Carotenoids are yellow, orange, and red pigments synthesized by plants and algae. They are responsible for the colours in fruits and vegetables, like the redness in tomatoes. When consumed, these pigments are used by many animals to produce brightly coloured displays. Examples range from the orange patches on guppies, the pink feathers of flamingos, the yellow-orange underside of common lizards, to the orange exterior of ladybird beetles.
In many species, carotenoid colouration is used by females to select their mating partners. Early studies on guppies for example, showed that females preferred to mate with males with bright carotenoid colouration. Similar results have since been found in other fishes and various birds and reptiles.
Evolutionary theories suggest that carotenoid colouration is attractive because it signals good health. Besides influencing colour, carotenoids are thought to support health as antioxidants. Carotenoids cannot be synthesized in the body, and can only be obtained through carotenoid rich foods. Healthier individuals require less carotenoids to support health. Therefore, they can devote more carotenoids to colouration, thus signalling their superior health. However, the hypothesis that carotenoids support health has received mixed support in animal studies. While some studies found that carotenoids reduce oxidative stress, enhance immune function, and increase male reproductive function, others have found null results.
Recent studies suggest that carotenoid colouration may also function as an attractive signal of health in humans. It has been shown that individuals prefer faces that are yellower and redder. To examine the causal effect of carotenoids on facial appearance and health in humans, we conducted an experimental dietary supplementation study. Men were given a 12-week dose of either the carotenoid beta-carotene or placebo. We took photographs of each man’s face before and after supplementation. Skin colour was measured from the photographs. A separate group of women was then asked to select the more attractive face out of the before and after photos for each man. We also took a number of health measures, including oxidative stress, immune function, and semen quality prior to, and after the 12-week study.
So did beta-carotene positively affect the health and appearance of the men in the study? Our results support the idea that carotenoid colouration affects human attractiveness. Beta-carotene consumption enhanced the yellowness and redness in the men’s faces and when women were asked to rate faces for attractiveness they found those of carotene supplemented men more attractive.
While beta-carotene seemed to have made the men look more attractive, it did not have an effect on three measures of health; immune function, oxidative stress, and semen quality. The null health findings suggest that carotenoid colour might not be a signal of health in humans. However, it is worth noting that the male participants in the study were all relatively healthy and young individuals. Therefore, it is possible that they did not need additional carotenoids to support their health and were able to use all of the supplemented beta-carotene to enhance skin colour. Also, our study focused only on one specific carotenoid: beta-carotene. It is possible that beta-carotene affects health only when it is coupled with other nutrients.
Overall, our results suggest that a healthy diet of carotenoid rich fruit and veggies can improve our attractiveness by enhancing our skin colour. But how much fruits and veggies do we have to eat? A previous study by Whitehead and colleagues using psychophysical methods to determine the minimum amount of fruits-and-vegetables associated colour change required to make our skin look more attractive estimated that we would need to eat three extra portions of fruits and veggies a day. In short, time to start eating your fruits and veggies.
Featured image credit: Fruits Vegetables by Domokus. CC0 Public Domain via Pixabay.