What onion preference reveals about the risk of diabetes and hypertension, according to a study

A study conducted by an international team of researchers suggests that a preference for the smell and taste of onions may be associated with a lower risk of type 2 diabetes and high blood pressure. The findings, published by Science Alert, are based on an unusual method: analysing the genes responsible for taste and smell.

Genetic epidemiologist Daniel Hwang, from the University of Queensland, Australia, explains that genes linked to taste and smell can be useful tools for studying the connections between diet and disease, contributing to stronger evidence regarding causal relationships in nutritional research. "We need better ways of understanding how diet influences diseases such as diabetes, heart disease and cancer," he states.

The researchers drew on data collected from more than 160,000 individuals aged between 37 and 73, from a UK health research database containing both genetic information and participants' dietary preferences.

The analysis identified hundreds of associations linked to 96 food preferences, including genetic variants for preferences for garlic, grapefruit, onion, horseradish or wasabi, broad beans, and adding salt to food. After examining 325 genes involved in taste and smell and 140 different foods, one association stood out in particular: that between a preference for onions and a variant of the olfactory receptor gene OR2T6.

This association was subsequently confirmed using a smaller database comprising younger individuals aged 25 and above, demonstrating that the genetic variant indicates a preference for onions regardless of age. This is significant because genes are fixed from birth and are not subsequently influenced by lifestyle or environmental factors — unlike dietary habits, which can change, for example, following a diagnosis of diabetes.

Building on this genetic link, the researchers separately analysed other genetic datasets and correlated the OR2T6 variant with health outcomes, finding a reduced risk of both high blood pressure and type 2 diabetes. The method used, known as Mendelian randomisation, involves using fixed genetic variants as indicators, rather than self-reported dietary data, which can be unreliable and may change over time.

Hwang notes that nutritional research based on Mendelian randomisation is expanding, with studies already clarifying the causal effects of coffee, alcohol, and milk consumption. Nevertheless, he cautions that analyses of this kind remain challenging due to difficulties in identifying genetic markers that accurately reflect what people actually eat.

For the time being, the link between a preference for onions and these favourable health outcomes does not demonstrate a direct cause-and-effect relationship. The researchers emphasise that the findings must be confirmed in larger and more diverse groups before any clinical conclusions can be drawn. However, they believe the bioactive compounds found in onions, which may explain this association, are worth investigating further.

The fact that a single food emerged so clearly from the analysis is regarded by the researchers as a sign that the method is sufficiently rigorous to highlight only genuine associations.

The study also notes that unhealthy diets are responsible, globally, for approximately 11 million premature deaths each year, owing to excessive consumption of sugary drinks or insufficient intake of fruit and vegetables.

"Establishing whether a particular food causes a disease or is merely associated with it represents a significant challenge in nutritional epidemiology," Hwang adds. "We have developed a framework based on taste and smell genes to help scientists better understand how diet contributes to the onset of chronic disease."