Onion: nutrition facts and health benefits

Onion

The onion is a bulbous vegetable plant which can be fresh (new onion) or dry (coated with a thin skin). It consists of white or pink leaves, fleshy and juicy, superimposed on each other. There are several varieties of dry onion: white onion, straw onion, red onion.

History of onion

Although the wild ancestor of the onion was not found, its first domestication centre could be the southwest Asia. This is certainly one of the most ancient cultivated vegetables. It is mentioned in texts of ancient Egypt dating back more than 4 000 years, as well as in the Bible where it is reported that during their exodus (1 500 years before our era), the Hebrews mourned his absence, as well as garlic and leek. In Greece and Rome, many varieties were already being cultivated. The Romans even devoted particular gardens to the onion, the Cepinae.

 

Nevertheless, even if some varieties were already cultivated in the 9th century, the onions will be really popular in Europe only in the Middle Ages. It will be one of the first European plants to be cultivated in America, first in the Caribbean, where Christopher Columbus introduces it. In the seventeenth century, it was established in the north of the United States as well as in Canada, where it was cultivated both by the settlers and by the Amerindians. The Europeans will introduce it to the east of Asia in the nineteenth century, although in these regions people still prefer to consume the many native species that are related to it. Today, it is produced in all temperate regions of the globe and tests to implant it in the semi-arid regions are underway.

Onion health profile

The onion is a universal aroma, consumed in all parts of the world. There are several varieties, some of which are particularly rich in antioxidants. The onion is part of the large family of alliaceous and, like garlic, it is attributed some beneficial properties to health.

Active principles and properties

Several prospective and epidemiological studies have shown high consumption of vegetables and fruit decreases the risk of cardiovascular diseases, certain cancers and other chronic diseases. More specifically, studies indicate that the consumption of vegetables from the family of alliums (onion, garlic, shallot, chives, green onion, leek) could have a protective effect against the cancers of the stomach and the intestine. To date, data are insufficient to link with other types of cancers (such as prostate, breast, esophagus, and lung cancers).

Cancer. Some epidemiological studies argue there is a link between the consumption of onion and the decrease in the incidence of different types of cancers. First, a synthesis of case-control studies carried out in Italy and Switzerland reveals the consumption of one to seven servings of onion per week decreases the risk of colon cancer, larynx and ovaries cancers. Added to this is a lower risk of esophageal cancer, oral cavity and pharynx with seven servings and more onion per week. The same trends are observed for cancers of the brain, stomach and esophagus (studies carried out in China).

Researchers in the Netherlands report an inverse relationship between onion consumption and the incidence of stomach cancer. Finally, prostate cancer mortality would be reduced by a high intake of onions. 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 ways of onion cooking, as well as the precise quantities consumed. In addition, some studies have failed to significantly demonstrate such protective effects against cancer.

The onion could act at different stages of cancer development. Indeed, studies show that onion extracts can inhibit mutation processes that trigger cancer. They also decrease the proliferation of cancer cells. These results come from in vitro studies and among animals. The compounds concerned and the specific mechanisms of action are better known and the research continues.

Cardiovascular disease. The onion contains compounds acting on different cardiovascular risk factors. The majority of studies on the subject were carried out in vitro or among animals, except for some preliminary studies among humans. The onion is known for its ability to decrease platelet aggregation in vitro, which is however 13 times lower than garlic. It should be noted platelet aggregation in the blood increases the risk of thrombosis and, therefore, cardiovascular disease. A recent study revealed the addition of raw onion to swine feeding for six weeks did not alter platelet aggregation, but significantly decreased their blood triglyceride levels. It should be noted the quantities used in these studies are equivalent to a daily consumption of one half to one and a half onion among humans.

A preliminary study among humans indicated the consumption of about three medium onions (500 g) in a soup decreased the ex vivo platelet aggregation (test carried out using the blood taken from the subjects). Still among humans, the daily consumption of about 220 g of onion cooked for two weeks did not allow to observe any positive effects on platelet aggregation. Some studies revealed the onions had less antiplatelet activity after cooking. The antiplatelet activity is due in part to the sulphur compounds and flavonoids (quercetin) of the onion. These two compounds could act synergistically. The specific modes of action still remain to be determined.

Antioxidants. Antioxidants are compounds protecting the body’s cells from damage caused by free radicals. The latter are highly reactive molecules that would be involved in the development of cardiovascular diseases, certain cancers and other diseases related to aging. The main classes of antioxidants in onion are anthocyanins and flavonols (specifically quercetin). The anthocyanins give the red color to certain onion varieties, and the flavonols color to the yellow onions. It should be noted that these antioxidant compounds are mainly lodged in the outer layers of onions. White onions contain few antioxidants compared with those of yellow and red color. In addition, the red onion varieties generally have antioxidant content and antioxidant activity above the pale-coloured onion varieties. A study at Cornell University in New York State indicates that the western yellow, New York bold, and northern red-type onions have antioxidant activity up to eight times higher than other varieties.

With tea and apple, the onion, especially yellow onion, is a major source of quercetin. The quercetin of the onion would be absorbed in appreciable quantity in the body, resulting in an increase in the antioxidant activity in the blood. This flavonoid and other onion antioxidant compounds may help to reduce the incidence of certain cancers. In addition, it is now well demonstrated that flavonoids, including quercetin, have a protective action against the oxidation of LDL-cholesterol (“bad” cholesterol). Thus, a high consumption of flavonols and flavones from the diet is associated with a lower risk of coronary heart disease.

Sulfur compounds. These substances are named because they contain one or more sulfur atoms in their chemical structure. As with garlic, the sulfur compounds form when the onion is cut. At this time, alliin (an inactive and odorless molecule of the onion) comes into contact with an enzyme and is transformed into precursors of the odour, flavour and lacrimal properties of the onion. A series of reactions ensue, whose final products are a complex mixture of sulfur compounds contained in the onion. Some of these compounds would limit the multiplication of cancerous cells, in addition to playing a role in the antiplatelet activity attributed to the onion.

Saponins. These substances have the ability to decrease blood cholesterol among animals and in vitro blood coagulation, two effects sought to better prevent cardiovascular disease. The role of these compounds is however little known among humans.

Selenium containing compounds. Like broccoli and garlic, onions have the ability to accumulate selenium from the soil, leading to the formation of selenium-containing compounds. Although it is difficult to quantify their benefits, these substances could contribute to the protective effect of the onion against cancer. The research is continuing in that direction.

Flavor and smell of onions

The pungent flavour and the strong smell of onions vary in intensity, depending on the variety. To a lesser extent, the environmental factors surrounding the onion crop (such as soil quality) also have an impact. Finally, the flavor and smell of onions become less strong plus the weight of the onion increases. Thus, the larger onions would be less pungent and less fragrant than the small ones.

Precautions

Irritable bowel syndrome

Irritable bowel syndrome is characterized by various disorders of the digestive system, including abdominal pain, flatulence and changes in defecation habits. This disorder may also be manifested by gastro-esophageal reflux or dyspepsia. Some people who are struggling with this syndrome may feel intolerant to various foods. Fermentable foods, such as onions, garlic and other vegetables from the alliaceous family are also implicated. The simple fact of limiting or avoiding their consumption often suffices to alleviate the symptoms. When symptoms are mild, or during periods of “remission”, it is sometimes possible to gradually reintegrate these foods, always respecting individual tolerance.

Organic gardening

 

For planting, it is always better to use young seedlings to transplant rather than seeds that are more susceptible to disease and are offered only in a few varieties. In contrast to the young plants, the choice is much greater, especially if you sow them yourself inside or in the greenhouse.

Its roots are superficial, the onion needs regular irrigation throughout the season. However, irrigation will be discontinued two or three weeks before harvest to allow the leaves and bulb to dry out, thereby promoting good conservation.

Avoid late applications of nitrogen fertilizers (e.g. surface applied compost), at the risk of delaying the maturation of the bulbs and causing a resumption of foliar growth. In this case, the collar of the bulbs thickens, which is not conducive to conservation, and there is formation of double or multiple bulbs.

Mulch the rows or the platform in order to keep the moisture and prevent the emergence of the weeds, because with its narrow leaves, the onion is poorly protected. If you choose to weed out the hoe, be careful not to damage the superficial roots.

By growing onions and carrots in alternate rows, the onion and carrot will be protected from flies.

Before storing the onions for the winter, let them dry a good week in a well ventilated area. Then keep them dry, in net bags.

Ecology and environment

The experts recognize the massive introduction of hybrid varieties in recent decades, adaptable to extremely varied climates, has led to a high genetic erosion of the Allium Cepa species. This resulted in a loss of organoleptic qualities which makes the reputation of many local varieties. In addition, hybrid varieties remain the property of large international companies selling them because they keep the secret of their filiation, forcing producers to buy them with new seeds each year.

In contrast, traditional local varieties belong to the consuming public and to producers who preserve their characteristics through ancestral know-how. They are the ones who multiply them by using seeds they harvest from their best plants and who cultivate them taking into account the geoclimatic conditions specific to their region. It is a fact people are just beginning to recognise: plant biodiversity is always accompanied by a diversity of flavours and culinary uses.

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