Given the desirability of those traits in humans, it might not surprise you to find out that we are but one of the many species expressing carotenoid pigmentation. Indeed, recent work by researchers at the University of St. Andrews' Perception Lab has revealed that even small variations in human carotenoid consumption--mainly in the form of fruits and vegetables--can lead to noticeable differences in skin color; further, these differences are associated with how attractive and healthy other people find us to be.
These findings were the result of a six-week study of 35 volunteers whose fruit/vegetable intake and skin coloration were recorded at three time points (the beginning of the study, 3 weeks in, and 6 weeks in). Coloration was measured using a spectrophotometer, which shines light onto a patch of interest and then records how much of that is reflected and what colors are present in the reflected light; this yields a metric called CIE L*a*b*, which indicates lightness, redness, and yellowness of the patch of skin (or, in this case, 7 different patches distributed across each participant's body).
(Diagram showing how a spectrophotometer works)
On average, volunteers reported eating 3.4 servings of produce per day--or 1.6 fewer servings than the UK-recommended 5 per day. Skin lightness decreased in participants who increased their produce consumption over the course of the study; in other words, these volunteers looked more tan, a pattern that was undoubtedly driven by significant increases in both redness and yellowness. This was only true for CIE L*a*b* measurements across all 7 skin patches, and not just a subset of the three on the face; additionally, these variations were only noticeable across the entire six-week period (e.g., week 0 to week 6) rather than over either of the 3-week sub-periods.
All volunteers who sunbathed, self-tanned, and applied makeup prior to the skin-tone analysis were all dismissed from the study. Still, the researchers wanted to ensure that the patterns observed here really did result from carotenoid consumption. To do this, they examined changes in skin reflectance over the range of wavelengths dominated by carotenoid coloration; experimental patterns were compared to the reflectance curves of three different carotenoids (alpha-carotene, beta-carotene, and lycopene), an average across all of these, and melanin--the pigment responsible for darkening our skin by conferring increasingly brown and black tones. The averaged carotenoid curve closely matched the reflectance curve describing the pattern observed in the study participants; this appears to have been driven primarily by lycopene, the carotenoid found in fruits such as tomatoes and papayas. The observed reflectance data were not, however, similar to the curve for melanin. Cumulatively, these findings indicate that increases in fruit and vegetable consumption--particularly of bright red produce--can result in noticeable changes in our complexions.
(Color spectrum. Carotenoids tend to absorb light most in the 400-540 nm range, but reflect light from 540 nm upwards--which is why they look yellow and red to us.)
In order to investigate the potential social ramifications of this, the researchers digitally manipulated photographs of four study subjects (two of each gender) to obtain 22 images per face. The middle face in the resulting continuum was unmanipulated and the entire spread represented a total dietary change of +/- approximately 5.6 produce portions per day. To determine which complexions were deemed most attractive, the researchers repeatedly exposed volunteers to two versions of the same face, one after the other; volunteers then picked which was yellower, healthier-looking, and more attractive. All evaluations were performed within a computer program that was aware of each photograph's actual color measurements; by keeping track of "wrong" answers, the program was able to calculate volunteers' discrimination thresholds--the point at which they were no longer able to perceive differences between photographs.
Faces that were classified as yellower, healthier, and more attractive were those with positive changes in fruit/veg consumption--in other words, those that reflected (no pun intended) higher carotenoid consumption. A difference of just under 2 fruit/veg portions per day was enough to permit discrimination of more yellow skin, while faces were perceived to be healthier and more attractive at 2.9 and 3.3 more produce portions per day, respectively.
(Tomatoes, which are a great source of the carotenoid lycopene.)
Cumulatively, the researchers emphasize that only a "relatively modest level of dietary change" is required to achieve a yellower, healthier-looking, and more attractive skin tone. Although baseline diet was not found to impact the amount of color change observed over the 6-week study, the authors point out that original skin tone might play an important role; Asian and African individuals, for instance, are already darker than Caucasians and may therefore require larger dietary changes in order to achieve comparably noticeable differences in skin tone. Interestingly, though, a previous study from the same lab found that people of all ethnicities found yellower skin healthier-looking and more attractive, regardless of the ethnicity of the person at whom they were looking. Thus, people of all ethnic backgrounds appear likely to gain similar social benefits--assuming, that is, that looking more attractive is beneficial--from eating more produce.
Eventually, the researchers hope that their results can be used to make healthier diets more appealing. Before they start any big marketing campaigns, though, there are some additional questions they'd like to answer. For instance, do people reach a "saturation point" at which increased carotenoid consumption no longer has positive chromatic benefits? How, exactly, do carotenoids change skin color--is it purely their activity as pigments, or might the patterns also result from interactions between carotenoids and polyphenols, leading to improved blood flow? Finally, will everyone be equally aware of these diet-induced color changes? The average age of volunteers in this study was 18.9, but color vision becomes less acute as people age, suggesting that larger carotenoid fluxes may be required to catch the attention of older viewers. Whatever the answers to these questions, one thing is clear: Our parents were right to admonish us to eat our veggies.
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Whitehead, R.D., Re, D., Xiao, D., Ozakinci, G., and Perrett, D.I. 2012. You are what you eat: within-subject increases in fruit and vegetable consumption confer beneficial skin-color changes. PLoS ONE 7(3):e32988.
Thanks to the following webpages for providing the images used in this post:
http://www.carotenoidsociety.org/
http://www.oilgae.com/ref/glos/spectrophotometer.html
http://cfialabioassignment.wikispaces.com/Identify+the+limited+range+of+wavelengths+of+the+electromagnetic+spectrum+detected+by+humans+
and+compare+this+range+with+those+of+other+vertebrates+and+invertebrates
http://www.fhsblog.com/?p=193
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