The myth about onions

Ripe onion on a white background

PETALING JAYA: Do onions absorb bacteria and keep us from getting sick?

A WhatsApp message has been circulating for years claiming that onions had saved a farmer from the influenza outbreak that killed 40 million people in 1919.

While many people would know that this is an outrageous claim, apparently, some think the claims could be true.

Nutritional sciences senior lecturer and Council Member of the Nutrition Society of Malaysia Dr Wong Jyh Eiin answers several myths related to onions.

Do onions absorb bacteria and prevent us from getting colds and flu?

Dr Wong: No, onions do not absorb bacteria. However, onions are rich in nutrients, flavonoids and other phytochemicals which boost body immune function, and thus, have chemoprotective effects on human health.

Would putting unpeeled onions in all rooms help prevent infection since they ‘absorb’ bacteria or viruses?

Dr Wong: No, onions do not absorb bacteria or viruses when left in a room.

 What about putting cut onions beside the bed when one is ill with pneumonia or another infection? Would that help?

Dr Wong: Pneumonia can be caused by many kinds of bacteria or viruses and can be highly contagious.

The risk of contracting pneumonia can be reduced by not smoking, practising good personal hygiene and getting vaccinated.

Putting cut onions beside the bed does not prevent pneumonia or other infections.

For more information, please see http://www.myhealth.gov.my/en/pneumonia/.

The WhatsApp post also says that we should not eat cut onions that have been left out in the open for some time. Would you say that is true? What should be the time limit?

Dr Wong: When cutting raw foods, cross contamination can occur due to poor food handling practices or kitchen utensils (such as cutting board and knives) that are not properly cleaned.

Therefore, all cut foods, including onions, should not be exposed to room temperature for more than two hours. Multiplication of microbes within this duration is high enough to cause food-borne disease.

Are contaminated cut onions more poisonous than other cut raw vegetables?

Dr Wong: All contaminated foods are not safe for consumption.

What are the benefits of onions and how do they benefit us? How should we eat them? Is it better to eat them raw or cooked?

Dr Wong: In general, onions are dietary sources of dietary fibre, vitamin C, vitamin B, and calcium.

Onions also contain flavonoids, namely anthocyanin and quercetin, that have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties.

Onions can be eaten either raw or cooked. When onions are sliced or chopped, they release enzymes (allinases) which break down amino acid sulfoxides to release propanethial-s-oxide.

This unstable volatile gas is rapidly converted to thiosulfonates that contribute to the distinct flavor and pungent smell of raw onions, which are also reported to have a range of anticarcinogenic and antiplatelet properties.

However, thiosulfinates also contribute to heat and burning sensation when onions are eaten raw (also irritation and tearing when chopping).

Cooking or heating onions reduces these sulphur compounds, which reduces its pungency and enables the onion flavours to mellow into something sweet and savoury.

While eating raw onions provides more beneficial sulphur compounds, the pungent odour of raw onions may be less acceptable or tolerable for many.

Depending on individual preferences, eating onions raw or cooked lightly will still provide many health benefits.

Why do onions cause flatulence, and can it be avoided?

Dr Wong: Onions contain fructans such as inulin and fructooligosaccharides, which are indigestible carbohydrates (dietary fibre) that pass through the upper gut.

In the large intestines, these carbohydrates are further fermented by intestinal bacteria which changes the gut microbiota and confers health benefits.

This fermentation process also produces gas released as flatulence.

To avoid flatulence caused by fructans, you can eliminate or restrict foods containing fructans such as wheat, onions and other members of the genus Allium (shallots, scallions, garlic).

Do you have other observations, comment or advice?

Dr Wong: I doubt the reliability of the information “Advantages and disadvantages of Onions” sent through social media.

For instance, I am sceptical that flu viruses were detected in onions when placed under a (light) microscope back in 1919.

This is because flu viruses are very small in size – approximately 80nm to 120nm in diameter – and could only be seen with an electron microscope, which was invented later in the 1930s.

The views Dr Wong expressed above is her personal view and do not reflect the opinions of the organisations’ she represents or where she works at.