thiosulfinate content, cooking by convection oven reduced the total thiosulfinate content of whole onions by >5-fold after 20–30 minutes and microwave heating similarly reduced thiosulfinate content within 0.75 seconds/g onion. Crushed onions retained the same levels of total thiosulfinate compounds as raw onions, even after convection oven or microwave heating 44). With alliinase destroyed from cooking, onions lost the enzymatic ability to form bioactive components, while many thiosulfinates formed prior to cooking remained stable.
Information on the bioavailability and pharmacokinetics of Allium-derived sulfur compounds in humans is limited. By measuring breath acetone and breath allyl methyl sulfide in humans following consumption of various garlic preparations or individual sulfur compounds, Lawson and Wang concluded that allicin and its metabolites are rapidly converted to allyl methyl sulfide after consumption 45). By measuring breath allyl methyl sulfide for 32 hours following consumption of crushed raw garlic and a commercially available garlic powder, Lawson and Wang found that the bioavailability of allyl thiosulfinates in human subjects was found to be similar for the two preparations 46). However, peak breath allyl methyl sulfide concentrations were reached 3.1 hours sooner in subjects consuming raw garlic (2.9 h vs. 6.0 h for raw garlic and garlic powder, respectively). At 32 hours, the breath concentrations of allyl methyl sulfide concentrations were less than 1% of the maximum concentration for both preparations.
Figure 1. Bioactive sulfur compounds in onions, garlic, shallots, leeks, and chives
Note: Bioactive sulfur compounds in Allium vegetables. S-alk(en)yl-L-cysteine sulfoxides (ASCOs) are the precursors to the biologically active compounds in Allium vegetables (garlic, onions, shallots, leeks, and chives). ASCOs are formed form γ-glutamyl-S-alk(en)yl-L-cysteines in the vegetables cells. When the vegetables are crushed, chopped, or chewed, the enzyme alliinase is released from vacuoles and catalyzes the conversion of ASCOs to highly reactive sulfenic acid intermediates. Thiosulfinate compounds are formed from the condensation of these intermediates. Thiosulfinate compounds and their metabolites are responsible for the characteristic flavors and odors of Allium vegetables. Predominant compounds of ASCOs and thiosulfinate compounds/metabolites in garlic and onions are listed.[Source 47)]
Onions, because they are consumed in larger quantities than other Allium vegetables, are a more significant dietary source of carbohydrates, fiber, potassium, iron, and vitamin C (Table 1) 48). Most onion cultivars are about 89% water, 9% carbohydrates (including 4% sugar and 2% dietary fibre), 1% protein, and negligible fat (table). Onions contain low amounts of essential nutrients and have an energy value of 166 kJ (40 Calories) in a 100 g (3.5 oz) amount. Onions contribute savory flavor to dishes without contributing significant caloric content.
Table 1. Onion (raw) nutrition facts
|Nutrient||Unit||Value per 100 g|
|Total lipid (fat)||g||0.1|
|Carbohydrate, by difference||g||9.34|
|Fiber, total dietary||g||1.7|
|Vitamin C, total ascorbic acid||mg||7.4|
|Vitamin B-12, added||µg||0|
|Vitamin A, RAE||µg||0|
|Vitamin A, IU||IU||2|
|Lutein + zeaxanthin||µg||4|
|Vitamin E (alpha-tocopherol)||mg||0.02|
|Vitamin E, added||mg||0|
|Vitamin D (D2 + D3)||µg||0|
|Vitamin K (phylloquinone)||µg||0.4|
|Fatty acids, total saturated||g||0.042|
|Fatty acids, total monounsaturated||g||0.013|
|Fatty acids, total polyunsaturated||g||0.017|
|20:2 n-6 c,c||g||0|
|20:5 n-3 (EPA)||g||0|
|22:5 n-3 (DPA)||g||0|
|22:6 n-3 (DHA)||g||0|
|Fatty acids, total trans||g||0|
|Proanthocyanidin polymers (>10mers)||mg||0|
Table 2. Green onion (tops only – raw) nutrition facts
|Nutrient||Unit||Value per 100 g||stalk 12 g||cup chopped 71 g|
|Total lipid (fat)||g||0.47||0.06||0.33|
|Carbohydrate, by difference||g||5.74||0.69||4.08|
|Fiber, total dietary||g||1.8||0.2||1.3|
|Vitamin C, total ascorbic acid||mg||13.4||1.6||9.5|
|Vitamin B-12, added||µg||0||0||0|
|Vitamin A, RAE||µg||200||24||142|
|Vitamin A, IU||IU||4000||480||2840|
|Lutein + zeaxanthin||µg||858||103||609|
|Vitamin E (alpha-tocopherol)||mg||0.21||0.03||0.15|
|Vitamin E, added||mg||0||0||0|
|Vitamin D (D2 + D3)||µg||0||0||0|
|Vitamin K (phylloquinone)||µg||156.3||18.8||111|
|Fatty acids, total saturated||g||0.165||0.02||0.117|
|Fatty acids, total monounsaturated||g||0.042||0.005||0.03|
|Fatty acids, total polyunsaturated||g||0.121||0.015||0.086|
|18:2 n-6 c,c||g||0.081||0.01||0.058|
|18:2 t not further defined||g||0||0||0|
|18:3 n-3 c,c,c (ALA)||g||0.04||0.005||0.028|
|18:3 n-6 c,c,c||g||0||0||0|
|20:2 n-6 c,c||g||0||0||0|
|20:5 n-3 (EPA)||g||0||0||0|
|22:5 n-3 (DPA)||g||0||0||0|
|22:6 n-3 (DHA)||g||0||0||0|
|Fatty acids, total trans||g||0||0||0|
|Fatty acids, total trans-monoenoic||g||0||0||0|
Table 3. Spring onions or scallions (includes tops and bulb – raw) nutrition facts
|Nutrient||Unit||Value per 100 g||tbsp chopped 6 g|
|Total lipid (fat)||g||0.19||0.01|
|Carbohydrate, by difference||g||7.34||0.44|
|Fiber, total dietary||g||2.6||0.2|
|Vitamin C, total ascorbic acid||mg||18.8||1.1|
|Vitamin B-12, added||µg||0||0|
|Vitamin A, RAE||µg||50||3|
|Vitamin A, IU||IU||997||60|
|Lutein + zeaxanthin||µg||1137||68|
|Vitamin E (alpha-tocopherol)||mg||0.55||0.03|
|Vitamin E, added||mg||0||0|
|Vitamin D (D2 + D3)||µg||0||0|
|Vitamin K (phylloquinone)||µg||207||12.4|
|Fatty acids, total saturated||g||0.032||0.002|
|Fatty acids, total monounsaturated||g||0.027||0.002|
|Fatty acids, total polyunsaturated||g||0.074||0.004|
|20:5 n-3 (EPA)||g||0||0|
|22:5 n-3 (DPA)||g||0||0|
|22:6 n-3 (DHA)||g||0||0|
|Fatty acids, total trans||g||0||0|
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