A Study in Purple: an Emerging Symbol for Healthy Food

By Michael Lim

March 16, 2018

A bunch of purple carrots

Purple has long been associated with royalty and wealth. Now it may also be a symbol of healthier food choices, says a research team led by Prof. Kelly Meckling in the Department of Human Health and Nutrition Sciences.

Purple varieties of foods such as carrots and potatoes get their colour from anthocyanins. Anthocyanins are plant pigments that have been linked to health benefits such as reduced risk of cardiovascular disease (CVD) and type II diabetes. These purple vegetables are not new; in fact, purple carrots and potatoes are the heirloom versions of the ‘common’ orange carrots and white potatoes that we consume today.

 “People often tell me that they see these purple vegetables and think they must be some genetically modified monster food,” said Meckling. “I’m hoping this research can help dispel some of those misconceptions.”

The anthocyanins in purple vegetables are thought to work their magic by altering gene expression. Changing the expression of genes associated with processes such as lipid metabolism (the breakdown of fat) can lead to reduced incidences of insulin resistance, a condition that is associated with the onset of diabetes. A few previous studies have examined the effects of anthocyanins in mammals, but it was unclear if substituting ‘common’ vegetables with their purple counterparts, could improve insulin resistance and high blood pressure.

To address this, Meckling and PhD student Hala Ayoub devised an experiment that involved raising adolescent rats on one of five diets. A control group was fed sucrose as its main source of carbohydrates, while four other groups were fed either purple carrots, orange carrots, purple potatoes, or white potatoes. Rats were then tested for a variety of factors associated with CVD and diabetes.

Excitingly, the study revealed several health benefits when rats were switched to a diet of purple vegetables compared to their less colourful counterparts. Rats fed purple carrots, for example, had lower ventricular pressure in the heart, which meant their heart was not working as hard, and an overall reduction in blood pressure.  Purple potatoes, meanwhile, led to an increase in glucose tolerance and higher insulin sensitivity – both indicators of a reduced risk of diabetes. While rats on a ‘common’ vegetable diet had improved health compared to those from a high sucrose diet, purple vegetables had health benefits above and beyond their lighter coloured brethren.

While you might be tempted to run to your nearest Farmer’s Market and pick up some purple vegetables, this study, and most before it, processed the vegetables in some way, such as via juicing or freeze-drying. Meckling notes that these processes may alter the ability for the body to absorb and utilize anthocyanins. In addition, the health benefits observed in rats may not necessarily occur in humans. To find out, Meckling and her team are examining the effects of whole purple vegetables on human test subjects; primary results suggest reduced fat and blood pressure levels. In the meantime, Meckling’s advice is that there’s no harm in incorporating more vegetables into your diet, purple or not.

“Carbohydrates like potatoes often have a bad reputation,” said Meckling. “I want people to realize that there are benefits of eating vegetables, even common potatoes, depending on how they’re prepared.”


Other contributors to the study include University of Guelph collaborators Prof. Jeremy Simpson, Mathew Platt (Human Health and Nutritional Sciences), Prof. Mary Ruth MacDonald and Prof. Alan Sullivan (Plant Agriculture), and Dr. Rong Tsao from Agriculture and Agri-Food Canada. Funding for this research came from the Ontario Ministry of Agriculture, Food and Rural Affairs. 


Read the full article in the Journal of Medicinal Foods.

Read about other CBS Research Highlights.