One of my favourite restaurants in Aberdeen is an Indian place close to where I used to live. At first, it does not even look like a restaurant. It is a single wooden door which is slightly broken and torn from years, maybe decades, of use, against a wall painted in bright blue, which is now a mixture of baby blue and green from the years of bad weather. As you walked through the door, there is just enough space to place the order. Take-away only, and you have to wait outside no matter what the weather is. Once I waited 10 minutes through a snowstorm – that's how good their food is. Trust me, it is exceptional!

I am food lover and curry is one of my go-to take-away orders. Depending on the curry you order, you might be eating a lot of good stuff, including some ingredients considered as super foods. This is not limited to curry, of course.  Many dishes around the world use a wide range of ingredients that can be classified as "super". Take turmeric, for example, which is a spicy renowned for its antioxidant properties and super food status, is perhaps one of the most common super foods worldwide. 

Many studies have shown the (potential) health benefits from turmeric and its main compound, Curcumin, from blood sugar levels to Alzheimer disease (e.g. [1]). Its status as super food is such that supplements are widely available and were found to modulate important aspects of human physiology, including the gut microbiome profile [2]. 

Because of its health benefits, I was happy to see that turmeric and another super food, spirulina, were the two components present in some natural food dyes, widely used in culinary. And while trying to develop an assay to measure physiological responses to diet, I decided that such natural food dyes were the perfect candidates for the job at hand. Little I know that super foods can also be super toxic. 

Using Drosophila melanogaster, we tested the effects of different food dyes on female oviposition and larval development. This was the first experiment, which I conducted as a preliminary data collection step to show that the food dyes were not resulting in major effects on the flies and that I could proceed to develop my assay. To our surprise though, the data showed that some dyes, such as green and blue, were extremely toxic to larval development, even if they did not change female oviposition behavior (Fig 1). 

Puzzled,  we decided to test the basic compounds that made up the green and blue dyes: spirulina and turmeric. But they are both considered super foods. We did not think much of it, and thought that the results would prove largely contradictory to our findings. However, we were surprised to find exactly what we were seeing in the food dyes. 

Turmeric was toxic to the developing larvae. Importantly, we could recapitulate the developmental delays of both green and blue diets at varying concentrations of turmeric, which we discovered using a dose-dependent response curve (Fig 2). 

Looking through the literature, we found that food dyes have been used in nutritional experiments in the laboratory in insects, which is surprising because literature also suggests that turmeric is used as a natural insecticide (e.g. [3]). Therefore, it is possible that food dyes used in laboratory experiments compromise the overall results. Interestingly though, our results confirmed the huge potential for turmeric to be used as natural pesticides, given that toxicity effects within the range of 2.5%-10% turmeric (w/v) are likely toxic to most insects but harmless to vertebrates including mammals and humans. 

Our study, published in the Journal of Insect Physiology [4], highlights new avenues for the potential use of turmeric as natural pesticides. Of course, the evidence is presented for a model species in a laboratory setting. Other studies in insect that are pests of agriculture or disease vectors, in large scale settings, must be conducted. But our results are promising, and can unleash new ways to control insect pest outbreaks in a cost-effective and environmental-friendly way. We are now working to understand how far and wide the toxic effects of turmeric are in other insect species.

More is coming, watch this space.

References

1. Singletary, K. (2010). Turmeric: an overview of potential health benefits. Nutrition Today, 45(5), 216-225.

2. Peterson, C. T., Vaughn, A. R., Sharma, V., Chopra, D., Mills, P. J., Peterson, S. N., & Sivamani, R. K. (2018). Effects of turmeric and curcumin dietary supplementation on human gut microbiota: A double-blind, randomized, placebo-controlled pilot study.

3. de Souza Tavares, W., Akhtar, Y., Gonçalves, G. L. P., Zanuncio, J. C., & Isman, M. B. (2016). Turmeric powder and its derivatives from Curcuma longa rhizomes: insecticidal effects on cabbage looper and the role of synergists. Scientific reports, 6(1), 34093.

4. von Hellfeld, R., Christie, C., Derous, D., & Morimoto, J.  (In press). Super food or super toxic? Turmeric and spirulina as culprits for the toxic effects of food dyes in Drosophila. Journal of Insect Physiology, 104600.