Cranberry sauce is perhaps a non-obvious star of the Thanksgiving dinner table. With its rich red color – whether homemade or from the can – the holiday favorite is actually part of the hardwiring in our brain: A new study finds that people favor red-colored foods over green ones, and consistently undervalue the caloric content of green foods over red ones. The overall effect means that most people are likely to be drawn to cranberry sauce over green beans this Thursday.
“The cranberry sauce should be more arousing,” says Raffaella Ida Rumiati of SISSA in Italy, senior author on a new paper in Nature Scientific Reports. “Arousal predicts wanting and mediates preparatory behavior,” she says. This is owed to “trichromatism” in people and other primates that allows us to visually evaluate food for nutritive and other properties based on color. The researchers hypothesized that the effect of color on visual evaluation of food comes from the relationship between energy-content and color food found in nature.
In the study, Rumiati, with Francesco Foroni and Giulio Pergola, asked healthy participants to rate how arousing they perceived a large set of food and non-food items. The researchers then independently estimated the calorie content of food stimuli and asked participants to rate the perceived calorie content. They also asked participants rate the level of transformation of different food images; the non-food images were a control.
They found that the color content of food specifically, and not non-food, predicts arousal and perceived calorie content. The brighter the red, the higher the arousal; and the brighter the green, the less the perceived calorie content. And this result held for even non-natural, “transformed” food – meaning you could evaluate green pesto, for example, as having fewer calories than a red marinara sauce.
CNS spoke with Rumiati about these results, their evolutionary links, and what it means for all of us this Thanksgiving and beyond into the holiday season.
CNS: How did you become personally interested in color and food evaluation?
Rumiati: I was invited to join an interdisciplinary research program on food for which my school received some money. I thus had the opportunity to build a small but very keen research group with Dr. Foroni, Dr. Pergola, and some Ph.D. students. Food soon became central to my interests because it cuts across several domains. Color popped out as an interesting aspect of food to investigate right from the beginning as we were collecting our database of stimuli to use in our experiments.
CNS: How do you define trichromatism?
Rumiati: Trichromacy is a feature of the visual system that allows us to ‘color’ the surrounding environment. Humans express three different receptors that feature different sensitivity to specific wavelengths, i.e. colors. Other primates, such as the lemurs, only have two of them. Thus, trichromacy is an attribute of species with three photoreceptors.
CNS: Why is trichromatic color important evolutionarily?
Rumiati: Trichromatic color vision is suggested to have evolved to improve foraging by distinguishing between red and green nuances. It has been hypothesized that this peculiarity of our visual system has represented an evolutionary advantage because more reddish nuances in fruits and leaves are generally associated with higher energy or greater protein content.
CNS: What have we known previously the relationship between trichromatic color and food evaluation?
Rumiati: Previous evidence showed that trichromatic primates are better off than dichromates in judging ripeness of fruits and edibility of leaves. What was missing is whether trichromatism is also associated with visual evaluation of nutritional and appetitive properties of food in humans.
CNS: What was the new insight you were seeking?
Rumiati: We aimed to ascertain whether color plays a role also in human food evaluation and more, whether this heuristic is generalized to all foods. The second part of out goal is very important, because nowadays it is possible to have, for instance, a green-colored cake that is much more energy dense than a strawberry – hence, this heuristic is no longer valid.
CNS: What were your most excited or surprised to find?
Rumiati: The most surprising finding in our view is that the effect emerges in spite of correcting for a number of possibly confounding variables, including other basic visual features of food, the level of food processing, the estimated calorie content, and even participants’ physiological state, such as hunger. It is a relatively small, but highly generalized effect, and this is what one would expect from a hard-wired heuristic. Moreover, the effect is only significant for food and there is no effect of general brightness (e.g., blue light does not matter). We were surprised to find a clean effect, when factoring out all confounders we had measured, particularly given the complexity of food evaluation.
“our participants appeared to systematically underestimate the energy content of greener food items, which is relevant for people observing dietary restrictions.”
CNS: What do you most want people to understand about this work? Does it change how we should think about food and our choices, especially when it comes to artificially colored food?
Rumiati: The main point is that because of the hardwired association between greenish nuances and lower energy content we tend to value less the food with higher green-color content. This has been a strong advantage when food was scarce and we needed to search for it. Nowadays this mechanism can backfire due to the abundance of food available and to the fact that our brain is generally more attracted to any food (natural or transformed) that has high reddish-nuances (as indexed by arousal assessments in our experiment). While for natural food this is a heuristic, for transformed food it can become a bias. Importantly, our participants appeared to systematically underestimate the energy content of greener food items, which is relevant for people observing dietary restrictions.
CNS: What’s next for this line of work?
Rumiati: In the next study, that should hopefully come out soon, and in which we investigate the categorical distinction between transformed and natural food, we are concerned with the influence not only of the intrinsic characteristics of the food on how our brain processes it, but also on characteristics intrinsic to the perceiver, such as the BMI. Food neuroscience is also becoming very useful in making sense of anomalies observed in food behaviours of patients with different neurological conditions including Parkinson Disease and primary dementia. Stay tuned!
CNS: Anything I didn’t ask you about that you’d like to add?
Rumiati: We need to understand better how food selection is mediated by brain mechanisms, and how this process changes along the lifespan. Understanding these mechanisms should help us in making better decisions in human nutrition.
-Lisa M.P. Munoz
