Food and diet

Onion study strengthens ‘5 a day’ message

Onions “cut heart disease risk” according to a report on the BBC website today. The article reports that some of the chemicals formed in the breakdown of quercetin, a compound found in tea, onions, apples and red wine, can reduce the inflammation of blood vessels that can lead to artery thickening (atherosclerosis), and eventually to heart disease.

This story is based on laboratory experiments looking at the effects of quercetin and the chemicals formed in its breakdown, on certain genes and proteins in the cells that line blood vessels. Although this study is interesting from a biological perspective, it may not be representative of what happens in the human body when quercetin-containing foods such as onions are eaten. Regardless of our understanding of the biological processes underlying how fruits and vegetables provide their benefits, the overall message that a diet rich in fruits and vegetables is best for your health still applies.

Where did the story come from?

Dr Sandra Tribolo with colleagues from the Institute of Food Research in Norwich and from universities in Madrid and Nottingham, carried out this research. The study was funded by the UK Biological Sciences Research Council. The study was published in the peer-reviewed medical journal Atherosclerosis.

What kind of scientific study was this?

This was a laboratory study into the effects of the flavonoid quercetin, and the chemicals that form in the body when quercetin is broken down (its metabolites), on the cells and processes involved in inflammation in blood vessels.

The researchers grew cells from the inside surface of human umbilical veins in the laboratory. Some of the cells had quercetin and three of its metabolites added to them at concentrations similar to those that might be seen in the body after eating quercetin-containing foods. Other cells were left untreated to be used as controls. After incubating the cells with those compounds they then added other chemicals which would provoke an inflammatory response. They then looked at how the presence or absence of quercetin and its metabolites affected the activity of three genes that produce proteins involved in inflammation, and the concentration of these proteins on the surface of the cells (which is where these proteins are usually found).

What were the results of the study?

Researchers found that quercetin reduced the activity of the three genes involved in producing the inflammatory response. The three metabolites of quercetin generally had less effect on the activity of these genes than quercetin itself, but they all reduced the levels of the protein found on the surface of the cells and produced by one of these genes.

What interpretations did the researchers draw from these results?

Researchers concluded that, at concentrations that might be seen in the body, both quercetin and its metabolites reduce the activity of important molecules involved in the early stages of atherosclerosis.

What does the NHS Knowledge Service make of this study?

This study does provide us with information about the possible effects of quercetin and its metabolites on the cells that line blood vessels when they are grown in the laboratory. However, the human body as a whole is obviously much more complex, and we cannot say for certain whether exactly the same processes reported here actually occur in a living human, or what effect, if any, they would have on the risk of atherosclerosis.

Overall, the message that a diet rich in fruits and vegetables is best for your health still applies, even if we don’t yet fully understand how it provides its benefits.

Sir Muir Gray adds...

Interesting, but I still won’t eat onions and will get my five a day in other ways. I don’t like onions, or garlic, and although I know garlic may be good for me, onions and their cousin the garlic don’t like me.

NHS Attribution